High resolution laser excitation spectroscopy of the B2E-X2A1 transitions of calcium and strontium monoborohydride.

High resolution spectra of the B2E-X2A1 transitions of CaBH4 and SrBH4 have been recorded using laser excitation spectroscopy in a laser ablation/molecular jet source. Because of rotational cooling in the molecular jet and nuclear spin statistics, transitions arising from only the K'=1<--K"=0, K'=2<--K"=1, and K'=0<--K"=1 subbands have been observed. For each molecule, an analysis of the data using 2E and 2A1 symmetric top Hamiltonians yielded rotational, spin-orbit, and spin-rotation parameters for the observed states. For both molecules the rotational constants compare well with those calculated for a tridentate borohydride structure. A large reduction in the spin-orbit splitting and in the metal-ligand separation for each molecule indicates an increase in the amount of d atomic orbital character in the first excited 2E states of the monoborohydrides as compared to the monomethyl derivatives. For each molecule no evidence of internal rotation of the BH4- ligand was found. A change in the magnitude and sign of the spin-rotation constant epsilon1 confirms an energy reordering of the first excited 2E and 2A1 states in both CaBH4 and SrBH4 as compared to CaCH3 and SrCH3. The data also suggest that the B2E1/2 rotational energy levels of CaBH4 may be perturbed by a vibronic component of the A2A1 state.