Absolute two-photon ionization yields for selected organic molecules at 248 nm

Absolute two-photon ionization yields for benzene, fluorobenzene, diethylaniline, and trimethylamine have been measured at 248 nm as a function of KrF laser fluence for two pulse lengths of approximately 8 and 20 nsec. We have also investigated NO ionization yields at 226 and 193 nm. The ionization is a resonant process that can be modeled as a four-level system. From the data, we derive ground-state absorption cross sections, intermediate-state ionization cross sections, and effective intermediate-state lifetimes. In some cases, a small contribution to the ionization yield from a direct nonresonant two-photon process can be inferred from the model.