Excitation methods

Several different excitation schemes can be used in laser spectroscopy. This is illustrated in Fig. 9.1 for the case of alkali atoms. 

Single-Step Excitation. Atoms are transferred directly from the ground state to the excited state using an allowed electric dipole transition. ThLs means an S-P transition for an alkali atom. 

Multi-Step Excitation. Since tunable lasers have high output powers ena-bling the saturation of optical transitions, stepwise excitation via short-lived intermediate states is possible. A two-step process has been indicated in the figure. For an alkah atom this may mean S-P-D transitions. Stepwise excitations give access to states that cannot normally be reached. 

In the cascade decay of a laser-excited state, lower-lying states are populated as illustrated in the figure for the case of alkali atoms. This provides further possibihties for laser spectroscopy investigations. 

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The simulation of the actual distortion of the eddy current flow caused by a crack turns out to be too time consuming with present means. We therefore have developed a simple model for calculating the optimum excitation frequencies for cracks in different depths of arbitrary test sarriples Using Equ. (2.5), we are able to calculate the decrease in eddy current density with increasing depth in the conductor for a given excitation method, taking into account the dependence of the penetration depth c on coil geometry and excitation frequency. 

Atomic Absorption/Emission Spectrometry. Atomic absorption or emission spectrometric methods are commonly used for inorganic elements in a variety of matrices. The general principles and appHcations have been reviewed (43). Flame-emission spectrometry allows detection at low levels (10 g). It has been claimed that flame methods give better reproducibiHty than electrical excitation methods, owing to better control of several variables involved in flame excitation. Detection limits for selected elements by flame-emission spectrometry given in Table 4. Inductively coupled plasma emission spectrometry may also be employed. 

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

We follow the convention adopted by the literature on coherent control via the one- versus three-photon excitation method, where the frequency of the three-photon field is denoted coi and that of the one-photon field is denoted C03. 

It is worth summarizing at this point the different excitation methods used for phosphors that will be referred to throughout this chapter. There are three types photoluminescence (PL) which is based on initial excitation by absorption of light, cathodoluminescence (CL) which is based on bombardment with a beam of electrons, as in a cathode ray tube (CRT) and electroluminescence (EL) which is based on application of an electric field (either a.c. or d.c.) across the phosphor. 

Non-destructive elemental analysis of solid or liquid samples for major and minor constituents. Used in routine analysis of metallurgical and mineral samples. Most suited to the determination of heavy elements in light matrices (e.g. Br or Pb in petroleum). Well suited for on-stream, routine analysis. Electron beam excitation methods valuable in surface studies in combination with electron microscopy. Detection limits generally in the range 10-100 ppm. Relative precision, 5-10%. 

The lifetime of the singlet excited state (the fluorescence lifetime TF) is of the order of picoseconds to 100 nanoseconds (10—12 - 10-7 seconds) and can now be measured accurately using pulsed laser excitation methods and other techniques. Since the radiative transition from the lowest triplet state to the ground state is formally forbidden by selection rules, the phosphorescence lifetimes can be longer, of the order of seconds. 

Clearly, this work both offers an exciting method for screening for... 

If there is no emission shift, but only variation in the fluorescence quantum yield on protonation, one follows the fluorescence intensity at one emission wavelength when the acid and the base forms of the probe are excited. All the advantages of the ratio method described with two emission wavelengths also apply to this fluorescence excitation method. 

Let us consider in detail a recent paper by Jones, Kearns, and Wing which represents one of the most thorough investigations of singlet-triplet transitions and reveals much of the power of the phosphorescence excitation method. 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

As discussed in the previous section, excited shear layers dispersed at a higher rate than the natural shear layer growth rate. The amount of increase depended on the excitation frequency and amplitude. It was difficult to assess the effect of amplitude due to the passive nature of the excitation technique, but the frequency effect was investigated by comparing the results obtained with various cavities [14]. The results will be discussed in this section along with two other issues. One deals with the compressibility effect such as extending the results to a higher convective Mach number and the other concerned with possible thrust penalty associated with the passive excitation method. 

As was the case for techniques based on potential excitation, current-excitation methods are best understood by studying the time-dependent concentration changes in solution caused by the excitation signal applied to the electrode. Concentration-distance profiles for the case of species O being reduced to R by a current-step excitation signal (application of constant current to the cell) are shown in Figure 4.2. Consider first the profiles in Figure 4.2A for the reactant, O. An important concept is the relationship between the applied current and the slope of the profile at the electrode surface as expressed by... 

The net effect of the presence of the solution resistance on potential excitation methods is that the potential seen by the electrode solution interface is different from the potential applied by the potentiostat. This difference is current-dependent and the current is itself potential-dependent. The resistance also makes it more difficult to separate current components arising from the double-layer capacitance from the faradaic process. Similar complications arise for current excitations. 

REACTANT EXCITATION METHOD REACTION STUDIED COMMENTS REFERENCE... 

Presumably because of the relatively high content of phenolic amino acids, products based on milk are strongly autofluorescent when viewed after either 360 nm or 490 nm wavelength excitation. Methods for fluorescence reduction such as prior staining with Toluidine Blue O or Evan s Blue can be employed, especially on sectioned material where artifactual collapse of the emulsion which is caused by the charged nature of the dye, is not an issue [22],... 

Luminescence processes may be categorized by the excitation method used with any particular luminescent molecule. Photoluminescence is the excitation process that involves the interaction of electromagnetic radiation with photons. The process is termed chemiluminescence when the exciting energy results from a chemical reaction. Any luminescence arising from an organism is referred to as bioluminescence. 

After consideration of the different measurement methods we now focus on typical measurement types which are all based on one of the voltage excitation methods but differ in the excitation and the electrical treatment signal in between repeated measurements. 

In this section, we will discuss signal excitation methods. We start with the commonly used pulse sequences before moving on to new and hybrid excitation schemes. 

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