Probing intramolecular vibrational energy re-distribution by using a pair of femtosecond-laser pulses: a theoretical model

We propose a method of probing intramolecular vibrational energy re-distribution (IVR) by using a pair of femtosecond laser pulses. This does not rely on the idea of localized wave packets and is applicable to polyatomic molecules which undergo rapid IVR. The first pulse produces a non-stationary wavefunction composed of several vibrational eigenstates. After a time interval, comparable with the time scale of IVR, some components of the non-stationary wavefunction are depleted by the second pulse. The populations in these eigenstates are detected by observing dispersed fluorescence. Numerical simulations based on a coupled oscillator model demonstrate that the second pulse depletes specific components selectively. The populations after the second pulse reflect how the IVR proceeds until the second pulse is introduced.