Vibrational relaxation in jet‐cooled alkyl benzenes. II. Fluorescence spectra

Dispersed fluorescence spectra of a series of laser‐excited n‐alkylbenzenes cooled in a supersonic free jet have been obtained under effectively collision‐free conditions. The series includes all members of the n‐alkylbenzene family from methyl‐ through n‐hexylbenzene. Pulsed laser excitation was cleanly made into the 000, 6b01, 1210, and 18a10 vibronic bands of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. Resonance fluorescence from the O00 excitation experiments permitted clear assignment of the six ring modes (6a, 6b, 1, 12, 18a, 9a) which in combinations and short progressions dominate the fluorescence spectra of these molecules. Except for 6a and 1, the frequency of these ground state vibrations is found to be constant throughout the alkylbenzene series to better than 1 part in 100. Fluorescence from the 6b10, 1210, and 18a10 excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) oc...