ITAMP WorkshopCold Alkaline-Earth AtomsOrganizers: Paul Julienne, Kalle-Antii Suominen, Nils AndersenSeptember 7-9, 2000 |
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Hemmerich |
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Dr. Andrei Derevianko Dr. Alan C. Gallagher Prof. Chris H. Greene
Dr. Paul Julienne
Dr. Svetlana A. Kotochigova Dr. Mette Machholm Dr. Chris Oates
Dr. Fritz W. Riehle Professor William C. Stwalley Prof. Kalle-Antti Suominen Dr. Jan Thomsen
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Strontium Trapping in a Vapor Cell
Strontium is loaded from vapor into a resonance-line (461 nm) MOT, within a sapphire-windowed, poly-crystalline-alumina cell. The cell and magnetic-field coils are within an oven operated at ~300 C, while an external Sr reservoir and ion pump provide ~ 109/cm3 Sr density and a much lower background-gas density. Recycling lasers pump from the 53P0 and 53P2 states to 63S1, to prevent "shelved" atom loss that results from the 51P1Æ41D2Æ53P2,1 branching. The trapped-atom velocity distribution is probed with a frequency-stabilized, intercombination-line (689 nm) laser. An intensity-dependent temperature, that approaches the Doppler limit (TD) at low intensity, is observed for this structure-free transition that does not undergo polarization-gradient cooling. Trapped-atom lifetimes (tt ) of 0.1-1s are observed by switched loading. The shape of the loading transient, and the intensity and Sr-density dependence of tt, indicate that tt is primarily limited by Sr-Sr* collisional loss induced by the trapping beams. Measurements versus these parameters yield a trap-loss rate coefficient, per Sr and Sr*, k(n) @6x10-9cm3/s at 40 MHz detuning, with about 30 % uncertainty due primarily to uncertainty in the trapped-atom density. This loss mechanism, which is roughly proportional to intensity, limits trapped-atom number to ~108and densities to ~1011/cm3. A calculation based on the semi-classical (Gallagher-Pritchard) model indicates that this loss results from exciting the 1Pgmolecular state, which has a high survival probability to small internuclear separation (R) since it does not radiate at close range. Excitation occurs at long range due to retardation of the coupling, so this provides an exceptionally efficient Sr-Sr* loss mechanism. The calculation, which includes this rapidly varying G(R) and the finite collision temperature, yields a k that is several times larger than the measured value. This model ignores bound states, and it is not clear if this is the cause of the discrepancy.
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Ab Initio Study of the Ground and First Excited Electronic States of the Strontium DimerS. Kotochigova, E. Tiesinga, and P. S. Julienne National Institute
of Standards and Technology
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Calculations on Cold Collisions Between Laser Cooled Alkaline Earth AtomsMette Machholm Paul S. Julienne Kalle-Antti Suominen Orsted Laboratory, NBIfAFG A laser red-detuned from the 1P0-1S1 cooling transition will cause trap loss from a sample of laser cooled and trapped earth alkaline atoms via three mechanisms: two by excitation to the attractive molecular singlet states at long range (1Su+ and 1Pg) followed by state change (SC) to a triplet molecular states at short range. The third mechanism is radiative escape (RE) after excitation to the 1Su+-state at long range, and spontaneous decay at shorter range. We use quantum scattering methods to calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 50 atomic linewidths to the red of the 1S0-1P1 cooling transition. The major isotopes of the alkaline earth atoms have no hyperfine structure to complicate the theoretical studies. We evaluate both the RE and SC mechanisms of trap loss. The RE mechanism via the allowed 1Su excitation is dominant for more than about one atomic linewidth detuning. Molecular vibrational structure due to photoassociative transitions to bound states begins to appear beyond about ten linewidths detuning. The SC trap loss via excitation at long range to the 1Su+-state and coupling to a triplet state at short range shows the same features as the RE trap loss, but is about one order of magnitude smaller. The SC trap loss via excitation to the 1Pg-state is smaller than via the two other mechanisms, except at very small detuning. The 1Pg SC have vibrational and rotational structure even at small detunings. For magnesium the short range molecular potentials are available, and show a simple structure, therefore we have chosen this system as our model system. Little is known about the short range molecular potentials for the other alkaline earth diatomics, and we simplify these systems by introducing one effective SC crossing. The coupling at the SC crossing is scaled by the magnitude of the fine-structure splitting of the 3P atomic state. The quantum calculations include the
3-dimensional aspects of the collisions by summing over branches
and partial waves, and take the effect of a finite temperature
into account by thermal averaging over collisions energies. Except
for uncertainties in the short range potentials and the SC coupling
strength the trap loss spectra for Mg are complete. The modelling
of the other species, allow us to discussed scaling of properties
throughout the The trap loss spectra presented are
for a weak probe laser, therefore independent of the cooling
laser. We span the temperature range from the Doppler limit for
the 1S0-1P1 cooling and 3 orders of magnitude down covering
also the temperatures reached by cooling on the |
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Experiments with Laser Cooled and Trapped 40Ca Atoms: Increased Trap Lifetimes and Optical Frequency MeasurementsC. W. Oates, E. A. Curtis, and L. W. Hollberg
We have been using diode lasers to cool and trap Ca atoms for use in an optical frequency standard based on the 657 nm intercombination line (natural linewidth = 400 Hz). We have constructed a compact magneto-optic trap with which we can load ~ 107 Ca atoms in a 20 ms loading time, which is limited by optical pumping through the 1D2 level to a dark state. Using a repumping diode at 672 nm to block this path, we have seen trap lifetimes longer than 200 ms, limited (we believe) by background collisions. With our trapped sample we have performed Bordé-Ramsey spectroscopy and have seen sub-natural linewidths as narrow as 200 Hz. Presently we are performing frequency measurements on the clock transition with a mode-locked laser system. This new measurement capability should enable detailed investigations of systematic shifts at near the Hz level, including those due to collisions between cold atoms. |
Quench Cooling - Heading
Towards Ultra-Cold
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Trapping and Cooling of Calcium: Achievements, Problems, Remedies and ApplicationsF. Riehle, G. Zinner,
U. Sterr, T. Trebst, T. Binnewies, G. Wilpers, J. Helmcke E. Tiemann The exceptionally narrow intercombination lines 3P1 - 1S0 of the alkaline earth metals can be used for high-precision experiments in various fields. We have developed a laser stabilized to the intercombination transition of laser cooled 40 Ca atoms. This laser represents an optical frequency standard with a relative uncertainty of 2.5 x 10-13[1] that can be used to realize the SI unit of length, the meter with highest accuracy[2]. This application and others e.g. time-domain atom interferometry or photoassociation spectroscopy are hampered by the existence of a loss channel to the 1D2 state limiting the number of trapped atoms and by the high Doppler limit of the 1P1- 1S0 transition (423 nm) of about 1 mK. When the leak to the metastable 1D2 state was closed by a repumping laser (672 nm) the storage time could be increased to allow for photoassociation spectroscopy of cold Ca atoms [3]. The observed Ca2 spectrum was used to identify the regular vibrational series characteristic of the 1/R3 asymptotic potential with the rotational substructure resolved for the deepest observed level. A method was devised based on the narrow intercombination transition[4] that enabled us to reduce the velocity spread of the Ca atoms in one dimension close to the recoil limit. The method uses the repeated selection and accumulation of slow atoms from the pre-cooled atomic cloud and the repeated rethermalization of the remaining atoms. Atom interferences with such a prepared ensemble showed an increased visibility. To obtain lower velocities of Ca atoms in three dimensions, several methods are possible that are based on the narrow intercombination transition or on transitions from the excited states. Examples will be discussed at the workshop. References [1] F. Riehle, H. Schnatz, B. Lipphardt, G. Zinner, T. Trebst, J. Helmcke, IEEE Trans. Instrum. Meas., IM 48, (1999), 613 [2] T. J. Quinn, Metrologia 30 (1993/94) 523 [3] G. Zinner, T. Binnewies, F. Riehle, E. Tiemann, to be published in Phys. Rev. Lett. [4] Tomas Binnewies, Uwe Sterr, Jürgen Helmcke, F. Riehle, Phys. Rev. A 62, (2000), 011601(R) |
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Spectroscopy and Potential Energy Curves of Mg2William C. Stwalley
The electronic spectroscopy of 24Mg2 was first studied by Balfour and Douglas1 in 1970. Shortly thereafter, the potential curve and long-range forces in the X 1Sg+ ground state were studied by Stwalley and Li.2,3 The implications of this and further work for the scattering lengths and potential BEC of 24Mg and 26Mg will be discussed.
Supported in part by the National Science Foundation. 1. W. J. Balfour and A. E.
Douglas, Can. J. Phys. 48, 901 (1970). |
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Semiclassical and Quantum Theories on Cold Collisions Applied to Magnesium AtomsKalle-Antti Suominen Department of Applied
Physics and Helsinki Institute of
Physics Alkaline earth atoms such as Mg, Ca and Sr have reasonably simple quasimolecule structures, which is expected to make it possible to test the prevailing trap loss theories against experimental data. The loss of atoms from traps in the presence of near-resonant light can be studied using either semiclassical or fully quantum mechanical models. I shall review the basic theoretical approaches and discuss the applicability of the semiclassical methods in the case of magnesium when the atoms are laser cooled using the 1P0-1S1 transition. A comparison between the semiclassical and quantum results indicates that around and below the Doppler cooling temperature (2 mK) the semiclassical theories fail to agree with the quantum results.
This work has been performed in collaboration
with Mette Machholm (National |
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Physics with Cold Magnesium AtomsJan W. Thomsen Niels Bohr Institute In Copenhagen we are currently investigating properties of cold magnesium atoms, especially collision properties. Most alkaline earth systems have a particularly simple level structure with no hyperfine structure. These systems are thus well suited for detailed quantitative quantum mechanical modeling [1] involving only a few potential curves in contrast to more complicated systems like, e.g., Na- Na with hundreds of potential curves in play. The magnesium atom has an additional advantage compared to other alkaline earth atoms, since no lower lying D-states are present close to the cooling transition. We have recently constructed an experimental setup for cooling and trapping magnesium atoms using the 285.2 nm 1S - 1P transition[2]. We trap more than 106 atoms with a density of about 1010 atoms cm3. The MOT temperature is about 2 mK. The first theoretical studies of loss mechanisms in the magnesium MOT has been carried out [1] and parallel experimental investigations are in progress.
[1] M. Machholm, K-A. Suominen and P.S. Julienne: Phys. Rev A 59 R4113 (1999) [2] K. Sengstock, U. Sterr, J.H. Muller, V. Rieger, D. Bettermann, W. Ertmer: Appl. Phys. B 59 99 (1999) |
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Photoassociation of Cold Ca and Its Prospect to Cold Ca2 Molecules 1E. Tiemann, 2G. Zinner, 2T. Binnewies, 2F. Riehle 1Institut für
Quantenoptik 2Physikalisch-Technische
Bundesanstalt We will report on the first successful
observation of photoassociation spectra of cold Ca. The experimental
conditions and the theoretical analysis of the asymptotic potential
are described. This is put into context with the existing molecular
data and derived potentials |
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Thursday, September 7, 2000 |
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Chair: Hossein Sadeghpour |
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| 8:30-9:00 a.m. | P. Julienne: Welcome and introduction to workshop |
| 9:00-9:45 a.m. | A. Gallagher: Strontium Trapping in a Vapor Cell |
| 9:45-10:30 a.m. | H. Katori: Experiments with Ultracold Strontium Atoms |
| 10:30-11:00 a.m. | Coffee |
| 11:00-11:45 a.m. | J. W. Thomsen: Physics with Cold Magnesium Atoms |
| 11:45-2:00 p.m. | Lunch |
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Chair: Kate Kirby |
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| 2:00-2:45 p.m. | M. Machholm: Calculations on Cold Collisions Between Laser Cooled Alkaline Earth Atoms |
| 2:45-3:30 p.m. | K.-A. Suominen: Semiclassical
And Quantum Theories On Cold Collisions Applied To Magnesium Atoms |
| 3:30-4:00 p.m. | Refreshments |
| 4:00-4:45 p.m. | E. Rasel: TBA |
| 4:45-5:30 p.m. | E. Tiemann: Photoassociation of Cold Ca and Its Prospect for Cold Ca2 Molecules |
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5:30 p.m. Reception in Perkin Lobby |
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Friday, September 8, 2000 |
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Chair: Kalle-Antti Suominen |
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| 9:00-9:45 a.m. | C. Oates: Experiments with Laser Cooled and Trapped 40Ca Atoms: Increased Trap Lifetimes and Optical Frequency Measurements |
| 9:45-10:30 a.m. | F. Riehle: Trapping and Cooling of Calcium: Achievements, Problems, Remedies and Applications |
| 10:30-11:00 a.m. | Coffee |
| 11:00-11:45 a.m. | A. Hemmerich: Production
of Ultracold Metastable Calcium Ensembles in a Bichromatic Magneto-Optic Trap |
| 11:45-12:30 p.m. | F. Cruz: A System for Cooling and Trapping of Calcium |
| 12:30-2:00 p.m. | Lunch |
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Chair: Alex Dalgarno |
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| 2:00-2:45 p.m. | C. Greene: Creation of Ultra-Long-Range Rydberg Molecules in a MOT or Condensate |
| 2:45-3:30 p.m. | A. Derevianko: Long-Range Interaction Of Two Alkaline-Earth Atoms: Accurate Ab Initio Relativistic Calculations |
| 3:30-4:00 p.m. | Refreshments |
| 4:00-4:45 p.m. | S. Kotochigova: Ab Initio Study of the Ground and First Excited Electronic States of the Strontium Dimer |
| 4:45-5:30 p.m. | Breakout discussion groups |
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Saturday, September 9, 2000 |
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Chair: Nils Andersen |
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| 9:00-9:45 a.m. | W. C. Stwalley: Spectroscopy and Potential Energy Curves of Mg2 |
| 9:45-10:30 a.m. | P. Julienne: Group II Atoms: A Cornucopia of Cold Collision Physics |
| 10:30-11:00 a.m. | Coffee |
| 11:00-12:00 noon | Wrapup discussions and summary |
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