EXCITATION TRANSFER AMONG, AND QUENCHING OF, THE BARIUM 6S5D D-3(J) METASTABLE LEVELS DUE TO COLLISIONS WITH ARGON, NITROGEN, AND BARIUM PERTURBERS

We describe a set of experiments that investigate excitation transfer among the barium 6s5d 3DJ metastable levels due to collisions with argon perturber atoms, and quenching of atoms in these levels by nitrogen perturber molecules. The metastable levels were populated through optical pumping of the 6s2 1S06s6p 3P1o intercombination transition with a pulsed laser, followed by stimulated emission or stimulated Raman scattering into the 6s5d 3D1,2 levels. Collisional mixing then distributed population throughout the 3DJ levels. Time-dependent absorption coefficients were obtained by measuring the transmission of a weak cw probe laser beam (after the pulsed laser fired) at several perturber number densities. Argon buffer gas was used to investigate fine-structure mixing, and nitrogen was used to study quenching. The time-dependent absorption coefficients were fitted to appropriate fitting functions, obtained by solving the set of coupled rate equations used to model the time evolution of the metastable level populations. The buildup rates of the initially unpopulated 3D3 and underpopulated 3D1 levels were used to determine the collisional mixing rates, while the measured decay rates of the 3DJ level populations yielded the quenching rates. From plots of buildup rates versus argon number density for the 3D1,3 states, we were able to determine four of the six fine-structure mixing rate coefficients kij. The results are k12=(1.710.18), k21=(1.390.13), k23=(1.420.13), and k32=(1.920.16)×10-13 cm3 s-1. The experiment is not sensitive to the values of k13 and k31. Estimated rate coefficients for excitation transfer among the 3DJ levels due to collisions with barium atoms are k21Ba=1.3×10-10 cm3 s-1 and k23Ba=5.8×10-10 cm3 s-1. A plot of the population decay rates versus N2 number density for each of the three metastable levels yielded the average nitrogen quenching rate coefficient kq=(3.140.19)×10-11 cm3 s-1. © 1994 The American Physical Society.