Effects of velocity-changing collisions on two-photon and stepwise-absorption spectroscopic line shapes

Abstract

We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3S_{\frac{1}{2}}\to 3P_{\frac{1}{2}}\to 4D_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3S_{\frac{1}{2}}\to 3P_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited $3P_{\frac{1}{2}}$ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-Störer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the $3P_{\frac{1}{2}}$ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium $3P_{\frac{1}{2}}$ atoms and helium, neon, and krypton perturbers.