FIRST OBSERVATION OF THE SPIN-ORBIT INTERACTION BETWEEN THE $\tilde{X}{^{1}A_{1}}$ AND THE $\tilde{a}{^{3}B_{1}}$ STATES of $SiH_{2}$ BY STIMULATED EMISSION PUMPING SPECTROSCOPY
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Date
2002
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Ohio State University
Abstract
The energy separation and order between the triplet and the singlet electronic states have been one of the central issues of $SiH_{2}$ radical from both chemical and spectroscopic points of view. However, any rotationally and/or vibrationally resolved observation of the triplet ($\tilde{a}{^{3}B_{1}}$) state has not yet been reported. Since the $\tilde{a}$ state is considered to be located $\sim 7000 cm^{-1}$ above the singlet ($\tilde{X}{^{1}A_{1}}$) state, it is expected that an effect of the singlet-triplet interaction appears among highly excited vibrational levels of the $\tilde{X}$ state. Thus, we have carried out the stimulated emission pumping (SEP) spectroscopy of $SiH_{2}$ in the vibrational energy region up to $10000 cm^{-1}$. 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 $4800-10000 cm^{-1}$. 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, $P = 2v_{1} + v_{2} + 2v_{3}$, construct polyad structures. The vibrational levels observed belong to polyads of $P = 5 - 10$. In the case of $P \leq 9$, 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 $\tilde{X}$ and the $\tilde{a}$ states based on the rotational dependence of this perturbation. The internal energy of the triplet state observed was about $9645 cm^{-1}$ measured from the (000) level of the $\tilde{X}$ state. This level is tentatively assigned as (030), based on the theoretical $calculation^{a}$. Details of the analysis will be discussed at the presentation.
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$^{a}$W. GABRIEL et al., Chem. Phys. 174, 45 (1993).
Author Institution: Department of Chemistry, Graduate School of Science, Tohoku University
Author Institution: Department of Chemistry, Graduate School of Science, Tohoku University