Monochromatic spectrum

An experimental technique that is useful for structure studies of biological macromolecules and other cry stals with large unit cells uses neither the broad, white , spectrum characteristic of Laue methods nor a sharp, monochromatic spectrum, but rather a spectral band with AA-/A, 20%. Because of its relation to the Laue method, this technique is called quasi-Laue. It was believed for many years that the Laue method was not useful for structure studies because reflections of different orders would be superposed on the same point of a film or an image plate. It was realized recently, however, that, if there is a definite minimum wavelength in the spectral band, more than 80% of all reflections would contain only a single order. Quasi-Laue methods are now used with both neutrons and x-rays, particularly x-rays from synchrotron sources, which give an intense, white spectrum. 

Lasers are used to deliver a focused, high density of monochromatic radiation to a sample target, which is vaporized and ionized. The ions are detected in the usual way by any suitable mass spectrometer to produce a mass spectrum. The yield of ions is often increased by using a secondary ion source or a matrix. 

Figure 8.14 The monochromatized AlATa carbon Is X-ray photoelectron spectrum of ethyltrifluoroacetate showing the chemical shifts relative to an ionization energy of 291.2 eV (Reproduced, with permission, from Gelius, U., Basilier, E., Svensson, S., Bergmark, T. and Siegbahn, K., J. Electron Spectrosc., 2, 405, 1974)...

Combined Soot W2O, and CO2 Radiation The spectral overlap of H9O and CO9 radiation has been taken into account by the constants for obtaining Ec- Additional overlap occurs when soot emissivity , is added. If the emission bands of water vapor and CO9 were randomly placed in the spectrum and soot radiation were gray, the combined emissivity would be Eg phis , minus an overlap correction g s- But monochromatic soot emissivity is higher the shorter the wavelength, and in a highly sooted flame at 1500 K half the soot emission hes below 2.5 [Lm where H9O and CO9 emission is negligible. Then the correction g s must be reduced, and the following is recommended ... 

To summarize Filtering is an effective way of producing intense monochromatic beams, but it is severely limited because it cannot be used at all wavelengths and cannot achieve high spectral purity at any wavelength. The analysis of a spectrum, that is, the selection of a line and the measurement of its intensity, requires Bragg reflection. 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

When accelerated sufficiently, amplitude-frequency modulation in the absence of dephasing results in signal monochromatization, just like in the case of pure frequency modulation. Before the spectrum collapses, exchange between branches causes their broadening, but after collapse it provides their coalescence into a single line at frequency... 

The Compton scattering cannot be neglected, but it is independent of molecular structure. Then, fitting experimental data to formulas from gas phase theory, the concentration of excited molecules can be determined. Another problem is that the undulator X-ray spectrum is not strictly monochromatic, but has a slightly asymmetric lineshape extending toward lower energies. This problem may be handled in different ways, for example, by approximating its spectral distribution by its first spectral moment [12]. 

Current instruments use complex and refined optical systems in order to radiate the cuvette with monochromatic light selected from the radiant spectrum of the light source. A radically different approach may be practical when tunable lasers become available at reasonable prices. 

To identify a certain pigment, monochromatic light is required, but the assessment of food quality by a human inspector or by a device requires normal daylight. The evaluation procedure, however, to be discussed in the next section, also involves decomposition into monochromatic (red, green, and blue) regions. Monochromatic dissection of a continuous spectrum by monochromators can be achieved in three ways ... 

The spectrum excited by an electrical discharge through gas at low pressme can be studied with two simple types of spectrum tube seen in Figure 75. These tubes also act as weak sources of certain radiations, such as monochromatic light for an optical spectrometer. The... 

Raman scattering spectroscopy is used to probe the vibrational excitations of a sample, by measuring the wavelength change of a scattered monochromatic light beam. This is usually performed by impinging a monochromatic laser beam to the sample surface, and by recording the scattered beam spectrum. 

XPS or ESCA (electron spectroscopy for chemical analysis) is a surface sensitive technique that only probes the outer atomic layers of a sample. It is very useful tool to study polymer surfaces [91]. An XPS spectrum is created by focusing a monochromatic beam of soft (low-energy) X-rays onto a surface. The X-rays cause electrons (photoelectrons) with characteristic energies to be ejected from an electronic core level. XPS, which may have a lateral resolution of ca. 1-10 pm, probes about the top 50 A of a surface. 

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