Ion-lattice relaxation processes and spectroscopic linewidths

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Abstract
Raman contributions to the width of a no-phonon line are calculated for a two level system. The existence of the semidiagonal process previously suggested is confirmed, and it connection with the stabilization of the lattice around an electronic excitation is examined. The theory also predicts a new self energy process, in which the Raman transitions eventually return the electronic excitation to its original state. This process demonstrates the inequivalence of the spin-lattice relaxation times as measured by population equilibration and from spectroscopic linewidths. The existence of such linewidth contributions suggests that the correct statement of the well known relation between spectroscopic linewidths and lifetime effects is that the full width at half power of the spectral line equals the sum of the inverse lifetimes of the two states (before and after the photon interaction) with respect to the broadening interaction.