Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

Abstract

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

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

“…In the recent experimental literature, the vibrational states are categorized in three groups: states containing excitation only in the totally symmetric modes 30,47 , the pure bending polyads 9 , and the stretch-bend combination polyads 10 . We organize our discussion accordingly.…”

“…The stretch-bend combination polyads demand a more detailed comparison, because a global model that accounts for their vibrational structure has not yet been developed, despite the existence of extensive assignments and rotational analyses 10 . Such a comparison is presented graphically in Fig.…”

“…Most recently, the comprehensive assignment of the low energy vibrational structure has led to the identification of several "extra" levels. These appear in the spectrum near 3000 cm −1 above the ground vibrational state of the trans conformer, just as the effective Hamiltonian (H eff ) models developed at lower energy begin to break down 10 .…”

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.…”