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 havi ng 5-50 atoms is considered in this article. In general, the interaction of intense radiation fields with molecules can result in both electron emissi on and subsequent dissociation. For the laser excitation Scheme employed he re, intact ions are observed in addition to dissociative ionization channel s for all classes of molecules investigated to date. An excitation mechanis m is considered where the electric field of the laser mediates the coupling between the radiation and the molecule. This field-induced ionization is c ompared with the more common frequency-mediated coupling mechanism found in multiphoton processes. Measurements of intense-laser photoionization proba bility are presented for several series of molecules. An outline of our str ucture-based model is presented to enable calculation of relative tunneling rates and prediction of the laser-molecule coupling mechanism. The relativ e ion yields for various series of hydrocarbon molecules are found to be in good agreement with that predicted by the structure-based tunnel ionizatio n model. Measurements of photoelectron kinetic energy distributions also su ggest that the ionization phenomena proceed to a large degree through a fie ld-mediated excitation process. The photoionization/dissociation products a re measured in an ion spectrometer and are interpreted in terms of a compet ition between electronic excitation and energy transfer to nuclear degrees of freedom. Evidence for field-induced dissociation is presented.