DIFFUSION OF BARIUM ATOMS IN THE 6S5D D-3(J) METASTABLE LEVELS AND THE 6S(2) S-1(0) GROUND-STATE THROUGH NOBLE-GAS PERTURBERS

We describe a set of experiments that investigate diffusion of barium 6s5d D-3(J) metastable level and 6s(2) S-1(0) ground-state atoms throu gh the noble gases: He, Ne, Ar, Kr, and Xe. The barium metastable leve ls were populated through optical pumping of the 6s(2) S-1(0)-->6s6p P -3(1)0 intercombination transition with a pulsed laser, followed by sp ontaneous or stimulated emission into the 6s5d D-3(1,2) levels. Collis ional mixing then distributed population throughout the 6s5d D-3(J) le vels. A weak narrow-band cw laser was tuned to appropriate transitions to probe the spatial distribution of metastable level atoms, well as the hole created in the ground-state spatial distribution, by the puls ed laser. Time-dependent absorption coefficients were obtained by meas uring the transmission of the probe laser at several different pump-pr obe spatial separations. From these absorption data, spatial number de nsity profiles were mapped out for various times following the firing of the pump laser. These density profiles were used to determine the d iffusion coefficient for each state at a particular buffer-gas pressur e and temperature. From these pressure- and temperature-dependent diff usion coefficients D, we determined the thermally averaged diffusion c ross sections sigma(D)(6s5d D-3(J)) and sigma(D)(6s(2) S-1(0)) for the metastable and ground-state atoms diffusing through the noble gases, respectively. We also report values for the scaled standard diffusion coefficients D-0. Our experimental values of the thermally averaged di ffusion cross sections sigma(D)(6s5d D-3(J)) and sigma(D)(6s(2) S-1(0) ) for the barium-helium system are compared with values calculated fro m published, theoretical barium-helium potentials and the agreement is excellent.