Low frequency vibrational anharmonicity and nuclear spin effects of Cl-(H2) and Cl-(D2).

Low frequency combination bands of 35Cl-(H2) and 35Cl-(D2) have been measured in the region between 600 and 1100 cm-1 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 35Cl-(H2) (35Cl-(D2)) spectrum contains three bands at 773 cm-1 (620 cm-1), 889 cm-1 (692 cm-1), and 978 cm-1 (750 cm-1) 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 v1 + v2, 2v1 + v2, and 3v1 + v2 for 35Cl-(H2) and 2v1 + v2, 3v1 + v2, and 4v1 + v2 for 35Cl-(D2), where v1 is the 35Cl-⋯H2 stretching fundamental and v2 is the Cl-(H2) bend. The observed asymmetric temperature dependent line shape of the v1 + v2 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 35Cl-(D2) shows a splitting of 7 cm-1 for the strongest band which can be attributed to the tunneling of the ortho/para states of D2.