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Bulletin of the American Physical Society

Ultracold photo-associative ionization collisions of Na atoms in a magneto-optical trap: Optical block equation calculations

Y. B. Band, Paul S. Julienne, Carl J. Williams

Published May 1, 1993• 0 citations
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Abstract

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

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Ultracold photoassociative ionization collisions in a magneto-optical trap: The optical-field-intensity dependence in a radiatively dissipative environment

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We report here measurements of two-body associative-ionization collisions between sodium atoms confined in a magneto-optic trap. These collisions represent a kind of ``open'' or dissipative collision for which the energy of the atom plus applied light field subsystem need not be conserved due to spontaneous-emission coupling to the vacuum modes of the radiation field. The experiment measures the photoassociative-ionization-rate constant as a function of the optical field intensity from about 40 to 260 mW ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. These results are in reasonable agreement with the predictions of an optical-Bloch-equation theory [Y. B. Band and P. S. Julienne, Phys. Rev. A 46, 330 (1992)], but differ strongly from the predictions of a local-equilibrium theory [A. Gallagher, Phys. Rev. A 44, 4249 (1991)].

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Effects of laser light on ultracold atom collisions

V. Sánchez-Villicaña

Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 $\\mu$K. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes $\\sp{85}$Rb and $\\sp{87}$Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold $\\sp{87}$Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^

•Jan 1, 1996•0 citations
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
We report here measurements of two-body associative-ionization collisions between sodium atoms confined in a magneto-optic trap. These collisions represent a kind of ``open'' or dissipative collision for which the energy of the atom plus applied light field subsystem need not be conserved due to spontaneous-emission coupling to the vacuum modes of the radiation field. The experiment measures the photoassociative-ionization-rate constant as a function of the optical field intensity from about 40 to 260 mW ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. These results are in reasonable agreement with the predictions of an optical-Bloch-equation theory [Y. B. Band and P. S. Julienne, Phys. Rev. A 46, 330 (1992)], but differ strongly from the predictions of a local-equilibrium theory [A. Gallagher, Phys. Rev. A 44, 4249 (1991)].
Collisions of ultracold trapped atoms ( T< 1mK) have opened a new field of atomic collisions with abundant opportunities for theory and experiments. Photo-associative ionization (PAI) in sodium atoms has been the first two-body collision Studied in optical traps3. In this process two ground state Na atoms absorb two photons during the course of the collisional encounter. The first absorption at long range put the colliding system on an attractive C3/R3 potential curve, and the two atoms begin to accelerate toward each other. A second absorption promotes the system to a doubly excited Na(3p)+Na(3p) potential curve from which the PAI process takes place at short range.
The production and research of ultracold heteronuclear molecules have aroused the great interest recently. On the one hand, these molecules are extremely popular in experiments for exploring the collision dynamic behaviors in threshold, photoassociative spectrum and strong dipole-dipole interactions. On the other hand, ultracold polar molecules populated at deeply bound levels in the singlet ground state are the right candidates to investigate quantum memories for quantum simulation, and to study strongly interacting quantum degenerate gases. The precise knowledge of cold collision processes between two different types of alkali atoms is necessary for understanding and utilizing ultracold heteronuclear molecules, sympathetic cooling, and thus formation of two species BEC. The goal of the present investigation is to study the collisions between ultracold sodium atoms and cesium atoms. We systematically demonstrate simultaneously trapping ultracold sodium and cesium atoms in a dual-species magneto-optical trap (MOT). The sodium atom trap loss rate coefficient Na-Cs is measured as a function of Na trapping laser intensity. At low intensities, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities, collisions involving excited atoms are more important. A strong interspecies collision-induced loss for Na atoms in the MOT is observed. In order to obtain the trap loss coefficient Na-Cs, we proceed in two steps. First, the Cs repumping laser is blocked to avoid the formation of ultraold Cs atoms. The loading process of Na atoms is recorded when the Cs trapping laser is on. Second, the loading curves of the Na atoms are obtained as Cs atoms are present with the repumping laser beams. The total losses PNa and PNa' are acquired by fitting the two loading curves of trapped Na atoms. Thus, the trap loss coefficient Na-Cs can be derived from the difference between total losses PNa and PNa' divided by the density of the Cs atoms. The coefficient Na-Cs decreases in a range of 5-10mW/cm2, which originates from the hyperfine-state changing (HFC) collision. A Doppler model is used to calculate the Na atom trap depth, in that the atom trap depth and exoergic energy determine the behavior of the collisional trap loss rate coefficient. The three corresponding calculated critical intensities of Na trapping laser are 7.98, 14.82, 16.2 mW/cm2 respectively in the Na-Cs HFC collision process. The first calculated critical intensity value agrees well with the experimental result. Our work provides a valuable insight into HFC collision between Na and Cs atoms and also paves the way for the production of ultracold NaCs molecules using Photoassociation (PA) technology. Furthermore, the experimental results lay a great basis for the obtainments of high sensitive heteronuclear NaCs molecular PA spectrum and the creation of deeply bound ground state molecules.
We study photo-associative ionization (PAI) for sodium atoms held in a magneto-optical trap (T < 1 mK). The process is investigated using one and two laser frequencies. For a single color PAI, we measure the rate constant as a function of light intensity and the result is compared with calculation based on semiclassical Bloch equation theory. The measured rate constant appears to saturate sooner than theory. In the two-color PAI (TCPAI) we investigate individual steps of the process. Different combinations of laser frequencies prepare different populations in each of the hyperfine ground state and the contribution of each one in this collisional process is investigated. The comparison with theory allows a fairly clear understanding of PAI and TCPAI.
We report here measurements of two-body photoassociative ionization collisions between sodium atoms within an ultranarrow velocity class selected from a well-collimated, thermal atomic beam. Doppler-shifted excitation of the Na(3s2{\mathrm{}}^{2}2S1/2{\mathit{S}}_{1/2}S1/2​;F=1)\ensuremath{\rightarrow}Na(5p2{\mathrm{}}^{2}2P3/2{\mathit{P}}_{3/2}P3/2​) transition at 285 nm by a single-mode laser defines the narrow velocity class that subsequently populates the F=2 hyperfine level of the Na ground state by optical pumping. Probe laser beam excitation tuned near the Na(3s2{\mathrm{}}^{2}2S1/2{\mathit{S}}_{1/2}S1/2​;F=2)\ensuremath{\rightarrow}Na(3p2{\mathrm{}}^{2}2P3/2{\mathit{P}}_{3/2}P3/2​;F=3) transition produced photoassociative ionization with an average collision energy EK{\mathit{E}}_{\mathit{K}}EK​ corresponding to a temperature of 5.3 mK (EK{\mathit{E}}_{\mathit{K}}EK​=3/2kT). We determine the rate constant for the process as a function of probe beam intensity and polarization.
Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^