Associative ionization in collisions of slowed and trapped sodium

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 ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$→${\mathrm{Na}}_2^{\mathrm{°}}$+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)].