We report on the photoionization and photofragmentation of benzene (C 6 H 6 ) and of the monohalobenzenes C 6 H 5 -X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range B10 13 W/cm 2 to B10 15 W/cm 2 . We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C 6 H 5 X + ) are observed. Our data is consistent with resonance-enhanced multiphoton ionization (REMPI) involving the neutral 1 pp* excited state (primarily a phenyl excitation): all of our plots of parent ion yield versus intensity display a kink when this excitation saturates. From the intensity dependence of the ion yield we infer that both the HOMO and the HOMOÀ1 contribute to ionization in C 6 H 5 F and C 6 H 5 Cl. The proportion of phenyl (C 6 H 5 ) fragments in the mass spectra increases in the order X = F, Cl, Br, I. We ascribe these substituent-dependent observations to the different lifetimes of the C 6 H 5 X 1 pp* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative 3 ns* state. This breaks the C-I bond in an early stage of the ultrashort pulse, which explains the abundance of fragments that we find in the iodobenzene mass spectrum. For the lighter X = F, Cl, and Br this dissociation is much slower, which explains the lesser degree of fragmentation observed for these three molecules.

Intense-field stability of photoionization and fragmentation are compared for aniline (C6H5–NH2) and nitrobenzene (C6H5–NO2), with amine vs. nitro substitutions explored.

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