Process laser based techniques

Lasers are the precision tools of photochemistry and they have been used to both pump (initiate) and probe (analyse) chemical processes on time-scales that are short enough to allow the direct observation of intramolecular motion and fragmentation (i.e. on the femtosecond time-scale). Thus, laser-based techniques provide us with one of the most direct and effective methods for investigating the mechanisms and dynamics of fundamental processes, such as photodissociation, photoionization and unimolecu-lar reactions. Avery wide variety of molecular systems have now been studied using laser techniques, and only a few selected examples can be described here. 

Because chemical and structural properties of natural and artificial gems are very similar in this case, the possibilities of Raman and LIBS methods are rather limited. It was found that other laser-based techniques could be very effective for rapid spectroscopic discrimination between natural and synthetic emeralds, rubies, and alexandrite (Armstrong et al. 2000). The first one is DRIFTS (Diffuse Reflectance Fourier Transformed Infra-Red Spectroscopy) and the second one is NIR (Near Infra-Red) Spectroscopy. In some cases it was even possible to discriminate between gems made by different synthetic processes. Once again, there is a significant benefit to having two independent methods available. 

The above described methodologies have allowed us to conduct spectroscopic studies with a finesse which is not easily attainable using conventional techniques. These laser based techniques in some combination or variation have provided us with the means to resolve or to deconvolute intrinsic features connected with the energy level structure of the systems of interest. These features have so far been assumed to remain static as a function of time. This is, of course, not always the case as various interactions of centers with their environs produce dynamical changes in the excited state and lead to relaxation, optical energy dissipation and transfer. These processes are not generally radiative. 

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Since the calibration factors for these techniques are known and relatively stable (depending on constant optical and geometrical parameters), the calibration process for laser-based measurements is simple or can be ignored. 

Schubert J, Schdning MJ, Schmidt C, Siegert M, Mesters St, Zander W, Kordos P, Liith H, Legin A, Mourzina YG, Seleznev B, Vlasov YG (1999) Chalcogenide-based thin film sensors prepared by pulsed laser deposition technique. Appl Phys A Mater Sci Process 69 803-805... 

Laser-based X-ray sources have played a significant role in previous research in this field ever since the pioneering experiments conducted in 1997 using this technique [10]. This initial work was followed by extended experiments [11—17]. On the other hand, accelerator-based approaches have more recently provided the first results at the femtosecond time scale [18], Despite this, their compactness and relatively inexpensive infrastructures as well as their perfect synchronization down to the femtosecond timescale with the process under study represent key advantages of the laser-based schemes. 

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