Ultrafast photoionization of toluene and the di- and trimethylbenzene isomers: the effects of orbital symmetry on intense-field stability

Our recently published work on the femtosecond intense-field photoionization of azabenzene and the diazabenzene isomers (J. Chem. Phys. 136, 054309 (2012)) presents the four molecules as a systematic series of perturbations to the ring structure of benzene. In these molecules, we have observed correlations between molecular orbital symmetry and stability against intense-field ionization and fragmentation. We further our exploration of this phenomenon with preliminary results on the ultrafast photoionization of seven similarly-structured substituted benzenes -- toluene [$C_{6}H_{5}CH_{3}$], and the di- and trimethylbenzene isomers [$C_{6}H_{4}(CH_{3})_{2}$ and $C_{6}H_{3}(CH_{3})_{3}$] -- divided into four molecular series related through various degrees of symmetry. Ions are created with 50 fs, 800 nm pulses under single-molecule conditions. Ion yields are collected with a time-of-flight ion mass spectrometer, and recorded as a function of laser intensity without the focal volume effect (Phys. Rev. Lett. 100, 023002 (2008)).