Observing the transition from a multiphoton-dominated to a field-mediated ionization process for polyatomic molecules in intense laser fields

The photoelectron spectra for benzene, naphthalene, and anthracene ionized using a 780 nm, 170 fs laser pulse of intensity 3.8 x 10(13) W cm(-2) reveal a decrease in the discrete features associated with above threshold ionization with increasing molecular size. A corresponding exponential increase in the intensity of a broad featureless distribution indicates that field ionization dominates the coupling mechanism for the larger molecules. A coupling model based on a zero-range potential predicts that only multiphoton ionization should occur. This model, incorporating both the ionization potential and electronic delocalization of the molecule, correctly predicts the observed transition. [S0031-9007(98)07853-3].