Effect of hydrogen bonding on far-ultraviolet water absorption and potential implications for 193-nm ArF excimer laser-tissue interaction

The mechanisms causing transient 1 93-nm optical absorption of collagen during ablative-fluence ArF excimer pulses are poorly understood. The preponderance of hypotheses proposed to explain this phenomenon - such as ultrafast secondary-structure denaturation of proteins and transient free radical formation - focus on the protein matrix and ignore potential cOntributions from other tissue components such as water. A substantial body of spectroscopic literature places 193 nm adjacent to a steep absorption edge ofwater that rises to 60,000 cm^-1 at 163 nm; other evidence shows that this absorption edge shifts toward 1 93 nm upon hydrogen-bond breakage. In this paper we show that heating of water from 20 - 100°C increases the liquid's absorption coefficient. Further investigations using an infrared pump laser show a significant increase in absorption by water of a 1 93-nm probe beam. Based on this evidence, we speculate that 1 93-nm laser ablation of tissue may contain a photothermal component related to dynamic absorption of incident radiation by water.