LIF excitation spectra for S0 → S1 transition of anthranilic acid: Detailed studies

Laser-induced fluorescence (LIF) excitation spectra of jet-cooled molecules of anthranilic acid were investigated. In order to distinguish bands belonging to the monomer from those belonging to the dimer species three different spectra were recorded under conditions which differed in the value of partial pressure of vapours of anthranilic acid. Narrow bandwidth of rotational profiles in the acquired spectra made it possible to resolve new interesting spectral features and to analyse the rather small anharmonicity in progressions of several low-frequency vibrations. Eleven fundamental bands up to ca. 1400 cm⁻¹ and five overtones of out-of-plane vibrations were assigned. Simulation based on approximate relations of the experimental band intensities of overtones and combination bands with respect to fundamental bands was carried out. This simulation was found a simple and practical tool in analysis of the spectra and an aid to verify the proposed assignment.Harmonic and anharmonic frequencies of vibrational modes were calculated using the HF/6-31G∗∗ and CIS/6-31G∗∗ methods for the S₀ and S₁ state, respectively. Modelling of band intensities using displacement parameters derived from the results of the ab initio calculations was performed. This modelling significantly underestimates the displacement parameters. The Dushinsky matrix derived from the results of the ab initio calculations was used to check the validity of the models and to find modes which undergo large frequency changes and mode mixing upon electronic excitation.