Studies On Photodissociation Dynamics Of Halogenated Compounds Using Velocity Map Imaging Method

Photodissociation dynamics of halogenated compounds of C2H5Br,C2H5I,o,m,p-C6H4FBr and C6H5Br etc have been investigated under the radiation of UV light using ion velocity map imaging coupled with resonance-enhanced multiphoton ionization techniques. Under the UV light radiation, alkyl halides of C2H5Br and C2H5I are excited to a repulsive potential surface and lead to a direct dissociation of C-Br bond, while aryl halides of o,m,p-C6H4FBr and C6H5Br are excited to S1 state, a bound delocalized singlet (π,π*) state, which can interact with a repulsive triplet (n,σ*) state localized on the C-Br bond via spin-orbit coupling. In the wavelength range of 245-283nm, there is a avoided crossing between 3Q0 and 1Q1 states of , C2H5Br and the probability of nonadiabatic transition increases with the decreasing of wavelength. For the photodissociation of C2H5I in the wavelength range of 245-283nm, we measured the photofragment ion images of I , I* , C2H5 and radical, and found the I* channel is accompanied by a colder internal state of C2H5, while the I channel is with a hotter C2H5. The curve crossing probability rises gradually with the wavelength, suggesting that the conical intersection be located close to the exit channel. Br is the primary product of the photodissociation of fluorobromobenzene and bromobenzene near 266nm and it is generated through two channels. Comparing with the photodissociation of bromobenzene, the extra fluorine atom of fluorobenzene accelerates the rate of the internal conversion from the (π,π*) state to the (n,σ*) state, and the different positions on the phenyl ring also have distinct effect on it.In order to investigate the molecular behavior on the repulsive potential surface during photodissociation, we built a photoelectron imaging setup with a high resolution of 1.5% with the calibration experiment of oxygen. A noncollinearly phase-matched parametric amplifier is set up, and on the basis of the 800nm femtosecond laser, it can make the laser wavelength tunable through the visible of 450-620nm and the ultraviolet of 210-350nm. So we will investigate ultrafast dynamics of molecular photodissociation using femtosecond time-resolved pump-probe technique coupled with photofragment ion imaging or photoelectron imaging in future work.