Double-resonance spectroscopy of the high Rydberg states of HCO. III. Multiple pathways in the vibrational autoionization of the bending overtone

Ab initio structure and vibration-rotation dynamics of the formyl and isoformyl cations, HCO+/HOC. J Chem Phys 2019;150:154307. [PMID: 31005073 DOI: 10.1063/1.5089718]

Accurate structure and potential energy surface of the formyl and isoformyl cation system, HCO⁺/HOC⁺, in its ground electronic state X̃ ¹Σ⁺ have been determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. Both the isomers are confirmed to be linear at equilibrium, with the total energy minimum of HOC⁺ lying 14 120 cm^-1 above that of HCO⁺ and the HCO⁺ → HOC⁺ isomerization energy barrier being 26 870 cm^-1 (in the Born-Oppenheimer approximation). The equilibrium structural parameters for HCO⁺ are estimated to be r_e(HC) = 1.0919 Å and r_e(CO) = 1.1058 Å, whereas those for HOC⁺ are estimated to be r_e(HO) = 0.9899 Å and r_e(CO) = 1.1544 Å. The vibration-rotation energy levels were predicted for various isotopologues using a variational approach and compared with the experimental data. For the spectroscopically well characterized formyl cation, the observed vibration-rotation energies and the effective rotational constants are reproduced to within about 2.3 cm^-1 and 1.7 MHz, respectively. The role of the core-electron correlation, higher-order valence-electron correlation, scalar relativistic, and adiabatic effects in determining the structure and vibration-rotation dynamics of both the isomers is discussed.

Mode dependent vibrational autoionization of Rydberg states of NO2. II. Comparing the symmetric stretching and bending vibrations

Triple-resonance excitation and high-resolution photoelectron spectroscopy are combined to characterize the mode selectivity of vibrational autoionization of the high Rydberg states of NO2. Photoelectron spectra and vibrational branching fractions are reported for autoionizing Rydberg states converging to the NO2+ X 1Sigmag +(110) state, that is, with one quantum in the symmetric stretch, nu1, and one quantum in the bending vibration, nu2. These results indicate that autoionization proceeds most efficiently through the loss of one quantum from the symmetric stretch rather than from the bending vibration. The implications of this result are discussed in terms of the autoionization mechanism.

Vibrational autoionization in rydberg states of ammonia

Rotationally resolved photoelectron spectroscopy is used to elucidate the mechanism for vibrational autoionization via the nontotally symmetric umbrella vibration of ammonia ( NH3). This mechanism depends strongly on the character of the autoionizing level, and the data demonstrate that both the electronic and rotational degrees of freedom can compensate for the change in vibrational symmetry that occurs for odd changes in the quantum number of the umbrella vibration. This result has implications for models of the mode dependence of vibrational autoionization in polyatomic molecules.

Experimental Characterization of the Higher Vibrationally Excited States of HCO(+): Determination of omega(2), x(22), g(22), 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' = 0 and N' = 2 rotational levels of the Sigma(-) Renner-Teller vibronic component of the 3ppi (2)Pi (030) complex define individual series converging to rotational levels, N(+) = 1 through 5 and 3 through 5 of the HCO(+) vibrational states (03(1)0) and (03(3)0), 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, omega(2) = 842.57 cm(-1), the vibrational angular momentum splitting constant, g(22) = 3.26 cm(-1), and the diagonal bending anharmonicity, x(22) = -2.53 cm(-1), separated from the off-diagonal contribution, x(12), by reference to ab initio calculations. Results of experiment on the higher vibrationally excited states of HCO(+) are compared with recent theoretical predictions. Copyright 2000 Academic Press.