Curve-crossing study of methyl halide in A-band

Abstract

The methyl halide A-band photodissociation process $CH_3X$ → $CH_3 + X(^2 P_{3/2})$, $X^* (^2 P_{1/2})$, (X=Br, I) was studied near 277 and 234 nm. Full three dimensional speed and angular distributions of the nascent fragments was recorded using [2+1] resonance-enhanced multi-photon ionization (REMPI) and velocity imaging. By combining the $X^*$ quantum yield and anisotropy parameters for both spin-orbit excited state and ground state, the curve-crossing probabilities of methyl iodide, nethyl bromide and their isotopically substituted counterparts are determined. The comparison of $CH_3I$ and $CD_3I$ at 277 nm is contrary to the results at 248 nm. This imply that a recoil velocity of fragment is still important when the molecule is in $C_s$ symmetry where the Landau-Zener equation could not be applied.