Single-vibronic–level fluorescence spectra of aniline and aniline–argon clusters in a supersonic jet

Abstract

A supersonic-jet apparatus has been constructed employing a continuous nozzle and large pumping capacity. A set of concave mirrors in a ‘Welsh’ arrangement is used inside the vacuum chamber for efficient collection of the fluorescence. The fluorescence excitation spectrum of the 000 band of the A–X system of aniline (AN) shows a characteristic rotational contour, and computer simulation, with type-B rotational selection rules, shows that the rotational temperature is ca. 10 K. A type-A contour computed with similar parameters shows that low-resolution type-A and -B contours may not be easy to distinguish and at low temperatures. The single-vibronic-level fluorescence (SVLF) spectrum of AN with 000 excitation shows the 6b01 band very weakly. Vibration ν6b is of b2 symmetry, and this type of vibronic coupling, which is much stronger in, for example, toluene, fluorobenzene and phenol, has been demonstrated in AN for the first time. SVLF spectra following excitation into the 16a20 and 10b20 bands, which are 6 cm–1 apart, confirms their assignments. Since ν16b and ν10b are of different symmetry, a2 and b1, respectively, and they have a similar wavenumber, there is a Darling–Dennison resonance between the 16a2 and 10b2 vibronic levels. SVLF spectra obtained by tuning the laser to the 000 band of the van der Waals complex AN–Ar1 show that the fluorescence occurs not only from the 00 level of AN–Ar1 but also from the 00 level of bare AN. It is suggested that the bare AN fluorescence originates from the pseudo-continuum which underlies the 000 band of AN–Ar1 and is due to larger AN–Arn clusters, which may be unstable with respect to Ar bending motions in the A state.