Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

The properties of pulsed IR-laser ablation of biological soft tissue (porci ne cornea) were studied in vitro systematically and quantitatively with a f ree-electron laser in the wavelength range 6 less than or equal to lambda l ess than or equal to 20 mu m at fluences ranging from 3.1 to 9.4 J/cm(2). D ynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation thre shold, the ablated mass, and the particle size were investigated. The ablat ion plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm(2) the average initial velocity of the ejected particles was deduc ed from the extension of the plume to range from 120-400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopica lly determined absorption coefficient of cornea alpha(cornea). Where absorp tion due to proteins is high (at lambda = 6.2 and 6.5 mu m), ablated mass, velocity and recoil momentum behave according to a(cornea). For the first t ime, variations of the ablation plume from pulse to pulse were observed. Th ose, as well as the particle size, not only depend on the absorption coeffi cient, but also on the predominant absorber.