Contents Spectroscopy with Lasers W. Demtr6der ............... Spectroscopy with Lasers Prof. Dr. W. Demtr~ler Universittit Trier-Kaiserslautern, Fachbereich Physik, Kaiserslautern Contents Introduction ................ Characteristic Features of Lasers as Spectroscopic Light Sources ................ III. Spectroscopic Applications of Lasers ........ 1. Absorption Spectroscopy .......... 2. Fluorescence Spectroscopy .......... 3. Measurements of Exited-State Lifetimes ...... 4. Spectroscopic Investigations of Collision Processes 5. Photochemistry and Laser Photolysis ....... 6. Raman Spectroscopy ............ 7. Light Scattering Observations ......... 8. Plasma Spectroscopy ............ 9. Chemical Microanalysis and Spectrophotometry 10. Nonlinear Optics and Solid-State Spectroscopy IV. High-Resolution Spectroscopy Based on Saturation Effects . 1. Different Kinds of Saturation Effects ...... 2. Lamb Dip Spectroscopy ........... 3. Frequency Stabilization by Saturated Absorption 4. Investigation of Collision Processes from Line Shape Measurements .............. V. Spectroscopy of Laser Media ........... 1. Infrared and Submillimeter Wave Spectroscopy 2. Excitation Mechanisms and Collision Processes in Gas Discharges ............... 3. Solid-State and Semiconductor Lasers ...... 4. Chemical Lasers .............. VI. Conclusion ................ VII. Zusammenfassung ............... VIII. References ................. I. II. 3 5 12 12 19 23 27 32 41 48 51 56 57 60 60 64 68 70 72 72 74 76 78 84 86 88 I. Introduction The development and technical improvement of many different laser systems during recent years has greatly enlarged the scope of possible laser applications in various fields of physics and chemistry. Laser oscillations have so far been obtained at several thousand different wavelengths, covering the range from submillimeter waves to the ultraviolet region, thus lasers have proved especially useful for spectroscopic investigations. Here they may solve problems where conventional light sources fail, and experiments which can hardly be done with spectral lamps often turn out to be much easier when a laser is used as the light source. Some of these spectroscopic applications of lasers will be discussed in this review article, the experiments described exemplifying the laser's potential as a tool for spectroscopists. It will be shown that the laser not only exceeds spontaneous light sources in intensity and spectral resolution, but, thanks to its special characteristics, makes possible investigations of a new and fundamental kind. The first section explains the characteristics of the laser which are important for spectroscopic investigations and the differences as compared to spontaneous light sources. The next section, which is the most extensive, deals with the various kinds of spectroscopic experiments which have been done or can be done with lasers as external light sources, where \external\ means that the probe under investigation is placed outside the laser resonator. Together with a brief description of the experiments and a summary of the results, there is a discussion of the main qualities of the laser which made these special investigations possible. Section IV explains a new approach to high resolution spectro- scopy based on various kinds of saturation effects. Some of the experiments are performed inside the laser resonator, which implies the presence of coupling phenomena between the absorbing molecu- les under investigation and the laser oscillation itself. These feedback effects can be used for high-precision frequency stabilization and to measure frequency shifts and line profiles with an accuracy never Introduction previously obtained. By illuminating basic problems of the inter- action of optical radiation with matter, these measurements may therefore serve as a check on theoretical approximations in this field. The last section reports the results of some experiments with spectroscopy of the various laser media. This field is, of course, closely related to the understanding of the laser action itself, for instance, the elucidation of excitation mechanisms in different laser types, term assignments for various laser transitions or investigations of competing collision processes. Spectroscopy of laser media has therefore been performed by numerous laboratories; it is shown here that our knowledge of the spectroscopic properties of gaseous, liquid and solid-state media, especially in the infrared region, has been increased considerably by taking advantage of the laser charac- teristics described in Section II. Especially interesting for chemists is the investigation of chemical lasers, where chemical energy from exothermic reactions is converted into light. Since the field of spectroscopic laser applications is so vast and the number of published papers exceedingly large, this review cannot be complete. However, the author has tried to give a reasonable survey of what has been done and to offer some ideas about what can be done in modern spectroscopy 1) with such an interesting and stimulating invention as the laser (Light A_mplification by Stimulated Emission of _Radiation). II. Characteristic Features of Lasers as Spectroscopic Light Sources There are five main qualities which make the laser an at Lractive spectroscopic light source 2): 1. Its comparatively high output power on a single transition. 2. The spatial coherence of the induced emission which renders it possible to focus the laser output into a nearly parallel light beam. 3. Its temporal coherence, causing spectral linewidths of the in- duced emission to be smaller by several orders of magnitude than those of fluorescence lines emitted by spectral lamps. 4. The possibility of tuning the frequency of a laser line inside a frequency range by several methods, depending on the laser type. 5. The fact that several lasers can generate very short light pulses (down to 10- 13 sec duration) with high peak powers (up to 1013 watts) 16 a), which can be used to investigate short term transitions (e. g. lifetime measurements, flash photolysis, etc.). In view of their importance, these five points will now be discus- sed in more detail: 1. Table 1 gives wavelengths and output powers for some impor- tant laser types operated in a continuous-wave (cw) or pulsed mode. The pulsed lasers normally have much higher peak powers but there are technical or theoretical limitations of the maximum repetition frequency, which means that their time-averaged intensity is often below that of the cw lasers. For comparison the output power of a high-pressure mercury lamp (Osram HBO 200) also is listed. The reader has to consider, however, that the mercury lamp radiates this power into the unit solid angle (= 60 °) distributed over the spectral range from 2000 to 6000.A, whereas the laser intensity is concentrated at a single wavelength and collimated in a beam with a very small divergence between 10- 4 and 10- 7 sterad. With respect to intensity, most of the laser transitions therefore are superior to conventional lamps, especially for experiments which demand a narrow spectral range or a small solid angle of the incoming light.
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