High-Power Laser Experiments

After the introduction of frequency resolved CARS by Maker and Terhune [1], time resolved experiments became possible with the invention of high power lasers with femtosecond resolution. Leonhardt [2] and for example Hayden [3] performed femtosecond CARS experiments in liquids. A first femtosecond time resolved CARS experiment in gas phase was performed by Motzkus et. al. [4] where the wave packet dynamics of the dissociation of Nal was monitored. The first observation of wave packet dynamics in gaseous iodine was reported by Schmitt et al. [5]. They were able to observe dynamics in both, the ground and excited state with the same experiment. A summary of high resolution spectroscopy in gas phase by nonlinear methods is given by Lang et al. [6]. 

The main prerequisite to observing a Raman signal excited with the evanescent wave is the use of a high power laser. In the experiments described here the blue line (488 nm) of a cw Argon -Ion laser with a power output of approx. 1.3 watts was used preferentially. 

Taniguchi, et al. [99] have tested the one- and two-photon excitation for [Cr(acac)3]3- for several irradiation wavelengths but found no indication of a superlinear dependence of photoracemization or photodecomposition. This seems to be in line with the experiments of Gunde and Richardson [6] on Na3[Gd(02C-CH2-0-CH2-C02)3] 2NaC104 6H20 crystals. The possibility discussed in several papers [4-6] that in high-intensity radiation fields a pure electric dipole 2-photon absorption may lead to CD effects adds a new possibility of rationalization of the high-power laser results. 

For most proteins, intrinsic triplet lifetimes of aromatics are quite short at room temperatures and thus triplet dye labels must be used. Our own experience has been with the use of triplet probes and triplet anisotropy decay. Since this is somewhat of a new, and we believe under-utilized, field, we will stress it here. Because the triplet yield is usually quite small and either low sensitivity absorption techniques or very low quantum yield phosphorescence measurements must be made, a reasonably high-powered laser is necessary for this kind of experiment. 

The use of high-power lasers with short wavelengths allows generation of the most extreme shock pressures in laboratories [15], but only in conjunction with extremely short pulse durations in the nanoseconds range. Laser irradiation shock experiments may be regarded as adequate simulations of hypervelocity impacts of microgram-mass micrometeorites onto atmosphere-free bodies such as the Moon [21] as well as onto spacecraft. Such impacts do not occur on Earth, as high-speed micrometeorites bum up in the atmosphere. 

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