Stimulated emission pumping of acetylene: Evidence for quantum chaotic behavior near 27 900 cm−1 of excitation?

Abstract

Clumps of narrowly spaced lines (∼10 lines per ${\mathrm{cm}}^{\text{−1}}$ in a 2 ${\mathrm{cm}}^{\text{−1}}$ wide clump) appear in the stimulated emission pumping spectrum of the acetylene ${\mathrm{X̃\hspace{0.17em}}}^1{\sum }_g^{+}$ state near 27 900 ${\mathrm{cm}}^{\text{−1}}$ of vibrational excitation above the vibrationless level. All of the lines in a given clump arise from transitions into levels with the same lower state $\mathrm{J″}$ value. Thus, the fine structure within each clump is of a purely vibrational nature. Rotationally resolved and assigned spectra at such high vibrational excitation have not been previously reported for a polyatomic molecule. The unique qualities of these spectra make it possible for the first time to apply one of the suggested criteria of quantum chaos to a real molecular spectrum. Analysis of the spacings between adjacent lines in several of these clumps shows that they approximately follow a Wigner distribution rather than a Poisson distribution, a characteristic which is expected for chaotic behavior within the molecule.