Femtosecond laser interactions in the condensed phase : application to transient absorption and materials processing
Solid State laser systems are now capable of generating temporal pulses of sub-10 fs duration and facilitate the study of ultrafast phenomena in chemical, biological and physical systems. The high field intensities inherent in these pulses, means that highly non-linear processes may be observed. Studies of solution phase transient absorption and femtosecond pulse laser ablation have been undertaken and in each case an experimental apparatus has been developed. Ultrafast pump-probe studies of room temperature solutions of 3-hydroxyflavone and a number of its derivatives have been undertaken. Measurement of transient absorption spectra attributable to the excited tautomer arising from ESIPT was observed throughout the visible wavelength range (400 - 700 nm). In cyclohexanea nd acetonitriles olutions, ESIPT was found to be so rapid that it was only possible to assign a time constant < 35 fs to the process. In ethanol, however, a time constant of 60 fs was determined. The slower ESIPT in this solvent is attributed to the greater strength of the solute-solvent interactions. The influence of substituents in the 4' position of the phenyl ring of 3-hydroxyflavone has also been investigated and we observe unusual behaviour for a cyano substituent in polar solvents and for dimethylamino and aza-Crown substituents. We attribute this behaviour to charge transfer followed by rapid solvent re-organisation. The possibility of the formation of TICT states is also discussed. Ultrafast laser ablation has been studied for a variety of dielectric, semiconducting and metallic substrates with emphasis on the development of the technique for rapid prototyping of micro-fluidic devices. Some fundamental observations of the ablation mechanisms and residual structures is presented along with results for prototyped devices for molecular diffusion and macro-filter applications.