15min:

H. ISHIKAWA, Y. MURAMOTO AND N. MIKAMI, Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.

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~³\!B₁) state has not yet been reported. Since the a state is considered to be located 7000 cm^-1 above the singlet (X~¹\!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 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. \par We have observed fifty-one vibrational levels in the vibrational energy region of 4800--10000 cm^-1. Due to strong 1 ₁:2 ₂ Fermi and 2 ₁:2 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 9645 cm^-1 measured from the (000) level of the X state. This level is tentatively assigned as (030), based on the theoretical calculation. Details of the analysis will be discussed at the presentation.