Brumer

Chu

Corkum

 Doerner

Eberly

Fisch

 Gaarde

 Greene

Gross

Hawrylak

Maquet

 Neumark

Ramunno

Rau

Reitze

 Rost

Santra

Schneider

 Seideman

 Stockman

Adaptive feedback has been applied to the control of a host of molecular processes. However, the complicated optimal incident field that results offers little insight into the mechanism by which control is achieved. In this talk we provide a theoretical analysis of two liquid state adaptive feedback experiments and demonstrate the simplicity of the underlying mechanisms. The limitations imposed by narrow band lasers, and hence the possible utility of attosecond
sources, is noted.

I will describe some recent developments of ab initio and self-iteration-freetime-dependent density functional theories and computational methods for nonperturbative treatment of multiphoton processes of multi-electron atomic and molecular systems in intense ultrashort laser fields. The extension of these methods to the exploration of detailed mechanisms responsible for very-high-order strong-field phenomena in the attosecond time scale will be presented.

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Re-collision electrons dominate strong field atomic and molecular science. If we measure the high harmonics or attosecond optical pulses produced by re-collision electrons, we are observing the interference between the bound state wave (parent) function or wave packet and the continuum wave packet. As with any interferometer, this allows a self-referential measurement of the spatial and temporal structure of the waves involved ­ the bound state electron and the continuum electron wave packet. I will show a measured image of the bound state orbital of N2 [2] and describe the dependence of the alignment dependence of the high harmonic signal from aligned C6H6. This is the first step towards tomographic orbital imaging of an organic molecule. I will also show how attosecond bound state electron wave packets can be measured [3]. If we measure, instead, the electron, we are observing the electron scatter from the ion. The scattering pattern contains information on atomic positions in the parent molecule [4]. The Re-collision electron offers the potential for full characterization of the electronic vibrational properties of molecules on any time scale.

[1] undergraduate student from Ottawa University
[2] J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pepin, J. C. Kieffer, P. B. Corkum and D. M.
Villeneuve, "Tomographic Imaging of Molecular Orbitals", Nature 432, 867 (2004)
[3] H. Niikura, D. M. Villeneuve and P. B. Corkum, "Mapping Attosecond Electron Wave Packet
Motion", Phys. Rev Lett. 94, 083003 (2005)
[4] T. Zuo, A. D. Bandrauk and P. B. Corkum, "Laser Induced Electron Diffraction: A New Tool for
Probing Ultrafast Molecular Dynamics", Chem. Phys. Lett. 259, 313 (1996)

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Today multi particle imaging allows us to measure in coincidence the momentum vectors of all fragments of an atomic or molecular ionization process.In combination with short laser pulses this allows for attosecond time resolution of the fragmentation process without attosecond pulses.

Conceptual pictures have had a key role in guiding experimental extensions from high-field one-electron science (ATI and HHG) into the multi-electron regime of controlled attosecond behavior. Almost all of these conceptual pictures are classical or at least quasi-classical. We will present the conclusions that can be drawn from a purely classical theory in the regime of intensities and pulse durations of interest. One advantage is that the theory is completely Hamiltonian, starting at t=0 with reasonable initial conditions prior to single ionization, and proceeding by direct integration of two-electron Newtonian equations to a description of high-field double ionization. No tunneling event is assumed, and all major features of non-sequential double ionization are reproduced, extending to identification of the four stages [1] of NSDI and a rationale explaining the intensity dependence of the observed ion momentum lineshapes [2].
[1] P.J. Ho, R. Panfili, S.L. Haan and J.H. Eberly, PRL {94} 093002 (2005)
[2] P.J. Ho, PRA (submitted)

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Mainly due to the method of chirped pulse amplification, laser intensities
have grown remarkably during recent years. However, the attaining of very much
higher powers is limited by the material properties of gratings. These limitations
might be overcome through the use of plasma, which is an ideal medium for
processing very high power and very high total energy. Plasma might be
irradiated by a long pump laser pulse, carrying significant energy, which is then
quickly depleted in the plasma by a short counter propagating pulse. This counter
propagating wave effect has already been employed in Raman amplifiers using
gases or plasmas at low laser power. Of particular interest are the new effects,
which enter in high power regimes. Focused intensities can then be contemplated
that are several orders of magnitude higher than what is currently available
through chirped pump amplifiers. Techniques that involve current high power
lasers employ micron to quarter micron light, with the most important high
power applications arising in picosecond to femtosecond compression. This talk
reviews the experimental and theoretical basis for these ideas, as well as speculations concerning the applicability of plasma to compress sub-femtosecond light.

Attosecond pulse trains (APTs) are a natural tool for studying and controlling strong field processes driven by an infrared (IR) laser. This control originates in the short duration of the individual pulses in the train, and their periodicity, which is half the IR laser period. This allows us to use the APT to fix the ionization to a particular time during the IR laser cycle, and therefore to select which quantum paths are available for the ionized electron to follow. We will discuss how this APT-induced control of the electron continuum dynamics represents a new and valuable paradigm in strong field physics. In particular, we will show that both the time-frequency characteristics and the yield of harmonics generated by a single atom, as well as by a macroscopic number of atoms, are strongly influenced by the timing of the initial ionization step selected by the APT. We will also show how an electron wave packet created by APT-driven ionization gains energy from the strong IR field, and demonstrate that the energy-resolved angular distributions of the ionized electrons depend on the timing and the time-frequency characteristics of the APT.

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The subject of xenon atoms or clusters exposed to short-wavelength radiation from a free electron laser will be addressed. Of particular interest are the mechanism of energy absorption in clusters, and the origins of high charge state production in both the atomic and the cluster experiments. We stress the importance of utilizing an improved description of the inverse bremsstrahlung process. In particular, it is important for any quantitative theoretical description to include a realistic treatment of the basic atomic physics, notably the screening of the nuclear charge by inner shell electrons. Then, in addition, it is vital to include the Debye screening effects associated with the liberated electrons that form a plasma. When recombination and electron impact ionization are included, in conjunction with a model for the free plasma expansion after the laser pulse is finished, improved agreement for the charge state distributions in the cluster experiment is achieved. Some remarks about interesting future experiments worth pursuing will also be presented.

For large many-particle systems, the wave function is an illegitimate scientific concept [Walter Kohn, Nobel Lecture], illegitimate in the sense that it can neither be calculated nor stored. Modern density functional theory is based on the surprising fact that knowledge of the ground-state density alone is sufficient to calculate all physical observables of a stationary quantum system. In this lecture, a time-dependent generalization of density functional theory (TDDFT) will be presented which treats both electrons and nuclei quantum mechanically. TDDFT calculations of molecular excitation spectra, of high harmonic generation and of the Coulomb explosion will be presented. Furthermore, a time-dependent generalization of the so-called electron localization function (ELF) will be shown. This quantity allows one to visualize the degree of localization of the electron distribution and provides, in the static limit, a topological classification of chemical bonds. The time-dependent version of the ELF contains an additional term arising from the phases of the time-dependent Kohn-Sham orbitals. Movies of the time-dependent ELF allow the time-resolved observation of the formation, the modulation, and the breaking of chemical bonds, and can thus provide a visual understanding of complex reactions involving the dynamics of excited electrons [1]. Movies of a laser-induced pi-pi* transition and of a proton-molecule scattering process will be presented. Finally, optimal-control theory will be generalized to treat time-dependent targets. In this way, the time-dependent density may be controlleddirectly [2].

[1] T. Burnus, M.A.L. Marques, and E.K.U. Gross, Phys. Rev. A (Rapid Comm.) 71, 010501 (2005).
[2] I. Serban, J. Werschnik, E.K.U. Gross, Phys. Rev. A, (2005, in press); quant-ph/0409124.

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There is currently interest in localizing electrons in semiconductor quantum dots and exploiting their spin for quantum computing. Quantum computing implies both existence of electronic correlations as well as real time operations on time scales shorter than the relevant decoherence times. We will review some of the decoherence mechanisms, including photon scattering, and how they can be observed in transport spectroscopy. The dephasing times on the scale of nanoseconds suggest that attosecond pulses may be required for the operation of quantum gates. Quantum operations require the understanding of the driven electron system, with one and two-electron systems as simplest examples of qubits and gates. As a first step, we will discuss electronic properties of one and two interacting electrons trapped in semiconductor quantum dots as a function of strong magnetic field and strong AC driving field using direct integration of the time dependent Schrödinger equation in configuration space as well as the "dressed atom" picture. We will next move to discuss coded qubits composed of larger electron numbers, and potential application of short pulses for their operations.

The advent of new harmonic generation-based devices, delivering ultra-short pulses of coherent XUV (or soft-x-ray) radiation, makes feasible the observation in real time of atomic or molecular electronic phenomena that were so far accessible only indirectly, through sophisticated high resolution spectroscopy measurements. In fact, these measurements relied on the capabilities of the most recent (3 rd -generation and beyond) synchrotron facilities.
In this progress report, we shall address several questions related to the physics of this class of phenomena, including multicolor atomic ionization (ATI in the simultaneous presence of a comb of harmonics and of the pump laser) and the process of harmonic generation itself.
One of these questions is related to the potential applications of the Dirac combs of frequencies that are produced when harmonics are generated. More precisely, the question is how to exploit the temporal structure of the (Fourier-transformed) pulse train that is generated, each wagon within the train having duration within the attosecond range?Another question is related to the coherence properties of the harmonics emitted by an ensemble of atoms. This point is of importance when the harmonic signal itself is used to analyze the time-dependent response of atomic or molecular systems in the presence of an intense laser pulse.We shall report on the recent results of numerical simulations that shed light on the dynamics of multicolor ATI and of the harmonic generation process.

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The attosecond science program currently under development at the University of California, Berkeley, will be described. Much of the focus of this program is to extend attosecond science beyond atomic physics to the study of dynamics in molecules, clusters, and condensed phase systems. Examples to be presented include interatomic Columbic decay in rare gas clusters and metal-ligand complexes.

The interaction of a VUV free electron laser of unprecedented peak intensity with noble gas clusters was recently investigated at DESY. The experiment showed unexpectedly high charge states. Our computational analysis shows that strongly coupled plasma is created, and that this is the key to understanding these high charge states. Strongly coupled plasmas occur in the limit of high particle density and low temperature. The degree to which strong coupling occurs determines whether collective (plasma oscillation) or collisional processes dominate the many-body dynamics, and determines on which time scales they occur. Our analysis reveals that by varying the laser parameters, one may tailor the plasma coupling parameter and therewith the interplay between collective and collision-type processes. We will show how ultrafast technology will allow us to probe the transition between weakly and strongly coupled plasmas by resolving the destruction of collective effects on a sub-femtosecond time scale.

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Intense and short pulse laser interactions with atoms call for the study of the time evolution of atomic states. The mutual interplay of the intense field and electron correlations will be of increasing interest in this area of study. An analytical approach to handling time-dependent operator equations will be presented and applied to a toy model of two electrons bound by harmonic forcesand with a harmonic coupling between them.

Current methods of attosecond pulse generation utilize high order harmonic generation (HHG) or stimulated Raman scattering to produce trains of pulses. If the optical driving field is sufficiently short in an HHG process, it is possible to produce isolated attosecond pulses as short as 650 as. Both sources offer the ability to generate high intensity coherent light, and as such offer unprecedented opportunities for time-resolved investigations of atomic, molecular, and solid state systems. In this talk, I discuss applications of HHG attosecond pulses in solid-state materials spectroscopy and surface modification. In particular, the coherence of the light can be exploited for surface patterning and modification. With wavelengths of 20-40 nm, even nanojoules of energy can produce intensities in excess of 1019 W/cm 2 . As such, attosecond pulses offer interesting possibilities for performing surface modification with high spatial resolution. I also discuss the possibility of using simple femtosecond oscillators and photonic crystal fibers (PCFs) with dispersion properties tailored to generate almost pure self-phase modulated spectra with >1000 nm bandwidths. Unlike super continuum from standard PCFs, the spectral amplitude and phase characteristics of light generated in specially designed dispersion-flattened PCFs are in principle compressible to less than 1 fs.

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The attosecond pulses available now and in the near future cannot be used to pump-probe bound electronic motion in atoms analogously to vibrational motion in molecules pumped and probed by femtosecond pulses. On the other hand, extended electronic systems, such as clusters or quantum dots provide ample, interesting and feasible applications for probing electron dynamics with attosecond pulses. This is of interest, since there has been no way so far to experimentally access details of these electron dynamics in time and is feasible since the typical binding energies in extended systems and the corresponding period of bound electron motion is within the reach of attosecond pulses. In the talk, I will specifically address the question what can be learnt about the dynamics in rare gas clusters by shining attosecond pulses on them.

Electronically excited cations, generated by inner-valence ionization of small molecules, relax in general by dissociation and, eventually, photon emission. Typically, autoionization is forbidden for energetic reasons. Using extensive ab initio calculations, it has been predicted that the situation changes fundamentally in an inner-valence ionized cluster, which releases its excess energy by emitting an electron. This novel process, referred to as Intermolecular
Coulombic Decay (ICD), is characterized by an efficient Coulombic energy transfer mechanism between monomers in the cluster. The decay phenomenon is ultrafast, taking place on a femtosecond timescale. This talk provides a basic overview of the phenomenon of ICD. The most important theoretical predictions are presented, together with recent experimental evidence for ICD in neon clusters.

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The Finite Element Discrete Variable Method (FEDVR) may be combined with a novel propagation algorithm based on the Lie-Trotter-Suzuki (LTS) formula to produce a new algorithm which scales as O(N), is quite accurate and parallelizes in a very natural fashion. The method is a generalization of ideas introduced by DeRaedt and relies on the structure of the FEDVR Hamiltonian to reduce the propagation to first constructing time-propagators for the small, finite element submatrices and then using the LTS second or fourth order algorithm to propagate the initial state from time t to time t + delta. Both the second and fourth order methods have been programmed and tested in a number of simple problems in order to examine the accuracy and efficiency of the method. Numerical results will be presented for problems including the release of a confined, one-dimensional Bose-Condensate in an optical lattice and a simple model for multi-photon ionization of an atom.

The transport properties of single-molecule devices continue to fascinate
solid-state physicists, material scientists, chemists, bio-physicists and
engineering alike, both due to the fundamentally new physics associated with
conductance in the molecular scale and due to a range of already demonstrated
and projected applications. The ultrafast electronic dynamics in molecular-scale
junctions differs qualitatively from those in solids, surfaces and isolated
molecules. It takes place in an open system that is remote from equilibrium and
is subject to a strong electric field. Interestingly, in many cases the electronic
motion is strongly coupled to internal modes of the molecular moiety of the
device, giving rise to current-driven phenomena such as vibration, rotation, inter-mode
energy flow and reaction. At present molecular-scale electronics are probed
in the laboratory using steady state techniques (conductance measurements) and
hence most theoretical studies have focused on time-independent formulations,
where the electronic dynamics are concealed. In the talk, I will not argue that attosecond pulses could make a route to dynamical studies of molecular-scale devices by capturing the electronic time-scales. Rather, I will suggest that, if such time-domain probes became available, they would open a range of new and exciting opportunities for spectroscopy and control.

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Nanoplasmonics has recently attracted great attention due to a multitude of enhanced optical phenomena induced by resonant optical fields localized at the nanoscale in nanostructured systems. Among such phenomena are surface-enhanced Raman scattering (SERS), fluorescence, generation of harmonics, photoelectron emission, etc. We discuss ultrafast phenomena on the nanoscale induced by femtosecond laser pulses. One of the focus points of the talk will be the coherent control of the spatial distribution of optical fields on the nanoscale. The phase degree of freedom of ultrashort laser pulses provides a unique possibility to control the nanometer-femtosecond spatiotemporal dynamics in nanostructures. We will focus on two-photon excitation of fluorescence and two-photon electron emission. There is also a pronounced dynamics of the nanoscale distribution of the local optical fields within fractions of the optical period, i.e., on the attosecond time scale. For a complex, disordered nanosystem, such a dynamics leads to giant attosecond fluctuations of the local optical fields. For nonlinear optical processes, coherent control allows one to define and study the dynamics of optical excitation on the scale of ~100 as. We will discuss recent experiments and numerous prospective applications of the ultrafast nanoplasmonic effects.

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