Vibrational coherence effects in the pump–probe studies of photochemical predissociation

We examine how the results of pump–probe experiments with short pulses are affected by the coherence of the excitation process, in the particular case when the pump causes the predissociation of the molecule. To document and understand this process we perform exact quantum mechanical calculations and examine simple models. The exact calculations are performed on a model of CH3ONO photodissociation. We examine the effect of coherence when the probe excites the transition complex and when it excites the NO molecule produced by photodissociation. In both cases the vibrational coherence leads to oscillations in the dependence of the two pulse laser-induced fluorescence (LIF) signal on the delay time between the pulses. In the first case the effect is due to the coherent excitation of the predissociation resonances. In the second the important factor is the coherence of the NO vibrational state produced by photodissociation. The physical factors affecting this process are discussed in detail. One of the interesting findings is that the relative translational energy distribution affects in an important way the manner in which vibrational coherence modifies the two pulse LIF signal.