Photoexcitation, ionization, and dissociation of molecules using intense near-infrared radiation of femtosecond duration

The coupling mechanism between an intense (similar to 10(13) W cm(-2), 780 nm) near-infrared radiation field of duration 50-200 fs with molecules having 5-50 atoms is considered in this article. In general, the interaction of intense radiation fields with molecules can result in both electron emission and subsequent dissociation. For the laser excitation Scheme employed here, intact ions are observed in addition to dissociative ionization channels for all classes of molecules investigated to date. An excitation mechanism is considered where the electric field of the laser mediates the coupling between the radiation and the molecule. This field-induced ionization is compared with the more common frequency-mediated coupling mechanism found in multiphoton processes. Measurements of intense-laser photoionization probability are presented for several series of molecules. An outline of our structure-based model is presented to enable calculation of relative tunneling rates and prediction of the laser-molecule coupling mechanism. The relative ion yields for various series of hydrocarbon molecules are found to be in good agreement with that predicted by the structure-based tunnel ionization model. Measurements of photoelectron kinetic energy distributions also suggest that the ionization phenomena proceed to a large degree through a field-mediated excitation process. The photoionization/dissociation products are measured in an ion spectrometer and are interpreted in terms of a competition between electronic excitation and energy transfer to nuclear degrees of freedom. Evidence for field-induced dissociation is presented.