The electronic states of Boron Difluoride (BF₂) free radical was investigated in cooperation between Atomic Physics Group I at Institute of Modern Physics, Chinese Academy of Sciences (IMP) and the group of Professor Dennis J Clouthier at University of Kentucky, USA.

The near-ultraviolet band system of the jet-cooled BF₂ free radical has been studied by a combination of laser-induced fluorescence  (LIF)  and  single  vibronic  level  wavelength  resolved  emission spectroscopies.

The radical was produced in a supersonic discharge jet using a precursor mixture of 1%–3% of BF₃ or 10BF₃in high pressure argon. A large number of bands were found in the 340–286 nm region and assigned as transitions from the  ground state to the lower Renner-Teller component of the  excited state, based on our previous ab initio potential energy surface predictions, matching the emission spectra Franck-Condon profiles of ¹¹BF₂and ¹⁰BF₂, and comparison of observed and calculated boron isotope effects.

Several high resolution LIF bands have been rotationally analyzed providing ground state structural parameters of r₀(BF) = 1.3102(9) Å and θ₀)(FBF)= 119.7(6)°. The ground state totally symmetric vibrational energy levels of both boron isotopologues have also been measured and assigned up to energies of more than 8000 cm^-1.

Although BF₂ might be considered to be a “simple” free radical, understanding the details of its electronic spectrum remains a major challenge for both theory and experiment. It is indeed fortunate that the calculations predicted the pattern of K=0(∑) levels with such high fidelity that they allowed us to assign the very complex vibronic structure in the electronic spectrum of BF₂ for the first time.

The results have been published in J. Chem. Phys. 135, 094305, 2011.

Weblink：http://jcp.aip.org/resource/1/jcpsa6/v135/i9/p094305_s1?isAuthorized=no

Fig. 1: The Franck-Condon pattern of the emission spectra of bands in the  LIF progression. The transitions down to the bending overtone levels in the ground state show a regular intensity pattern, which is similar for both isotopologues (¹¹BF₂ on the left, ¹⁰BF₂ on the right), aiding in the determination of the vibronic assignments of the LIF bands(Image by IMP).

Fig. 2: The high resolution LIF spectrum of the band  level of ¹¹BF₂(top) with assignments. Each line in the spectrum is split into two components, due to the presence of the unpaired electron. The bottom spectrum is a simulation of the observed spectrum, using the constants obtained from the rotational analysis (Image by IMP).