BH{sub 2} revisited: New, extensive measurements of laser-induced fluorescence transitions and ab initio calculations of near-spectroscopic accuracy

• Published in: The Journal of chemical physics 2015-05, Vol.142 (17)
• Main Authors: Sunahori, Fumie X., Gharaibeh, Mohammed, Clouthier, Dennis J., Tarroni, Riccardo
• Format: Article
• Language: English
• Subjects: ATOMIC AND MOLECULAR PHYSICS; BORANES; BORN-OPPENHEIMER APPROXIMATION; BORON 10; BORON 11; BORON HYDRIDES; COUPLING; DEUTERIDES; ELECTRON CORRELATION; ELECTRONS; EXCITED STATES; FLUORESCENCE; GROUND STATES; HYBRIDIZATION; LITHIUM FLUORIDES; POTENTIAL ENERGY; RADICALS; SPIN
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4919094

The spectroscopy of gas phase BH{sub 2} has not been explored experimentally since the pioneering study of Herzberg and Johns in 1967. In the present work, laser-induced fluorescence (LIF) spectra of the A{sup ~} {sup 2}B{sub 1}(Π{sub u})−X{sup ~2}A{sub 1} band system of {sup 11}BH{sub 2}, {sup 10}BH{sub 2}, {sup 11}BD{sub 2}, and {sup 10}BD{sub 2} have been observed for the first time. The free radicals were “synthesized” by an electric discharge through a precursor mixture of 0.5% diborane (B{sub 2}H{sub 6} or B{sub 2}D{sub 6}) in high pressure argon at the exit of a pulsed valve. A total of 67 LIF bands have been measured and rotationally analyzed, 62 of them previously unobserved. These include transitions to a wide variety of excited state bending levels, to several stretch-bend combination levels, and to three ground state levels which gain intensity through Renner-Teller coupling to nearby excited state levels. As an aid to vibronic assignment of the spectra, very high level hybrid ab initio potential energy surfaces were built starting from the coupled cluster singles and doubles with perturbative triples (CCSD(T))/aug-cc-pV5Z level of theory for this seven-electron system. In an effort to obtain the highest possible accuracy, the potentials were corrected for core correlation, extrapolation to the complete basis set limit, electron correlation beyond CCSD(T), and diagonal Born-Oppenheimer effects. The spin-rovibronic states of the various isotopologues of BH{sub 2} were calculated for energies up to 22 000 cm{sup −1} above the X{sup ~} (000) level without any empirical adjustment of the potentials or fitting to experimental data. The agreement with the new LIF data is excellent, approaching near-spectroscopic accuracy (a few cm{sup −1}) and has allowed us to understand the complicated spin-rovibronic energy level structure even in the region of strong Renner-Teller resonances.