Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Abstract
The photoelectron spectra for benzene, naphthalene, and anthracene ionized using a 780 nm, 170 fs laser pulse of intensity 3.8 × 1013 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. ©1998 The American Physical Society