Knowledge Bank

The $28000cm-1$ region of the $X~1\Sigma g+$ state af HCCH is examined by Stimulated Emission Pumping (SEP) from the $v3=2$ and $3,K=1$ levels of the trans-bent $A~1Au$ excited state. In contrast to the situation for high overtone spectroscopy which accesses primarily C-H overtones of u-symmetry. SEP samples C-C and trans-bend levels of g-symmetry. Among the most striking features in the SEP spectra are - $2cm-1$ wide dumps (about 1 clump per $10cm-1$) each consisting of - 20 $0.02cm-1$ wide lines. The clumps appear to be $K\prime =1\to K\prime \prime =0$ sub-bands. Although no correlation has been found between the fine structure of clumps observed for consecutive $J\prime \prime$ values., the centers of gravity for all observed clumps appear to be well described by a rotational constant B = $1.166cm-1$ which is surprisingly similar to the value B = $1.177cm-1$ for the vibrationless level. Density of state calculations suggest that each clump corresponds to a combination of the three totally symmetric normal modes (in $C2h$ these are C-C stretch, symmetric H-C stretch, and trans-bend) and that the lines within each clump correspond to all states of $Ag(\Sigma g+)$ symmetry.