VIBRATIONALLY HIGHLY EXCITED ACETYLENE AS STUDIED BY DISPERSED FLUORESCENCE AND STIMULATED EMISSION PUMPING SPECTROSCOPY

Author Institution: Department of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology; Department of Pure and Applied Sciences, The University of TokyoDispersed Fluorescence (DF) spectra of $C_{2}H_{2}$ in the $\widetilde{A}$ state were measured at $\sim 30 cm^{-1}$ resolution by exciting single rovibronic levles of the $\widetilde{A}$ state. The DF spectra are mainly composed of progressions in the $V_{2}$ and $V_{4}$ modes. The Frank-Condon bright features have vibrational quantum numbers of $V_{1}=0,1, V_{2}=0-5, V_{3}=0, V_{4}=0.30, V_{5}=0-4$. A total of 117 vibrational bright states, distributed between 5700 and $23,400 cm^{-1}$ were indentified. The vibrational term values of the assigned features are represented by a third order anharmonic expansion to within experimental error. The determined expansion coefficients are consistent with literature values. The vibrationally highly excited region between 16,500 and $21,000 cm^{-1}$ was also investigated with higher resolution $(0.4cm^{-1})$ by Stimulated Emission Pumping spectroscopy. A comparison of the SEP and DF spectra having the same upper rovibronic levels reveals that a single feature assigned to one bright state in the DF spectrum is composed of $>10-40$ peaks. The considerable couplings revealed in the vibrationally highly excited region are evidenced by the splittings resolved under high resolution