Knowledge Bank

The energy separation and order between the triplet and the singlet electronic states have been one of the central issues of $SiH2$ radical from both chemical and spectroscopic points of view. However, any rotationally and/or vibrationally resolved observation of the triplet ($a~\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$) state has not yet been reported. Since the $a~$ state is considered to be located $\sim 7000cm-1$ above the singlet ($X~\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$) state, it is expected that an effect of the singlet-triplet interaction appears among highly excited vibrational levels of the $X~$ state. Thus, we have carried out the stimulated emission pumping (SEP) spectroscopy of $SiH2$ in the vibrational energy region up to $10000cm-1$. In this paper, we will report an observation of a small but a definitive perturbation due to the singlet-triplet interaction in the SEP spectrum. We have observed fifty-one vibrational levels in the vibrational energy region of $4800-10000cm-1$. Due to strong $1\nu 1:2\nu 2$ Fermi and $2\nu 1:2\nu 3$ Darling-Dennison resonances, vibrational levels having the same polyad quantum number, $P=2v1+v2+2v3$, construct polyad structures. The vibrational levels observed belong to polyads of $P=5-10$. In the case of $P\le 9$, all the vibrational energies observed were fitted very well by the effective Hamiltonian model in which the above resonances were considered. In the case of the $P=10$ polyad, however, an unexpected splitting of the band was observed. It was confirmed that this splitting is due to the spin-orbit interaction between the $X~$ and the $a~$ states based on the rotational dependence of this perturbation. The internal energy of the triplet state observed was about $9645cm-1$ measured from the (000) level of the $X~$ state. This level is tentatively assigned as (030), based on the theoretical $calculationa$. Details of the analysis will be discussed at the presentation.