The infrared-ultraviolet dispersed fluorescence spectrum of acetylene: New classes of bright states

Abstract

Single rotational levels of ungerade vibrational levels, 2ν3′+ν6′ and 3ν3′+ν6′ (both with bu symmetry), in the à 1Au electronically excited state of acetylene were excited by an IR-UV double resonance scheme via the ν3″ fundamental level in the X̃ 1Σg+ state, and the rotationally resolved dispersed fluorescence (DF) spectra were recorded at 3.2–4.5 cm−1 resolution. The term values of the new ungerade levels were determined within an accuracy of 0.56 cm−1(1σ) through careful calibration achieved by frequency standard atomic Fe and Hg lines. A total of 111 new ungerade vibrational levels with Σu+, Σu−, and Δu symmetry below 10 000 cm−1 was identified in the high-resolution IR-UV-DF spectra, which provide access to new classes of X̃ 1Σg+ bright states: (i) (0,v2″,0,v4″1,1−1)Σu+, (0,v2″,0,v4″1,11)Δu, and (0,v2″,0,v4″3,1−1)Δu, which are the Franck–Condon (FC) bright levels from the nν3′+ν6′ (n=2,3) levels in the à 1Au state; (ii) (0,v2″,0,v4″−1,11)Σu− levels which appear through the a-axis Corioris interaction between nν3′+ν6′ and nν3′+ν4′ (n=2,3) in the à 1Au state; and (iii) (0,v2″,1,v4″0,0)Σu+ and (0,v2″,1,v4″2,0)Δu levels which gain transition intensity from the Duschinsky effect associated with the bent-linear ÖX̃ transition. All observed ungerade term values and previously determined gerade and ungerade term values below 10 000 cm−1 were fitted by two effective model Hamiltonians, i.e., a pure-bend effective Hamiltonian and a stretch–bend effective Hamiltonian. The stretch–bend effective Hamiltonian is expressed in terms of 31 Dunham expansion parameters and 11 anharmonic resonance parameters associated with (i) five stretch–bend anharmonic resonances; (ii) one stretch–stretch and two bend–bend Darling–Dennison resonances; and (iii) one vibrational l resonance. The parameters in this Hamiltonian were determined from a least-squares fit of 287 vibrational term values (111 new ungerade levels, 128 levels from absorption, 1 level from stimulated Raman, 13 levels from stimulated emission pumping (SEP), and 34 levels from UV-DF spectroscopy) below 10 000 cm−1 with a standard deviation of σ=1.21 cm−1. The FC patterns for the v4″=odd ungerade levels, (0,v2″,0,v4″,1), in the IR-UV-DF spectra were derived, and the nodes along the v4″ trans-bend mode were found at v4″=11 via the 2ν3′+ν6′ upper state, and at v4″=9 and 15 via the 3ν3′+ν6′ upper state, which is consistent with the ν3′ dependence of the FC patterns observed in previous UV-DF studies.

Bending Dynamics of Acetylene: New Modes Born in Bifurcations of Normal Modes

“…The first (a) is the same Hamiltonian of ref 15 as we used for Figure 6, except that the high-order 15 in our bifurcation analysis because it fits higher energy experimental levels, so we believe it more accurately reflects highly excited bending dynamics.) The third Hamiltonian (c) was developed by Hoshina et al 24 by combining available stretch-bend data from dispersed fluorescence and absorption spectra. The fourth Hamiltonian (d) is the fit of Herman and co-workers 33 of stretch-bend spectra, constructed to reflect data from a variety of spectral techniques.…”

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

“…Many experiments of this type have been carried out in order to characterize the higher vibrational levels of the ground state, [19][20][21][22][23][24][25][26][27][28] the Rydberg states, [29][30][31] and the ground state of the cation. [32,33] The first rotational analyses of the à # X system, over 50 years ago, [34][35][36] showed that C 2 H 2 , though linear in its ground electronic state, becomes trans-bent (point group C 2h ) in the à state.…”

Full dimensional Franck-Condon factors for the acetylene $\tilde{\mathbf {A}}$Ã 1 A u—$\mathbf {\tilde{X}}$X̃ $\mathbf {^1\Sigma _g^+}$Σg+1 transition. I. Method for calculating polyatomic linear—bent vibrational intensity factors and evaluation of calculated intensities for the gerade vibrational modes in acetylene

“…In the current work, we have chosen the columns of l to be consistent with the parameters of theX-state effective Hamiltonian reported by Herman and co-workers 21,23 and expanded by Jacobson and co-workers. 24,28,67 The columns of l are chosen to have positive overlap with the corresponding columns of l . We have chosen l R and l R so that positive displacement corresponds to right-handed rotation around the axes as defined in Figure 3.…”