ABSORPTION-SPECTROSCOPY OF SIH2 NEAR 640 NM

The (H) over tilde B-1(1)-(X) over tilde (1)A(1) absorption spectrum o f SiH2 has been observed using intracavity laser absorption spectrosco py with an equivalent path length of up to 13.0 km and the (A) over ti lde B-1(1)(0, 0, 0)-(X) over tilde (1)A(1)(0, 0, 0) band near 640 nm r ecorded for the first time. The silylene radical was generated in a co ntinuous discharge in a flowing mixture of silane in argon, giving a c oncentration of the order of 10(10) SiH2/cm(3). The spectrum spans the region between 15350 and 16100 cm(-1). Rotational transitions have be en assigned to levels up to J=16 and K-a=9, with Delta K-a up to 5, De lta 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, th e strength of the irregular perturbations affecting the rotational sta tes of (A) over tilde B-1(1)(0,0,0) state is found to be much weaker t han that affecting the other (0, upsilon(2)', 0) levels previously stu died. The analysis of the spectrum has allowed the determination of th e rotational constants of the B-1(1) (0,0,0) level, and a new estimati on of those of the vibrational and electronic ground state. The geomet ry of the excited electronic level has also been determined for the fi rst 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 Ph ysics. [S0021-9606(98)01815-7].