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Physical Review A

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

Vanderlei Salvador Bagnato, Luís Gustavo Marcassa, Yang Wang, J. Weiner, Paul S. Julienne, Y. B. Band

Published Oct 1, 1993• 15 citations
Publisher

Abstract

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)].

DOI
10.1103/physreva.48.r2523

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Bulletin of the American Physical Society•May 1, 1993•0 citations
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 cm\ensuremath−2{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}cm\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)].
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 present precise near-dissociation limit measurements of ultracold collision rate constants (trap loss, fine-structure changing, and photoassociative ionization) as a function of trap intensity from Isat{I}_{\mathrm{sat}}Isat​ to $I\ensuremath{\approx}{40I}_{\mathrm{sat}}$ for Cs and Na atoms cooled and confined in a magneto-optical trap. We find strong saturation for all processes at intensities approximately 10 times the free atom saturation level Isat.{I}_{\mathrm{sat}}.Isat​. Our results provide a precise reference point for testing and comparing different theoretical models used to analyze ultracold collisions.
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.
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.