A theoretical study of the resonant multiphoton ionization from a molecular system: Relaxation effects

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Abstract
Effects of the vibrational and electronic relaxations on the resonant two-photon ionization from molecules are theoretically studied. Taking into account the relaxations in the resonant state, an expression for the ion current is derived by using the master equation involving zero-, one-, and two-photon processes in the Markoff approximation. The resultant ion current is expressed by the time-dependent and -independent terms. The time-dependence of the ion current is originated from the longitudinal relaxation, and on the other hand the time-independent term consists of both the coherent interaction of molecules with laser fields, and the elastic interaction between the molecules and the heat bath. Approximate expressions for the ion current for the slow and fast vibrational relaxation rates compared with the optical transition rate are also derived by using the matrix perturbation method. Model calculations of the ion current are performed to demonstrate the longitudinal relaxation effect for a model system characterized by the two-level resonant state.