TheS1thinsp;rarr;thinsp;S0transition of acetylene has been studied by spontaneous and stimulated emission. In the spontaneous emission studies, we report new data on lowhyphen;resolution dispersed fluorescence from an unrelaxed single rotational level ofS1. Transitions intoS0levels from the zerohyphen;point level up to sim;28thinsp;000 cmminus;1of vibrational energy are reported. Earlier investigations of the emission from relaxed acetylene in a neon matrix and of partially relaxed fluorescence from a radio frequency discharge in acetylene have been reexamined and reinterpreted in the light of the present work. The stimulated emission study employed stimulated emission pumping (SEP) for highhyphen;resolution investigations of two regions ofS0energies, one near 9550 cmminus;1and another near 27thinsp;900 cmminus;1. Definite vibrational assignments were possible for most of the features observed in the lower energy region. This allowed the determination of two new vibrational constants,y224andy244. The region near 27thinsp;900 cmminus;1reveals a hitherto unobserved kind of vibrational behavior. A series of broad features (sim;1.5 cmminus;1wide) appear at intervals of sim;10 cmminus;1in broadband SEP scans. At higher resolution, each of these is found to consist of many individual lines with DUMPhyphen;laser limited linewidths. This structure is discussed in terms of the Franckndash;Condon principle for theS1thinsp;rarr;thinsp;S0transition and the possible relevance of the observed behavior to theories of ergodicity in quantum systems. A detailed treatment of the data, using various measures of quantum ergodicity, appears in the following paper.
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