Electronic Structure of the BF 2 Radical Determined by ab Initio Calculations and Resonance-Enhanced Multiphoton Ionization Spectroscopy Dean B. Atkinson, ² Karl K. Irikura,* ,‡ and Jeffrey W. Hudgens* ,§ Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: July 31, 1996; In Final Form: NoVember 15, 1996 X We report the first electronic absorption spectrum of the boron difluoride radical. This spectrum appeared in mass-selected multiphoton ionization spectra between 235 and 420 nm. Strong bent-linear structure changes prevented observations of electronic origin bands. EOM-CCSD ab initio calculations suggest that the observed vibrational bands arise from A ˜ 2 B 1 r X ˜ 2 A 1 (T vert ) 35 100 cm -1 ) one-photon absorption and from B ˜ 2 A 1 (3s) rr X ˜ 2 A 1 (T vert ) 59 100 cm -1 ) and C ˜ (3p) rr X ˜ 2 A 1 (T vert ) 63 100 cm -1 ) two-photon transitions. Ab initio calculations predicted the geometries and vibrational frequencies of the ground states of the BF 2 radical, cation, and anion. Ab initio calculations also predicted the vertical transition energies to the excited electronic states from the ground state radical. QCISD(T) calculations estimate ionization potentials for BF 2 radicals of IP a ) 8.66 eV and IP v ) 10.44 eV and adiabatic and vertical electron detachment energies for BF 2 - of EA ) 1.14 eV and VDE ) 1.64 eV. We estimate these ionization and detachment energies to be reliable to about 0.05 and 0.10 eV, respectively. Introduction We report the first electronic absorption spectra of BF 2 radicals. These spectra were observed using resonance- enhanced multiphoton ionization (REMPI) spectroscopy. To support the analyses of these spectra, we also report high-level ab initio calculations for BF 2 , BF 2 + , and BF 2 - . The REMPI spectra presented here form the basis of a very sensitive optical detection method for gas-phase BF 2 radicals. These results have practical application for studies of chemical reactions involving BF 2 , many of which have commercial importance to the semiconductor industry. Simple compounds of boron, including BF 2 species, are used in a variety of applications relating to chemical vapor deposition (CVD). BF 2 also appears during the bulk deposition of boron nitride as a hardener on semiconductor surfaces. 1-3 More recently, BF 2 + ion implantation has been used to form very thin p + /n semiconductor junctions. 4-7 Limited spectroscopic data are available for BF 2 . Infrared spectra of BF 2 radicals and cations entrained in cryogenic matrices have established most of the fundamental vibrational frequencies. 8,9 The ESR spectrum reported by Nelson and Gordy indicates that BF 2 (X ˜ 2 A 1 ) has a bent structure with ∠F- B-F ) 112°; 10 however, most electronically excited states of BF 2 are linear. The bent-linear geometry change that ac- companies electronic excitation frustrates spectroscopic studies by making the vibrational structure complex and the origin band intensity vanishingly weak. To date, electronic spectra attributed to BF 2 have been obtained by high-energy excitation of BF 3 molecules followed by observation of the dispersed fluores- cence. 11-14 Creasey et al. 11 recorded an emission spectrum of BF 2 between 220 and 290 nm. They attributed the spectrum to emissions from the 1 2 B 1 state and suggested that the spectrum may include contributions from the 2 2 A 1 state. The energy of the emitting states was not determined. Most information on the structure and excited states of the BF 2 molecule has come from ab initio theoretical calculations. 15-22 In this work we use the results of ab initio calculations to deduce spectroscopic assignments of REMPI spectra. We present high- level ab initio calculations which predict the geometries, vibrational frequencies, and energetics of ground state BF 2 , BF 2 + , and BF 2 - and also the vertical excitation energies from the ground state of BF 2 to its lower energy valence and Rydberg states. To aid the spectroscopic assignments, we also make use of the multireference singles and doubles configuration interac- tion (MRD-CI) calculations by Peric ´ and Peyerimhoff, who have predicted the vibrational frequencies of the lower valence and Rydberg states. 15,16 The MRD-CI calculations explicitly ac- counted for some of the many perturbations among the different vibrational levels of the electronic states. Computational Procedures and Results The ground-state geometries and harmonic vibrational fre- quencies of BF 2 , BF 2 + , and BF 2 - were determined using the 6-311+G* basis set (66 contracted basis functions, cGTOs) at the frozen-core QCISD(T) 23,24 and hybrid density-functional B3LYP 25 levels of theory. Vertical excitation energies in the neutral radical, at the B3LYP geometry, were computed in the frozen-core approximation at the well-correlated EE-EOM- CCSD level of theory 26 and also at the uncorrelated, singles- only configuration interaction (CIS) level. Rydberg states have a simple electronic structure and are well described using such single-reference theories, but valence states may be more complicated. Thus the vertical excitation energies to valence states were also calculated at the multireference CASPT2- (13,13) level 27 (involving more than 300 000 configuration state functions in the CASSCF reference). The weight of the CASSCF reference was about 0.92 in each CASPT2 wave function obtained. Valence-state excitation energies were also calculated using the B3LYP method; density functional proce- dures are often surprisingly effective in systems with substantial nondynamical correlation. The aug-cc-pVTZ basis sets, 28,29 supplemented by a set of diffuse d-functions (R) 0.016) on boron, which we denote as aug-cc-pVTZ+d(B) (143 cGTOs for BF 2 ), were used for the excitation energy calculations. The diffuse d-functions were added to improve the description of ² NIST/NRC Postdoctoral Associate. ‡ E-mail address: karl.irikura@nist.gov. § E-mail address: jeffrey.hudgens@nist.gov. X Abstract published in AdVance ACS Abstracts, February 15, 1997. 2045 J. Phys. Chem. A 1997, 101, 2045-2049 S1089-5639(96)02325-0 This article not subject to U.S. Copyright. Published 1997 by the American Chemical Society