Stimulated photon echo for angular analysis of elastic and depolarizingYb/emph>–noble-gas collisions

Elastic and depolarizing collisions between excited ytterbium atoms (6s6p ${}^3$ $P_1$ level) and noble gases (He, Ne, and Xe) are investigated using a stimulated-photon-echo technique. The method is sensitive to collisional velocity changes concerning both the total population ${\rho }^0$ and the alignment ${\rho }^2$ of the level. More specifically, it enables us to compare precisely the velocity changes associated with elastic scattering and with alignment-preserving scattering. This represents an improvement with regard to previous works. A detailed theoretical discussion of depolarizing collisions in laser spectroscopy is presented. A simplified density-matrix equation is derived under appropriate approximations. Stimulated photon-echo data for elastic scattering are interpreted within the limits of small-angle scattering in a long-range van der Waals potential; total elastic scattering cross sections are derived from comparison between experiment and calculation: ${\mathit{\sigma }}_{\mathrm{He}}^{\mathrm{el}}$=220(20) A${̊}^2$, ${\mathit{\sigma }}_{\mathrm{Ne}}^{\mathrm{el}}$=320(20) A${̊}^2$, ${\mathit{\sigma }}_{\mathrm{Xe}}^{\mathrm{el}}$=1240(60) A${̊}^2$. Total rates for destruction of alignment are measured in an auxiliary standard photon-echo experiment; the corresponding cross sections are ${\mathit{\sigma }}_{\mathrm{He}}^{\mathrm{}\left(2\right)\mathrm{}}$=94(3) A${̊}^2$, ${\mathit{\sigma }}_{\mathrm{Ne}}^{\mathrm{}\left(2\right)\mathrm{}}$=123(7) A${̊}^2$, ${\mathit{\sigma }}_{\mathrm{Xe}}^{\mathrm{}\left(2\right)\mathrm{}}$=300(20) A${̊}^2$. Stimulated photon-echo data for depolarizing collisions are successfully compared with a calculation that we develop starting from a long-range anisotropic model potential.