Generalized Franck-Condon principle for resonant photoemission

A generalized Franck-Condon (GFC) principle for resonant x-ray Raman scatte ring and for resonant photoemission in particular is derived and numericall y investigated. The GFC amplitudes differ from ordinary FC amplitudes by th e presence of photon and photoelectron phase factors which describe the cou pling-or interference-of the x-ray photons or Anger electrons with the nucl ear motion. With the GFC amplitudes, a Kramers-Heisenberg relation is obtai ned for vibronic transitions that corrects the so-called lifetime-vibration al interference formula. For resonant photoemission in the soft-x-ray regio n involving typical bound potential surfaces, the generalization gives a co ntribution to the FC factors that can amount to 20%. For core excitation ab ove the dissociation threshold, the GFC principle relates to Doppler effect s on the ejected photoelectron both for the so-called "molecular'' and "ato mic" bands. The role of the GFC principle in direct photoionization is brie fly discussed. [S 1050-2947(99)08 809-5].