## Abstract

Stimulated-emission pumping spectra of acetylene-
${h}_{2}\tilde{X}{\phantom{\rule{0.2em}{0ex}}}^{1}{\Sigma}_{g}^{+}$ for *J*^{″} ≤ 10 and *l*^{″} (≈*K** _{a}*) = 0, 2 at 11 400–15 700 cm

^{−1}above the zero-point level exhibit a puzzling combination of contradictory symptoms of both regularity and severe mixing. The stimulated-emission pumping spectra are subjected to a variety of analyses in order to characterize the extent of vibrational mixing and the separability of rotation and vibration. These schemes include reduced term-value plots (from which 31 subband origins and

*B*values were derived), screening for coincidences between term values accessible from successive upper-state (

*Ã*

^{1}

*A*

*) rotational levels, polarization diagnostics, comparisons between observed, calculated total, and calculated Franck–Condon permitted vibrational densities of states, and various types of level-spacing statistics that are frequently used in studies of the quantum analog of classical chaos. The most remarkable observations are (i) the profound difference between the properties of*

_{u}*l*

^{″}= 0 versus

*l*

^{″}= 2 levels, in particular the persistence of relatively perturbation-free term value plots for

*l*

^{″}= 0 levels but not for

*l*

^{″}= 2 levels, (ii) the negligibly small parity doubling of

*l*

^{″}= 2 levels, (iii) evidence of vibrational mixing (but not

*l*

^{″}mixing) within both

*l*

^{″}= 0 and

*l*

^{″}= 2 submanifolds, and (iv) nearly Poisson fluctuations of the level spacings rather than the spectral rigidity that underlies some forms of extremely chaotic classical dynamics.

© 1990 Optical Society of America

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