Collapse of vibrational structure in spectra of resonant x-ray Raman scattering

Extreme narrowing, or collapse, of electron-vibrational bands is predicted as a new phenomenon in resonant radiative and nonradiative x-ray scattering (RXS) spectra. It is shown that in the inelastic scattering case, that is, when the potential surfaces of ground and final states are different, a considerable narrowing of the vibronic RXS band results from detuning the excitation photon frequency $\omega$ away from the absorption resonance. By fine tuning the frequency, this may under special circumstances also occur in the region of strong photoabsorption. In the case of elastic Rayleigh scattering, that is, when the potential surfaces of ground and final states are identical, the narrowing results in a total collapse to a single resonance by detuning the frequency. The theory predicts how the differencies in vibrational structure of resonant and nonresonant photoemission spectra depends on the excitation frequency. The effect of frequency detuning on complex spectral multilevel structures due to multiplet and spin-orbit splittings is discussed qualitatively. It is shown that the notion of duration time for the x-ray scattering process plays a crucial role in the understanding of RXS spectra.