Communication: Observation of local-bender eigenstates in acetylene

Abstract

We report the observation of eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene by stimulated emission pumping spectroscopy via vibrational levels of the S1 state involving excitation in the non-totally symmetric bending modes. The N(b) = 14 level, lying at 8971.69 cm(-1) (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the N(b) = 16 level, lying at 10 218.9 cm(-1), is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.

Size-Dependent πg + πu Combination Band Intensities of Polyynes C2nH2 (n = 1–9) Analyzed by the Local CCH Bending and the Linear Response Functions

“…It uses a single internal coordinate as the vibrational coordinate; it has been widely applied to stretching vibrations of CH and OH with high independence of other modes. − Hence, many researchers have investigated CH vibrations of triple-bonded carbons using acetylene as a target molecule. For example, Child et al studied CH stretching vibrations, McCoy and Sibert studied C 2 H 2 and HCN, − Henry et al − studied overtone levels of CH stretching and CCH bending vibrations for C 2 H 2 , and Field et al demonstrated − the applicability of the one-dimensional LM model to high-order large-amplitude CCH bending vibrations. Particularly, Field et al showed that a one-dimensional LM could incorporate anharmonicity with higher accuracy and lower calculation cost than multidimensional NM vibrations; it easily draws a physical picture from the calculation results.…”