The relative photoionization/dissociation probabilities are presented for the molecules benzene, naphthalene, and anthracene upon interaction with 780 nm laser radiation of duration 170 fs and intensity 3.8 X 10(13) W cm(-2). Both the ionization probability and the dissociation yield increase exponentially from benzene to anthracene as measured by time-of-flight mass spectra. A structure-based model is presented for the excitation of large polyatomic molecules by intense laser irradiation with pulse widths on the time scale of molecular vibration (100 fs) and with peak field strengths of 1-2 V Angstrom(-1). The model accounts for molecular structure and is able to accurately predict the transition from multiphoton ionization (MPI) to tunnel ionization. It is also demonstrated that this structure-based model can quantitatively predict the experimentally measured ionization probabilities. In comparison, models employing the more conventional zero-range potential do not accurately predict either the transition or the relative ion yield measured experimentally.