MQDT analysis of radiationless decay rates of autoionizing rydberg states of polyatomic molecules

Fast intramolecular radiationless decay processes which compete with autoionization in the decay of high Rydberg levels of polyatomic molecules are taken into account by an absorptive imaginary part of the reactance matrix in the multi-channel-quantum-defect-type (MQDT) formalism. Calculations for a simple model involving two electronic ionization continua and a single vibrational mode reveal a dramatic impact of the decay processes on the near-threshold ionization cross section, in particular, a strong suppression of autoionizing series. The model calculations provide a semiquantitative explanation of the characteristic features of autoionization spectra recently obtained by two-color near-threshold photoionization in several polyatomic molecules.