Knowledge Bank

We have been investigating the HCP $⟷$ CPH isomerization'' of HCP molecule by the stimulated emission pumping (SEP) spectroscopy. In the previous study, we recorded SEP spectra in the energy region up to 26800 cm$^{-1}$ in the $\tilde{X}$ state from the vibrational ground level. This value is very close to the height of the isomerization'' barrier (27400 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) predicted by the theoretical study. It was found that several vibrational levels exhibit complicated perturbation patterns due to the Coriolis type interaction}. Strong Coriolis perturbations in the energy region near the isomerization'' barrier are predicted by Jacobson and Child based on their spherical pendulum model} \underline{\textbf{114}}, 262 (2001).}. They also pointed out that the parity splitting in $\ell \ne 0$ levels should be a good spectroscopic indication of the isomerization'' in addition to the rotational constants. \par Until now, due to the rotational selection rules, only the $\ell =0$ levels were observed in the energy region above 20000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. However, using a perturbed level in the $C~$ state as an intermediate, we have succeeded in recording SEP spectra which have sampled $\ell =1$ levels in the 26250 -- 27300 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ region in the $X~$ state, in the present study. As a result, we have found a good relation between the rotational constants and the parity splittings. Larger-$B$ levels exhibit larger parity splittings. For instance, A vibrational level having a $Bave.$ of 0.76 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ exhibits $\Delta B=0.13cm-1$, while another level having a $Bave.$ of 0.62 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ exhibits $\Delta B=0.03cm-1$. The $\Delta B$ of the former level is about 4 times larger than that of the latter. It is confirmed that the parity splitting is one of the spectroscopic indications of the ``isomerization'' of the HCP/CPH system as expected. \par