Experimental Study of Hyperfine State Changing Collisions in the Cs(6\textit{P}$_{1/2}$) State using Argon Perturbers

A two-step excitation experiment has been employed to measure the collisional rate coefficient and to study the velocity distribution of Cs(6$P_{1/2})$ atoms that have undergone a single hyperfine state changing collision with Ar. First, a single-mode, cw Ti:Sapphire laser is tuned to line center of the 6$S_{1/2}(F$ = 4) $\to $ 6$P_{1/2}(F'$ = 3 or 4) transition. Then, the frequency of a single-mode cw dye laser is scanned over the various 6$P_{1/2} \quad \to $ 8$S_{1/2}$ hyperfine transitions to measure relative populations in the 6$P_{1/2}$ hyperfine levels. Absorption of the probe laser is monitored by detecting 8$S_{1/2} \quad \to $ 6$P_{3/2}$ fluorescence. The experiment is conducted at room temperature, where the Cs density is low ($n \quad \sim $ 3.4 $\times $ 10$^{10}$ atoms cm$^{-3})$,and thus Cs-Cs collisions are negligible. The Ar pressure is varied from 0 -- 3 Torr, and Cs-Ar collisions cause transfer of population from the directly excited 6$P_{1/2}(F')$ level to the other 6$P_{1/2}$ hyperfine level. The data are analyzed in the single collision, strong pump, and weak probe limits to yield the rate coefficient for Cs(6$P_{1/2})$-Ar hyperfine state changing collisions. In addition, the one-dimensional velocity changing collision kernel for Cs(6$P_{1/2})$ atoms prepared with $v_{z}$ = 0 that undergo $F'$ = 3 $\leftrightarrow \quad F'$ = 4 hyperfine state changing collisions is reported, as is the 6$P_{1/2} \quad \to $ 8$S_{1/2}$ argon pressure broadening rate.