VIBRONIC SPECTRA OF THE MATRIX-ISOLATED VAN-DER-WAALS COMPLEXES NH-CENTER-DOT-N2 AND NH-CENTER-DOT-CO

NH.N-2 and NH.CO van der Waals complexes have been prepared in rare ga s matrices by in situ photolysis of the precursor molecules HN3 and HN CO, and by in situ photolysis of NH3 in rare gas matrices doped with N -2 and CO, respectively. NH.CO has been detected in absorption on the A (3) Pi <-- X (3) Sigma(-) transition. Its a --> X fluorescence yield is very low, but not zero. The perturbing N-2 neighbour red shifts th e NH b (1) Sigma(+) <-> X (3) Sigma(-) transition by 0.07%, and the NH a (1) Delta <-> X (3) Sigma(-) transition by 0.25%, from their origin s in purl argon matrices. Rotation of the NH moiety in argon is suppre ssed by the N, neighbour. High resolution spectra of the a (1) Delta < -- X (3) Sigma(-) transition consist of 10 closely spaced lines in arg on (widths between 0.18 and 0.35 cm(-1)), and of 6 lines in krypton. T he lines are assigned to five double substitutional sites which host t he NH.N-2 van der Waals molecule in constant proportions of 3.6:3.2:1. 2:1:1 in argon, and to three sites in krypton, where the proportions a re 4.9:3.0:2.1. Each site contributes a doubler, which is due to elect ronic degeneracy lifting in the a (1) Delta state. The relative abunda nces of the sites and the splittings (ranging from 2 to 7.6 cm(-1)) ar e practically unaffected by deuteration. The number of sites can be re lated with our previous conclusion that NH is trapped in substitutiona l sites of (local) O-h and D-3h symmetry. Annealing experiments reveal that NH is perturbed by N-2 only in its first coordination shell. Loc al modes, including site-specific modes of very low frequencies, are t entatively interpreted by means of model calculations.