Picosecond observations of quantum-beat phenomena in 2,5-diphenylfuran and argon complexes

Fluorescence time profiles have been measured for vibronically excited PPF and its 1:1 van der Waals complexes with Ar and CH4. Fluorescence quantum beats (685 MHz) are exhibited in PPF at the low energy of 256 cm−1. Overtones and some combination levels involving the 256 cm−1 mode also show quantum beats, ranging in frequency from 160 MHz to 3.7 GHz, whereas excitation into other levels up to 877 cm−1 shows little vibrational coupling. Experiments with PPF/Ar1 and PPF/(CH4)1 show that this low-energy level mixing in the bare molecule provides a channel for extensive coupling (τIVR = 300 ps (Ar), 115 ps (CH4) from 2561). On the other hand, excitation of either complex at 867 cm−1 reveals weaker coupling, despite the large relative increase in energy; significantly, argon-atom predissociation at this energy is preceded by beat-modulated resonance fluorescence.