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AIP conference proceedings

Observation of associative ionization of ultra-cold laser-trapped sodium atoms

P. L. Gould, Paul D. Lett, William D. Phillips, Paul S. Julienne, H. R. Thorsheim, J. Weiner

Published Jan 1, 1988• 2 citations
Publisher

Abstract

We have observed associative ionization of laser cooled and trapped sodium atoms. The measured rate coefficient for the process at a temperature of (0.75±0.25) mK is (1.1+1.3−0.5)×10−11 cm3/sec, which implies a cross section of 8.6×10−14 cm2. This is three orders of magnitude larger than the cross section measured in previous experiments at higher temperatures.

DOI
10.1063/1.37375

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We have observed associative ionization of laser cooled and trapped sodium atoms. The measured rate coefficient for the process at a temperature of (0.75±0.25) mK is (1.1+1.3−0.5)×10−11 cm3/sec, which implies a cross section of 8.6×10−14 cm2. This is three orders of magnitude larger than the cross section measured in previous experiments at higher temperatures.
We have observed associative ionization of laser-cooled and -trapped sodium atoms. The measured rate coefficient for the process at a temperature of 0.75\ifmmode\pm\else\textpm\fi{}0.25 mK is (1.1${\ifmmode\pm\else\textpm\fi{}}_{0.5}^{1.3}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which implies a cross section of 8.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ cm2{\mathrm{cm}}^{2}cm2. This is 3 orders of magnitude larger than the cross section measured in previous experiments at higher temperatures. The measurement involves the use of a new kind of laser trap.
Recent advances in optical stopping and trapping of particles now permit the investigation of atom collision dynamics at temperatures below 1 mK. We discuss three specific examples of inelastic processes appropriate for study in this new energy domain: radiative association; collisional heating; and associative ionization. Theoretical study of radiative association and collisional heating demonstrate new quantal features apparent in the total inelastic cross section due to the small number of partial waves contributing to the nuclear motion. We present the first experimental observation of an inelastic process, associative ionization between excited sodium atoms, and report a measure of the absolute cross section ~3 orders of magnitude larger than the cross section measured in conventional conditions.
The results of an experiment on the associative ionization collision between sodium and cesium excited atoms are reported. The atoms are excited by two resonant cw lasers to the 3P3/2{\mathit{P}}_{3/2}P3/2​ and 6P3/2{\mathit{P}}_{3/2}P3/2​ levels, respectively, and the formation of the NaCs+{\mathrm{NaCs}}^{+}NaCs+ molecular ion is observed. The rate constant of such process has been measured relative to the known rate constant of sodium associative ionization and its value resulted in K=(5.4\ifmmode\pm\else\textpm\fi{}3.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.
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.
Collisional and thermal ionization of sodium n S and n D Rydberg atoms with n = 8–20 has been studied. The experiments were performed using a two-step pulsed laser excitation in an effusive atomic beam at atom density of about 2 × 10 10 cm −3 . Molecular and atomic ions from associative, Penning and thermal ionization processes were detected. It was found that the atomic ions were created mainly due to photoionization of Rydberg atoms by photons of blackbody radiation at the ambient temperature of 300 K. Blackbody ionization rates and effective lifetimes of Rydberg states of interest were determined. The molecular ions were found to be from associative ionization in Na( nL ) + Na(3S) collisions. Rate constants of associative ionization have been measured using an original method based on relative measurements of Na + 2 and Na + ion signals.
We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1, ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1$ hyperfine state was determined to be $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm2{\mathrm{m}}^{2}m2, which implies a positive value of the scattering length.