Advantages of Lasers in Spectroscopy

In order to illustrate the advantages of absorption spectroscopy with tunable lasers, we first compare it with conventional absorption spectroscopy, which uses incoherent radiation sources. Figme 1.1 presents schematic diagrams for both methods. 

It can be measured by comparison with a reference beam with power Pr = Pq. which can be realized, for instance, by shifting the absorption cell alternatively out of the light beam. This gives the absorption spectrum 

The spectral resolution is generally limited by the resolving power of the dispersing spectrometer. Only with large and expensive instruments (e.g., Fourier spectrometers) may the Doppler limit be reached [1]. 

The detection sensitivity of the experimental arrangement is defined by the minimum absorbed power that can still be detected. In most cases it is limited by the detector noise and by intensity fluctuations of the radiation source. Generally, the limit of the detectable absorption is reached at relative absorptions AP/P 10 -10 . This limit can be pushed further down only in favorable cases by using special sources and lock-in detection or signal-averaging techniques. 

This signal should be larger than the noise equivalent power NEP (this is the input power of the detector which gives the same detector output as the noise). 

Contrary to radiation sources with broad emission continua used in conventional spectroscopy, tunable lasers offer radiation sources in the spectral range from the UV to the IR with extremely narrow bandwidths and with spectral power densities that may exceed those of incoherent light sources by many orders of magnitude (Sects. 5.7,5.8). 

In several regards laser absorption spectroscopy corresponds to microwave spectroscopy, where klystrons or carcinotrons instead of lasers represent tunable coherent radiation sources. Laser spectroscopy transfers many of the techniques and advantages of microwave spectroscopy to the infrared, visible, and ultraviolet spectral ranges. 

The advantages of absorption spectroscopy with tunable lasers may be summarized as follows  

The fact that lasers are replacing conventional spectral lamps to an increasing extent in numerous applications demonstrates their superiority over incoherent light sources for many experiments. To illustrate the advantages and limitations of lasers in the present state of the art, we pick out five characteristic properties. 

1) The large spectral power density p (v) attainable from many laser types may exceed that of incoherent light sources by many orders of magnitude. 

This may significantly reduce noise problems caused by detector noise or background radiation. Furthermore, the large intensity allows new nonlinear spectroscopic techniques such as saturation spectroscopy (Sect.10.2), or multiphoton processes (Sect.10.5), which open new possibilities of studying molecular transitions not accessible to linear spectroscopy. 

V = 5x 10 cm/s and t = 10 s, we obtain d = 5 x 10 cm = 0,5 ym, which shows that the fluorescence from excited molecules with lifetimes t 10 s is.essentially emitted from the interaction zone where the molecules are excited. Diffusion of the excited molecules out of this zone becomes noticeable only for t 10 s. In these cases either the diffusion has to be reduced by increasing the pressure or the field of observation has to be enlarged. 

4) The possibility of continuously tuning the wavelength of such narrow-band lasers has certainly opened a new area of spectroscopy. A single-mode tunable laser represents a device which is a combination of an intense light source and an ultrahigh resolution spectrometer. Tunable lasers and tuning techniques are therefore covered in a separate chapter. 

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This brief overview of some of the advantages of lasers in spectroscopy will be outlined in more detail in the following chapters and several examples wiU illustrate their relevance. 

In the future, we can expect the development of novel experimental techniques in solid-state NMR spectroscopy for investigation of functioning catalysts. Important goals are (i) the enhancement of the sensitivity of solid-state NMR spectroscopy, for example, by a selective enhancement of the nuclear polarization taking advantage of laser-polarized xenon, (ii) increases in the temperature range accessible for the characterization of solid-catalyzed reactions, and (iii) the coupling of NMR spectroscopy with other techniques such as mass spectrometry. Furthermore, modern two-dimensional techniques of solid-state NMR spectroscopy such as MQMAS NMR spectroscopy will be applied to improve the resolution of the spectra. 

Cavity Ring-Down Spectroscopy was introduced in 1988 by O Keefe and Deacon as a spectroscopic method for absorption measurements (O Keefe and Deacon, 1988). It is a versatile high sensitivity absorption technique. One of the most essential advantages of CRDS in contrast to usual absorption methods is that the CRDS signal is not affected by intensity fluctuations of the laser since only the decay time of the signal, which does not depend on the laser intensity, is detected. 

Another advantage of the SERS spectroscopy is to obtain vibrational spectroscopic informations in electrolyte solution under conditions close to the real biological situation. The continuous development of laser sources with new excitation wavelength lines renders it possible to expand the study of adsorbed biomolecules on different metal surfaces which can also be chemically or electrically modified to adjust specific adsorption properties. Such a crucial event in medical applications as the behaviour of implants in contact with blood can be thus envisaged by the study of the adsorption of blood proteins and its physiological consequences. The possibility to monitor the interfacial electric field of the electrode surface can also be used to... 

The Chapter 1 contains the basic definitions of the main scientific terms, such as spectroscopy, luminescence spectroscopy, luminescent mineral, luminescent center, luminescence lifetime, luminescence spectrum and excitation spectrum. The state of the art in the steady-state luminescence of minerals field is presented. The main advantages of laser-induced time-resolved technique in comparison with steady-state one are shortly described. 

The experimental advantages of laser spectroscopy regarding spectral power density and spectral resolution have brought about a great variety of applications in many scientific and technical fields. The selection of examples presented in this chapter is by far not complete but intends to illustrate the impact of laser spectroscopy on the development of new experimental techniques in chemistry, biology, and environmental sciences. The importance of laser spectroscopic applications is emphasized by the publication of many monographs, review papers, or conference proceedings on this subject. For more detailed information the reader is therefore referred to the cited literature [1.17,19 14.1-6]. 

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