Chemical timing 3. The picosecond dynamics of intramolecular vibrational redistribution from 11 levels in S1 p-difluorobenzene vapor

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Abstract
Collisional quenching of S1–S0 fluorescence (chemical timing) has been used to study the dynamics of intramolecular vibrational redistribution (IVR) from 11 initial vibrational levels in S1 p-difluorobenzene (pDFB). The levels range in energy from 1615 to 3310 cm−1 with total state densities ranging from about 10 to 2000 per cm−1. The IVR follows intermediate case kinetics of radiationless transition theory for levels below about 2500 cm−1 and statistical case kinetics for higher levels. The IVR involves coupled states spanning an energy width of 0.02–0.3 cm−1. The IVR lifetimes vary from about 15 to 300 ps with much variability among initial levels. A low frequency out-of-plane mode appears to be an accelerating mode for IVR. Average matrix elements for the level coupling associated with the IVR are found by analysis of the kinetic data to be in the range 0.01–0.1 cm−1. Vibration-rotation coupling appears to be an important contributor to IVR, at least for the lower levels.

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