Darling–Dennison resonance and Coriolis coupling in the bending overtones of the ÃAu1 state of acetylene, C2H2

Abstract

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.

Reduced dimension discrete variable representation study of cis–trans isomerization in the S1 state of C2H2

“…We find that all but the lowest-lying states contain at least a few percent trans character, using the crude metric 2 π 0 2π π ∞ 0 |ψ| 2 dR CC dφ 1 dφ 2 , but even these states are predicted to have non-negligible intensity in one of the four spectra discussed in Ref. [9]. These results are partially summarized in Table V.…”

“…Beginning with the discovery that acetylene changes shape from linear to trans-bent upon electronic excitation 1-3 , theà ←X spectrum has exhibited a string of unexpected phenomena, including axis switching 4 , triplet perturbations [5][6][7] , as well as unusually strong Coriolis interactions and Darling-Dennison resonance between the bending vibrations 8,9 . Most recently, the comprehensive assignment of the low energy vibrational structure has led to the identification of several "extra" levels.…”

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

“…4 The transition is now known to be a π * ← π excitation from the linear ground state to a trans-bent excited state, S 1 (à 1 A u ). Further detailed analyses by Watson et al, [5][6][7] the Crim group, 8,9 Yamakita and Tsuchiya, [10][11][12] and Merer et al [13][14][15][16][17] have led to an essentially complete assignment of the vibrational and rotational structure up to 4500 cm −1 above the zero-point level of theà state. One of the most interesting results of these analyses has been the discovery 17,18 of bands that belong to the cis-bent isomer of theà state, the zero-point level of which lies about 2670 cm −1 higher in energy than that of the trans-bent isomer.…”

“…The levels of the bending vibrations, ν 4 (torsion, a u ) and ν 6 (cis bend, b u ) interact strongly by Darling-Dennison 26 and Coriolis resonances, 9 such that the vibrational structure of these modes forms polyads, which are groups of levels with the same value of v 4 + v 6 . Their rotational structure is highly irregular, 9,14,16 though it can be modeled with an effective Hamiltonian. Simultaneous excitation of ν 3 (trans bend, a g ) progressively destroys the bending polyad structure 16 because the saddle point on the potential surface at the isomerization barrier results in very large anharmonicity in the 3 m 6 n combination levels.…”

A study of the bending motion in tetratomic molecules by the algebraic operator expansion method

“…[1][2][3][4][5][6][7][8] Also a molecule may change its point group symmetry upon electronic excitation as shown many years ago [9][10][11] and as demonstrated recently. 12 A commonly used method to describe tetratomic molecules is the force-field method. 13 This method is an expansion in terms of curvilinear internal coordinates, three stretching and four bending for tetratomic molecules.…”