Vibrationally highly excited acetylene as studied by dispersed fluorescence and stimulated emission pumping spectroscopy: Vibrational assignment of the feature states

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Abstract
The dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectra of acetylene originating from single rovibronic levels of the à 1Au state were measured with resolutions of 30 and 0.5 cm−1, respectively, in order to examine the vibrational level structure of the electronic ground X̃ 1Σ+g state. The SEP spectra revealed that the number of vibrational levels under each peak in the DF spectra increases with vibrational energy from a single vibrational level below 8000 cm−1 to as many as ten vibrational levels above 16 500 cm−1. Taking account of the fact that a peak in the DF spectrum in the high energy region is composed of more than one level, a DF peak is called a feature state (or a feature). In the DF spectra from two trans-bending levels (v3=2 and 3) of the à state a total of 140 DF features between 5 700 and 21 200 cm−1 were detected and long progressions in the trans bend (v″4=6 –18) and CC stretch (v■2=0 –6) were identified. Below 14 000 cm−1, 26 out of the 50 observed features were unambiguously assigned to these two modes and represented by a second order anharmonic expansion within the ∼20 cm−1 experimental error. At least three additional trans-bend progressions built on excitation in third vibrational mode were identified. Possible assignments of the third mode to the CH stretch (ν″1) and the cis bend (ν■5) are compared. The Darling–Dennison (DD) resonance between the two degenerate bending modes (trans and cis) was proposed as a mechanism to lend Franck–Condon (FC) intensity to the ν″5 mode. The vibrational analysis of the DF features shows that the DF features correspond to the zero-order FC bright basis states. Each feature represents a group of levels which share the character of a zero-order FC bright level. Above 14 000 cm−1, characteristic groups of DF features with a width of around 300 cm−1 appear in the DF spectra originating from both v3=2 and v′3=3. The relative intensity patterns within each group of features in the two DF spectra are nearly identical. Three anharmonic resonances, including the DD resonance, are proposed as a plausible mechanism which splits a single FC bright state into several DF features. The SEP measurement revealed that a single DF feature splits further into several features with widths around 0.5 cm−1. The characteristic nested level structure identified in the DF and SEP spectra are explained in terms of a stepwise energy flow via a series of anharmonic resonances from the initially excited CC stretch/trans-bend vibrations to the remaining vibrational modes.