Collisional relaxation of excited-state Zeeman coherences in atomic ytterbium vapor

We present the results of an experiment designed to probe the collision-induced relaxation of excited-state Yb Zeeman coherences. Our experiment is sensitive not only to the depolarization-type relaxation mechanisms often observed in optical-pumping experiments, but also to the effects of collisionally induced velocity changes. Our measurements were performed in the 6s6p ³{P}₁$ excited state of the even (I=0) isotopes of gas-phase atomic Yb by means of a trilevel photon-echo technique. Depolarization rates, velocity-change spectra, and total scattering cross sections of Yb-He (Yb-Ar) collisions were measured (deduced). Both rank-1 and rank-2 spherical tensor components of the ³P₁ state play a role in our experiment, and our results represent averages of potentially different relaxation rates attributable to these quantities. Comparison of our results with rank-0 results obtained elsewhere reveals interesting similarities as well as differences.