Molecular quantum dynamics in a thermal system: Fractional wave packet revivals probed by random-phase fluorescence interferometry

The method of coherence observation by interference noise (COIN) [Kinrot , Phys. Rev. Lett. 75, 3822 (1995)] has been shown to be a useful tool for me asurements of wave packet motion at the quantum-classical border. We presen t the first systematic interferometric study of fractional vibrational revi vals in the B state of thermal iodine (I-2) vapor. Experimental COIN interf erograms ranging from 200 fs to 40 ps are presented for various excitation wavelengths. The complex temporal structure of the observed fluorescence in cludes rapid initial damping in the short-time regime and the appearance of quarter- and half-revivals on the quantum-mechanical long-time scale. Thes e features arise from a delicate balance between rotational and vibrational molecular coherences. The clear observation of the wave packets on the lon g time scale is possible due to the long-time stability of the COIN interfe rometer. Lowest-order perturbative solutions nicely recover the experimenta l results, and closed-form analytical expressions based upon the factorizat ion approach and the Poisson summation give insights into the nature of dep hasing and rephasing of vibrational wave packets subject to rotational inho mogeneous broadening. (C) 2001 American Institute of Physics.