Sources of Radiation

There are two important types of light sources used in spectroscopy: polychromatic and monochromatic sources. The polychromatic sources span a wide range of frequencies. For instance, many commercial spectrometers use light sources which emit radiation similar to black-body radiation from hot materials. For far-infrared spectral range within $35\, cm^{-1} < \bar{\nu} < 200\, cm^{-1}$, a typical source is a mercury arc inside a quartz envelope where most of the radiation is generated by the hot quartz. Either a Nernst filament (ceramic with $ZrO,YtO,TlO$, ets.), or a globar ($SiC$) is used as a source of mid-infrared radiation within $200\, cm^{-1} < \bar{\nu} < 4000\, cm^{-1}$. Tungsten filaments is used in the visible and near infrared spectral range $320\, nm < \lambda < 2500\, nm$.

Different types of gas discharge lamps are common light sources in ultraviolet and visible spectral region $180\, nm < \lambda < 800\, nm$. In a xenon discharge lamp an electrical discharge excites xenon atoms to excited states which then emit ultraviolet radiation. At high pressure (several kPa) the output of a xenon lamp consists of sharp lines superimposed on a board intense background due to emission from plasma. High pressure xenon lamps are widely used as a source of radiation which is similar to the black body radiation at 6000 K. There are also many other widely used lamps which use discharge in deuterium, mercury, neon, krypton and produce polychromatic many-line radiation at different wave lengths.

Very important modern source of radiation from IR till X-rays spectral range is synchrotron radiation. A typical synchrotron storage ring consists of a high energy electron beam travelling in a circular path many meters in diameter. The electrons move around the circle under influence of a strong magnetic field which is perpendicular to the circle plane and generate radiation in very wide spectral range. The polychromatic beam is dispersed by a diffraction grating and quasi monochromatic radiation is separated by a slit. Except in the microwave region, synchrotron radiation is much more intense than can be obtained by most other conventional sources.

The monochromatic sources span a very narrow range of frequencies around a central value. Convenient monochromatic sources in the near IR-UV spectral range ( $200\, nm < \lambda < 800\, nm$) are electrodeless lamps containing a small amount of a chemical element at the pressure of about $10^{-3}$ Pa and a few Pa of inert gas $Ne$. Radio frequency discharge in the lamp results in emission of extremely narrow bond resonance spectral lines of the element. There are large number of various chemical elements which can be used in these lamps, for instance $Cs, Rb, K, Na, Ag, Hg, Sn, Be, Sr$, and others. These monochromatic sources can be used for detection of many small amounts of elements and find wide application in the atomic absorption spectroscopy.

However, the most widely used monochromatic sources of radiation are lasers which revolutionary transformed the whole world. The basic principle of laser operation is stimulated emission from the sample with the population inversion. Independent from the construction in detail, lasers contain the following important components: an active medium with an upper and a lower working energy levels, a pumping source, and a resonator.

The population inversion between the upper and the lower working levels is necessary for obtaining the amplification of the stimulated emission in the sample. As shown in eq.(13) the Einstein coefficient of spontaneous emission, $A_{mn}$ is proportional to the third order of the radiation frequency, $\nu^3$. Therefore, in the visible and even in the UV spectral range intensity of spontaneous emission is much larger than the intensity of the stimulated emission. For instance, for the radiation wavelength of 500 nm (green light) and at a temperature of 300 K, the ratio between the intensities of the spontaneous and stimulated transitions becomes $F = e^96 = 5\cdot10^{41}$. Therefore, among the $5\cdot10^{41}$ transitions, only one is due to induced emission. In contrast, for the radiation frequency $\nu=1\,GHz$ (microwave spectral range) and the same temperature the ratio is $F = 1,6\cdot10^{-4}$. Therefore, nearly all transitions are induced. Thus, it is not astonishing that the first laser generated microwaves. It was called the maser.

The first laser system developed by Maiman in 1960 was a ruby laser based on the three level scheme. Nowadays, the four level laser scheme is used for almost all lasers systems because it provides more effective way for producing the population inversion.

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