Interaction of Slow Electrons with Molecular Solids and Biomolecules

October 16-18, 2003

Organizers: Ilya Fabrikant and Leon Sanche

 Home

 Workshops

Participants

Online Talks

Abstracts

  Program

Azria

Bass

Bowen

Burrow

Compton

Dupuis

Fabrikant

Gay

Gianturco

Gutowski

Hunting

Huo/Babb

Illenberger

Inokuti

Le Sech

Lu

McCurdy

 Naaman

Orlando 

Rescigno 

 Sanche

Sevilla

 Swiderek

  Teillet-Billy

 Online registration for invited participants:

http://cfa-www.harvard.edu/cgi-bin/conf_reg/reg.pl?conf_id=ITAMPOCT03

Workshop Participants

Dr. Roger Azria
Université Paris-Sud
Bt 351
Orsay,91405 France
azria@lcam.u-psud.fr
 
Dr. Andrew D. Bass
Faculty of Medicine
Univ of Sherbrooke
Sherbrooke, J1H 5N4 Canada
Andrew.Bass@Usherbrooke.ca
 
Prof. Kit H. Bowen
Department of Chemistry
Johns Hopkins Univ
Baltimore, MD 21218
kbowen@jhu.edu
 
Prof. Paul D. Burrow
Department of Physics and Astronomy
Univ of Nebraska--Lincoln
Lincoln, NE 68588-0111
pburrow1@unl.edu
 
Prof. Robert N. Compton
Department of Chemistry
The University of Tennessee
Knoxville, TN 37996
rcompton@utk.edu
 
Dr. Michel Dupuis
Pacific Northwest National Laboratory
P.O. Box 999 MS: K1-83
Richland, WA 99352
michel.dupuis@pnl.gov
 
Prof. Ilya Fabrikant
Department of Physics and Astronomy
University of Nebraska-Lincoln
Lincoln, NE 68588-0111
ifabrikant1@unl.edu
 
Prof. Tim Gay
Behlen Laboratory of Physics
University of Nebraska-Lincoln
Lincoln, Nebraska 68588-0111
tgay1@unl.edu
 
Prof. Franco A. Gianturco
Dipartimento di Chimica
Università di Roma "La Sapienza"
Città Universitaria, 00185 Rome, Italy
fa.gianturco@caspur.it
 
Dr. Maciej Gutowski
Battelle for the US DOE
P.O. Box 999
Richland, WA 99352
maciej.gutowski@pnl.gov
 
Prof. Darel J. Hunting
Dept. Nuclear Medicine Radiobiology, Faculty of Medicine
University of Sherbrooke
3001,12th Avenue North
Sherbrooke, PQ, Canada J1H 5N4
Darel.Hunting@USherbrooke.ca
 
Dr. Winifred Huo
NASA Ames Research Center
Mail Stop T27B-1
Moffett Field, CA 04035-1000
whuo@mail.arc.nasa.gov
 
Prof. Eugen Illenberger
Institut fuer Chemie
Freie Universitaet Berlin
Takustrasse 3
D-14195 Berlin, Germany
iln@chemie.fu-berlin.de
 
Dr. Mitio Inokuti
Argonne National Laboratory
Argonne, IL 60439
inokuti@anl.gov
 
Dr. Cornelius E. Klots
Oak Ridge National Laboratory
1028 West Outer Drive
Oak Ridge, TN 37830
Klotsce@post.harvard.edu
 
Prof. Claude Le Sech
Univ Paris-Sud
LCAM Bat 351
91405 Orsay Cedex, France
lesech@lcam.u-psud.fr
 
Dr. Qing-Bin Lu
California Institute of Technology
Arthur Amos Noyes Laboratory for Chemical Physics
Mail Code 127-72
Pasadena, CA 91125
qblu@caltech.edu
 
Dr. William McCurdy
Lawrence Berkeley National Laboratory
One Cyclotron Road, MS-50B-4230
Berkeley, CA 94720
cwmccurdy@lbl.gov
 
Prof. Ron Naaman
Department of Chemical Physics
Weizmann Institute
Herzel St.
Rehovot, Israel 76100
ron.naaman@weizmann.ac.il
 
Prof. Thomas M. Orlando
Georgia Institute of Technology
School of Chemistry and Biochemistry
770 State Street
Atlanta, GA 30332-0400
Thomas.Orlando@chemistry.gatech.edu
 
Dr. Thomas N. Rescigno
Lawrence Berkeley National Lab
1 Cyclotron Road, MS-50F
Berkeley, CA 94720
tnrescigno@lbl.gov
 
Prof. Léon Sanche
Département de médecine nucléaire
et de radiobiologie
Faculté de médecine
Université de Sherbrooke
3001,12e ave nord, Sherbrooke
QC,Canada J1H 5N4
Leon.Sanche@USherbrooke.ca
 
Prof. Michael D. Sevilla
Department of Chemistry
Oakland University
Rochester, MI 48309
sevilla@oakland.edu

Prof. Petra Swiderek
Institut für Physikalische Chemie
Universität zu Köln
Luxemburger Str. 116
50939 Köln, Germany
acp35@uni-koeln.de
Dr. Dominique Teillet-Billy
Universite Paris-Sud
Batiment 351
91405 Orsay Cedex, France
billy@lcam.u-psud.fr

 Home

 Workshops
 Top

Workshop Program

October 16, 2003, Thursday

Pratt Conference Room (all day)
   
 8:30 Introductory remarks
   

Session I: Introduction
 8:45 L. Sanche: Interaction of Low Energy Electrons with Molecular Solids and Biomolecules : Theory, Experiments and Applications [Abstract]
 9:20
D. J. Hunting:  Understanding DNA: Pitfalls and Perturbations [Abstract]
 9:55 COFFEE BREAK
   

Session II: Electron Interaction with Polyatomics
10:20 T. Rescigno: Vibrational Excitation of Polyatomic Molecules by Slow Electrons [Abstract]
10:55 W. Huo: Dissociative Ionization of Aromatic and Heterocyclic Molecules [Abstract]
11:30 W. McCurdy: Dissociative Attachment of Electrons to Water: An Ab Initio Study of the Electronic and Nuclear Dynamics [Abstract]
12:05 LUNCH BREAK
   

Session III: Clusters and Surface Effects
14:00 K. Bowen: Negative Ions with Diffuse Excess Electrons[Abstract]
14.35 M. Gutowski: Valence and Dipole-Bound Anionic States of Pyrimidine Nucleic Acid Bases. Intermolecular Proton Transfer Induced by Excess Electron Attachment [Abstract]
15:10 REFRESHMENTS
   

Session IV: Theory of Electron Interaction with Condensed Atoms and Molecules
15:30 M. Inokuti: Approaches to Slow-Electron Transport in Condensed Matter [Abstract]
16:05 F. Gianturco: Radiation Damage Mechanisms in a DNA strands : Quantum Dynamical Decay after Secondary Electron Captures [Abstract]
16:40 General Discussion
17:30 RECEPTION IN PERKIN LOBBY
 

October 17, 2003, Friday

Phillips Auditorium (all day)
   

Session V: Electron Attachment to Biomolecules
8:45 E. Illenberger: Electron-Induced Processes: From Unimolecular Reactions to Complex Processes in Biologically Relevant Systems
9:20 P. Burrow: Temporary Anion States and Dissociative Attachment in DNA Bases and Amino Acids [Abstract]
9:55 COFFEE BREAK
   

 Session VI: Damage to Biomolecules
10:20 A. Bass: Electron Induced Fragmentation of Bio-molecules [Abstract]
10:55 C. Le Sech: Enhancement of X Rays-Induced Breaks of DNA and Cells Death Rate Due to Excitation of Platinum Atoms by Secondary Electrons. A Possible Application to Protontherapy [Abstract]
11:30 M. Sevilla: DFT Theory Treatment of Low Energy Electron Effects on Biomolecules [Abstract]
12:05 LUNCH BREAK
   

 Session VII: Theory of Electron Interaction with Condensed Atoms and Molecules
14:00 I. Fabrikant: Condensed-Matter Effects in Electron Attachment to Molecules [Abstract]
14:35 D. Teillet-Billy: Resonant Electron Scattering by Molecules in a Solid Environment [Abstract]
15:10 REFRESHMENTS
   

Session VIII: Condensed-Phase Reactions
15:30 P. Swiderek: Modification of Thin Molecular Films by Low-Energy Electrons [Abstract]
16:05 M. Dupuis: Electronic Structure and Reactivity in Solution:
Illustration of the QM/MM-pol-vib and Dielectric Continuum Models for Excited States and Dissociative Electron Attachment [Abstract]
16:40 Q-B. Lu: Large Enhancements in Dissociative Electron Attachment of ~0 eV Electrons to Chlorine-Containing Molecules Adsorbed on H2O Ices: Implications for Atmospheric Ozone Depletion [Abstract]
   

October 18, 2003, Saturday

Phillips Auditorium (all day)
   

Session IX: Condensed-Phase Reactions
8:45 T. Orlando: Low-Energy Electron Interactions with Nanoscale Water Films and DNA Interfaces [Abstract]
9:20 R. Azria: Electron Interactions with Condensed Molecules on Hydrogenated Silicon and Diamond Surfaces: Investigation of Substrate H- Desorption and Vibrations [Abstract]
9:55 COFFEE BREAK
   

Session X: Interaction of Low-Energy Electrons with Organic and Biomolecules
10:15 R. Compton: Attachment of Free and Quasi-free Electrons to Polar and Quadrupolar Molecules [Abstract]
10:50 T. Gay:  Scattering of Chiral Electrons by Chiral Molecules [Abstract]
11:25 R. Naaman: Interaction of Low Energy Electrons with Organized Layers of DNA [Abstract]
12:00 General Discussion and Conclusions
13:00 Adjourn

Abstracts

Azria

Bass

Bowen

Burrow

Compton

Dupuis

Fabrikant

Gay

Gianturco

Gutowski

Hunting

Huo

Inokuti

Le Sech

Lu

McCurdy

 Naaman

Orlando 

Rescigno 

 Sanche

Sevilla

 Swiderek

  Teillet-Billy

 Electron Interactions with Condensed Molecules on Hydrogenated Silicon and Diamond Surfaces: Investigation of Substrate H- Desorption and Vibrations

R. Azria and A. Lafosse

Laboratoire des Collisions Atomiques et Moléculaires (UMR 8625)
Bât 351
Université Paris Sud
F-91 405 Orsay Cedex, France

Surface modification for producing useful surface properties is currently achieved by wet chemical or plasma methods. Alternative methods using low energy electrons (LEE) may be used since such electrons, in particular in the energy range of dissociative attachment process, can initiate in many adsorbates highly selective bond cleavage with high cross sections. However, the knowledge of LEE interactions with the adsorbate and with the substrate to be functionalized, on which the adsorbate is deposited, is required for optimization and good control of the different steps of the processes involved.

Following this idea, we have investigated low energy electron interaction with 1H-Si(100) phase of hydrogenated silicon and with hydrogenated polycrystalline diamond surfaces..

The experimental setup consists of a double stage UHV chamber separated by a valve. Electron stimulated desorption (ESD) measurements were carried out in the upper stage, which consists of a hemispherical electron monochromator as electron gun and a hemispherical energy analyzer in line with a quadrupole mass filter for kinetic energy analysis and identification of desorbing ions. The surface vibrations were studied by means of a high resolution electron energy loss (HREEL) spectrometer (Omicron, model IB500) housed in the lower chamber.

Specular elastically back-scattered electron yield (reflectivity curve) and surface vibrations excitation as function of incident electron energy will be presented in addition to H- desorption. Energy band structure effects will be emphasized.

 Home

 Workshops
 Top

 Electron Induced Fragmentation of Bio-molecules

Andrew D. Bass and Léon Sanche

Groupe des IRSC en Sciences des Radiations
Dépt. de Médecine Nucléaire et de Radiobiologie
Faculté de Médecine
Université de Sherbrooke
Sherbrooke, Quebec, CANADA

Recent experiments show that electrons of energies less than 20 eV can induce both single and double strand breaks in plasmid DNA [1 ] with energy dependencies indicative of the formation and decay of transient negative ion states. The magnitude of the apparent cross sections for this damage is similar to that observed when samples are irradiated with electrons of 100eV [ 2]. Here, we compare strand break yields to measurements of the electron stimulated desorption (ESD) of anions (principally H-, O-, OH-) from similarly prepared films of plasmid and short-chain linear DNA and from thin films of H2O, DNA bases and the dioxyribose analogue tetrahydrafuran (THF) [ 3]. Similarities between the ESD strand break and data emphasises the importance of dissociative electron attachment in the damage process and suggests that reactive scattering by fragment species contributes to the desorbed anion yield. We will describe newly adopted experimental procedures in DNA purification that have resulted in a greater sample sensitivity to low energy electron bombardment and present results showing significant yields of single-strand breaks in plasmid DNA at energies as low as 0.6 eV. In separate experiments on short-chain single-strand oligo-nucleotides chemically bound to a gold substrate [4 ], the ESD yield of neutral species is found to depend not only on the incident electron energy, but on base sequence. Possible reasons for this sequence specificity will be discussed.

[1] B. Boudaiffa, P. Cloutier, D. Hunting, M.A. Huels, L. Sanche Science 287, 1658 (2000)
[2] M.A. Huels, B. Boudaiffa, P. Cloutier, D. Hunting, L. Sanche, J. Amer. Chem. Soc. 125, 4467, (2003)
[3] X. Pan, P. Cloutier, D. Hunting, and L. Sanche., Phys. Rev. Lett. 90, 208102 (2003)
[4] H. Abdoul-Carime, L. Sanche Radiat. Res. 156,151, (2001)

 Negative Ions with Diffuse Excess Electrons

Kit H. Bowen, Jr.

Department of Chemistry,
Johns Hopkins University
Baltimore, MD 21218, USA

Here, we report on our anion photoelectron spectroscopic studies of two types of highly diffuse excess electron states: dipole bound anions and double Rydberg anions.

Dipole bound anions Monopolar positive charges are well known to trap electrons, viz. the nuclei of atoms. When the next term in the multipole expansion, i.e., the dipole moment, binds an electron, a so-called dipole bound anion is formed. The excess electron cloud of dipole bound anions is highly diffuse, residing, in their ideal manifestations, well outside of the nuclear framework of the polar molecule or cluster that trapped them. Interestingly, dipole bound anions exhibit the distinctive signatures of diffuse electron states in their photoelectron spectra. We have studied many ground state, dipole bound anions, beginning with (H2O)2- almost twenty years ago. In this talk, we present our studies of dipole bound anions of biological molecules and clusters. These will include nucleic acid bases and amino acids, both hydrated and unhydrated. (Bob Compton will discuss the topic of quadrupole bound anions.)

Double Rydberg Anions One can envision forming a so-called double Rydberg anion by replacing the proton nucleus within H- with a closed-shell, molecular cation. While this analogy captures the basic concept, it fails to include the important feature that, unlike in H-, the outer two electrons in double Rydberg anions are highly diffuse. We have discovered several long-lived, double Rydberg anions and solvated, double Rydberg anions, all based on ammoniated cations. As in the case of dipole bound anions, double Rydberg anions, which are otherwise completely different, exhibit the characteristic photoelectron spectral signature of extremely diffuse electron states. In this talk, we will present results on NH4-, N2H7-, N3H10-, N4H13-, N5H16-, and NH4-(NH3)1.

Acknowledgment: Our work on these topics has been aided by the calculations of L. Adamowicz, of V. Ortiz, and of M. Gutowski. We have also benefited from on-going conversations with P. Burrow, R. Compton, and M. Sevilla.

 

Temporary Anion States and Dissociative Attachment in DNA Bases and Amino Acids

P.D. Burrow, A.M. Scheer, K. Aflatooni and G.A. Gallup


Department of Physics and Astronomy
University of Nebraska-Lincoln
Lincoln, NE 68588-0111
pburrow1@unl.edu

The energy required to attach an electron to a given compound is a fundamental molecular property that is relevant to a variety of chemical interactions. The characteristics of the temporary anion states that are produced by such attachment, that is, their energies, lifetimes and molecular charge distributions, play a key role in the dynamics of the electron/molecule interaction, leading to selective vibrational excitation and even bond breaking at low electron energies. An efficient way to locate such anion states in the gas phase is by examining resonance structure in the total electron scattering cross section [1]. Over the last 30+ years, a variety of molecular families have been studied this way [2], and generally this work has been accompanied with theoretical results, at various levels of sophistication, supporting the anion state assignments. In spite of the volume of work, some 217 papers over this period, very little of it is related to molecules of biological interest. It wasn't until 1998 that molecules as fundamental as the DNA bases were studied [3], and until 2001 for just a handful of the amino acids [4].

In this presentation, I will review the latter work briefly but will focus mostly on connecting it to the recent flurry of measurements of dissociative electron attachment (DEA) in such molecules. In particular, using our new results in the halouracils and aided by quantum chemical calculations, I will show how all of the sharp structures found at low electron energies (< 4 eV) can be assigned either to specific vibrational Feshbach resonances associated with dipole bound states or to p* shape resonances in these compounds. The molecular orbital properties responsible for this behavior will be stressed. A clear distinction between the two mechanisms for bond breaking is important to make since it is unlikely that the contribution from the dipole bound states will occur in DNA itself, because of the diffuse nature of the dipole state wavefunction.

Finally a few comments on electron induced strand breaking in DNA will be made regarding the theoretical proposal of Barrios et al. [5] in which damage is inflicted at a site "remote" from the DNA base that captures the incident electron. Examples of such effects occurring in other molecules will be presented [6].

[1] L. Sanche and G.J. Schulz, Phys. Rev. A 5, 1672 (1972).
[2] A bibliography of this work may be found in http://physics.unl.edu/directory/burrow/Files/burrow.htm
[3] K. Aflatooni, G.A. Gallup and P.D. Burrow, J. Phys. Chem. A 102, 6205 (1998).
[4] K. Aflatooni, B. Hitt, G.A. Gallup and P.D. Burrow, J. Chem. Phys. 115, 6489 (2001).
[5] R. Barrios, P. Skurski and J. Simons, J. Phys. Chem. B 106, 7991 (2002).
[6] D.M. Pearl, P.D. Burrow, J.J. Nash, H. Morrison, D. Nachtigallova and K.D. Jordan, J. Phys. Chem. 99, 12379 (1995).

 Home

 Workshops
 Top

 

Attachment of Free and Quasi-free Electrons to
Polar and Quadrupolar Molecules

Robert N. Compton

Departments of Chemistry and Physics
The Univ. of Tennessee
Knoxville, TN 37996

Recent experimental and theoretical studies in the field of negative ion physics have shown that in some cases it is useful to describe the binding of an electron to a molecule as a result of the dominant multipole moment of that molecule (e.g., dipole, quadrupole, etc.) together with the polarizability attraction. These anions are exceedingly weakly bound and are subject to collisional detachment as well as detachment by modest electric fields or low energy photons. In cases where a molecule has a dipole or quadrupole moment of sufficient strength to permanently bind an extra electron but also possesses a bound valence anion, the multipole-bound state and valence state can interact to form a coupled system. In this sense the dipole (or quadrupole) states can act as entrance channels or "doorway" states to the formation of the more strongly bound valence anion states. Dipole- and quadrupole-bound anions are readily formed through charge exchange collisions between atoms in high Rydberg states and polar or quadrupolar molecules. The Rydberg electron transfer (RET) is most efficient with molecules entrained in supersonic nozzle expansions which gives rise to kinetic energy in the center-of-mass system of the colliding species and, more importantly, rotationally cold molecules. The RET process is a "resonance" type electron transfer and there exists a maximum in the cross section for RET as a function of the effective quantum number (n*max) of the Rydberg atom. In this sense the electron being attached is quasi-free. Electron affinities (EA) of the polar and quadrupolar molecules are determined from the semi-empirical relationship EA = [23/ n*max2.8eV] as well as from electric field detachment thresholds. We have applied these methods to the study over 30 molecules1 and will discuss the measured binding energies of these anions in relation to their multipole moments, shape, conformation, polarizability, etc. EA's have also been calculated at various levels of approximation and will be compared with experiment. Finally, we will present compelling evidence that the succinonitrile molecule can support both a dipole-bound anion in its gauche form (EA ~ 108 meV) and a quadrupole-bound anion in its trans form (EA ~ 20 meV)2. Some of these RET experiments will be compared with free electron attachment to form valence anion states studied with a new trochoidial electron spectrometer coupled to a time-of-flight mass spectrometer. These experiments will be discussed with relation to new ideas involving electron transport in biological systems.

1. N.I. Hammer et al. J. Chem. Phys. 119, 3650(2003).

2. C. Desfrancois et al. (submitted).

 

Electronic Structure and Reactivity in Solution:
Illustration of the QM/MM-pol-vib and Dielectric Continuum Models for Excited States and Dissociative Electron Attachment

Michel Dupuis

Molecular Interactions & Transformations
Chemical Sciences Division
Pacific Northwest National Laboratory
Richland WA 99352

Michel.dupuis@pnl.gov

We are interested in studying the electronic structure and reactivity of molecules in the condensed phase in the context of ab initio quantum chemical methods. Approaches to the treatment of solvation effects include models in which the solvent molecules are represented explicitly, and models in which the solvent is represented by a dielectric continuum surrounding a molecular cavity.

An explicit solvent representation allows one to get a detailed understanding of the effects and the role of the solvent molecules on the electronic structure of the solute and on the reaction mechanism in the condensed phase. We use the method of direct ab initio molecular dynamics (MD) in conjunction with hybrid representations QM/MM-pol-vib/CONT of the solute and the solvent. In this presentation we will highlight the model and selected applications to activated processes in aqueous solution (SN2 reactions, proton transfers), and to non-adiabatic processes (electronic excitation and electron transfer).

Dielectric Continuum models are computationally very practical. Continuing challenges remain with the cavity definition for consistent level of accuracy. Their application to the dissociative electron attachment in chloroethenes will be highlighted.

This work is supported in part by the U.S. Department of Energy's Office of Biological and Environmental Research, and by the Office of Basic Energy Sciences, Chemical Physics Program. The Pacific Northwest National Laboratory is a multi-program national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO-1830.

Condensed-Matter Effects in Electron Attachment to Molecules

Ilya I. Fabrikant

Department of Physics and Astronomy
University of Nebraska-Lincoln
Lincoln, NE 68588

Many of electron attachment processes occur in a condensed-matter or cluster environment whereas the theory is developed mainly for gas-phase molecules. Therefore it is important to develop theoretical approaches which take into account the environmental effects. The
most important of these is the polarization of the surrounding medium by the temporary negative-ion state which might strongly affect the magnitude of the attachment cross sections.

In the past few years we studied these effects in two types of processes. The first is the dissociative electron attachment to molecules condensed on surfaces of rare-gas films and burried in the films. Our results [1,2] show very strong enhancement of the attachment cross sections due to the polarization effect and due to the change of the density of states in the bulk medium as compared to that in vacuum.

The second class of studied processes is the electron attachment to molecular Van der Waals clusters. They are often strongly affected by vibrational Feshbach resonances (VFR) corresponding to the temporary vibrational excitation of an individual molecule in the cluster with the simultaneous capture of the electron by the long-range field of the cluster. Two very different types of VFRs were detected at the University of Kaiserslautern. The first type, observed in electron attachment to methyl iodide dimers and trimers [3], has its origin in
dissociative attachment to the monomer. With increasing number of monomers in a cluster, the energy of VFR is rapidly shifting away from the vibrational excitation threshold and the resonance width is rapidly growing. Essentially no structure is left in the attachment spectrum for the trimer. This phenomenon was explained by the effects of solvation in dissociative attachment, similar to the polarization effect in medium. In contrast, the VFRs observed in electron attachment to carbon dioxide clusters [4] remain sharp with increasing number of molecular units N. The position of VFRs in these systems can be explained by simple model calculations involving polarization interaction of the electron with the cluster environment.

This model has been recently developed further by incorporating a coupling between the vibrational excitation channels and nondissociative attachment channel. The latter channel is due to electron attachment to the extended affinity states with the following redistribution of the excess energy among the phonon modes [5]. The calculations show that the position of the VFR is red-shifted with N, in accordance with the observations and the theoretical model of Ref. [3]. Whereas the VFR width does not change significantly with N, the resonance amplitude is growing with N.

This work has strongly benefited from collaboration with L. Sanche and H. Hotop.

[1] I. I. Fabrikant, K. Nagesha, R. Wilde, and L. Sanche, Phys. Rev. B 56, R5725 (1997).
[2] K. Nagesha, I. I. Fabrikant, and L. Sanche, J. Chem. Phys. 114, 4934 (2001).
[3] J. M. Weber, I. I. Fabrikant, E. Leber, M.-W. Ruf, and H. Hotop, Eur. Phys. J. D 11, 247 (2000).
[4] E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf, and H. Hotop, Eur. Phys. J. D 12, 125 (2000).
[5] M. Tsukada, N. Shima, S. Tsuneyuki, H. Kageshima, and T. Kondow, J. Chem. Phys. 87, 3927 (1987).

 Scattering of Chiral Electrons by Chiral Molecules*

T.J.Gay, A.S.Green and M.A.Rosenberry

Behlen Laboratory of Physics
University of Nebraska
Lincoln, Nebraska 68588-0111
tgay1@unl.edu

The scattering of longitudinally-polarized electrons by chiral molecules should exhibit phenomena that are analogous to those associated with light scattering. Passage of a beam of light through a chiral target will cause any linear polarization it has to rotate ("optcal activity"), and unpolarized light to become circularly polarized ("circular dichroism") [1]. The analogous effects in electron scattering are, respectively, the rotation of the direction of transverse spin polarization in the plane perpendicular to the electronic momenta, and the production of longitudinal polarization in an initially unpolarized beam [2]. Despite some initial false starts, one experiment has observed electron circular dichroism to date [3]. In this talk I will discuss the basic phenomenology associated with polarized electron ­ chiral molecule scattering, review the experimental situation, and consider possible models that explain the dynamics responsible for electron circular dichroism [4]. In addition to elastic or total scattering measurements, other collision channels that might enhance chiral effects will be considered.

* Work supported by the NSF Grant PHY-0099323.

[1] K.Blum and D.G.Thompson, Adv.At.Mol.Phys. 38, 39-86 (1997).
[2] D.W.Walker, J.Phys.B 15, L289-L292 (1982).
[3] C.Nolting, S.Mayer, and J.Keßler, J.Phys.B 30, 5491-5499 (1997).
[4] T.J. Gay, M.E. Johnston, K.W. Trantham, and G.A. Gallup, in Selected Topics in Electron Physics, Proceedings of the Peter Farago Symposium on Electron Physics, H. Kleinpoppen and M.C. Campbell, eds. (Plenum, New York, 1996).

 Home

 Workshops
 Top

 

RADIATION DAMAGE MECHANISMS IN DNA STRANDS : QUANTUM DYNAMICAL DECAY AFTER SECONDARY ELECTRON CAPTURE


F.A. Gianturco

Department of Chemistry, The University of Rome 'La Sapienza
and INFM Piazzale A. Moro 5, 00185 Rome, Italy.

It is by now well known that high energy radiation can induce damage in liquids and solids via the produc-tion of a wide variety of intermediate species that are being formed within nanoscopic volumes along the ionizing tracks. In qualitative terms, one may say that the primary photon interaction (i.e. the absorbed and scattered pho-tons from the impinging radiation) removes electrons from a very broad range of molecular states and of molec-ular aggregates, stripping them from outer shells and down to the core levels[1]. Such shower of charged particles, however, now looses energy by further causing ionization in the environment through which they are slowed down. They can thus undergo multiple collisions and generate multiple emissions of further electrons which then con-stitute the secondary electrons emitted at lower energies and, like the primary ones, over a very broad energetic span. One of the most effec-tive mechanisms for driving the extra-energy deposition into the molecules composing the DNA basis, and follow-ing the impact of low-energy electrons on thin molecular films, can be related to similar mechanisms existing in the gas phase [2]. At the elementary level, therefore, the for-mation of a Transient Negative Ion (TNI) by temporary electron attachment to the molecule is followed by energy redistribution and different pathways to dissociative at-tachment (DA) and dipolar dissociation (DD). They lead in turn to the production of different fragments, both neutral and ionic.


On the strength of our earlier studies on the structure of scattering precursors to dissociative attachment processes mediated by trapped resonant states [3,4,5] we have begun to analyse possible TNI formation in biomolecules, beginning with Uracil, Thymine and Guanine [6].

At the meeting we therefore intend to show that it is indeed possible to carry out essentially ab initio treatments of the quantum scattering dynamics of low-energy electrons trapped by a complicated biomolecule to form specific TNI transient states that provide clear signatures of precursor structures to possible DA decay channels in such systems.
References
[1] J. Ward, Advances in Radiation Biology 5 (Academic Press, New York, 1995)
[2] B. Boudaiffa, P. Cloutier, D. Hunting, M.A. Huels and L. Sanche, Science, 287, 1658 (2000)
[3] R.R. Lucchese and F.A. Gianturco, Int. Rev. Phys. Chem. 15, 429 (1996)
[4] F.A. Gianturco et al., J. Phys. B 34, 59 (2001)
[5] F.A. Gianturco and R.R. Lucchese, Phys Rev. A 64, 32706 (2001)
[6] A. Grandi, N. Sanna and F.A. Gianturco, submitted (2003).

 

Valence and Dipole-Bound Anionic States of Pyrimidine Nucleic Acid Bases. Intermolecular Proton Transfer Induced by Excess Electron Attachment

Maciej Gutowski(a), Iwona D_bkowska(a,b), Maciej Haranczyk(a,b), Rafal Bachorz(a,b), Janusz Rakb

(a)Chemical Sciences Division
Pacific Northwest National Laboratory
Richland, WA 99352, USA

(b)Department of Chemistry
University of Gda´nsk
Sobieskiego 18
80-952 Gda´nsk, Poland

Shoujun Xu(c), J. Michael Nilles(c) Dunja Radisic(c) Sarah T. Stokes(c) Kit H. Bowen(c)

(c)Department of Chemistry
Johns Hopkins University
Baltimore, MD 21218, USA

Relative stability of dipole-bound and valence anionic states in pyrimidine nucleic acid bases will be discussed. Spectra of some anionic complexes of pyrimidine bases with X (X= amino acid, carboxylic acid, alcohol, purine base) reveal broad features with maxima around 2 eV. These features cannot be associated with the anion of an intact pyrimidine base solvated by X. Our computational results suggest that the excess electron attachment can induce a proton transfer from X to the O8 atom of uracil or thymine and N3 of cytosine, leading to a hydrogenated pyrimidine base - a radical. The radical supports a bound anionic state upon an excess electron attachment. The barriers for breaking the sugar-phosphate bond are very small in nucleotides containing such an anion.

 Understanding DNA: Pitfalls and Perturbations

Darel J. Hunting

Department of Nuclear Medicine and Radiobiology,
Faculty of Medicine, University of Sherbrooke,
Sherbrooke (Québec) Canada J1H 5N4

In 1953, three landmark papers were published in the same issue of Nature describing the structure of DNA (1,2,3), a molecule which appears deceptively simple but which is remarkably complex. Fifty years later, the 3 billion pair human genome has been sequenced and yet we are still discovering new aspects of DNA structure, function and susceptibility to damage.

One of the dilemmas in the reductionist approach to biology is to determine the minimum level of complexity necessary to predict the behavior of a complex biological system. Can we model the susceptibility of DNA to low energy electrons using individual DNA bases and nucleosides? Is a double stranded oligonucleotide a better model? Does a 3000 base pair supercoiled plasmid reliably represent the situation found in a n actively transcribed chromatin domain, in which the DNA is also supercoiled? Our research group uses all of the above model systems and is developing new ones in an attempt to understand the mechanisms by which low energy electrons damage DNA in cells.

1. J.D. Watson and F.H.C. Crick, "A structure for deoxyribose nucleic acid", Nature, 4356, 737 (1953).
2. M.H.F. Wilkins, A.R. Stokes and H.R. Wilson, "Molecular structure of deoxyribose nucleic acids", Nature, 4356, 738 (1953).
3. Rosalind E. Franklin and R.G. Gosling, "Molecular configuration in sodium thymonucleate", Nature, 4356, 740 (1953).

 

Dissociative Ionization of Aromatic and Heterocyclic Molecules

Winifred M. Huo

Applications Branch
NASA Ames Research Center
Moffett Field, CA 94035-1000

Space radiation poses a major health hazard to humans in space flight. The high-energy charged particles in space radiation ranging from protons to high atomic number, high-energy (HZE) particles, and the secondary species they produce, attack DNA, cells, and tissues. Of the potential hazards, long-term health effects such as carcinogenesis are likely linked to the DNA lesions caused by secondary electrons in the 1 ­ 30 eV range.

Dissociative ionization (DI) is one of the electron collision processes that can damage the DNA, either directly by causing a DNA lesion, or indirectly by producing radicals and cations that attack the DNA. To understand this process, we have developed a theoretical model for DI. Our model makes use of the fact that electron motion is much faster than nuclear motion and assumes DI proceeds through a two-step process. The first step is electron-impact ionization resulting in a particular state of the molecular ion in the geometry of the neutral molecule. In the second step the ion undergoes unimolecular dissociation. Thus the DI cross section saDI for channel a is given by

saDI = saI PD,

with saI the ionization cross section of channel a and PD the dissociation probability. This model has been applied to study the DI of H2O, NH3, and CH4, with results in good agreement with experiment [1]. The ionization cross section saI was calculated using the improved binary encounter-dipole model [2] and the unimolecular dissociation probability PD obtained by following the minimum energy path determined by the gradients and Hessians of the electronic energy with respect to the nuclear coordinates of the ion.

This model is used to study the DI from the low-lying channels of benzene and pyridine to understand the different product formation in aromatic and heterocyclic molecules. DI Study of the DNA base thymine is underway. Solvent effects will also be discussed.

[1] W.M. Huo, C.E. Dateo, G.D. Fletcher, and D.Y. Wang, XXIII ICPEAC Abstracts of Contributed Papers, Fr078 (2003).
[2] W.M. Huo, Phys. Rev. A 64, 042719 (2001).

 Approaches to Slow-Electron Transport in Condensed Matter

Mitio Inokuti

Physics Division
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439-4843


The de Broglie wavelength l of an electron with kinetic energy T = 150 eV is 0.1 nm, which is comparable with distances between neighboring atoms in an ordinary molecule and in usual condensed matter. In general, l = 0.1 x (150 eV / T ) 1/2 nm becomes greater at lower T, and eventually reaches 7.7 nm at thermal energy at room temperature. Although this notion is well known for eight decades, some of current calculations on transport of slow electrons in condensed matter remain based upon the classical trajectory of an electron. Examples include track-structure calculations extended to electrons of low energies and analyses of electron recombination with a geminate ion. Yet approaches are possible for carrying out some calculations consistent with quantum mechanics. Let us briefly survey several such possibilities.

The Feynman path-integral method [1] is akin in algorithm to a Monte Carlo simulation, and is straightforward at least in principle to program on a computer. Indeed, it has been applied for example to the hydration of an excess thermal electron in water [2-4]. An obstacle in extending the treatment to electrons of even eV's and higher lies in an extremely rapid increase of the number of paths to be examined, resulting in a prohibitive computation time. It is therefore desirable to find some short cut, even as a sacrifice in accuracy, perhaps somewhat similar in spirit to the condensed-history method in particle-track simulations.

The Wigner phase-space function method [5-7] is attractive, because it provides an extension of the distribution function in the sense of Boltzmann. Thus, connections to the track length distribution, or the degradation spectrum [8] of the familiar electron-transport theory will be clear. A major challenge in an application of this method is the complexity of what corresponds to collision terms in the Boltzmann equation. It involves amplitudes rather than differential cross sections for individual collision processes. Thus, the preparation of input data will have to be far more extensive than in a classical case. One way for dealing with this issue will be to start with a potential for electron-molecule interactions as used in the path-integral method [9] and to generate amplitudes on a computer as needed.

Some of the methods using electron orbital functions also can be adapted to electron-transport analysis in radiation physics. An example is the theory of LEED (low-energy electron diffraction) [10-12]. It focuses on the treatment of multiple scattering of electrons of moderately high energies by spatially fixed atomic lattices, and thus need to be adapted to lower energies with some account of atomic vibrations. In this respect, the method of Car and Parrinello [13] and the END (electron-nuclear dynamics) [14] are highly suggestive and encouraging.

Concerning electron recombination with a geminate ion in liquid, it is appropriate to view the electron-ion system as being in a perturbed Rydberg state interacting with many degrees of freedom of surrounding molecules, in the spirit of the multi-channel quantum-defect theory [15]. Prototypes of such a treatment are seen in studies on Rydberg states of loosely bound dimers [16] and of large molecules [17]. A similar idea about geminate-ion recombination was indeed presented by Schiller [18], though technical details remain to be developed further. A key point in this line of thoughts is to distinguish electron interactions with an ion into a short range and a long range. Properties of short-range interactions can be deduced from existing knowledge about collisions and spectra in the gas phase, while effects of long-range interactions are intrinsically governed by condensed-phase properties. The goal of theory then is to link the two elements in a coherent way.

Work was supported by the U. S. Department of Energy, Office of Science, Nuclear Physics Division, under Contract No. W-31-109-Eng-38.

References

1. R. P. Feynman and A. R. Hibbs, Quantum Mechanics and Path Integrals (McGraw-Hill, New York, 1965).
2. J. Schnitker and P. J. Rossky, J. Chem. Phys. 86, 3471 (1987).
3. J. Schnitker, et al., Phys. Rev. Lett. 60, 456 (1988).
4. B. J. Schwartz and P. J. Rossky, Phys. Rev. Lett. 72, 3282 (1994).
5. E. Wigner, Phys. Rev. 40, 749 (1932).
6. J. E. Moyal, Proc. Cambridge Philos. Soc. 45, 99 (1949).
7. W. B. Brittin and W. R. Chappell, Rev. Mod. Phys. 34, 620 (1962).
8. M. Kimura, M. Inokuti, and M. A. Dillon, in Advances in Chemical Physics, edited by I. Prigogine and S. A. Rice, 84, 193 (1993).
9. J. Schnitker and P. J. Rossky, J. Chem. Phys. 86, 3462 (1987).
10. J. B. Pendry, Low Energy Electron Diffraction: the Theory and Its Application to Determination of Surface Structure (Academic Press, London, 1974).
11. M. A. Van Hove and S. Y. Tong, Surface Crystallography by Low Energy Electron Diffraction: Theory, Computation and Structural Results (Springer-Verlag, Berlin, 1979).
12. M. A. Van Hove, et al., Low-Energy Electron: Experiment, Theory and Structure Determination (Springer-Verlag, Berlin, 1986).
13. R. Car and M. Parrinello, Phys. Rev. Lett. 55, 2471 (1985).
14. E. Deumens, et al., Rev. Mod. Phys. 66, 917 (1994).
15. U. Fano and A. R. P. Rau, Atomic Collisions and Spectra (Academic Press, Orlando, 1986).
16. N. Y. Du and C. H. Greene, J. Chem. Phys. 90, 6347 (1989).
17. M. Thoss and W. Domke, J. Chem. Phys. 106, 3174 (1997).
18. R. Schiller, J. Chem. Phys. 92, 5527 (1990).

 Home

 Workshops
 Top

Enhancement of X Rays-Induced Breaks of DNA and Cells Death Rate Due to Excitation of Platinum Atoms by Secondary Electrons. A Possible Application to Protontherapy

C. Le Sech

Laboratoire des Collisions Atomiques et Moléculaires (URA 281)
Bât 351
91405, Orsay Cedex, France

Complexes made of DNA and chloroterpyridine platinum (PtTC) bound to plasmid DNA, were placed in aqueous solution and irradiated with monochromatic X-rays, tuned to the resonant photoabsorption energy of the LIII shell of the platinum atoms. The number of single and double strand breaks (ssb and dsb) induced by irradiation on supercoiled DNA plasmids was measured by the production of circular-nicked and linear forms. In order to disentangle the contribution of the direct effects imparted to ionization, and the indirect effects due to a free radical attacks, experiments were performed in the presence of a hydroxyl free radical scavenger, dimethyl sulfoxide (DMSO).

An enhancement of the number of ssb and dsb is observed when the plasmids contain the Pt intercalating molecules. A quantitative analysis is performed in order to evaluate the respective contributions of the direct effects (Auger effect) and the indirect effects (free radical attack) to the number of DNA strand breaks.

Even when off-resonant X-rays are used, the strand break efficiency remains higher than expected based upon the absorption cross section, as if the Pt bound to DNA is increasing the yield of strand breaks. A mechanism is suggested, involving secondary photoelectrons generated from the ionization of water which efficiently ionize Pt atoms and generate ssb and dsb. If this mechanism is correct, then heavy atoms, with a large cross section for ionization by electrons that are bound to the DNA should behave as a radiosensitizer.

Experiments on leaving cells loaded with Pt atoms (Chinese Hamster Ovary CHO) by incubation in a solution containing PtTC molecules (3.5x10-4M) during 6 hours, show also an enhancement in cells death rate when compared to cells that do not contain Pt atoms. Further experiments on leaving cells made in presence of DMSO suggest that the main contribution is free radicals mediated. This result can be analyzed and understood with the mechanism proposed to explain the increase of DNA damages by secondary electrons presented above.

These findings may provide an insight to understanding the effects of new radiotherapy protocols, associated chemotherapeutic agents such as cisplatin and ordinary radiotherapy for tumoral treatments. A possible way to deliver selectively the dose in a well-defined volume is suggested using the properties of the Linear Energy Transfer of atomic ions interacting with matter.

 Large Enhancements in Dissociative Electron Attachment of ~0 eV Electrons to Chlorine-Containing Molecules Adsorbed on H2O Ices: Implications for Atmospheric Ozone Depletion

Q.-B. Lu(a) and L. Sanche(b)

(a) Arthur Amos Noyes Laboratory for Chemical Physics
M. C. 127-72
California Institute of California
Pasadena, CA 91125, USA

(b)
CIHR Group in the Radiation Sciences
The Faculty of Medicine
University of Sherbrooke
Sherbrooke, Quebec, Canada J1H 5N4

Atmospheric ozone depletion is an issue of global concern. It was generally accepted that the photolysis of chlorofluorocarbons (CFCs) by UV photons from sunlight causes CFC destruction in the atmosphere, and that heterogeneous chemical reactions of inorganic chlorine compounds (mainly HCl and ClONO2) on ice surfaces in polar stratospheric clouds (PSCs) are responsible for the formation of the ozone hole. However, our recent studies [1-3] showed that dissociative electron attachment (DEA) of nearly zero eV electrons to CFCs and other chlorine-containing molecules adsorbed on H2O ice is largely enhanced compared with the gas phase results. This enhancement has been identified to be due to electron transfer from the precursor to the fully solvated state in polar molecular ice to a chlorine molecule that then dissociates via DEA. This effect should be very efficient in the winter polar stratosphere, where low-energy electrons can be produced by cosmic-ray ionization and trapped in PSC ice [4]. In this talk, we present our laboratory measurements on absolute DEA cross sections of chlorine molecules adsorbed on H2O ice, and discuss the physics of these processes. The implications of these observations for the formation of Antarctic / Arctic ozone holes, and correlations with data obtained by field measurements (satellite, balloon and ground station), will also be discussed.

[1] Q.-B. Lu & L. Sanche, Physical Review B63, p.153403 (2001).
[2] Q.-B. Lu & L. Sanche, J. Chem. Phys 115, 5711(2001).
[3] Q.-B. Lu & L. Sanche, J. Chem. Phys. 119, 2658 (2003); ibid, submitted.
[4] Q.-B. Lu & L. Sanche, Phys. Rev. Letts 87, 078501 (2001).

 

Dissociative Attachment of Electrons to Water: An Ab Initio Study of the Electronic and Nuclear Dynamics

C. W. McCurdy

University of California and Computing Sciences
Lawrence Berkeley National Laboratory
Berkeley, CA 94720

 Abstract PDF

 Home

 Workshops
 Top

 The Interaction of Low Energy Electrons with DNA Monolayers

S.G. Ray(a), S. S. Daube(b), R. Naaman(a)*

(a) Department of Chemical Physics
(b) Chemical Research Support
Weizmann Institute, Rehovot 76100, Israel


Low energy electron (< 2 eV) transmission through self-assembled monolayers of short DNA-oligomers was investigated. A clear inverse-correlation is found between the number of guanines (G) bases in a single stranded DNA-oligomer and the yield of electron transmission. The results could be modeled only by taking into account the wave-like nature of the electrons. The electron transmission through layers made of double stranded DNA is by a factor of 2 to 4 more efficient than through layers of single stranded DNA-oligomers.

Based on these observations, a model is suggested that may provide an explanation to the fact that the DNA of species with high resistivity to radiation contains a high G content.

Two photons photoelectron spectroscopy studies support the conclusion that the G bases are good capturer of electrons.

 

Low-Energy Electron Interactions with Nanoscale Water Films and DNA Interfaces

Thom Orlando, Y. Chen, N. Hud, and C. Santai

School of Chemistry and Biochemistry and School of Physics
Georgia Institute of Technology
Atlanta, GA 30332

It is well recognized that high-energy particles, such as g­rays, lose energy primarily via ionizing and exciting the passage media. For media such as condensed molecular solids, liquids and cells, these energy loss channels produce reactive ions and radicals as well as a large number of low-energy (1-100 eV) secondary electrons. Recent work has demonstrated that the inelastic scattering of these low-energy secondary electrons leads to efficient single and double strand breaks in DNA [1]. Previously, we have examined low-energy electron-stimulated processes in pristine crystalline and amorphous nanoscale ice films and have observed DEA of interfacial D2O water by monitoring the production and reactive scattering of negative ion (D-) fragments [2-4]. The three peaks observed in the D- yield are identified as arising from excitation of 2B1, 2A1 and 2B2 core-excited Feshbach resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion-pair formation or to DEA resonances involving the 2a1 deep valence level of water. The 2B1, 2A1 and 2B2 with the thickness, temperature and morphology of the D2O film. The D- yield generally increases with temperature but deviates noticeably from this trend at temperatures corresponding to structural phase transitions in ice. The D- temperature dependence is remarkably similar to that observed for electron-stimulated desorption of D+ [5]. These results have been discussed in terms of re-orientation of surface (interfacial) water molecules, reduction of hydrogen bonding interactions and return of atomic ­p character in the unoccupied "molecular" orbitals of the surface molecules [5,6].
Since the DEA and proton ESD yields are remarkably sensitive to the local scattering potential, we have extended this work to examine the role of interfacial water and counter-ions in the low-energy electron-induced damage of DNA/water interfaces. We spin-coat DNA containing well-characterized amounts of structural water molecules on mica or graphite and examine the charged and neutral particles produced and desorbed during low-energy electron bombardment. We have developed a custom ultrahigh vacuum surface science chamber for this purpose which is equipped with a time-of-flight spectrometer for neutral product detection using frequency tripling VUV photoionization techniques. The sample(s) can also be removed for post irradiation gel-electrophoresis studies. Our results on pristine water films and preliminary results on the role of intrinsic water, counter ions and phosphate anions in DNA damage will be discussed.

References:
1.)B. Boudaiffa, et. al. Science, 287, (2000) 1658.
2.)W. C. Simpson, et. al. J. Chem. Phys. 107, (1997) 8668.
3.)W. C. Simpson, et. al. J. Chem. Phys. 108, (!998) 5027.
4.)W. C. Simpson, et. al. Surf. Sci. 390, (1997) 86.
5.)M. T. Sieger, W. C. Simpson and T. M. Orlando, Phys. Rev. B. 56, (1997) 4925.
6.)J. Herring, A. Alexandrov, and T. M. Orlando, submitted, Phys. Rev. Lett.

 Vibrational Excitation of Polyatomic Molecules by Slow Electrons

T. N. Rescigno

Computing Sciences
Lawrence Berkeley National Laboratory
Berkeley, CA 94720

Vibrational excitation can be a significant energy loss mechanism in electron-molecule collisions at collision energies below the thresholds for electronic excitation. These processes are of fundamental interest in understanding how electronic energy is channeled into nuclear motion. The cross sections, however, are generally small unless resonances are involved or threshold effects are enhanced through various mechanisms.

Resonant vibrational excitation can generally be treated using the local complex potential or 'boomerang' model and the required resonance parameters can be extracted from first-principles fixed-nuclei scattering calculations. Moreover, as we have recently shown for the case of CO2[1], the local complex potential model can be extended to treat coupled resonances in multiple nuclear dimensions with time-dependent wavepacket techniques.

Peculiarities in threshold vibrational excitation cross sections were first observed in the diatomic hydrogen halides several decades ago. More recently, such structures have been observed in both polar and non-polar polyatomic targets. We have developed a virtual state model, based on the zero-range theory of Gauyacq, Dube and Herzenberg, that can be used to describe these effects and that can be implemented non-empirically. The principal result of this treatment is the derivation of a nuclear wave equation, reminiscent of the boomerang model, that governs the nuclear dynamics. This theory gives, for the first time, a quantitatively accurate description of the struture and selectivity recently observed in the excitation of the Fermi polyads in CO2 .[2]

[1] C. W. McCurdy, W. A. Isaacs, H.-D. Meyer and T. N. Rescigno, Phys. Rev. A 65, 032716 (2002).
[2] M. Allan, Phys. Rev. Lett. 87, 033201 (2001); J. Phys. B 35, L387 (2002).

Interaction of Low Energy Electrons with Molecular Solids and Biomolecules : Theory, Experiments and Applications

Léon Sanche

Groupe des Instituts de recherché en Santé du Canada
Faculté de médicine
Université de Sherbrooke
Sherbrooke (Québec) Canada J1H 5N4

Low energy electrons (LEEs) with energies in the range 0-30 eV can induce, at interfaces and within condensed matter, specific reactions which are of relevance to applied fields such as nanolithography, dielectric aging, radiation waste management, radiation processing, astrobiology, planetary and atmospheric chemistry, radiobiology, radiotherapy and ballistic electronics. Understanding these processes requires knowledge of the interaction of LEEs within condensed matter and/or at surfaces. A number of experimental techniques have been devised to obtain experimental data on such interactions and the reactions induced by LEEs. These will be described at the workshop with a discussion of the advantages and problems associated with each technique. Results will be presented on reactions induced be LEEs in polyethylene and DNA. The challenges that lie ahead to obtain a theoretical description of electron-molecule interaction in the condensed phase will also be mentioned and suggestions made to treat electron scattering from large biomolecules such as DNA. The role of LEE interactions in some of the above applications will be discussed at the workshop with emphasis on the nanoscopic aspects of radiobiology and radiotherapy.

 DFT Theory Treatment of Low Energy Electron Effects on Biomolecules

Michael D. Sevilla

Department of Chemistry
Oakland University
Rochester, Michigan 48309, USA

     Our recent work with DFT theoretical calculations of excess electron addition to biomolecules will be presented that confirm that low energy electrons (LEES) can induce dehalogenation reactions and DNA strand breaks.1-3 Such reactions are significant because LEEs are produced in very large quantity along all the tracks of ionizing radiation. Our calculations show that each of the halouracil anion radicals is found to have two thermally accessible electronic states of differing symmetries, i.e., p*(A") and s*(A').1 The potential energy surface begins with the bonding p* state and crosses to the s* state which is dissociative. The activation energy barriers toward dehalogenation are 21, 4 and 2 kcal/mole for FU, ClU and BrU, respectively. The overall enthalpy changes show the dehalogenations are exothermic for ClU and BrU anions. For the F-U anion the lowest energy path is not the loss of fluoride ion but the detachment of HF. The sensitivity of the halouracils to LEE is found to be in the order of BrU ClU >> FU, in agreement with experimental observations. It was found that the gas phase adiabatic EA of halogenated base pairs21-3 are higher than that of AU,3 and slightly higher or comparable to the base pair guanine-cytosine.3 Base pairing with adenine slightly decreases the EA of the halouracils, in contrast to the substantial increase in EA on base pairing of natural bases. As a result, the probability of electron capture by halouracils when in ds DNA is expected to be substantially reduced. Our results suggest that the radio-sensitization properties of halouracils should be less effective in double strand than in single stranded DNA as is found experimentally. DNA strand breaks are also experimentally found from low energy electron attack and this was tested in our calculations. We employ a model that consists of two deoxyribose (sugar) rings connected by a phosphate in which the sites of bases on the sugar are replaced by amino groups, and the 3'- or 5'-endings are terminated with hydrogens, and both gas phase and solvated cases were treated.4 DFT calculations show that a very large exothermic energy release on strand breaking in the gas phase that is augmented by solvation of the systems.4 Since the phosphate backbone is not the preferred site for attachment of a thermal electron,3 the LEE must encounter the backbone and the dissociation must occur before transfer to the DNA base. We note that ESR observation of the radicals expected from DNA phosphate cleavage by LEEs has been reported in experimental efforts with irradiated DNA.5

1. Xifeng Li, Leon Sanche and Michael D Sevilla, J. Phys. Chem. B. 2002, 106, 11248. (2002)
2. Xifeng Li, Michael D Sevilla and Léon Sanche , J. Amer. Chem. Soc. 2003, 125, 8916-8920 ()
3. Xifeng Li, Zhongli Cai and M. D. Sevilla, J. Phys. Chem. B. 2002, 106, 9345.
4. Xifeng LiÝ, Michael. D. Sevilla and Léon Sanche, submitted for publication
5. David Becker, Amanda Bryant-Friedrich, CherylAnn Trzasko, and Michael D. Sevilla, Radiation Research, 2003, 160, 174.

 Home

 Workshops
 Top

Modification of Thin Molecular Films by Low-Energy Electrons

P. Swiderek

Institut für Physikalische Chemie, Universität zu Köln, Luxemburger Str. 116,
50939 Köln, Germany
Fachbereich 02, Universität Bremen, Leobener Str., NW2, 28359 Bremen, Germany

Interest in the chemical modification of adsorbates and thin molecular films at surfaces by exposure to low-energy electrons is increasing. This concerns not only the curing of coatings and lithographic techniques such as proximity printing, i.e. exposure of a surface to low-energy electrons through a mask [1], but also reactions induced by the current under the tip of the scanning tunneling microscope (STM) [2]. Examples have shown that products are sometimes formed with surprising selectivity. On the other hand, the underlying elementary reaction steps are often not well established. Therefore, basic mechanistic studies are definitely needed to complement present and to form a foundation for future technical developments. This contribution will adress the following aspects of the subject:

Examples relevant to emerging technologies. This includes nanostructuring by chemical lithography [3] as well as reactions induced by the STM [2]. Open questions relating to these examples will be raised.

Experimental techniques that are useful for the study of electron-induced reactions. An overview will be given and the merits and limitations of these techniques will be discussed.

Recent results of work aiming at a more profound insight into the mechanism of the reactions. This concerns the identification of products of electron-induced reactions in cyclopropane [4] and attempts to elucidate the elementary steps of complex electron-induced reactions in materials containing nitro groups. Furthermore, examples of experiments aiming at the measurement of cross sections for the formation of specific products in thin molecular films will be presented [5,6].

References
[1] C.David, H.U.Müller, B.Völkel, M.Grunze, Microelectric Engineering 30, 57 (1996).
[2] S.-W.Hla, L.Bartels, G.Meyer, K.-H.Rieder, Phys.Rev.Lett. 85, 2777 (2000).
[3] W.Eck, V.Stadler, W.Geyer, M.Zharnikov, A.Gölzhäuser, M.Grunze,
Adv.Mat. 12, 805 (2000).
[4] P.Swiderek, M.C.Deschamps, M.Michaud, L.Sanche,
J.Phys.Chem. B 107, 563 (2003).
[5] M.Lepage, M.Michaud, L.Sanche, J.Chem.Phys. 107, 3478 (1997), 113, 3602 (2000).
[6] P.Swiderek, M.C.Deschamps, M.Michaud, L.Sanche, to be published.

 Resonant Electron Scattering by Molecules in a Solid Environment

D.Teillet-Billy, C. Marinica and J.P.Gauyacq

Laboratoire des Collisions Atomiques et Moléculaires
UMR CNRS-Université Paris-Sud 8625
Bât.351
Université Paris-Sud
91405 Orsay Cedex, France

Molecular resonances formed by transient electron capture play a very important role as reaction intermediates in inelastic processes induced by electron impact on molecules at low energy. The characteristics of these resonances and of the inelastic processes they induce can be strongly modified by the presence of a surrounding medium. Different physical environment are considered corresponding to a probe molecule i/ physisorbed on solid surfaces or ii/ embedded in van der Waals clusters. In such systems, the probe molecule is only slightly perturbed by the surrounding medium but the interaction of the scattering electron with the environment can induce strong effects in the resonant scattering properties. The role of an insulating surrounding medium is studied, by considering atomically thin layers of rare gas physisorbed on a metal surface and clusters of a few to hundreds of rare gas atoms. Calculations have been performed for systems involving resonances of nitrogen and oxygen molecules and Ar or Ne rare gases. Striking effects show up ; a few examples will be presented at the conference, illustrating that the adsorption site influences the static properties of resonances, leading to lifetime increased or reduced by large factors depending on the local environment of the probe molecule. Structures have also been shown to appear in the resonant excitation of a probe molecule embedded in a rare gas cluster reflecting the electronic properties of the cluster and the quantisation of the electron motion inside a finite size object. The theoretical calculation is based on an effective description of the interaction of the electron with the probe molecule and includes a microscopic description of the scattering of the electron by the surrounding atoms. Our approach proposes a realistic theoretical description of simple adsorbed or embedded systems. The extreme importance of the environment for electron induced processes is emphasized.

 Home

 Workshops
 Top

ACCOMMODATIONS

Listed below are the names and, where possible, the 800 numbers of hotels and a bed and breakfast agent to assist you in getting accommodations for the upcoming Workshop.

It is especially important that you book a room for the workshop right away as the fall is a very busy time in Cambridge, and you might not be able to get one of the cheaper bed and breakfasts. As housing is expensive in Cambridge/Boston, you may wish to get together with a friend and share a room.

The hotels are within walking distance of the Institute, the Sheraton a short walk and the other two longish walks. They all are on bus routes:

Best Western Homestead Inn, 220 Alewife Brook Parkway, Cambridge,
MA 02138; (617) 491-1890 or 1 (800) 528-1234

Harvard Manor House, 110 Mt. Auburn St., Cambridge, MA 02138
(617) 864-5200

Sheraton-Commander, 16 Garden St., Cambridge, MA 02138; (617) 547-4800 or 1 (800) 325-3434

Boston Reservations/Boston Bed & Breakfast, Inc., 1643 Beacon St., Suite
23, Waban, MA 02168; (617) 332-4199; Fax: (617) 332-5751; e-mail: bostonreservations@bostonreservations.com

All of this information plus more is on the ITAMP web page at http://www.cfa.harvard.edu/itamp under "living accommodations."

We recommend your booking through Boston Reservations/Boston Bed & Breakfast, as in most cases they can get you a room at lower cost than a cold call will get you. If you tell them you are attending a workshop at the Harvard Observatory, they will make every effort to book you at a bed and breakfast, or hotel if you wish, in close proximity. They have many comfortable accommodations in the surrounding neighborhood, and previous workshop participants have been very satisfied with their rooms.

We also strongly advise your not bringing or renting a car. There is no visitor parking at the Observatory and most on-street parking in Cambridge is designated for Cambridge residents only. There are few places in Cambridge and Boston that aren't easily accessible by public transportation and we recommend it highly.
--

 Home

 Workshops
 Top