The light source

The deuterium arc lamp presents a good continuity and intensity in the ultraviolet region, and a low noise level, but a poor continuity, in the visible range (Fig. 3.6). The intensity of the emitted light of these lamps decreases constantly in time and generally it becomes half of the initial one in circa 1,000 h. 

The tungsten filament-halogen lamp presents a good continuity, intensity and stability over a part of the UV and the whole visible range (Hg. 3.6), moreover, it is able to guarantee a low noise and a lifetime of circa 10,000 h. 

An alternative to these two light sources can be the xenon lamp, that is continuous and intense over the whole UV-vis region (Fig. 3.6) but with the important drawback of a much higher noise. For this reason it is generally mounted only in dedicated instrumentation for applications needing a very high intensity. 

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Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

The light source must be highly monocln-omatic so that the Raman scattering occurs at a well-defined... 

As it stands, the picture of dynamics from Eq. (29) is derived from the interaction of molecules with a continuous light source, that is, the system is at equilibrium with the oscillating light field. It is also valid if the light source is an infinitely short laser pulse, as here all frequencies are instantaneously excited. 

If the light-source function is "tuned" to peak near co = cof i, and if g(co) is much broader (in... 

The fact that the probability of excitation from i to f grows linearly with the time T over which the light source is turned on implies that the rate of transitions between these two states is constant and given by ... 

If the light source is switched on and off and held for long periods of equal duration in either light or darkness, then the radical concentration in the system will consist of an alternation between the situation described in Figs. 6.5a and b. Because we have specified that the duration of each phase is long, the net behavior is essentially a series of plateaus in which the illumination is either Iq or zero and the radical concentration is either [M], or zero, with brief transitions in between. This is illustrated in Fig. 6.5c. The concentration of radicals is consistent with Iq, but is present only half of the time hence the rate of polymerization is only half what it would be for the same illumination operating continuously. 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Commercially, the irradiation of the 5,7-diene provitamin to make vitamin D must be performed under conditions that optimize the production of the previtamin while avoiding the development of the unwated isomers. The optimization is achieved by controlling the extent of irradiation, as well as the wavelength of the light source. The best frequency for the irradiation to form previtamin is 295 nm (64—66). The unwanted conversion of previtamin to tachysterol is favored when 254 nm light is used. Sensitized irradiation, eg, with fluorenone, has been used to favor the reverse, triplet-state conversion of tachysterol to previtamin D (73,74). 

Molten zones are also formed by radiant heating (71). The light source may be focused carbon arcs, xenon lamps, sunlight, or lasers. Very high temperatures have been achieved with all of these. For example, sapphire has been float-zoned in this manner, at over 2000°C. 

The arrangement illustrated in Figure 1 is commonly used for angular characterization of scattered light. The light source is usually a laser. The incident beam may be unpolarized, or it can be linearly polarized with provisions for rotating the plane of polarization. Typically the plane of polarization is perpendicular to the plane of... 

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