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

Abstract

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) occurring in the ¹B₂(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 6b¹₀ excitation (vibrational energy, E_v=530 cm⁻¹) the IVR process is first observable in n‐(t)‐butylbenzene where the IVR rate, k_vr, is ⩾5>10⁶ sec⁻¹. This rate increases to ⩾4×10⁷ sec⁻¹ and ≳10⁹ sec⁻¹ in 6b¹₀ excited n‐(t)‐pentyl‐ and n‐(t)‐hexylbenzene, respectively. For 12¹₀ excitation (E_v=933 cm⁻¹) k_vr⩾10⁸ sec⁻¹ for n‐(t)‐propylbenzene and k_vr×10⁹ sec⁻¹ for all higher n‐(t)‐alkylbenzenes. Vibrational relaxation is found to be substantially slower for alkylbenzenes in the gauche conformation: k_vr5 sec⁻¹ for 6b¹₀‐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.