Vibrational relaxation in jet-cooled alkylbenzenes. II. Fluorescence spectra

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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, 12 01, and 18a01 vibronic bands of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. Resonance fluorescence from the O 00 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 6b01, 1201, and 18a 01 excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) occurring in the 1B 2(ππ*) state prior to emission. By comparing the relative intensity of the resonance and relaxed fluorescence, a lower limit to the rate of the IVR process has been measured as a function of the type of system mode excited (6b1, 121, or 18a1) and as a function of the size and nature of the bath (the alkyl chain in its various conformations). As expected this IVR rate increases with increasing alkyl chain length. For 6b01 excitation (vibrational energy, E ν=530 cm-1) the IVR process is first observable in n-(t)-butylbenzene where the IVR rate, kνr, is ≥5>10 6 sec-1. This rate increases to ≥4×107 sec-1 and > 109 sec-1 in 6b£ excited n-(t)-pentyl- and n-(t)-hexylbenzene, respectively. For 1201 excitation (Eν=933 cm-1) k νr≥108 sec-1 for n-(t)-propylbenzene and kνr×109 sec-1 for all higher n-(f)-alkylbenzenes. Vibrational relaxation is found to be substantially slower for alkylbenzenes in the gauche conformation: kνr<6× 105 sec-1 for 6b01-excited n-(g)-pentylbenzene. With increasing alkyl chain length the spectrum of the relaxed fluorescence becomes sharper and closer in frequency and pattern to the resonance fluorescence of the 000-excited molecule. This indicates an increasing involvement in the IVR process of bath modes in the alkyl chain which are far removed from the benzene ring. © 1980 American Institute of Physics.

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The Journal of Chemical Physics

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