Model study of near-threshold photoionization of large molecules: The effect of vibrational relaxation

The influence of intramolecular vibrational relaxation in large molecules on near-threshold photoabsorption and photoionization processes is investigated. The vibrational relaxation results from coupling of the initially excited modes to a large number of inactive bath modes. Starting from a model Hamiltonian including all vibrational modes, the bath degrees of freedom are eliminated within the Markov approximation using Hilbert-space projection-operator techniques. Additional Feshbach projection techniques and a threshold expansion of Coulomb Green's function are used to cast the resulting expressions into a numerically tractable form. Predissociation channels are included in a phenomenological manner. The numerical results allow us to study the characteristic effects of vibrational relaxation on absorption and ionization spectra. In particular, the competition between autoionization, predissociation and vibrational relaxation of the ion core is investigated. The suppression of the ionization quantum yield above the lowest ionization threshold as a result of these two decay channels is demonstrated. It is shown that the quenching of autoionization by vibrational relaxation or predissociation can be experimentally distinguished on account of the different scaling behaviour of both processes.