Optical coherent transient measurements of velocity-changing collisions in SF_6

Abstract

Elastic velocity-changing collisions have been observed for the first time to our knowledge in a nonpolar molecule by photon-echo studies of SF₆ using a frequency-switched CO₂ laser that we have developed. The photon-echo decay is nonexponential and reveals the velocity-changing collision cross section to be σ_vc = 180 Ų, with an average velocity change per collision of Δu = 155 cm/sec. Using delayed optical nutation, we also determine the cross section for the population decay rate (1/T₁) to be σ_pop = 335 Ų. To within experimental accuracy, we find that phase-interrupting collisions are not important (T₁ ≃ T₂). A comparison of these cross sections with those for the polar molecule ¹³CH₃F is made.