Noble Gas Induced Relaxation of the Li 3S-3P Transition Spanning the Short Term Impact Regime to the Long Term Asymptotic Regime
Photon echoes have a Doppler free character which allows one to study relaxation processes which would otherwise be hidden in the inhomogeniously broadened spectral profile. It has recently been shown, for example, that contrary to expection, a radiating atom in a linear superposition of dissimilar electronic states can undergo identifiable velocity changing collisions . Studies of this nature require an examination of the sub-Doppler region of the spectral line shape. The effect manifests itself, in the case of photon echoes, in a dependence of the effective relaxation cross section σeff on the excitation pulse separation τ. In this paper we report measurements in Li vapor where τ can be increased into the regime where σeff once again becomes independent of τ. In the limit τ=0 we measure σ0 which is the phase changing cross section as calculated by Baranger  while in the large τ limit we measure σ∞ the average total scattering cross section of the ground and the excited states. Our data at intermediate values of τ is used to determine the form of the scattering kernel and the average velocity change per collision. These measurements are for the 2S–2P superposition states in atomic Li perturbed by each of the noble gases. For He perturbers the scattering kernel is found to be Lorentzian, for the other perturbers it is Gaussian.
KeywordsVelocity Change Superposition State Pockels Cell Collision Kernel Photon Echo
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