Molecular wave packet revivals probed by phase-randomized fluorescence (COIN) interferometry

Summary form only given. In traditional wave packet interferometry, the dynamics of a quantum wave packet is studied by preparing in the same system a second well defined time delayed probe wave packet, and detecting the resulting interference signal between them. This observation imposes stringent requirements on the stability of the delay (including the relative phase) between the pulses. A new method recently introduced for coherence observation by interference noise (COIN), where a pair of time delayed, randomly phased pulses is used for the excitation, avoids the stability problem by measuring the fluctuations about the average signal, instead of the interference signal itself. We investigate molecular iodine at finite temperatures as a model for molecular systems. A compact analytical expression was derived for the time-dependence of the interference noise excited from a thermal mixture of vibrational and rotational states of the ground electronic potential. Despite the rotational broadening leading to an apparent decay of the COIN signal, a clear interference signal reflecting the motion of the vibrational wave packet can be seen. Moreover, although the initial coherences vanish within several vibrational periods, a revival of the signal can be observed later, after many vibrational periods. Due to the rotational recurrence, remnants of vibrational (fractional and hill) revivals can be observed long after the initial rotational decay. These structures-on the short time scale as well as on the long time scale-can be interpreted as involving both rotational and vibrational coherences. Detailed numerical simulations have been compared to the results of femtosecond experiments on molecular iodine in a room temperature vapor cell, showing good agreement on most features (qualitative as well as quantitative) of the observations.