Source, light

An alternative to the use of a broadband white light source is the use of laser illumination that can be tightly focused on the area of interest In contrast to organic fluorophores, PRPs do not photobleach and even at very high light intensities, the 

Electrodeless discharge lamps (EDLs) are also available commercially. As the name implies, these emission sources provide a glow discharge by virtue of the fact that a sealed bulb, containing a monatomic gas at low pressure, and 

A number of multi-element lamps are also available commercially. Various metals, in powdered form, are mixed in predetermined ratios, pressed and sintered to produce the cathode material. However, only certain combinations are practicable. The obvious advantages of this format is that fewer lamps are required and the time required to switch from one element to another is minimised, however, the intensity of the emissions are generally lower than from the corresponding single element lamps. 

Electronic excited states of a chemical species are generally produced by absorption of photons in the near UV (180-400 nm) and the visible (400-800 nm) spectral regions the light source must therefore provide radiations in this range. In practice, the most used sources are incandescent lamps, or arc lamps containing mercury or xenon as emitting gas in particular cases, laser sources can also be used. 

Incandescent lamps. They are the usual tungsten or tungsten-halogen lamps, which emit in a continuous manner from the near UV to the IR. When UV radiation is required, the bulb of the lamp must be of quartz or fused silica. The intensity of the emitted radiation depends on the electric power applied. 

Low pressure 10 -10 Torr) Hg lamps. The emission spectrum of mercury lamps depends on the pressure of the gas. At low pressure, the emission consists almost exclusively of 254 and 185 nm radiations, in a ratio which depends on Hg pressure, temperature and arc current, but is generally around 10 1. Since the 185 nm radiation is only transmitted by a special type of quartz (see Sect. 4.1.4), lamps having bulbs of normal quartz are commonly used as sources of monochromatic 254 nm light. 

Medium pressure 1-10 bar) Hg lamps. These lamps emit several spectral lines in the UV and in the visible, over a very weak continuum emission background. The main Unes are listed in Table 4.1, together with their relative intensities, expressed as power (watt) or as number of photons. Small differences in the 

High pressure ( 20 bar) Hg lamps. An high gas pressure causes an increase of the continuum background, that may also overcome the lines, and an increase of the relative emission intensity in the visible these lamps are therefore used as a source of visible radiation. It is to be noted that they also emit IR radiation, and so they often require an efficient cooling of the system. 

However, outdoor weathering tests are not quite reproducible due to the instability of the climate. Moreover, they are invariably slow, longer and longer exposure periods being required for the evaluation of new polymers with improved stability. This gradual increase in exposure time and a tendency towards more standardized methods has led industries to develop more powerful light sources and artificial weathering devices. 

The principal sources of ultraviolet radiation are carbon arcs, xenon arcs, mercury arcs and derived fluorescent lamps. Comparison of the emission spectra of these light sources with sunlight is shown in Fig. 10 [15]. Energy characteristics of various light sources and their relation to the weatherability of plastics have been discussed by Hirt and Searle [16]. 

Xenon arc gas discharge lamps are recognized to approximate the spectrum of sunlight better than any other commercially available light sources. For this reason they are extensively used in commercial equipment for the indoor testing of polymers. Experiments with these instruments have been found to correlate well with outdoor tests. 

Mercury arcs do not give an accurate reproduction of the solar spectrum. Their energy output is concentrated in the mercury emission 

Energy distribution of various artificial ultraviolet sources, (a) Sunlight, (b) xenon arc, (c) carbon arc, (d) mercury lamp, (e) fluorescent lamp [reproduced with permission from Ref. 15], 

Due to its relevance in the operational mechanism of various lamps, let us begin with a brief description of the main features of thermal radiation. 

---

Shearography monitors the speckular 2D interference pattern of an unpolished surface illuminated by a coherent light source, and is therefore a metliod that lends itself to the testing of industrial materials. Small surface, or near-surface defects may produce localised strain on... 

The view of this author is that knowledge of the internal molecular motions, perhaps as outlined in this chapter, is likely to be important in achieving successfiil control, in approaches that make use of coherent light sources and quantum mechanical coherence. However, at this point, opinions on these issues may not be much more than speculation. 

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

The foremost of the modem teclmiques is tlie use of lasers as spectroscopic tools. Lasers are extremely versatile light sources. They can be designed with many usetlil properties (not all in the same instmment) such as high intensity, narrow frequency bandwidth with high-frequency stability, tunability over reasonable frequency ranges, low-divergence beams which can be focused into very small spots, or pulsed beams with... 

A iiseUfl light source is the helium resonance lamp which produces light of wavelength 58.4 nm or a photon energy of 21.2 eV, enough to ionize any neutral molecule. Often several peaks can be observed in the photoelectron spectnim... 

In the most general temis, an infrared spectrometer consists of a light source, a dispersmg element, a sample compartment and a detector. Of course, there is tremendous variability depending on the application. 

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... 

Two scans are required to obtain an absorption spectrum. First, a blank reference scan is taken that characterizes the broadband light source. Then a scan with the sample in place is recorded. The ratio of the sample power spectrum to the reference power spectrum is the transmission spectrum. If the source has stable output, then a single reference scan can be used with many sample scans. 

A8, which leads to D, = 1/(2A8). The factor of two arises because a minimum of two data points per period are needed to sample a sinusoidal wavefonn. Naturally, the broadband light source will detennine the actual content of the spectrum, but it is important that the step size be small enough to acconunodate the highest frequency components of the source, otherwise they... 

A point light source is imaged onto the specimen by the objective and the transmitted light collected by the collector lens and detected by a broad-area detector in the case of reflection microscopy, the objective lens also serves simultaneously as a collector (see figure Bl.18.10. The resolution is solely detennined by the objective lens, because the collector has no imaging fimction and only collects the transmitted light. The... 

Fischer U Ch 1985 Optical characteristics of 0.1 pm circular apertures in a metal film as light sources for scanning ultramicroscopy J. Vac. Sc/. Technol. B 3 386... 

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

Perhaps the ultimate femtosecond light source, the OPA exploits a nonlinear parametric process to amplify a portion of... 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

---