Nonresonant femtosecond ionization/dissociation of aromatic hydrocarbons

The coupling mechanism between an intense (∼1013 W cm--2, 780 nm) near-infrared radiation field of duration 125 fs with molecules having 12 to 32 atoms is considered in this dissertation. The time-of-flight mass spectra are reported for the molecules benzene (C 6H6), biphenyl (C12H10), diphenylmethane (C13H12), and diphenylethane (C14H 14). The ionization of these molecules is consistent with a structure-based tunneling model. The model correctly predicts the order of relative ion yields: benzene < diphenylmethane < diphenylethane. The experimental:calculated yields for benzene, biphenyl, diphenylmethane, and diphenylethane are 1:1, 27:257, 59:113, and 134:467 at 1.2 V A--1. The ionization probabilities are not correlated with those predicted by the ADK model. The ADK model predicts relative probabilities of 1 (benzene): 4.4 (biphenyl): 2.1 (diphenylmethane): 1.3 (diphenylethane). In addition, comparison of the Keldysh adiabaticity parameter, gamma, to the molecular adiabaticity parameter, gamma(Psi), at 1 V/A returns ratios for gamma(Psi)/gamma of 0.68, 0.35, 0.42, 0.29 for benzene, biphenyl, diphenylmethane, and diphenylethane. These ratios suggest that tunneling ionization occurs at a lower intensity than that predicted by the Keldysh adiabaticity parameter. A comparison of the femtosecond mass spectra to 70 eV mass spectra shows that the dissociation occurs in a statistical manner for the molecules biphenyl, diphenylmethane, and diphenylethane for a specific laser intensity. The similarity in these mass spectra also suggest that the probability distribution function, P(E), is similar for these very different excitation methods. The molecule p-terphenyl does not show any resemblance to the 70 eV electron impact mass spectra for any laser intensity. This molecule appears to fragment in a nonstatistical manner. A mechanism of energy deposition consistent with the structure-based coupling model includes the characteristic length of the molecule. Calculation of a "potential drop" across the molecule and predicts greater energy deposition into p-terphenyl compared to biphenyl, diphenylmediane, and diphenylethane. The possible excess energy absorbed by p-terphenyl may be cause for the extensive fragmentation observed in this molecule. The large size of p-terphenyl also confers a larger ionization probability of 244 relative to benzene at 1.2 V A--1. The interdependence of the ionization and dissociation of p-terphenyl is discussed.