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Physical Review Letters

Hyperfine effects on associative ionization of ultracold sodium

Mark E. Wagshul, Kristian Helmerson, Paul D. Lett, S. L. Rolston, William D. Phillips, R. Heather, Paul S. Julienne

Published Apr 5, 1993• 68 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.70.2074

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Bulletin of the American Physical Society•May 1, 1993•0 citations
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.
Associative ionization (AI) in sodium (Na + Na + 2 hv → Na2+ + e−) has been a convenient reaction in which to study excited state collisions under "ultracold" conditions (≤1 mK) because the reaction product is easily identified and collected with high signal-to-noise. Previous studies [1] have revealed evidence of molecular intermediate-state resonances under high-intensity laser conditions. Theoretical analysis [2] has associated this structure with singly- and doubly-excited Na2 intermediate states. In this work, we present a new technique utilizing a switched magneto-optical trap (MOT) to make clean measurements of the spectrum of associative ionization in a regime where we can identify the peaks as being associated with states of the Na2 molecule. In addition, our technique reveals new structure in the spectrum of AI versus laser frequency, pointing out the previously unappreciated importance of atomic hyperfine structure in the reaction.
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.
Specific molecular mechanisms are proposed for associative ionization collisions of ultracold sodium atoms in a hybrid optical trap. When an intense, strongly detuned optical trap laser is on, the ionization rate is modulated by molecular bound-state resonances which are strongly affected by field dressing. When the weak, slightly detuned optical molasses lasers are on to provide cooling, an excitation mechanism which produces the two excited atoms at large internuclear separation in different hyperfine states accounts for the lower observed ionization rate.
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 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
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.