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 $3.8\times {10}^{13}\mathrm{W}{\mathrm{cm}}^{-2\mathrm{}}$ 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.