We report the development of a heterogeneous resin-tube model to study the influence of blood vessels on the apparent absorption of the system, mu a(sys), using a time-resolved technique. The experimental results show that mu a(sys) depends on the absorption inside the tubes, mu a(tube), tube diameters, and tube-to-sample volume ratios. A mathematical expression relating mu a(sys) and mu a(tube) is derived based on the experimental results and is verified by time-resolved Monte Carlo simulations for heterogeneous models. This analytical formula predicts that the apparent absorption coefficient measured on a biological organ is a volume-weighted sum of the absorption coefficients of different absorbing components. We present some apparent absorption coefficients measured in vivo in animals and humans and discuss improved algorithms that calculate the hemoglobin saturation by including background-tissue absorption and blood vessel distribution.

The ultraviolet and visible emission spectra from excimer laser-produced silicon plasmas were studied and the ablation rate measured as a function of laser energy density and wavelength. A spectroscopic investigation of the laser-produced plasma showed Si I, Si II, and Si III spectral lines with higher laser intensity causing a higher degree of ionization in the plasma. Both timeintegrated and time-resolved spectroscopic studies showed electronic transitions superimposed on a weak continuum over the entire range from 250 to 640 nm. The photoablation rate of Si was independent oflaser wavelength (193 or 248 nm), and had an energy density threshold of;:::; 1.3 J/ cm 2 • The threshold was almost independent of the buffer gas pressure between vacuum and 1000 Torr. These results are described in the framework recently developed for excimer laser ablation of metals.

The results of a study of afIerglow in the vuv spectral region of discharges in xenon, krypton and argon (at pressures of 0.2 to 6 Torr) are reponed. Values have been obtained for recombination coefficients and f o m i o n rates of molecular ions from atomic ions, as well as for ceaain collisional excihtion lransfer constants for the 3pS4s'P1 level of argon by comparing the experimental pressure dependence of resonance-line afterglow intensities with approximate analytical solutions of balance equations. The time dependence of Xenon resonance lines in mixtures of helium neon. argon and luypton were also compared with approximate solutions of the balance equations. The rate conslant for the formation of xenon molecular ions in three-body collisions of Xe+ with two Xe aoms was measured to .be k,.(Xe) = 0.8(+0.2) x IO-" cm6 St; with Ne as the third atom the Same reaction had a constanl of k+(Ne) = 3.0(*0.8) x IO-" cm6 SKI and with Ar as the third atom k+(Ar) = 0.8(*0.2) x cm6 5 -l . The Kri formation constant in lhree-body collisions of Krt with two Kr atoms was measured to be 0.53(*0.15) x IO-'] cm6 s-' and, for Ar: formtion in a similar mction with two ugon atoms, it is 1.9(+0.5) x cm6 5-l. The recombination constants U for the atomic ions were obtained rrl(Xe) = 1.5(10.5) x IO-y, olt(Kr) = 0.8(+0.2) Y and q ( A r ) = 1.0(*0.2) x cm3 s-'.

The gain characteristics of an electron-beam pumped XeF(C-+ A) excimer amplifier operating in the bluegreen spectral region were investigated for several laser pulse lengths. Saturation energy densities of 50 and 80 mJ/cm2 were measured for injected laser pulse durations of 250 fs and-100 ps, respectively. A gain bandwidth of 60 nm was observed with-100 ps pulse injection. Using an optimized unstable resonator design, the laser amplifier has produced 275 mJ pulses with a pulse duration of 250 fs and a 2.5 times diffraction limited beam quality, making the XeF(C-+ A) amplifier the first compact laser system in the visible spectral region to reach peak powers at the terawatt level.

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