A novel laser system has been developed to study the effects of multiple laser pulses of differing wavelengths on cutaneous blood vessels in vivo, using the hamster dorsal skin flap preparation and in vitro, using cuvettes of whole or diluted blood. The system permits sequenced irradiation with well‐defined intrapulse spacing at 532 nm, using a long‐pulse frequency‐doubled Nd:YAG laser, and at 1064 nm, using a long‐pulse Nd:YAG laser. Using this system, we have identified a parameter space where two pulses of different wavelengths act in a synergistic manner to effect permanent vessel damage at radiant exposures where the two pulses individually have little or no effect. Using a two‐color pump–probe technique in vitro, we have identified a phenomenon we call green‐light–induced infrared absorption, where a pulse of green light causes photochemical and photothermal modifications to the chemical constituents of blood and results in enhanced infrared absorption. We identify a new chemical species, met‐hemoglobin, not normally present in healthy human blood but formed during laser photocoagulation which we believe is implicated in the enhanced near‐infrared absorption.

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A novel laser system has been developed to study the effects of multiple laser pulses of differing wavelengths on cutaneous blood vessels in vivo, using the hamster dorsal skin flap preparation and in vitro, using cuvettes of whole or diluted blood. The system permits sequenced irradiation with well-defined intrapulse spacing at 532 nm, using a long-pulse frequency-doubled Nd:YAG laser, and at 1064 nm, using a long-pulse Nd:YAG laser. Using this system, we have identified a parameter space where two pulses of different wavelengths act in a synergistic manner to effect permanent vessel damage at radiant exposures where the two pulses individually have little or no effect. Using a two-color pump-probe technique in vitro, we have identified a phenomenon we call greenlight-induced infrared absorption, where a pulse of green light causes photochemical and photothermal modifications to the chemical constituents of blood and results in enhanced infrared absorption. We identify a new chemical species, met-hemoglobin, not normally present in healthy human blood but formed during laser photocoagulation which we believe is implicated in the enhanced near-infrared absorption.

in absorption in the vicinity of 83 nm. Predissociative widths as narrow as 0.3 cm have been measured, and profile analyses based on the theory of Pano are performed on these predissociated states. The importance of overlap integrals in the predissociation of B" is shown, and the absorption cross section o = (7.2 +0. 7) &(10 " cm' for the B"~X(2,0) is determined. Anomalous behavior observed in the D (v = 5) states of 0, is analyzed in terms of an accidental perturbation by the D" (v = 0) states.

The nonlinear coupling of 193-nm radiation to a range of atomic and molecular materials has been experimentally explored up to a maximum intensity on the order of -10' W/cm . Studies of collision-free ion production clearly exhibit anomalous behavior which strongly implies that the atomic shell structure is the principal determinant in the observed response. On the basis of the observed coupling strength and the measured atomic-number (Z) dependence, the experimental evidence points to a coherent atomic motion involving several electrons, possibly an entire shell, as the main physical mechanism enabling the scale of energy transfers seen. Therefore, states representing multiple excitations appear to play a central role in the coupling, a consideration that fundamentally distinguishes the nonlinear interaction of a multielectron atom from that of a single-electron system. Comparison of the experimental findings with standard theoretical treatments, of either a perturbative or nonperturbative nature, does not produce satisfactory agreement. Conversely, the formulation of a simple classical estimate qualitatively conforms to several features of the observed behavior including the shell character of the interaction, the maximum energy transfer, the dependence of the average energy transfer on the intensity of irradiation, the frequency dependence of the observed energy transfer, and the weak influence of polarization.

Intense vacuum-ultraviolet stimulated emission in molecular hydrogen, on both the Lyman and Werner bands, following excitation by two-quantum absorption at 193 nm on the X 'Xg~E,F 'Xg transition, has been observed. The shortest wavelength seen in the stimulated-emission spectrum was 117. 6 nm corresponding to the C 'H"~X 'Xs+ (2-5) Q(2) transition. The C 'II"state appears to be populated with a mechanism involving electron collisions. The radiative cascade mechanism found to lead to the vacuum-ultraviolet emissions on the Lyman band also causes strong infrared stimulated emission to occur on the E,F 'Xg~8 'X+"band. Two entirely separate radiative excitation channels are observed to play important roles in the state-selective molecular population of the E,F 'Xg level. One involves two 193-nm quanta in the X~E,F amplitude while the other process combines a 193-nm quantum with a first Stokes-shifted photon in H2. The optical Stark effect was seen to play a significant role in the excitation process with shifts of molecular resonances as large as -45 cm . Substantial deviations from Born-Oppenheimer behavior, resulting in a dramatic shift of the stimulated spectrum depending upon the excited-state rotational quantum number, were clearly observed for molecular levels close to the potential maximum separating the inner and outer wells of the E,F 'Xg state. The maximum energy observed in the strongest stimulated line was -100 pJ, a value corresponding to an energy conversion efficiency of -0.5 Jo. The pulse duration of the stimulated emission is estimated from collisional data to be -10 ps, a figure indicating a maximum converted vacuum-ultraviolet power of -10 MW.

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