Nonadiabatic dynamics in the photodissociation of $ICH_2CN$ at 266 and 304nm studied by velocity map ion imaging technique

Abstract

Velocity map ion imaging technique has been established. Especially, photon sources, a vacuum chamber, electronics, sample injection devices, detection devices and a system control unit are well-operated and stabilized. Then, machine was calibrated by taking molecular oxygen photodissociation images at 224.999 nm. Photodissociation dynamics of iodoacetonitrile ($ICH_2$CN) have been investigated by using velocity map ion imaging. First, C-I bond scission dynamics of iodoacetonitrile(($ICH_2$CN) have been studied at pump wavelengths of 266 and 304 nm. The prompt C-I bond rupture takes place on the repulsive excited states to give I (^2P$_{\frac{3}{2}}$) and I$^{*}$(^2P$_{\frac{1}{2}}$), and their speed and spatial distributions were simultaneously measured. The recoil anisotropy parameter ( β) at 266 nm was determined to be 1.10 and 1.60 for I and I*, respectively, while it is found to be much higher at 304 nm to give β = 1.70 and 1.90 for I and I$^{*}$, respectively. Branching ratios for I$^{*}$/I channels are measured to be 0.724 and 0.136 at 266 and 304 nm, respectively. These give insights on nonadiabatic curve crossings and relative oscillator strengths of optically accessible transitions of ICH₂CN. Accordingly, relative oscillator strengths of parallel/perpendicular transitions and nonadiabatic curve crossings among excited states are quantitatively characterized. A large portion of the available energy (42-49%) goes into internal energy of the CH₂CN fragment. A modified impulsive model in which the CH₂CN fragment is assumed to be rigid predicts the energy disposal quite well. Delocalization of an unpaired electron of the CH₂CN radical during the C-I bond cleavage, leading to a large structural change of the CH₂CN moiety, may be responsible for internally hot fragments.