Spectroscopy of Laser Media

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

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Contents Spectroscopy with Lasers Introduction. Characteristic Features of Lasers as Spectroscopic light Sources. Spectroscopic Applications of Lasers. High-Resolution Spectroscopy Based on Saturation Effects. Spectroscopy of Laser Media. Conclusion. Zusammen-fassung. (418 references). 

Richards-Kortum R (1995) Fluorescence spectroscopy of turbid media. In Welch AJ and van Gemert MJC (eds) Optical-Thermal Response of Laser-Irradiated Tissue. New York Plenum. 

Because Raman signals are typically weak, intense lasers in combination with sophisticated tight collection must be used. Although intense laser radiation can potentially harm the delicate structures in the visual system, ocular tissue has been found to be a very suitable target for Raman spectroscopy for two reasons. First, the ocular media (cornea, lens and vitreous) generally have good optical clarity, which enables high penetration of laser excitation and optical detection of scattered... 

The simplicity and robustness of the method makes it well suited to a number of practical analytical applications, such as sensitive noninvasive in vivo disease diagnosis, security screening and the quality control of pharmaceutical tablets. The concept is also potentially applicable to fluorescence spectroscopy, NIR tomography of turbid media and other general applications, where the enhanced coupling of laser radiation into a turbid medium is beneficial an example is the case of photodynamic therapy in cancer treatment of subsurface tissues. 

The medium infrared spectral region contains typically vibrational transitions of molecules and their rotational substructure. Therefore it is obvious, that one can use vibration rotation transitions in a laser medium itself, provided there is an inversion mechanism available. However, in the gas phase such transitions are fairly narrow and therefore will not be the ideal source for spectroscopy, where one would like to have a continuously tunable laser source in order to scan across a series of vibration-rotation transitions of the molecular gas to be investigated. Although we can make use of it for very special situations e.g.for the spectroscopy of paramagnetic molecules, where Zeeman-tuning of the molecular transition can be achieved, we must use other types of gain media for a tunable infrared laser. 

ABSTRACT. Tunable coherent radiation in the ultraviolet and vacuum ultraviolet has been generated by stimulated Raman scattering, by anti-Stokes Raman lasers, and by frequency mixing processes in nonlinear media. The theory and experimental progress in the development of these laser-driven sources is reviewed, and examples of available systems and their characteristics are discussed. Various applications in spectroscopy of radiation tunable in the wavelength region 200-90 nm are presented. 

In June 1986, Berthod started a sabbatical stay at the Department of Chemistry at the University of Florida with Prof J. D. Winefordner to study the hyphenation of laser spectroscopy with LC [9]. In December 1986, Dr. Garcia-Alvarez-Coque joined Winefordner s research group to investigate room-temperature phosphorescence in micellar media [10]. The exchange of ideas between both Berthod and Garcia Alvarez-Coque inspired their collaboration on fluorescence in microemulsions and reversed micelles [11]. 

Many different types of lasers use liquids or solids as amplifying media. Since the spectral characteristics of such lasers play a significant role in applications of laser spectroscopy, we briefly outline the spectral linewidths of optical transitions in liquids and solids. Because of the large densities compared with the gaseous state, the mean relative distances R(A, B ) between an atom or molecule A and its surrounding partners By are very small (typically a few tenths of a millimeter), and the interaction between A and the adjacent partners Bj is accordingly large. 

Many different types of lasers use liquids or solids as amplifying media. Since the spectral characteristics of such lasers play a significant role in applications of laser spectroscopy, we briefly outline the spectral linewidths of optical transitions... 

The techniques discussed here in connection with combustion diagnostics can clearly also be used for the monitoring of other reactive media. The techniques have been found to be valuable in the characterization of chemical vapour deposition (CVD) processes for semiconductor fabrication [10. 47]. The examples mentioned here illustrate the power of laser spectroscopic techniques in studying chemical processes. Numerous other examples of chemical applications of laser spectroscopy can be found. The field was covered in [10.48,2]. 

While matrix isolation spectroscopy is experimentally simple, it only allows unimolecular processes. Because diffusion is severely restricted, no bimolecular process can occur at low temperatures in a matrix. To allow such reactions, the upper excited-state chemistry needs to be conducted at room temperature in isotropic solution media. The high-intensity light sources required to produce high concentrations of excited states are achieved through the use of lasers. 

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