Absorption spectroscopy of SiH2 near 640 nm

The (H) over tilde B-1(1)-(X) over tilde (1)A(1) absorption spectrum of SiH2 has been observed using intracavity laser absorption spectroscopy with an equivalent path length of up to 13.0 km and the (A) over tilde B-1(1)(0, 0, 0)-(X) over tilde (1)A(1)(0, 0, 0) band near 640 nm recorded for the first time. The silylene radical was generated in a continuous discharge in a flowing mixture of silane in argon, giving a concentration of the order of 10(10) SiH2/cm(3). The spectrum spans the region between 15350 and 16100 cm(-1). Rotational transitions have been assigned to levels up to J=16 and K-a=9, with Delta K-a up to 5, Delta K-c up to 4. Perturbations have been detected in the spectrum, due to Renner-Teller and spin-orbit interactions between both electronic states and the B-3(1) state, predicted to be between them. However, the strength of the irregular perturbations affecting the rotational states of (A) over tilde B-1(1)(0,0,0) state is found to be much weaker than that affecting the other (0, upsilon(2)', 0) levels previously studied. The analysis of the spectrum has allowed the determination of the rotational constants of the B-1(1) (0,0,0) level, and a new estimation of those of the vibrational and electronic ground state. The geometry of the excited electronic level has also been determined for the first time from accurate experimental data. A change in the structure of this molecule takes place with this transition, the equilibrium angle opening from 92 degrees to 122.4 degrees, while the bond distance is reduced from 1.51 to 1.485 Angstrom. (C) 1998 American Institute of Physics. [S0021-9606(98)01815-7].