Multiorbital effects in strong-field ionization and dissociation of aligned polar molecules CH3I and CH3Br

Controlling the molecular axis offers additional ways to study molecular ionization and dissociation in strong laser fields. We measure the ionization and dissociation yields of aligned polar ${\mathrm{CH}}_{3}X$ ($X=\mathrm{I}$, Br) molecules in a linearly polarized femtosecond laser field. The current data show that maximum ionization occurs when the laser polarization is perpendicular to the molecular $\mathrm{C}\ensuremath{-}X$ axis, and dissociation prefers to occur at the laser polarization parallel to the $\mathrm{C}\ensuremath{-}X$ axis. The observed angular distributions suggest that the parent ions are generated by ionization from the HOMO. The angular distribution of fragment ions indicates that dissociation occurs mainly from an ionic excited state produced by ionization from the HOMO-1.