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Nonperturbative rates of harmonic generation or frequency mixing and rates of multiphoton ionisation were obtained for atomic hydrogen and for a one-electron model of the negative hydrogen ion, using the Floquet-Sturmian method. The following cases were investigated: elliptically polarised, monochromatic incident field, two-colour incident field with incommensurable frequencies, coherent super position of an incident field and its third harmonic, and superposition of a laser field and a static electric field. For elliptical polarisation, the ellipticity angle of the harmonics differ from the ellipticity of the incident laser and we established that there is an offset angle between the major axis of the incident laser's polarisation ellipse and that of the harmonics generated. The variation is greatest in the plateau region. Resonance-enhanced ionisation was studied when a high frequency field was applied in addition to a fundamental field. In contrast to these changes in the ionisation rate only small variations have been obtained in the harmonic generation rate. Results for frequency-mixing exhibit a difference in the strength of the harmonics, although the behaviour is the same in terms of the length of the plateau and the cut-off order. Also we found a marked change between sum-frequency and difference frequency processes, which depends on the harmonic order. In addition to a strong production of even harmonics, it was found that the variation of the rate of ionisation and harmonic generation in the presence of a static field oscillates as a function of the strength of the latter. In the case of hydrogen, we observed dc-stark shift induced resonance enhancements. The results obtained for H(^-) are in very good agreement with those obtained for two-colour (w, 3w) mixing. We concluded that the application of a static field in general leads to a reduction in the conversion efficiency.

The modes of action of a diamine salt of fatty acid with a general structure of [RNHz+(CHz)JNH3+(R'COO-)z], referred to as a multifunctional additive (MFA) in carbon black and silica filled natural rubber (NR) compound have been studied. The mechanical properties have been enhanced by incorporating the MFA. Optimum mechanical properties for the natural rubber filled with carbon black compound are obtained at an MFA level which is estimated to give complete monolayer coverage of the carbon black surface. Studies on silica filled NR show a much higher optimum MFA level than for carbon black filled compounds, which is attributed to a different orientation of the MFA molecules at the silica-rubber interface. The MFA molecules may stand on the filler surface with their polar sites attached to its surface, giving rise to a much higher monolayer value. Filler dispersion, measured by computer-aided image analysis and scanning electron microscopy (SEM), showed that MFA gives improved dispersion at equivalent mixing time. The effect of the MFA on crosslink density of carbon black filled NR compounds was found to be small, in contrast to silica filled NR compounds, where the increase in crosslink density enhanced properties significantly. Techniques to examine the properties of compounds with and without MFA at equivalent levels of filler dispersion have been developed. Results show that, in addition to the improvement in filler dispersion, increased energy dissipation at the rubber-MFA-filler interfaces improves tear and tensile strength. A mechanism of adsorption to explain the mode of action of the MFA at the elastomer/filler interface has also been proposed.