Monochromatic source

Radiation exits the monochromator and passes to the detector. As shown in Figure 10.12, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite effective bandwidth. The choice of... 

Until the advent of lasers the most intense monochromatic sources available were atomic emission sources from which an intense, discrete line in the visible or near-ultraviolet region was isolated by optical filtering if necessary. The most often used source of this kind was the mercury discharge lamp operating at the vapour pressure of mercury. Three of the most intense lines are at 253.7 nm (near-ultraviolet), 404.7 nm and 435.7 nm (both in the visible region). Although the line width is typically small the narrowest has a width of about 0.2 cm, which places a limit on the resolution which can be achieved. 

The use of vibrational Raman spectroscopy in qualitative analysis has increased greatly since the introduction of lasers, which have replaced mercury arcs as monochromatic sources. Although a laser Raman spectrometer is more expensive than a typical infrared spectrometer used for qualitative analysis, it does have the advantage that low- and high-wavenumber vibrations can be observed with equal ease whereas in the infrared a different, far-infrared, spectrometer may be required for observations below about 400 cm. ... 

An incident photon, hvo, from an essentially monochromatic source, such... 

The superposition principle is illustrated further with the Michelson interferometer. Light is divided between two arms at a beamsphtter, recombined and the resulting intensity is observed. For a monochromatic source, the on-axis intensity is a superposition of the two recombined beams, and varies cosinu-soidally with the difference in path lengths Az... 

For a unit intensity monochromatic source, S u) = (r o). which gives by the inverse Fourier transform Fn (r) = resulting in the famihar cos-... 

Like many of the topics discussed in this book, photochemical reactions are most likely to be used in niche applications for commercial and environmental reasons. Unless there is a major breakthrough in reactor and lamp design, widespread use of this technology is unlikely. Perhaps the best hope of producing high-intensity monochromatic sources of radiation rests with lasers, but currently equipment costs are too high to justify their use for commercial chemical production. 

The basic layout of Raman sensors is similar to fluorescence probes. The common sensor form is that of a fibre optic probe, with excitation and collection fibres. As the excitation light comes from a monochromatic source no excitation filter is required, but a spectrally matched emission notch filter blocking the excitation wavelength is almost always part of the sensor head. 

Similar to IR sensors, Raman sensors have profited from miniaturisation and improvement of light sources and optics. Essentially, a Raman sensor consists of (i) a monochromatic source, a (ii) sensor head, a (iii) filter separating the Raman lines from the excitation radiation and Rayleigh scattering and a (iv) spectral analyser. 

Sensitive materials, such as certain metal salts or organometallic compounds used as catalyst precursors, may decompose during XPS analysis, particularly in equipment with standard X-ray sources. Heat and electrons generated by the source are usually responsible for damage to samples. In these cases the monochromatic XPS offers a solution. For example, reliable spectra of the organoplati-num complexes in Figs. 3.4 and 3.5 could only be obtained with a monochromatic source. Under the standard source the Pt(IV) complex indicated in Figs. 3.4 and 3.5 decomposed into the Pt(II) precursor and Cl2 gas. 

A bit of a chemical application arose in our attempts to find a monochromatic source or absorber. Since EuF. > has fiuorite structure, there should be no quadrupole splitting. When a spectrum was taken of a commercial sample of EUF2 it was found that the europium present was essentially completely trivalent. The experimental spectrum is shown in Figure 9. 

Most optical spectral measurements, where the measurement of multiple wavelengths is required, will feature some type of polychromatic or broadband light source. There are a few exceptions here, such as tunable laser sources and source arrays. In such instances, the source is effectively "monochromatic at a given point in time. These sources are covered separately under monochromatic sources. 

In surface-enhanced Raman spectroscopy (SERS) samples are adsorbed onto microscopically roughened metal surfaces. Spectra are the intensities and frequencies of scattered radiation originating from a sample that has been irradiated with a monochromatic source such as a laser. SERS spectra are of molecules that are less than 50 A from the surface. 

Now let us assume that a monochromatic source of flux is placed in the plane of the entrance slit so that there is no constant phase relationship between the fields at any two given points in the slit. This, in itself, is a contradiction, because a perfect source monochromaticity implies both spatial and temporal coherence. By definition of coherence, a constant phase relationship would result. To eliminate the possibility of such a relationship, we must require the source spectrum to have finite breadth. Let us modify the assumption accordingly but specify the source spectrum breadth narrow enough so that its spatial extent when dispersed is negligible compared with the breadth of the slits, diffraction pattern, and so on. Whenever time integrals are required to obtain observable signals from superimposed fields, we evaluate them over time periods that are long compared with the reciprocal of the frequency difference between the fields. We shall call the assumed source a quasi-monochromatic source. 

Fig. 13 Simulation of a monochromatic source with a finite arm displacement of the interferometer, (a) Truncated cosine function (30 discrete data points), (b) Its Fourier transform, the sine function, which simulates the infrared spectral line.

Fig. 23 Interferogram and spectrum of two monochromatic sources that differ slightly in wave number, (a) Noise-free interferogram, understood to repeat indefinitely, (b) The two closely spaced spectral lines.

Fig. 24 Interferogram of the two monochromatic sources that would be obtained for a finite maximum path difference of the interferometer. (a) Finite interferogram. (b) Recorded spectrum. The two lines are completely merged into one.

Fig. 26 Interferogram and spectrum of the superposition of four monochromatic sources.

Fig. 27 Finite interferogram of the four monochromatic sources of Fig. 26 with Gaussian noise of rms amplitude 0.1 superimposed and the resulting degraded spectral lines, (a) Interferogram of 30 data points, (b) Merged and distorted spectral lines.

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