Femtosecond Raman-induced polarisation spectroscopy of coherent rotational wave packets: D2, N2 and NO2

Femtosecond Raman-induced polarisation spectroscopy of N2, D2 and NO2 was used to study the dynamics of coherent rotational wave packets in the ground electronic state with a time resolution ∼50 fs. The periods of the observed signal beats agree with theoretical predictions. Special attention was given to the effect of the pump light pulse chirp upon this dynamics. For the N2 and D2 molecules, the sign and value of the chirp of the pump pulse affect the amplitude and form of the observed signal, while for NO2 influence of the chirp was not seen. This shows that the chirp is significant for molecules with a large rotational constant. Evidence was obtained that for NO2 at short delay times the contribution to the signal is given by the vibrational wave packet in the excited electronic state on the background determined by the rotational dynamics in the ground state.