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

摘要

The dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectra of acetylene originating from single rovibronic levels of theAtilde;thinsp;1Austate were measured with resolutions of 30 and 0.5 cmminus;1, respectively, in order to examine the vibrational level structure of the electronic groundXtilde;thinsp;1Sgr;+gstate. 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 cmminus;1to as many as ten vibrational levels above 16thinsp;500 cmminus;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 afeaturestate(or a feature). In the DF spectra from twotranshyphen;bending levels (v3=2 and 3) of theAtilde; state a total of 140 DF features between 5thinsp;700 and 21thinsp;200 cmminus;1were detected and long progressions in thetransbend (vlsquo;4=6 ndash;18) and CC stretch (vsquflg;2=0 ndash;6) were identified.Below 14thinsp;000 cmminus;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 sim;20 cmminus;1experimental error. At least three additionaltranshyphen;bend progressions built on excitation in third vibrational mode were identified. Possible assignments of the third mode to the CH stretch (ngr;lsquo;1) and thecisbend (ngr;squflg;5) are compared. The Darlingndash;Dennison (DD) resonance between the two degenerate bending modes (transandcis) was proposed as a mechanism to lend Franckndash;Condon (FC) intensity to the ngr;lsquo;5mode. The vibrational analysis of the DF features shows that the DF features correspond to the zerohyphen;order FC bright basis states. Each feature represents a group of levels which share the character of a zerohyphen;order FC bright level. Above 14thinsp;000 cmminus;1, characteristic groups of DF features with a width of around 300 cmminus;1appear in the DF spectra originating from bothv3=2 andvrsquo;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 cmminus;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/transhyphen;bend vibrations to the remaining vibrational modes.