Absolute measurements intensity

The absolute measured intensity of a spectral line depends upon many factors previously discussed in Chaps. 2 and 3. IWo dominant factors controlling the production of fluorescent x-rays are the fluorescent yield and the excitation overvoltage (V—0) - . These two factors may be used to obtain reliable, rapid semiquantitative analyses from a qualitative identification spectrum. This will be discussed subsequently. Figures 8.5 through 8.7 illustrate the atomic number-dependence of the K fluorescent yield [ci>k]> the L fluorescent yield [o>l], and a combined critical excitation overvoltage fluorescent yield factor for K and L spectra. 

The absolute measurement of areas is not usually usefiil, because tlie sensitivity of the spectrometer depends on factors such as temperature, pulse length, amplifier settings and the exact tuning of the coil used to detect resonance. Peak intensities are also less usefiil, because linewidths vary, and because the resonance from a given chemical type of atom will often be split into a pattern called a multiplet. However, the relative overall areas of the peaks or multiplets still obey the simple rule given above, if appropriate conditions are met. Most samples have several chemically distinct types of (for example) hydrogen atoms within the molecules under study, so that a simple inspection of the number of peaks/multiplets and of their relative areas can help to identify the molecules, even in cases where no usefid infonnation is available from shifts or couplings. 

Practically it is more convenient to measure intensity ratios instead of absolute intensities. Thus, e.g., Cu may serve as a reference material, relative to which the ion intensities back-scattered from the atoms of the surface under consideration are measured ... 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

All measured intensities can be put on absolute scale by proceeding as follows. At high angles the scattering pattern can be considered as arising from a collection of noninteracting gas molecules rather than from a liquid sample. 

In this circumstance, it is more reliable to measure the absolute integrated intensity of the substrate peak and compare this with the integrated intensity from an equivalent crystal of the substrate material on which no layer has been grown. In the angular position for diffraction from the substrate, the layer will not diffract and the substrate peak intensity will be simply reduced by normal photoelectric absorption. For a symmetric reflection, it is easy to see that the integrated intensity 1 of the substrate peak with the layer of thickness t present is related to the integrated intensity of the bare substrate / o by... 

McMahon s group reported room temperature ZTRID values for dissociation of a number of cluster ions. Particularly interesting for thermochemical analysis are recent measurements of the temperature dependence of several reactions, because the temperature dependence is directly related to the dissociation energy. A further promising development is the ab initio calculation of the absolute infrared intensities for the vibrational modes of two of these complexes, which allows an independent derivation of from the data. For one of the complexes, (H20)3Cr,... 

In the first and obvious approach, "absolute" signal intensities are measured. Since very long data collection times are required, this method is only useful in studying equilibrated, i.e. nonchanging, systems. 

Absolute measurements of the 2PA cross section of chromophores are experimentally very demanding and difficult to implement for routine studies of new materials. This is mainly due to the fact that the 2PA excitation rate in a sample does not depend simply on the square of the average intensity, (f(r, t)), but on the average of the square of the intensity, (f(r, t) ) (r and t are the special and temporal coordinate) [63,64,78]. These two quantities are not the same if the laser piflse has substructure in space and time [85,86]. Thus, absolute 2PA determinations that rely on the measurements of signals proportional to the excitation rate, such as the 2P-induced fluorescence... 

Kummler68 reports a private communication from Mahan which suggests that the rate constant for the reaction between 02(1A9) and ethylene is slower, but not more than ten times slower, than the reaction between atomic oxygen and ethylene. Thus the rate constant should be between about 2 x 107and2 x 108 liter mole-1 sec-1. Absolute measurements of the reactivity of singlet oxygen with olefins have recently been made by Broadbent et al.,51 using both emission intensity measurements and the rate of reactant removal, to follow the reaction. A preliminary rate constant of 2.3 x 105 liter mole-1 sec-1 has been obtained for the reaction... 

When the energy of the charged particle beam is too large to easily stop the beam in a Faraday cup, the beam intensity is frequently monitored by a secondary ionization chamber. These ion chambers have thin entrance and exit windows and measure the differential energy loss when the beam traverses them. They must be calibrated to give absolute beam intensities. If the charged particle beam intensity is very low (<106 particles/s), then individual particles can be counted in a plastic scintillator detector mounted on a photomultiplier tube. 

A complete set of measured intensities often includes distinct blocks of data obtained from several (or many) crystals and, if data are collected on film, from many films. Because of variability in the diffracting power of crystals, the intensity of the X-ray beam, and the sensitivity of films (if used), the crys-tallographer cannot assume that the absolute intensities are consistent from one block of data to the next. An obvious way to obtain this consistency is to compare reflections of the same index that were measured from more than one crystal or on more than one film and to rescale the intensities of the two blocks of data so that identical reflections are given identical intensities. This... 

Krynauw and Schutte 178-181> in a series of papers discussed the infrared spectra of solid solutions of CI04 and C103 in alkali-halide lattices, measured the absolute absorption intensities and discussed the lattice-dependence of the v3 and v4 -modes of the C104 ion in these lattices. 

From the heats of formation of starting materials and of products one can calculate the chemical energy change (i.e. AH for thermal processes). The absolute measurement of fluorescence and of phosphorescence is difficult, particularly for gas phase reactions, but in principle this quantity can be obtained. The energy as heat is difficult to measure if the incident intensity is low and in fact it is very rarely determined. 

Emission Spectra. The chance to obtain vibrational fine structure is usually higher for luminescence than for absorption spectra since in emission, absolute light intensities with high sensitivities are measured rather than intensity differences. However, few compounds exhibit luminescence intense enough to measure a reliable... 

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