Local-mode overtones. Ultrashort pulse excitation, intramolecular relaxation and unimolecular reactions

Abstract

The dynamics of the preparation of local-mode overtone states in model systems have been considered where the excitation competes with relaxation of the initial state into either a reaction continuum or an intramolecular quasi-continuum. The overtone excitation process is many orders of magnitude slower than either vibrational relaxation or unimolecular decay, so that the prepared state is actually not localized. We demonstrate that when a continuum or quasi-continuum is present the prepared state is a sensitive function of the duration of the excitation pulse. Correspondingly, the absorption spectrum depends on the pulse duration, particularly when the pulse length is comparable to the IVR or reaction time. As such, ultrashort laser pulses are capable of probing the detailed intramolecular vibrational dynamics associated with decay of localized excitations. In the case of intramolecular relaxation into a quasi-continuum of vibrational bath states, the pulse-length-dependent spectrum reveals a sequential appearance of states which contribute to IVR. We have labelled these states as IVR resonances. In the case of unimolecular decay into a reactive continuum, the photochemical yield can be a non-monotonic function of the pulse length. This indicates the simultaneous competition amongst ultrashort pulse excitation, intramolecular dynamics and unimolecular reaction.