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.