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Physical Review A

Ultracold photoassociative ionization collisions in an atomic beam: Optical field intensity and polarization dependence of the rate constant

Chun-Cheng Tsao, Reginaldo de Jesus Napolitano, Yang Wang, J. Weiner

Published Jan 1, 1995• 11 citations
Publisher

Abstract

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(3s${\mathrm{}}^{2}$${\mathit{S}}_{1/2}$;F=1)\ensuremath{\rightarrow}Na(5p${\mathrm{}}^{2}$${\mathit{P}}_{3/2}$) transition at 285 nm by a single-mode laser defines the narrow velocity class that subsequently populates the F=2 hyperfine level of the Na ground state by optical pumping. Probe laser beam excitation tuned near the Na(3s${\mathrm{}}^{2}$${\mathit{S}}_{1/2}$;F=2)\ensuremath{\rightarrow}Na(3p${\mathrm{}}^{2}$${\mathit{P}}_{3/2}$;F=3) transition produced photoassociative ionization with an average collision energy ${\mathit{E}}_{\mathit{K}}$ corresponding to a temperature of 5.3 mK (${\mathit{E}}_{\mathit{K}}$=3/2kT). We determine the rate constant for the process as a function of probe beam intensity and polarization.

DOI
10.1103/physreva.51.r18

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

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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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Collisional modification of the atomic-Na 3s 2 S1/2 –5s 2 S1/2 two-color two-photon polarization spectrum

LeRoy F. Cook, D. A. Olsgaard, Mark D. Havey +1 more

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Physical Review A•Jan 1, 1993•16 citations
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(3s{\mathrm{}}^{2}$${\mathit{S}}_{1/2};F=1)\ensuremath{\rightarrow}Na(5p{\mathrm{}}^{2}$${\mathit{P}}_{3/2}) 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(3s{\mathrm{}}^{2}$${\mathit{S}}_{1/2};F=2)\ensuremath{\rightarrow}Na(3p{\mathrm{}}^{2}$${\mathit{P}}_{3/2};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.
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We have measured the rate of production of Na2+{\mathrm{Na}}_{2}^{+}Na2+​ ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the 3S(f=2)3S(f=2)3S(f=2) level (``bright MOT'') or the 3S(f=1)3S(f=1)3S(f=1) level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of Na2,{\mathrm{Na}}_{2},Na2​, we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly resonant excitation process with levels in the ${0}_{g}^{\ensuremath{-}}$ potential (asymptotically connected to 3S1/2+3P3/2){3S}_{1/2}{+3P}_{3/2})3S1/2​+3P3/2​) as the intermediate states, and levels in the ${0}_{u}^{\ensuremath{-}}$ and 1u{1}_{u}1u​ potentials (asymptotically connected to 3P3/2+3P3/2){3P}_{3/2}{+3P}_{3/2})3P3/2​+3P3/2​) as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.
We report cross-section measurements of the associative-ionization process, Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)+Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)\ensuremath{\rightarrow}Na2+{\mathrm{Na}}_{2}^{+}Na2+​+e as a function of velocity, alignment, and orientation in the collision energy range from 300 to 5 K. Two independently tunable laser beams select narrow velocity classes of excited sodium in a well-collimated atomic beam. Circular and linear polarization of the excitation lasers permits alignment and orientation of the excited atoms in the two velocity classes. The excitation functions show pronounced differences reflecting different molecular pathways to the final product, and they bridge the energy range between conventional and ``cold'' collision dynamics.
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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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