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

Abstract

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 ∼10¹³ W/cm² to ∼10¹⁵ W/cm². 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 ¹ππ* 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 ¹ππ* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative ³nσ* 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.