BH2 revisited: New, extensive measurements of laser-induced fluorescence transitions and ab initio calculations of near-spectroscopic accuracy

Abstract

The spectroscopy of gas phase BH2 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 Ã(2)B1(Πu)-X̃ (2)A1 band system of (11)BH2, (10)BH2, (11)BD2, and (10)BD2 have been observed for the first time. The free radicals were "synthesized" by an electric discharge through a precursor mixture of 0.5% diborane (B2H6 or B2D6) 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 BH2 were calculated for energies up to 22 000 cm(-1) above the X̃ (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(-1)) and has allowed us to understand the complicated spin-rovibronic energy level structure even in the region of strong Renner-Teller resonances.

A stimulated emission study of the ground state bending levels of BH2 through the barrier to linearity and ab initio calculations of near-spectroscopic accuracy

“…Our measured bending levels occur at 973.53, 1912.49, 2859.45, and 3868.25 cm 1 (v 2 = 1-4, 0 0,0 ). These values are all very slightly higher than our theoretical predictions, 8 which have a maximum error of 2.05 cm 1 (0.05%) for v 2 = 4 (see Table IV). Previously reported vibronic term values for the (0,4,0) state from empirically adjusted ab initio potentials are 3888 cm 1 (Ref.…”

“…The 11 BH 2 free radical was produced in a discharge free jet expansion 9 using precursor mixtures of 0.5%-0.1% diborane (B 2 H 6 ) in high pressure argon, as discussed in more detail elsewhere. 8 Low-resolution LIF spectra were recorded by exciting the jet-cooled radicals with the collimated beam of a pulsed tunable dye laser (Lumonics HD-500, linewidth 0.1 cm 1 ) and imaging the resulting fluorescence signals onto the photocathode of a high gain photomultiplier (EMI 9816QB). The signals were sampled with a gated integrator and recorded with LabVIEW-based data acquisition software.…”

“…These have been described in detail in our previous work. 8 Briefly, potential energy surfaces of the bent ground X 2 A 1 and the linear A 2 B 1 first excited states of BH 2 were generated from a series of coupled cluster singles and doubles with perturbative triples [CCSD(T)] level of theory single point calculations with the aug-cc-pV5Z basis set. These surfaces were then corrected in a systematic fashion for core correlation, complete basis set extrapolation, electron correlation beyond CCSD(T), and diagonal mass dependent Born-Oppenheimer effects.…”

“…In our previous work, 8 we obtained the energy difference (or barrier to linearity) between the energy minima of the analytical potential energy surfaces of 11 BH 2 without the mass dependent corrections as 2655.7 cm 1 . Since our emission and SEP data samples ground state rovibronic bending levels from 0.0 up to a maximum of 4597.7 cm 1 [(0,5,0) J = 5.5, 5 1,4 ], the measurements straddle the regions below, through, and above the barrier.…”

“…Very recently, 8 we showed that cold BH 2 radicals could be produced in a discharge jet using a precursor mixture of diborane (B 2 H 6 ) in high pressure argon and detected by laser induced fluorescence. 8 We were able to extend the 11 BH 2 spectrum up to 21 000 cm 1 , spanning bending levels from v 2 = 10 to 19 along with the detection of a few stretch-bend combination levels for the first time. We also studied the spectrum of 11 BD 2 , detecting bands in the bending progression from 2 14 0 to 2 23 0 and some stretch-bend combinations.…”