Experimental Characterization of the Higher Vibrationally Excited States of HCO⁺: Determination of ω₂, x₂₂, g₂₂, and B(030)

Analyses of high Rydberg series of HCO converging to the (030) vibrational state of the cation establish rovibrational state-detailed thresholds for HCO⁺. UV-visible laser double resonance isolates series for assignment. Strongly vertical Rydberg-Rydberg transitions from photoselected N′ = O and N′ = 2 rotational levels of the ∑^- Renner-Teller vibronic component of the 3pπ ²Π (030) complex define individual series converging to rotational levels, N⁺ = 1 through 5 and 3 through 5 of the HCO⁺ vibrational states (03¹0) and (03³0), respectively. Extrapolation of autoionizing series locates the positions of these rovibrational states to within ±0.01 cm^-1. The use of this information combined with precise ionization limits for lower vibrational states determined from earlier Rydberg extrapolations and spectroscopic information available from infrared absorption measurements enables an estimate of the force-field parameters for HCO⁺ bending. These parameters include the harmonic bending frequency, ω₂ = 842.57 cm^-1, the vibrational angular momentum splitting constant, g₂₂ = 3.26 cm^-1, and the diagonal bending anharmonicity, x₂₂ = -2.53 cm^-1, separated from the off-diagonal contribution, x₂₂, by reference to ab initio calculations. Results of experiment on the higher vibrationally excited states of HCO⁺ are compared with recent theoretical predictions.