Uses of lasers in spectroscopy

From 1960 onwards, fhe increasing availabilify of intense, monochromatic laser sources provided a fremendous impetus to a wide range of spectroscopic investigations. The most immediately obvious application of early, essentially non-tunable, lasers was to all types of Raman spectroscopy in the gas, liquid or solid phase. The experimental techniques. 

Laser radiation is very much more intense, and the line width much smaller, than that from, for example, a mercury arc, which was commonly used as a Raman source before 1960. As a result, weaker Raman scattering can now be observed and higher resolution is obtainable. 

A useful way of changing fhe wavelengfh of some lasers, for example fhe CO2 infrared laser, is fo use isofopically subsfifufed maferial in which fhe wavelengfhs of laser fransitions are appreciably alfered. 

If may be apparenf fo fhe reader af fhis sfage fhaf, when lasers are used as specfroscopic sources, we can no longer fhink in terms of generally applicable experimenfal mefhods. A wide variefy of ingenious techniques have been devised using laser sources and if will be possible fo describe only a few of fhem here. 

A useful way of changing the wavelength of some lasers, for example the C02 infrared laser, is to use isotopically substituted material in which the wavelengths of laser transitions are appreciably altered. 

In regions of the spectrum where a tunable laser is available it may be possible to use it to obtain an absorption spectrum in the same way as a tunable klystron or backward wave oscillator is used in microwave or millimetre wave spectroscopy (see Section 3.4.1). Absorbance (Equation 2.16) is measured as a function of frequency or wavenumber. This technique can be used with a diode laser to produce an infrared absorption spectrum. When electronic transitions are being studied, greater sensitivity is usually achieved by monitoring secondary processes which follow, and are directly related to, the absorption which has occurred. Such processes include fluorescence, dissociation, or predissociation, and, following the absorption of one or more additional photons, ionization. The spectrum resulting from monitoring these processes usually resembles the absorption spectrum very closely. 

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