| Several topics in excimer laser corneal ablation remain unresolved, ranging from fundamental to practical. The roles that photothermal and photochemical processes play in the mechanism of corneal ablation remain a topic of research, including laser-tissue coupling below the ablation threshold. Goals of the present work are to investigate the mechanism of photoablation and to assess whether bovine corneal ablations generated at laser repetition rates up to 400 Hz are comparable to ablations performed at current clinical rates (60--100 Hz).;A combination of experiments was implemented, including ablation plume dynamics, corneal ablation profiles and high-resolution microscopy. Using white-light interferometry analysis, no statistical difference was found between corneal ablation profiles created at 60 Hz and 400 Hz. Using plume imaging and transmission studies, the bulk ablation plume was found to dissipate on a time-scale less than the pulse-to-pulse separation for repetition rates up to about 400 Hz. A persistent, diffuse component of the ablation products was observed to be comparable at both rates. Microscopy did not reveal signs of thermal tissue damage for repetition rates up to 400 Hz. Ablations performed on PMMA did not reveal repetition rate effects. Ablation pattern algorithm reversal and plume extractor addition were analyzed for potential effects on the clinical outcome. Increasing laser repetition rates for clinical applications appear feasible.;Sub-ablative fluences utilizing 193-nm and 355-nm perturbations yielded insight into photochemistry of collagen and amino acids. Amino acid solutions were not permanently altered by either wavelength. For collagen solutions, an average of 28 photons at 193 nm was required to break a peptide bond. 355-nm perturbations resulted in an average of 508 photons required to break a peptide bond. A dynamic photobleaching occurs in both amino acid and collagen solutions at both wavelengths and is greater at 193 nm than at 355 nm, resolving by more than tens of nanoseconds but less than tens of seconds. Permanent changes induced in the collagen samples are due to scission of peptide bond. Mass spectrometry experiments analyzed the ablation products in the ablation plume. The experiments indicate a primarily photochemical ablation mechanism with peptide bonds being the primary chromophore. |
