Photodissociation Dynamics of Allyl Chloride at 200 and 266 nm Studied by Time-Resolved Mass Spectrometry and Photoelectron Imaging

The photodissociation dynamics of allyl chloride at 200 and 266 nm has been studied by femtosecond time-resolved mass spectrometry coupled with photoelectron imaging. The molecule was prepared to different excited states by selectively pumping with 400 or 266 nm pulse. The dissociated products were then probed by multiphoton ionization with 800 nm pulse. After absorbing two photons at 400 nm, several dissociation channels were directly observed from the mass spectrum. The two important channels, C–Cl fission and HCl elimination, were found to decay with multiexponential functions. For C–Cl fission, two time constants, 48 ± 1 fs and 85 ± 40 ps, were observed. The first one was due to the fast predissociation process on the repulsive nσ*/πσ* state. The second one could be ascribed to dissociation on the vibrationally excited ground state which is generated after internal conversion from the initially prepared ππ* state. HCl elimination, which is a typical example of a molecular elimination reaction, was found to proceed with two time constants, 600 ± 135 fs and 14 ± 2 ps. We assigned the first one to dissociation on the excited state and the second one to the internal conversion from the ππ* state to the ground state and then dissociation on the ground state. As we excited the molecule with 266 nm light, the transient signals decayed exponentially with a time constant of ∼48 fs, which is coincident with the time scale of C–halogen direct dissociation. Photoelectron images, which provided translational and angular distributions of the generated electron, were also recorded. Detailed analysis of the kinetic energy distribution strongly suggested that C₃H₄⁺ and C₃H₅⁺ were generated from ionization of the neutral radical. The present study reveals the dissociation dynamics of allyl chloride in a time-resolved way