Rotational analyses are reported for a number of newly-discovered vibrational levels of the S 1 -trans ( Ã 1 A u ) state of C 2 H 2 . These levels are combinations where the Franck-Condon active ! 2 " and ! 3 " vibrational modes are excited together with the low-lying bending vibrations, 3

Rotational analyses have been carried out for the overtones of the nu(4) (torsion) and nu(6) (in-plane cis-bend) vibrations of the A (1)A(u) state of C(2)H(2). The v(4)+v(6)=2 vibrational polyad was observed in high-sensitivity one-photon laser-induced fluorescence spectra and the v(4)+v(6)=3 polyad was observed in IR-UV double resonance spectra via the ground state nu(3) (Sigma(+) (u)) and nu(3)+nu(4) (Pi(u)) vibrational levels. The structures of these polyads are dominated by the effects of vibrational angular momentum: Vibrational levels of different symmetry interact via strong a-and b-axis Coriolis coupling, while levels of the same symmetry interact via Darling-Dennison resonance, where the interaction parameter has the exceptionally large value K(4466)=-51.68 cm(-1). The K-structures of the polyads bear almost no resemblance to the normal asymmetric top patterns, and many local avoided crossings occur between close-lying levels with nominal K-values differing by one or more units. Least squares analysis shows that the coupling parameters change only slightly with vibrational excitation, which has allowed successful predictions of the structures of the higher polyads: A number of weak bands from the v(4)+v(6)=4 and 5 polyads have been identified unambiguously. The state discovered by Scherer et al. [J. Chem. Phys. 85, 6315 (1986)], which appears to interact with the K=1 levels of the 3(3) vibrational state at low J, is identified as the second highest of the five K=1 members of the v(4)+v(6)=4 polyad. After allowing for the Darling-Dennison resonance, the zero-order bending structure can be represented by omega(4)=764.71, omega(6)=772.50, x(44)=0.19, x(66)=-4.23, and x(46)=11.39 cm(-1). The parameters x(46) and K(4466) are both sums of contributions from the vibrational angular momentum and from the anharmonic force field. For x(46) these contributions are 14.12 and -2.73 cm(-1), respectively, while the corresponding values for K(4466) are -28.24 and -23.44 cm(-1). It is remarkable how severely the coupling of nu(4) and nu(6) distorts the overtone polyads, and also how in this case the effects of vibrational angular momentum outweigh those of anharmonicity in causing the distortion.

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The LIF and H-atom action spectra of the A ˜-X ˜transition of acetylene in a supersonic jet were observed in the excitation-energy range of 47 000-50 600 cm -1 , where 60 vibrational states were identified in the A state through rotational analyses of respective bands. Most of these states originate from anharmonic couplings of the Franck-Condon-allowed 2 m 3 n states with the 4 i 6 j states where i + j ) even. This fact is consistent with the determined rotational constants of respective states whose values are mostly between those of the 2 m 3 n (m e 1, n e 4) and the 4 1 or 6 1 state. The line intensities of the H-atom action spectra increase at the higher excitation energies, indicating more efficient predissociation. The lifetimes of respective excited states were evaluated from the observed spectral line widths. In a higher excitation-energy region, the lifetime is shortened from 100 ps at 47 000 cm -1 to 25 ps at 50 600 cm -1 , which exceeds the adiabatic dissociation threshold of the A ˜state by 933 cm -1 and is the wavelength limit (197 nm) of the third harmonic of the dye laser. To excite acetylene in the higher excitation energies, vibrationally excited acetylene, which was prepared by irradiation with an IR laser, was excited further by a UV laser. Utilizing this IR-UV double-resonance method, acetylene could be excited up to 53 500 cm -1 , which corresponds to 187 nm. Beyond 50 600 cm -1 , the lifetime of excited acetylene becomes longer, around 100 ps at 51 100 cm -1 . This sudden increase in the lifetime is attributed to the predissociation mechanism changes from the triplet state-mediated dissociation to the direct dissociation on the A ˜state surface. This fact is also supported by the observed average kinetic energies of H atoms estimated from the Doppler line widths of the REMPI spectral lines of H atoms; below 50 600 cm -1 , the kinetic energy is in proportion to the excitation energy, whereas beyond 50 600 cm -1 , the observed average kinetic energies are around 1000-2000 cm -1 , which are a little smaller than the excess energy for the production of C 2 H(A ˜) + H. It was found that the predissociation mechanism or efficiency is not dependent on the vibrational parity, gerade or ungerade, of the initial excited state. This is attributed to the anharmonic coupling between the ν 3 ′ and ν 4 ′/ν 6 ′ vibrational modes.

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