Dynamics and yields for CHBrCl2 photodissociation from 215–265 nm

Abstract

We investigate the Ã-band photodissociation of CHBrCl₂ at 215, 225, 235, 245, 255, and 265 nm. Following C–Br bond cleavage, resonance enhanced multiphoton ionization and time of flight mass spectrometry provide selective detection of the two product channels, from which we quantify the relative quantum yield of Br/Br* production. Velocity-map imaging of the photofragments allows us to determine the energy partitioning as a function of the photolysis energy for different exit channels. The anisotropy present in the imaging data suggests that absorption to the ³Q_0⁺(A′) state is important throughout the entire region we study, though competition with other excited states is evident. The ³Q_0⁺(A′) state forms an avoided crossing with the ¹Q₁(A′) state, and we find that the propensity for adiabatic passage through this crossing region dictates the Br yield at longer wavelengths. At shorter wavelengths, Br production from excited states not subject to the crossing is more evident. While we find that spin–orbit excitation comes largely at the expense of the CHCl₂ internal energy, both channels still produce highly excited CHCl₂ photofragments. Impulsive modeling and comparison with similar halomethane dissociations suggests that a high degree of rotational excitation is present, dictated by the torque inherent in C_s-symmetry dissociation and the angular dependence of the potential.