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Physical Review A

Associative ionization in collisions of slowed and trapped sodium

Alan C. Gallagher

Published Oct 1, 1991• 30 citations
Publisher

Abstract

Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range ${\mathrm{Na}}^{\mathrm{*}}$-Na interaction initiates inward motion that is completed as ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].

DOI
10.1103/physreva.44.4249

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Associative ionization in collisions between Na(3 P3/2 ) and Cs(6 P3/2 )

C. Gabbanini, M. Biagini, Silvia Gozzini +2 more

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 3${\mathit{P}}_{3/2}$ and 6${\mathit{P}}_{3/2}$ levels, respectively, and the formation of the ${\mathrm{NaCs}}^{+}$ 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{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.

Physical Review A•Mar 1, 1991•6 citations
Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗-Na interaction initiates inward motion that is completed as Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗\ensuremath{\rightarrow}Na2\ifmmode∘\else\textdegree\fi{\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}Na2\ifmmode∘\else\textdegree\fi​+e. Thus low-temperature associative ionization does not result from Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].
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
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.
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 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.
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 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.