The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Abstract

Forty three vibronic levels of C2H2+, X 2Pi u, with upsilon4 = 0-6, upsilon5 = 0-3, and K = 0-4, lying at energies of 0-3520 cm(-1) above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1' two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the A1Au, 4nu3 (K = 1-2), 5nu3 (K = 1), nu2+4nu3 (K = 0), and 47,206 cm(-1) (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (upsilon4 = 0-3, K = 0-2) had been reported previously by Pratt et al. [J. Chem. Phys. 99, 6233 (1993)]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with upsilon4 + upsilon5 < or = 2, except for the unobserved upper (2Pi u component of the 2nu4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm(-1). The vibronic coupling parameter epsilon5 (about whose magnitude there has been controversy) was determined to be -0.0273(7). For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the upsilon4 + upsilon5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with upsilon4 + upsilon5 < or = 5, an rms deviation of 0.34 cm(-1) was obtained using 20 parameters. An interesting finding is that Hund's spin-coupling cases (a) and (b) both occur in the Sigmau components of the nu4 + 2nu5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91,953.77 +/- 0.09 cm(-1) (3sigma).

Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

“…Many experiments of this type have been carried out in order to characterize the higher vibrational levels of the ground state, [19][20][21][22][23][24][25][26][27][28] the Rydberg states, [29][30][31] and the ground state of the cation. [32,33] The first rotational analyses of the à # X system, over 50 years ago, [34][35][36] showed that C 2 H 2 , though linear in its ground electronic state, becomes trans-bent (point group C 2h ) in the à state. Consistent with this, the most obvious feature of the vibrational structure in absorption is a long progression in the trans-bending vibration, !…”

The Renner-Teller effect in HCCCl+(X̃2Π) studied by zero-kinetic energy photoelectron spectroscopy and ab initio calculations

“…Until recently, the most studied linear tetra-atomic molecules with RT effect are HCCS(X 2 Π), HCCH + (X 2 Π), and HCCO(B 2 Π). For HCCH + (X 2 Π), the rotationally resolved vibronic energy levels have been determined using high resolution photoelectron spectroscopy, [16][17][18] and the ab initio calculations have been found to be in good agreement with the experimental data. [19][20][21][22][23][24] However, there is no direct cross-mode vibronic coupling due to the inversion (i) symmetry.…”

Excited state corrections to looped adiabatic-to-diabatic transformation phases

“…The particular case of the acetylene molecular ion C 2 H þ 2 , with its ground state X 2 P u in the linear form, has been extensively treated, both theoretically and experimentally [26][27][28][29][30]. An interesting theoretical development was initiated by a series of papers [31][32][33][34], which considered distorted situations, such that the extremal two hydrogen atoms were bent away from the collinear axis.…”

“…The analytic result is finally assessed by comparing it to a solvable case (Section 4) and to a numerically obtained solution (Section 5). These arise from a Renner-Teller-type molecule [20][21][22][23][24][25][26][27][28][29][30], whose NACT aspects were recently considered [31][32][33][34][35][36].…”