Studying vibrational wavepacket dynamics by measuring fluorescence interference fluctuations

The principle of coherence observation by interference noise [COIN, Kinrot , Phys. Rev. Lett. 75, 3822 (1995)] has been applied as a new approach to measuring wavepacket motion. In the COIN experiment pairs of phase-randomized femtosecond pulses with relative delay time tau prepare interference fluctuations in the excited state population, so the correlated noise of fluorescence intensity-the variance varF(tau)-directly mimics the dynamics of the propagating wavepacket. The scheme is demonstrated by measuring the vibrational coherence of wavepacket motion in the B-state of gaseous iodine. The COIN interferograms obtained recover propagation, recurrences and spreading as the typical signature of wavepackets. The COIN measurements were performed with precisely tuned excitation pulses which cover the bound part of the B-state surface up to the dissociative limit. In combination with preliminary numerical calculations, comparison has been made with results from previous phase-locked wavepacket interferometry and pump-probe experiments, and conclusions drawn about the limitations of the method and its applicability to quantum dynamical research. (C) 2000 American Institute of Physics. [S0021-9606(00)01011-4].