Low frequency vibrational anharmonicity and nuclear spin effects of Cl(H) and Cl(D).

Low frequency combination bands of Cl(H) and Cl(D) have been measured in the region between 600 and 1100 cm by infrared predissociation spectroscopy in a cryogenic 22-pole ion trap using a free electron laser at the FELIX Laboratory as a tunable light source. The Cl(H) (Cl(D)) spectrum contains three bands at 773 cm (620 cm), 889 cm (692 cm), and 978 cm (750 cm) with decreasing intensity toward higher photon energies. Comparison of the experimentally determined transition frequencies with anharmonic vibrational self-consistent field and vibrational configuration interaction calculations suggests the assignment of the combination bands v + v, 2v + v, and 3v + v for Cl(H) and 2v + v, 3v + v, and 4v + v for Cl(D), where v is the Cl⋯H stretching fundamental and v is the Cl(H) bend. The observed asymmetric temperature dependent line shape of the v + v transition can be modeled by a series of ∑-∏ ro-vibrational transitions, when substantially decreasing the rotational constant in the vibrationally excited state by 35%. The spectrum of Cl(D) shows a splitting of 7 cm for the strongest band which can be attributed to the tunneling of the ortho/para states of D.