Near-infrared femtosecond photoionization/dissociation of cyclic aromatic hydrocarbons

Pulses of 780 nm light of duration 170 fs and power densities up to 3.8×1013 W cm−2 are used to study the photoionization/dissociation processes in the series of gas phase, cyclic aromatic hydrocarbons including benzene, naphthalene, phenanthrene, and anthracene. The near-infrared ionization process leads to the production of intact molecular ions for all of the molecules studied. Measurements of the ion intensity as a function of laser fluence revealed the order of the ultrafast ionization process to be 8.0±0.1 for anthracene, 6.9±0.1 for phenanthrene, 8.5±0.1 for naphthalene, and 8.1±0.1 for benzene. The relative femtosecond photoionization cross section decreased from 1.0 for anthracene to 0.2 for phenanthrene to 0.1 for naphthalene to ∼0.005 for benzene. The relative order and cross section of the femtosecond ionization processes suggest that a field ionization mechanism is operative.