The direct observation, assignment, and partial deperturbation of the ν4 and ν6 vibrational fundamentals in Ã 1A u acetylene (C2H2)

Abstract

A pulsed-laser double resonance technique provides previously unavailable spectroscopic data on the rovibrational structure of à 1Au acetylene (C2H2). Our assignment and analysis of transitions to the à state ν4′ (torsion) and ν6′ (antisymmetric in-plane bend) vibrational fundamentals uncovers a strong Coriolis interaction between these two nearly degenerate modes and weaker Coriolis interactions between the ν4′/ν6′ pair and remote à state rovibrational levels. We deperturb the direct Coriolis interaction between ν4′ and ν6′ to obtain vibrational frequencies, Coriolis coupling constants and partially deperturbed rotational and centrifugal distortion constants for these previously unobserved fundamentals. Parity selection rules for the Ã←X̃ band permit an unambiguous assignment of the vibrations (ν4′=764.9±0.1 cm−1 and ν6′=768.3±0.2 cm−1). We use these new experimental values to reassign several à state vibrations and to assign previously unidentified à state levels. We also identify two vibrational resonances that seem to be important in determining the rovibrational structure of à 1Au C2H2.

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

“…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.…”

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

“…12,14,[59][60][61][62][63][64][65][66] This state therefore provides optimal testing ground of the algebraic method. There are three fundamental bending vibrations of trans-S 1 with symmetry a u , b u , a g as shown in Fig.…”

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.…”

“…c) Email: rwfield@mit.edu structure of the S 1 state to become very complicated. 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.…”

“…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.…”