Resonance Fluorescence of Benzene. II

Abstract

Further study of the resonance fluorescence of benzene with an improved technique is described and the analysis of the spectrum given some time ago by Kistiakowsky and Nelles is amplified and corrected. The new assignment shows that, upon absorption of the 2536A mercury line, only the changes of the quantum number of one (ν1; ω = 990 cm—1) vibration of the nonexcited benzene molecule are unrestricted in fluorescence. In other vibrations the changes of the quantum numbers are not larger than unity (or two when required by selection rules). This interpretation of the resonance spectrum accounts for most of the line groups measured on the new plates. Five frequencies, besides ν1, or their multiples can be tentatively identified; they are: 432; 791; 1542; 2557 and 3174 cm—1. The fluorescence of hexadeuterobenzene has also been studied and the spectrum interpreted in the same manner but not as completely as that of benzene. The frequencies observed are: 944 (Δν unrestricted); 2460. The quenching of the resonance fluorescence of benzene has been investigated with five different gases. This fluorescence is quenched by being changed into high pressure fluorescence. There is very little, if any, loss of electronic excitation energy by benzene molecules on collisions. The quenching action is quite unspecific and appears to increase as the kinetic cross section of the foreign gas molecules. Some new high pressure fluorescence bands have been observed in benzene in the presence of foreign gases. These bands support V. Henri's analysis of the absorption spectrum as consisting of two close electronic levels of the excited molecule. Attempts to excite resonance fluorescence of benzene derivatives have, on the whole, been unsuccessful. Fluorobenzene and toluene, even at 0.01 mm pressure, emit spectra which, by using a spectrograph with a resolving power of better than 25,000, appear nearly continuous. Other derivatives tried have too little fluorescence to study it at low pressures.