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Bulletin of the American Physical Society

The role of hyperfine structure in ultracold atomic collision dynamics

Paul S. Julienne, Carl J. Williams

Published May 1, 1993• 0 citations
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Abstract

Recent experiments at NIST and the University of Maryland on cooled and trapped Na atoms have revealed several 1- and 2-color spectral features associated with the hyperfine structure of the Na atom. We have calculated the adiabatic molecular potential curves in a total angular momentum basis, including hyperfine structure, of the molecular states of the alkali dimer molecules that correlate to the {sup 2}P{sub 3/2}+{sup 2}S separated atoms. We have also calculated the hyperfine line strengths for optical coupling to the {sup 2}S +{sup 2}S ground molecular states. These are used to interpret the new experimental features for trapped Na as due to free-bound-bound or free-bound-free spectra of the Na{sub 2} molecule. New dynamical features illustrating optical control of ultracold reactions will be discussed. Supported in part by ONR.

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Bulletin of the American Physical Society•May 1, 1993•0 citations
Recent experiments at NIST and the University of Maryland on cooled and trapped Na atoms have revealed several 1- and 2-color spectral features associated with the hyperfine structure of the Na atom. We have calculated the adiabatic molecular potential curves in a total angular momentum basis, including hyperfine structure, of the molecular states of the alkali dimer molecules that correlate to the {sup 2}P{sub 3/2}+{sup 2}S separated atoms. We have also calculated the hyperfine line strengths for optical coupling to the {sup 2}S +{sup 2}S ground molecular states. These are used to interpret the new experimental features for trapped Na as due to free-bound-bound or free-bound-free spectra of the Na{sub 2} molecule. New dynamical features illustrating optical control of ultracold reactions will be discussed. Supported in part by ONR.
The photoassociation of colliding ultracold trapped atoms permits the observation of high resolution molecular spectra of high lying vibrational levels with low total angular momentum and outer turning points greater than 50 Å. This paper describes how the hyperfine structure of alkali dimer molecules at large internuclear separation can be used to assign the electronic character of the observed photoassociation spectra and how hyperfine structure helps to explain the observed photoassociation spectrum of the attractive electronic states of the Na2 molecule that correlate with Na(3 2S)+Na(3 2P3/2) atoms.
We report new calculations of two-body associative ionization collisions between sodium atoms confined in a magneto-optic trap at ultracold temperatures. These collisions represent a new kind of {open_quotes}open{close_quotes} or dissipative collision for which the energy of the atom + applied light field need not be conserved due to spontaneous emission coupling to the vacuum modes of the radiation field and require density matrix methods rather than wavefunction methods to describe the collision. In these collisions the Na atoms must absorb a photon when they are in close enough proximity that a significant fraction of the excited population survives relaxation back to the ground state. The Na atoms accelerate towards each other on the excited molecular state and absorbs a second photon to a doubly excited state which then proceeds to associatively ionize at close range. Our calculations incorporate a molecular picture of the atomic collision, laser optical-field dressing of the molecular states participating in the dynamics, and population and coherence decay due to spontaneous emission. The effect of excited state hyperfine structure, which plays a critical role in the dynamics, is simulated by simplified models with a reduced number of states based on the full set of molecular-hyperfine adiabatic potentialmore » curves. Comparison is made with recent measurements of photo-associative ionization. rates as a function of optical field intensity from about 20 to 130 mW cm{sup -2}.« less
High resolution photoassociation spectra of colliding ultracold trapped atoms contain detailed information about ground and excited state interactions. We calculate the asymmetric line shapes for ultracold Na2{\mathrm{Na}}_{2}Na2​ absorption using a resonance scattering expression, proportional to a free-bound Franck-Condon factor, that is justified by full quantum scattering calculations of a collision in a radiation field. The line shapes illustrate Wigner threshold law behavior, which is characteristic of the quantum limit as $T\ensuremath{\rightarrow}0$. Using an adiabatic hyperfine analysis, we calculate and compare a model spectra for J=1,2,3,and4J=1,2,3, \mathrm{and} 4J=1,2,3,and4 features of the Na2{\mathrm{Na}}_{2}Na2​ $^{1}\ensuremath{\Pi}_{g}(\ensuremath{\nu}=48)$ state to recent high resolution experimental data.
We observe a new resonance structure in the associative ionization spectrum of laser-cooled sodium due to the collision of two atoms in different ground hyperfine states. This associative ionization is due to doubly resonant excitation at moderate internuclear separation through intermediate molecular states. Substructure within this resonance is evidence for the role of molecular bound states in this process. We present calculations modeling the effect of ground-state hyperfine structure, the importance of which was unanticipated in earlier theoretical studies.
We have observed the spectrum of long range molecular states of singly excited Na2{\mathrm{Na}}_{2}Na2​ formed by photoassociation during ultracold associative ionization collisions of laser-cooled Na atoms. The low kinetic energy of the collision allows us to obtain very high resolution spectra and to observe states close to the dissociation limit. The spectrum shows regular vibrational series characteristic of the 1/R3{\mathit{R}}^{3}R3 long range portion of the 1g{1}_{\mathit{g}}1g​ potential (asymptotically S1/2{\mathit{S}}_{1/2}S1/2​+P3/2{\mathit{P}}_{3/2}P3/2​) as well as molecular hyperfine structure within these peaks. We extract potential parameters and compare with recent theory.
We present a new spectroscopic technique which allows the observation of long-range molecular states of ultracold sodium via the associative ionization (AI) reaction. Due to the ultracold conditions, we are able to achieve very high resolution spectra of free-bound molecular transitions which were previously observable only via bound-bound spectroscopy. Previous experiments using laser cooled sodium have revealed evidence of molecular spectra but were limited to a narrow tuning range and power broadened because the intense trapping laser was also used to excite the molecular transitions. By alternating the beams of a magneto-optical trap with a separate laser, we confine the atoms while leaving them unperturbed by the trapping light during the excitation period. The second laser excites free ground state atoms into the bound molecular states. With this scheme, we have been able to detect the vibrational series of molecular states in the intermediate AI excitation step, from just below threshold to 3 cm{sup {minus}1} deep. By resolving narrow structure within the spectra associated with molecular hyperfine splittings and comparing with theoretical calculations, we can unambiguously identify the molecular states involved.