The ùA_u state of acetylene: ungerade vibrational levels in the region 45,800–46,550 cm⁻¹

Abstract

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

“…Such an analysis of the 3 3 6 1 vibrational level is complicated by the fact that this level is Coriolis coupled to its polyad partner, 3 3 4 1 , and suffers additional local perturbations. 10,17 Such perturbations obscure the Kstaggering patterns that we observe more clearly in other isomerizing levels, such as cis 3 1 6 1 or cis 6 2 , that do not exhibit complications from Coriolis or Darling-Dennison interactions. However, with a sufficient number of observed K levels, it is possible to determine staggering parameters, even in highly distorted polyads, via effective Hamiltonian models.…”

“…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.…”

“…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 existence of the cis isomer had been predicted by several theoretical studies, [19][20][21][22][23][24] but it was thought that theà state of the cis isomer would be unobservable because it transforms as 1 A 2 in the C 2v point group; thus, transitions to it from the 1 Σ + g ground state are electric dipole forbidden.…”

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.…”

“…66,68 There are three fundamental bending vibrations of cis-S 1 , with symmetry a 2 , b 2 , a 1 , as shown in Fig. 12.…”