Vibrational relaxation in jet-cooled alkyl benzenes. I.Absorption spectra

Abstract

Fluorescence excitation spectra of a series of alkylbenzenes cooled in a supersonic free jet have been obtained for the first 1000 cm−1 region of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. The series includes all n-alkylbenzenes up to n-hexyl together with isopropyl- and tert-butylbenzene. As with toluene, the spectra in this region for all alkylbenzenes is found to be dominated by vibrations of five ring modes: 6a, 6b, 1, 12, and 18a. Three of these—the ’’system modes’’ (6b, 12, 18a)—are found to be largely invariant to changes in the alkyl chain length and type— the ’’bath.’’ For n-alkylbenzenes with chain length of three or higher, spectra of two distinct conformations are observed with roughly equal intensity. These conformations are distinguished by configuration about the 1–2 carbon–carbon bond of the alkyl chain. When this configuration is trans [n-(t)-alkylbenzene], the alkyl chain extends away from the phenyl ring leaving the ring free to van der Waals complex binding on both sides of the ring. The He2–n-(t)-alkylbenzene van der Waals complex is thus observed. When the conformation about the 1–2 bond is gauche [n-(g)-alkylbenzene] the alkyl chain partially covers one side of the phenyl ring. This produces a small red-shift of the spectrum (a self-induced solvent shift) and prohibits the formation of the ring-centered dihelium van der Waals complex.

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…The S 1 electronic state of Tol-h 8 has been the focus of a number of studies in the past as described above. The vibrational structure observed here compares well to the early jet-cooled fluorescence excitation spectrum of Hopkins et al, 54 and also to the later spectra reported by both Hickman et al, 3 and Doyle et al 24 Good agreement is also seen with the (1 + 1) REMPI spectrum recently reported by Davies et al 41 and the (1 + 1) REMPI spectrum recorded by Gunzer and Grotemeyer. 43 It is noted that the relative intensities of several bands observed in the (1 + 1) REMPI spectrum reported in the present work differ from those observed in previous studies, particularly the origin band; this is attributed to the varying UV intensity across the wavenumber range of this spectrum.…”

“…To higher and lower wavenumber there are weaker bands at 472.6 cm -1 and 451.8 cm -1 . In previous work, the assignment of the two most intense features to a Fermi resonance has been made; 3, 54 however, it is clear from other spectra, 24,54 and those presented here, that this region contains more than one contribution. Indeed, very recent time-resolved work (to be discussed further below), reports evidence for three contributions to the Fermi resonance.…”

“…Indeed, very recent time-resolved work (to be discussed further below), reports evidence for three contributions to the Fermi resonance. 41 In early work, 54 the two most intense features were assigned as being in Fermi resonance, but only one component was explicitly assigned (as M 11 ). In Ref.…”

The 700-1500 cm−1 region of the S1 ($\widetilde A{}^1B_2$Ã1B2) state of toluene studied with resonance-enhanced multiphoton ionization (REMPI), zero-kinetic-energy (ZEKE) spectroscopy, and time-resolved slow-electron velocity-map imaging (tr-SEVI) spectroscopy

“…As in the lower wavenumber range, the overall appearance of the Tol-h 8 spectrum is similar to previously reported jet-cooled spectra. 3,20,21 There are several intense bands observed above 700 cm −1 , which have mainly been assigned to vibrations of a 1 symmetry; however, several combination bands of overall b 2 symmetry are observed, which owe their intensity to Herzberg-Teller coupling to the S 2 electronic state.…”

“…Two features are observed at 734.4 and 753.2 cm −1 for Tol-h 8 , which are labeled "M" and "N"; these have been previously assigned as a Fermi resonance between two ZOS vibrations. 21,22 The assignment of the higher wavenumber vibrational eigenstate has been long-established as having a major contribution from the M 10 fundamental, 3,21,22 while Hickman et al 3 assigned the other ZOS of the Fermi resonance to the M 18 M 19 combination. This assignment was based on the vibrational activity in the dispersed fluorescence spectrum recorded from these intermediate levels, and is expected to form the major contribution to the lower wavenumber component of the Fermi resonance.…”