Observation of the c 1A1 state of methylene by optical-optical double resonance.

We report the observation of the rotationally resolved spectrum of the c ¹A₁ state of CH₂ via sequential single-photon absorptions at visible and near-infrared wavelengths. Direct absorption from the lowest singlet state ã ¹A₁ to c ¹A₁ occurs in the near UV, but it is weak because it corresponds to a two electron transition between the dominant single configuration approximations to the electronic wave functions. Some absorption lines in the c-ã system were originally reported in 1966 [G. Herzberg and J. W. C. Johns, Proc. R. Soc. London, Ser. A 295, 107 (1966)], but the weak spectra could not be assigned at the time. Interest in the c ¹A₁ state was rekindled by recent ab initio results [S. N. Yurchenko, P. Jensen, Y. Li, R. J. Buenker, and P. R. Bunker, J. Mol. Spectrosc. 208, 136 (2001)] which prompted the present work. The new spectra provide accurate energies for rotational levels in the v₂^linear=11,l=1 level of the state, and permit assignment of most of the line positions measured by Herzberg and Johns. The double-resonance technique used may be easily extended to the measurement of lower rovibrational levels in the electronic state and possibly also to access the d ¹A₂ state which is theoretically expected to lie at similar energies but, for symmetry reasons, is not accessible from the lowest singlet state in a single electric-dipole transition. [ABSTRACT FROM AUTHOR]