Spectroscopic characterization of the lowest Π and Σ bending states of ArHCN

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Abstract
The lowest excited bending states, SIGMA1 and PI1, of the ArHCN complex have been measured by millimeter-wave electric resonance optothermal spectroscopy. The principal molecular constants determined for the SIGMA1 state are v0 = 164 890.790(12) MHz; B = 1958-8571(37) MHz; D = -0.075 23(29) MHz; eq(aa)Q = 0.825(27) MHz; and mu(a) = -0.521(30) D. For the PI1 state, the constants are nu0 = 181 984.4126(47) MHz; B = 2031.3624(17) MHz; D = 0.153 35(16) MHz; eq(aa)Q = 0.904(11) MHz; and mu(a) = 0.273 02(63) D. The leading SIGMA1-PI1 coupling constants are the Coriolis coefficient beta0 = 1016.998 (13) MHz and the transition dipole moment mu(b) = 2.2535(57) D. The rotational constants for the two bending states indicate that the average separation between the argon and the HCN center of mass contracts by roughly 0.5 angstrom compared to the linear ground state. This is consistent with the nearly T-shaped average geometry for each state established by analysis of the dipole moments and quadrupole coupling constants. Agreement between this work and prior theory confirms attribution of the anomalous distortion and isotope effects in the ground state to extreme angular-radial coupling. The relative sign of the dipole moments for the SIGMA1 and PI1 states is resolved in this work, allowing an unambiguous interpretation of the angular information. Assuming Laguerre angular distributions, we obtain that the SIGMA1 state wave function has a maximum at an angle of 108-degrees with a halfwidth of 49-degrees, and that the PI1 state maximum is at 80-degrees with a halfwidth of 37-degrees. This estimate for the SIGMA1 state angular distribution indicates that although the state is not antilinear (ArNCH), as was expected, it does approach this configuration. The PI1 state is nearly a free rotor eigenstate, showing that the angular part of the potential surface near 90-degrees is extremely flat. The combined data from the ground, PI1, and PI1 states reflect virtually the entire angular coordinate along the radial minimum of the potential, and should provide a reliable benchmark for ab initio potential energy surfaces for ArHCN near the bottom of the well. We compare the data to predictions from available