Vibronic spectra of the matrix-isolated van der Waals complexes NH·N2 and NH·CO Original Research Article

NH · N2 and NH · CO van der Waals complexes have been prepared in rare gas matrices by in situ photolysis of the precursor molecules HN3 and HNCO, and by in situ photolysis of NH3 in rare gas matrices doped with N2 and CO, respectively. NH · CO has been detected in absorption on the A 3Π ← X 3Σ− transition. Its a → X fluorescence yield is very low, but not zero. The perturbing N2 neighbour red shifts the NH b 1Σ ↔ +X3Σ− transition by 0.07%, and the NH a 1Δ ↔ X 3Σ− transition by 0.25%, from their origins in pure argon matrices. Rotation of the NH moiety in argon is suppressed by the N2 neighbour. High resolution spectra of the a 1Δ ← X 3Σ− transition consist of 10 closely spaced lines in argon ( widths between 0.18 and 0.35 cm−1), and of 6 lines in krypton. The lines are assigned to five double substitutional sites which host the NH · N2 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 are 4.9:3.0:2.1. Each site contributes a doublet, which is due to electronic degeneracy lifting in the a 1Δ state. The relative abundances of the sites and the splittings (ranging from 2 to 7.6 cm−1) are practically unaffected by deuteration. The number of sites can be related with our previous conclusion that NH is trapped in substitutional sites of (local) Oh and D3h, symmetry. Annealing experiments reveal that NH is perturbed by N2 only in its first coordination shell. Local modes, including site-specific modes of very low frequencies, are tentatively interpreted by means of model calculations.