Highly resolved UV spectroscopy: structure of S1 benzonitrile and benzonitrile-argon by correlation automated rotational fitting

Highly resolved UV spectra of benzonitrile and benzonitrile-argon were measured by resonance enhanced two-photon ionization. Rotational constants are obtained by correlation automated rotational fitting (CARF) of the rotational fine structure in the spectra. For the benzonitrile monomer the observed line width of less than 100 MHz points to a relaxation rate smaller than 109 s−1. The main origin for the nonradiative relaxation is expected to be internal conversion to the S1 charge transfer state. The position of the Ar atom in electronically excited (S1) benzonitrile-argon is 3.48(1) Å above the molecular plane with a slight displacement of 0.535(4) Å towards the CN group.