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Physical Review A

Single-color photoassociative ionization of ultracold sodium: The region from 0 to -5 GHz

Arjen Amelink, K. M. Jones, Paul D. Lett, P. van der Straten, H G M Heideman

Published Jun 13, 2000• 8 citations
Publisher

Abstract

We have measured the rate of production of ${\mathrm{Na}}_{2}^{+}$ 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 $3S(f=2)$ level (``bright MOT'') or the $3S(f=1)$ level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of ${\mathrm{Na}}_{2},$ 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 ${3S}_{1/2}{+3P}_{3/2})$ as the intermediate states, and levels in the ${0}_{u}^{\ensuremath{-}}$ and ${1}_{u}$ potentials (asymptotically connected to ${3P}_{3/2}{+3P}_{3/2})$ 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.

DOI
10.1103/physreva.62.013408

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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 $3S(f=1)$ 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 0g\ensuremath−{0}_{g}^{\ensuremath{-}}0g\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 0u\ensuremath−{0}_{u}^{\ensuremath{-}}0u\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.
The rate coefficient for the laser-induced associative-ionization reaction of ultracold Na in an optical trap is calculated as a function of the trap laser detuning from the Na(2{(}^{2}(2S1/2{\mathit{S}}_{1/2}S1/2​,F=2)\ensuremath{\rightarrow}Na(2{(}^{2}(2P3/2{\mathit{P}}_{3/2}P3/2​,F=3) resonance. Aided by a knowledge of the excited-state potential-energy curves and of the nature of the free-atom optical-pumping process, we propose the following mechanism, which we call photoassociative ionization (PAI): 2Na(3{(}^{3}(3${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$)${\ensuremath{\rightarrow}}^{\mathrm{\ensuremath{\Elzxh}}\mathrm{\ensuremath{\omega}}}$Na2∗{\mathrm{Na}}_{2}^{\mathrm{*}}Na2∗​(${0}_{\mathit{g}}^{\mathrm{\ensuremath{-}}}$ and 1g{1}_{\mathit{g}}1g​)${\ensuremath{\rightarrow}}^{\mathrm{\ensuremath{\Elzxh}}\mathrm{\ensuremath{\omega}}}$Na2∗∗{\mathrm{Na}}_{2}^{\mathrm{*}\mathrm{*}}Na2∗∗​(1u{1}_{\mathit{u}}1u​)\ensuremath{\rightarrow}Na2+{\mathrm{Na}}_{2}^{+}Na2+​+${\mathit{e}}^{\mathrm{\ensuremath{-}}}$. Maxima in the calculated PAI rate coefficient occur at detunings that are simultaneously one-photon resonant with bound levels of the long-range ${0}_{\mathit{g}}^{\mathrm{\ensuremath{-}}}$ state and two-photon resonant with the lower rotational levels of a vibrational level lying 9 GHz below the dissociation threshold of the autoionizing 1u{1}_{\mathit{u}}1u​ state. The calculated PAI spectrum (PAI rate coefficient versus trap-laser detuning) displays a series of broad peaks between -0.5 and -4 GHz detuning and a cutoff at -5 GHz detuning, as does the experimentally measured spectrum of Lett et al. [Phys. Rev. Lett. 67, 2139 (1991)].The broad widths of the peaks in the PAI spectrum is due in part to the orientation averaging of the collision vector with respect to the electric-field vector and to the optical pumping of the Na2{\mathrm{Na}}_{2}Na2​\ensuremath{\rightarrow}Na2∗{\mathrm{Na}}_{2}^{\mathrm{*}}Na2∗​ rovibronic transition. The calculated PAI rate coefficient at -0.6 GHz detuning is a factor of 4 higher than the experimental value. Fine-structure-changing transitions play a role in the doubly excited states, because of the 1u{1}_{\mathit{u}}1u​ avoided crossing, but not in the intermediate states, because they have gerade symmetry and the only states that have been shown to undergo fine-structure-changing transitions with large probability have ungerade symmetry. Several predictions based on the proposed model and suggestions for future experiments are discussed.
We report two-color photoassociative ionization of sodium in a magneto-optical trap. The experimental results yield information on both singly and doubly excited states. We find that the highest bound vibrational levels ( v>20v>20v>20) of the singly excited ${0}_{g}^{\ensuremath{-}}$ state predissociate into the 32P3/2+32S1/2(Fg=1){3}^{2}{P}_{3/2}{+3}^{2}{S}_{1/2}({F}_{g}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1)32P3/2​+32S1/2​(Fg​=1) dissociation continuum due to avoided crossings of the hyperfine components of this potential with other molecular symmetries. Based on symmetry and energy consideration we argue that a doubly excited 1u{1}_{u}1u​ state remains autoionizing even when excited only a few GHz above the dissociation continuum.
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.
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