Collision-induced Ramsey resonances in Sm vapor

We report detailed studies of Ramsey-type resonances due to collisional velocity diffusion of Sm atoms within the Doppler-broadened ${}_{}{}^7{}_{}{}^{}\mathrm{F}_1^{}{}_{}{}^{}$→${\mathrm{}}^7$ ${\mathrm{F}}_0$ ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ transition at λ=570.68 nm in the presence of rare-gas perturbers (He,Ne,Ar,Kr,Xe). The experimental technique uses counterpropagating laser fields and relies on coherent resonance Raman processes to excite and detect sublevel coherence optically. A modulated pump laser field is used for velocity-selective resonance Raman excitation of oscillating Zeeman coherence in the J=1 ground state of Sm; the sublevel coherence is monitored by Raman heterodyne detection using a counterpropagating, unmodulated probe laser field. If the pump and probe fields interact with different atomic velocity subgroups, a signal only occurs in the presence of velocity-changing collisions which provide a transport of the Sm atoms with sublevel coherence in velocity space. The sublevel resonance curves turn out to be sensitive to both depolarizing and velocity-changing collisions; the measured line shapes show the characteristic features of Ramsey-type resonances. Our theoretical treatment is based on a novel four-level density matrix calculation; the analysis reveals that the temporal evolution of the collisional velocity-diffusion processes can simply be obtained by means of a Fourier transformation of the measured sublevel resonance curves. In the case of He perturbers the diffusion in velocity space can be described by a Fokker-Planck equation; here, the theoretical fits of the measured sublevel resonance curves directly yield the time constant for collisional thermalization and the cross section for depolarizing collisions. In the case of the heavier rare gases, however, the Fokker-Planck model fails to describe the velocity thermalization; here, the Keilson-Storer kernel yields a better agreement with the experimental data. We derive cross sections for depolarizing and velocity-changing collisions and values for the collisional velocity changes associated with orientation-preserving collisions.