Avoided Curve Crossing between the A₁ and B₁ States in CF₂Br₂ Photolysis at 234 and 265 nm

Abstract

The photodissociation dynamics of CF2Br2 has been studied using a two-dimensional photofragment ion image technique for the two excitation wavelengths 234 and 265 nm. At 234 nm, three dissociation channels, i.e., the radical CF2Br + Br (2P J ) (J = 1/2, 3/2), the three-body CF2 + 2Br (2P J ) (J = 1/2, 3/2), and the molecular elimination channel CF2 + Br2, were observed with quantum yields of 0.84, 0.15, and trace, respectively. The difference between β (CF2Br + Br (2P3/2)) = 0.65 and β (CF2Br + Br (2P1/2)) = 0.80 suggests that the excited A1 and B1 states are strongly correlated by avoided curve crossing with a probability of 0.78. At 265 nm, the radical channel, the observed major primary dissociation channel, shows several components due to the transition close to the curve crossing point. On the basis of observations at both wavelengths, we have proposed a photodissociation dynamics model of CF2Br2 after A-band excitation.

Photodissociation dynamics of vibrationally excited CH2Cl2 molecules

“…Since the b parameter of Cl à is larger than that of Cl, it is anticipated that the contribution of the A 00 state to the former channel is more effective. Absorption via more than one PES and curve crossings were suggested to account for the observed branching ratios and the anisotropies in other halocarbons as well, exemplified by CH 3 I [2], CH 3 Cl and CHD 2 Cl [15,16], CHFCl 2 [24,25], CH 3 CFCl 2 [28], CH 2 BrCl [18], CF 2 Br 2 [19], and CH 2 I 2 [20].…”

“…Numerous theoretical and experimental investigations on monohalogen alkyl molecules [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] have revealed the importance of non-adiabatic curve crossing dynamics in C-X (X ¼ Cl, Br, and I) bond rupture and in generation of ground state X( 2 P 3=2 ) [X] and spin-orbit excited X( 2 P 1=2 ) [X Ã ]. Fewer studies have been performed on dihalide alkyl molecules [7,[17][18][19][20], since the presence of two C-X bonds makes the structure of the electronically excited states and therefore of the UV absorption spectra more complex. Besides, the picture is further complicated by the greater number of potential dissociation pathways.…”

Matrix isolation and computational study of isodifluorodibromomethane (F2CBr–Br): A route to Br2 formation in CF2Br2 photolysis

“…22 At wavelengths below 260 nm, the nascent CF 2 Br radical spontaneously dissociates, producing CF 2 and a second Br atom. 8,11,21 Recent experiments have also indicated a small ͑0.04 quantum yield͒ contribution from a direct Br 2 elimination channel. 5 Consistent with these observations, we find very weak CF 2 Br absorptions following 240 nm photolysis of matrix isolated samples ͑Fig.…”

“…This was first shown for photolysis at 193 nm in a molecular beam by Huber and co-workers. 17 Later, in ion imaging experiments, Park et al 8 reported a "trace" amount of Br 2 photoproduct following excitation at 234 nm. More recently, Lin and co-workers 5 probed Br 2 directly using cavity ring down spectroscopy, and reported a quantum yield of 0.04 for the molecular channel at 248 nm.…”

Dissociative Ionization of Molecular CF2Br2 under 800 and 400 nm Intense Femtosecond Laser Fields

“…Their results suggest that the molecular Br 2 elimination reaction, CF 2 Br 2 → CF 2 + Br 2 , cannot occur under their experiment condition. Park et al [14] have studied the photodissociation dynamics of CF 2 Br 2 using 2D ion image technique. The difference of parameter β between channel (CF 2 Br + Br ( 2 P 3/2 )) and (CF 2 Br + Br ( 2 P 1/2 )) at 234 nm suggests that excited A 1 and B 1 states are strongly correlated by avoided curve crossing.…”