The IR−UV double resonance spectroscopy has been applied to observe the rovibronic level system of the ungerade nν3‘ + ν4‘ and nν3‘ + ν6‘(n = 2, 3) vibrational states in the Ã1Ag(S1) state of acetylene which are accessible from the selected rotational level J‘ ‘ of the ν3‘ ‘ state in the X̃1Σu + state. As was reported by Utz et al. [J. Chem. Phys. 1993, 98, 2742] for the ν4‘ and ν6‘ bands, the nν3‘ + ν4‘ and nν3‘ + ν6‘(n = 2,3) states are found to couple with each other by the a- and b-axis Coriolis interactions. The rotational analysis is performed taking the Coriolis interactions into account to determine the spectroscopic constants including the vibrational term values. The extent of the Coriolis interactions between nν3‘ + ν4‘ and nν3‘ + ν6‘(n = 2,3) is not so significant as that between ν4‘ and ν6‘. This is due to a larger anharmonic coupling of the in-plane trans-bending ν3‘ mode with the in-plane cis-bending ν6‘ mode than with the out-of-plane torsion ν4‘ mode, which causes a larger energy spacing between the pairs of the interacting levels as the ν3‘ quantum number increases. It is also found that most of rotational lines in the 3ν3‘ + ν6‘ band split into two or more peaks due to the S1−T3 interaction, while such rotational line splittings are not found in the 3ν3‘ + ν4‘ band. The present finding that the additional excitation in the out-of-plane torsion (ν4‘) mode suppresses the splittings suggests that the S1−T3 mixing occurs at the planar C 2 h or C 2 v geometry rather than at the nonplanar C2 geometry which is distorted along the torsional coordinate from the planar geometry.

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