Excess energy dependence of internal conversion in the S1 state of azulene

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low-frequency modes is offered.