Photodissociation dynamics of Br2 in wavelength range of 360-610 nm

Abstract

We study the photodissociation of Br2 in a wavelength range from 360 nm to 610 nm in the near-visible UV continuum band based on the calculation of time-dependent quantum wave packet including the rotational degree of freedom. We calculate four representative samples of two-dimensional (2D) slice images taken from photolysis of Br2 molecules, in which the different rings in the 2D slice images are corresponding to the different photodissiation channels. The radius of each 2D slice image ring is positively related to kinetic energy of photofragment. The maximum photofragment flux perpendicular or parallel to the photolysis polarization is also related to photodissiation channel. Furthermore, we calculate the total kinetic energy distribution P(E) and the P(E) distribution from the respective electronic excited states A, B and C in the wavelength range of 360-610 nm, from which we find that the wavelengths corresponding to the maximum dissociation probability from respective electronic excited states A, B and C are 510 nm, 469 nm, and 388 nm, respectively. As is well known, not only the total dissociation probability, but also the respective dissociation probability of electronic excited states is dependent on the laser wavelength. We also calculate the dissociation probabilities from electronic excited states A, B and C, respectively. We find that the dissociation probability of electronic excited state A is not significant when 480 nm and that the peak intensity of the dissociation probability to the A state is about 13.0\% of that to the C state, while that to the B state is about 43.4\%. In addition, because the electronic excited states A and C are related to the photodissociation channel Br + Br, and the electronic excited state B is corresponding to the photodissociation channel Br + Br*, the images which reveal the involvement of more than one product channel can be analyzed by the respective channel branching ratios. At the short wavelength ( 400 nm) the branching ratio (Br*/(Br+Br*)) is small, even near to zero, which reflects that electronic state C transition gives rise to many Br + Br over Br + Br*. However, within the wavelength range (=440-500 nm) Br + Br* photofragments are excess of Br + Br, so the electronic state B transition is dominant. At longer wavelength ( 530 nm) the branching ratio (Br*/(Br+Br*)) is also low, near to zero, indicating the prevalence of electronic state A transition. Ignoring the dissociation from electronic state C, the maximum dissociation probability 469 nm is consistent with branching ratio maximum 462 nm. Because the electronic excited state C is related to the photodissociation channel Br + Br, the branching ratio will be reduced. So the maximum wavelength of branching ratio is blue shifted.