Fluorescence and stimulated emission S1 → S spectra of acetylene: Regular and ergodic regions

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Abstract
The S1 → S0 transition of acetylene has been studied by spontaneous and stimulated emission. In the spontaneous emission studies, we report new data on low‐resolution dispersed fluorescence from an unrelaxed single rotational level of S1. Transitions into S0 levels from the zero‐point level up to ∼28 000 cm−1 of 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 high‐resolution investigations of two regions of S0 energies, one near 9550 cm−1 and another near 27 900 cm−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, y224 and y244. The region near 27 900 cm−1 reveals a hitherto unobserved kind of vibrational behavior. A series of broad features (∼1.5 cm−1 wide) appear at intervals of ∼10 cm−1 in broadband SEP scans. At higher resolution, each of these is found to consist of many individual lines with DUMP‐laser limited linewidths. This structure is discussed in terms of the Franck–Condon principle for the S1 → S0 transition 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.