Ultrafast REMPI in benzene and the monohalobenzenes without the focal volume effect

We report on the photoionization and photofragmentation of benzene (C6H6) and of the monohalobenzenes C6H5-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 similar to 10(13) W/cm(2) to similar to 10(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 (C6H5X+) are observed. Our data is consistent with resonance-enhanced multiphoton ionization (REMPI) involving the neutral (1)pi pi* 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 HOMO 1 contribute to ionization in C6H5F and C6H5Cl. The proportion of phenyl (C6H5) 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 C6H5X (1)pi pi* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative (3)n sigma* 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.