The direct observation, assignment, and partial deperturbation of ν5 and ν3+ν5 in A 1Au acetylene (C2H2)

Tobiason, J. D.; Utz, A. L.; Crim, F. F.
July 1993
Journal of Chemical Physics;7/15/1993, Vol. 99 Issue 2, p928
Academic Journal
A pulsed-laser double resonance technique (vibrational overtone excitation combined with laser-induced fluorescence detection) provides previously unavailable spectroscopic data on the rovibrational structure of A 1Au acetylene (C2H2). We collect fluorescence excitation spectra of transitions to vibronic levels lying between 2800 and 4300 cm-1 above the A state origin. In this region, we observe only two vibronic levels that are relatively unperturbed, which we assign to the A state antisymmetric C–H stretching fundamental vibration ν’5 and its combination with the trans-bending vibration, ν’3 + ν’5. Parity and symmetry selection rules for the A←X band, ab initio predictions for the ν’5 fundamental frequency, and the known frequencies of other A state vibrations permit an unambiguous assignment of the vibrations. The fit of ν’5 and ν’3 + ν’5 to a near-prolate asymmetric top Hamiltonian yields the observed vibrational frequencies (ν’5= 2857.4 ± 0.2 cm-1 and ν’3 + ν’5 = 3894.4 ± 0.1 cm-1) and rotational and centrifugal distortion constants. We deperturb the first-order a-axis Coriolis interactions of remote perturbers with ν’5 and ν’3 + ν’5 to obtain partially deperturbed a-axis rotational constants. We discuss other weak perturbations and present an energy level diagram for the vibrational states with ungerade symmetry in A state acetylene.


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