Millimeter-wave spectroscopy of the internal rotation hot band (j = 2-1) of the Ar-HCN complex

Millimeter-wave absorption spectroscopy combined with a pulsed jet expansion technique was applied to measure the internal rotation j = 2-1 hot band of the Ar-HCN complex in the frequency region of 147-287 GHz. In total 153 rovibrational lines, split into hyperfine components due to the nitrogen nucleus, were assigned to the ∑₂-∑₁, ∑ ₂-Π₁, Π₂-∑₁, Π ₂-Π₁, Δ₂-∑₁, and Δ₂-Π₁ subbands. A set of molecular constants for the ∑₂, Π₂, and Δ₂ internal rotation substates, including subband origins, rotational constants, nuclear quadrupole coupling constants, and Coriolis interaction constants, was determined. The internal rotation energy for the Σ₂ state, 412.8949 GHz, is higher than those for the Π₂ and Δ ₂ states, 392.3974 and 355.9570 GHz, by 20.498 and 56.938 GHz, respectively, in contrast to the ∑₁ state located by 17.094 GHz lower than the Π₁ state, the anisotropy of potential energy surface affecting the j = 2 and j = 1 states differently. The rotational and quadrupole coupling constants in the j = 2 excited state are quite different from those in the ground state, indicating drastic change in the average structure in the j = 2 state from the ground state. The determined molecular constants were compared with those calculated from the potential energy surface computed at the CCSD(T) level.