On the photofragmentation of SF2+: Experimental evidence for a predissociation channel

Abstract

In light of its potential importance in the semiconductor industry, the SF 2 reactive intermediate has recently received increased attention from both experimentalists 1,2 and computational chemists. 3 When compared with its neutral counterpart, the cationic SF 2 + species is, however, still far from being well characterized, with only a few computational 4-7 and only one prior experimental 8 study. The only experimental study available for the low-lying excited states of SF 2 + comes from the He I photoelectron ͑PE͒ spectrum, which was reported by de Leeuw et al. 8 three decades ago. The spectral features observed in the PE spectrum were assigned to ionizations from the X 1 A 1 ground state of SF 2 to the X 2 B 1 ground state and five low-lying excited states ͑Ã 2 A 1 , B 2 B 2 , C 2 B 2 , D 2 A 1 , and Ẽ 2 A 2 ͒ of SF 2 + . 8 A recent ab initio study employing the high-level RCCSD͑T͒ method was carried out by Lee et al. 6 to calculate the potential energy functions ͑PEFs͒ for the X 1 A 1 state of SF 2 and for the six states of SF 2 + listed above. Since the experimental research regarding SF 2 + has lagged far behind, any useful information on the lowlying states of SF 2 + from the experimental side is highly desirable. 6 In this letter, we report the first observation on the photofragmentation dynamics of SF 2 + by recording the resonance-enhanced multiphoton ionization ͑REMPI͒ spectra of SF 2 and its concomitant SF + fragment, from which a predissociation channel occurring in the low-lying excited states of SF 2 + is revealed to account for the observed fragmentation. In the present experiment, SF 2 molecules in their ground state were produced by the pulsed dc discharge in a pulsed molecular beam of SF 6 ͑30% in Ar͒. Details of the experimental apparatus and procedure can be found in our previous publications. 9,2 The upper part of Fig. 1 shows the ͑2+1͒ REMPI excitation spectrum of 32 SF 2 ͑m / z =70͒ in the laser wavelength range of 293-325 nm. Based on previous work, 9,10 we can readily assign this spectrum to the Ẽ ͑4p͒ ← ←X 1 A 1 and F ͑4p͒ ← ←X 1 A 1 transitions of SF 2 . ͑See Ref. 9 for details regarding the assignments given in Fig. 1.͒ The lower part of Fig. 1 shows the simultaneously recorded REMPI "daughter" spectrum of 32 SF + ͑m / z =51͒. Notably, it is only from a certain onset wavelength of ϳ310.9 nm that this daughter spectrum resembles its "parent" one. Such type of spectral resemblance usually implies that a mechanism of REMPI followed by dissociation ͑REMPID͒ takes effect. 2,[9][10][11] To facilitate the following discussions, we put all relevant elements in an energy level diagram ͑Fig. 2͒. The horizontal dotted lines in Fig. 2 denote the adiabatic ionization energies ͑AIEs͒ for the six cationic states. 8,6 The threephoton energies used in our experiment lie Ͼ1.5 eV below the cationic dissociation limit ͑SF + ͑X 3 ⌺ − ͒ +F͑ 2 P 0 ͒͒, hence it is natural to consider only the four-photon excitation, i.e., a ͑2+2͒ REMPID scheme is most likely to account for the observed fragmentation. Care ...