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 ÖX 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 ν_6b 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 ν_16b and ν_10b 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–Ar_n clusters, which may be unstable with respect to Ar bending motions in the à state.