The Resonance Fluorescence of Benzene

Abstract

The fluorescence of benzene vapor has been studied at pressures varying from 25 to 0.01 mm mercury using, as a monochromatic excitation source, the 2536A mercury line. At the lowest investigated pressures the emission spectrum consists of several narrow line groups. Their spacing shows that emission occurs in transitions from a definite vibrational level of the excited state to various vibrational levels of the normal state. The following vibrational frequencies of the normal benzene molecule have been identified: 160, 793 (uncertain), 988, 1200, 1354, 1663 and 3139 ${\mathrm{cm}}^{-1\mathrm{}}$. The fine structure of the line groups has not been fully resolved. They appear to consist each of two strong lines with several weaker ones grouped around them. The ordinary, high pressure, fluorescence of benzene vapor is interpreted as being due to transitions from the lowest vibrational level of the excited state (or states). This level is reached by molecules as the result of inelastic collisions upon excitation. An apparently continuous emission present at the long wave-length end of the fluorescence spectrum at higher vapor pressures is attributed to two causes, decreased intensity of bands heads owing to an increased moment of inertia of the molecule in higher vibrational levels of the normal state, and a crowding of vibrational levels when the total vibrational energy is large. This conclusion is supported by observations of the fluorescence of toluene, which appears to be almost wholly continuous.