Observation of the c1A1 state of methylene by optical-optical double resonance

We report the observation of the rotationally resolved spectrum of the c1A1 state of CH2 via sequential single-photon absorptions at visible and near-infrared wavelengths. Direct absorption from the lowest singlet state a1A1 to c1A1 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-a 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 c1A(1) 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 v2linear =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 d1A2 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.