Diffusion of barium atoms in the 6s5d 3DJ metastable levels and the 6s2 1S0 ground state through noble-gas perturbers.

We describe a set of experiments that investigate diffusion of barium 6s5d DJ metastable level and 6s S0 ground-state atoms through the noble gases: He, Ne, Ar, Kr, and Xe. The barium metastable levels were populated through optical pumping of the 6s S0→6s6p P 1 intercombination transition with a pulsed laser, followed by spontaneous or stimulated emission into the 6s5d D1,2 levels. Collisional mixing then distributed population throughout the 6s5d DJ levels. A weak narrow-band cw laser was tuned to appropriate transitions to probe the spatial distribution of metastable level atoms, as well as the hole created in the ground-state spatial distribution, by the pulsed laser. Time-dependent absorption coefficients were obtained by measuring the transmission of the probe laser at several different pump-probe spatial separations. From these absorption data, spatial number density profiles were mapped out for various times following the firing of the pump laser. These density profiles were used to determine the diffusion coefficient for each state at a particular buffer-gas pressure and temperature. From these pressureand temperature-dependent diffusion coefficients D , we determined the thermally averaged diffusion cross sections sD(6s5d DJ) and sD(6s 2 S0) for the metastable and ground-state atoms diffusing through the noble gases, respectively. We also report values for the scaled standard diffusion coefficients D0 . Our experimental values of the thermally averaged diffusion cross sections sD(6s5d DJ) and sD(6s 2 S0) for the barium-helium system are compared with values calculated from published, theoretical barium-helium potentials and the agreement is excellent. @S1050-2947~96!07706-2#