Types of excitations

In this paper we present simulations and measurements of several types of excitation coils, which match the special requirements for a SQUID based eddy current NDE system. We note however that all calculations presented here on penetration depths, current distributions and crack-detecting algorithms are also useful for conventional eddy current testing systems. 

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

As shown in Table 4.2, the most important contribution to the energy in a Cl procedure comes from doubly excited determinants. This is also shown by the perturbation expansion, the second- and third-order energy corrections only involve doubles. At fourth order the singles, triples and quadruples enter the expansion for the first time. This is again consistent with Table 4.2, which shows that these types of excitation are of similar importance. 

It is the n- n type of excitation which leads to significant fluorescence, the n-> z type producing only a weak fluorescence. The electronic transitions corresponding to charge-transfer bands also lead to strong fluorescence. 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

Lukeman, M. Wan, P. A new type of excited-state intramolecular proton transfer proton transfer from phenol OH to a carbon atom of an aromatic ring observed for 2-phenyl-phenol. J. Am. Chem. Soc. 2002, 124, 9458-9464. 

The sinoatrial (SA) node is located in the wall of the right atrium near the entrance of the superior vena cava. The specialized cells of the SA node spontaneously depolarize to threshold and generate 70 to 75 heart beats/ min. The "resting" membrane potential, or pacemaker potential, is different from that of neurons, which were discussed in Chapter 3 (Membrane Potential). First of all, this potential is approximately -55 mV, which is less negative than that found in neurons (-70 mV see Figure 13.2, panel A). Second, pacemaker potential is unstable and slowly depolarizes toward threshold (phase 4). Two important ion currents contribute to this slow depolarization. These cells are inherently leaky to sodium. The resulting influx of Na+ ions occurs through channels that differ from the fast Na+ channels that cause rapid depolarization in other types of excitable cells. Toward the end of phase... 

Chou PT (2001) The host/guest type of excited-state proton transfer a general review. J Chin Chem Soc 48 651-682... 

The Wallac Arthur multiwavelength system, shown earlier (Fig. 3), is an example of a recent polyvalent CCD-based instrument. Three light sources make it possible to perform several types of excitation top (fluorescence or reflectance),... 

During the many years that atomic emission spectrometry has been employed for chemical analysis a variety of types of excitation sources have been used. In earlier times electric discharges, dc-arcs and ac-sparks, found considerable favour. The inherent instability of the discharges has meant that as more stable alternatives have been developed they have been progressively replaced by them. Where electrical excitation is still employed it is achieved by an electrically controlled spark with far greater stability and much improved precision for the analysis. 

There are many ways to obtain information on the physico-chemical properties of materials. Figure 1.3 presents a scheme from which almost all techniques can be derived. Spectroscopies are based on some type of excitation, represented by the ingoing arrow in Fig. 1.3, to which the catalyst responds as symbolized by the outgoing arrow. For example, one can irradiate a catalyst with X-rays and study how the X-rays are diffracted (X-ray diffraction, XRD), or one can study the energy distribution of electrons that are emitted from the catalyst as a result of the photoe-... 

But there is a type of excitation that requires less energy, i.e. rotation. Irradiation with microwave radiation causes small molecules such as water to absorb energy, and thence rotate at high speed. These absorptions are allowed , quantum mechanically. 

An octahedral geometry (Oh symmetry) is, of course, an ideal case, which virtually none of our systems match. All have lower symmetry (typically >3 or Ci), which further splits the dlevels. However, the octahedral model is a starting point. Lowering the symmetries does not affect the basic nature of the types of excited states. Further, such important features as the d state energies are still dictated by the average A of the ligands. 

The optical absorption of some semiconductors or insulator materials shows a series of peaks or features at photon energies close to but lower than the energy gap (the pre-edge region). These features correspond to a particular type of excitation called... 

The major conclusion of the present study, as can be seen in Figures 1 and 2, is that the primary photodissociating states, in both nitrosyl ferrous and ferric heme complexes correspond to the d - d 2 excitations. The calculated energies also indicate that this dissociative channel can be activated independent of the excitation frequency in the range of Q to Soret band energies. This is the same type of excitation previously identified as the photoactive state involved in CO and O2 photodissociation from ferrous heme complexes. 

The photochemical results indicate that hydrogen abstraction proceeds from the 7171" singlet excited state of thiones 20a and 20b, and was followed by pho-tocyclization. Four parameters serve to define the geometry of intramolecular hydrogen atom abstraction d. A, 0, and co, which have the values shown in Table 5. Table 7 summarizes the ideal values of d. A, 0, and co for each type of excited state along with the crystallographically derived experimental values for compounds 20a,b. 

Little has been reported concerning the mechanism of the photocycloaddition reaction however, much is known about the photoreduction of carbonyl compounds.15,16 It has been shown that both hydrogen abstraction, leading to photoreduction, and most photocycloaddition reactions of carbonyl groups are characteristic of the same type of excited state reagent, that is, the carbonyl n,n state.17 Furthermore, much is known about the emission (phosphorescence and fluorescence) of carbonyl compounds, and all of this knowledge can be brought to bear upon the photocycloaddition reaction. 

One of the most important and difficult questions to answer for any photochemical reaction is which excited state is involved. Since these are the reagents, it is obviously important, if generalizations are to be made, to know which state is responsible for a given reaction. The question is difficult to answer because several different types of excited states, both singlet and triplet, are attainable even with the simplest of carbonyl compounds, and their reactivity may, in some cases, be similar. All of the discussion thus far has implied that the photocycloaddition reaction is characteristic of the n,n state. What is the evidence that this state can be involved and what is the character of this state which makes it reactive ... 

In general, carbonyl compounds that are reactive in the photocycloaddition reaction are also reduced upon irradiation in isopropyl alcohol.17 Subject to the limitation of triplet-triplet transfer to the olefin mentioned previously, the converse is also true. That is, carbonyl compounds that are photoreduced in isopropyl alcohol can form oxetanes unless their triplet energies are high enough for the olefins to act as quenchers. Thus, the two reactions are characteristic of the same type of excited state. (This is not an exclusive generalization.) The quenching experiments mentioned on pp. 308-311 provide evidence that the reactive state can be the triplet and, in some cases, only the triplet. Evidence for this state being n,ir comes from the fact that carbonyl compounds which are reactive usually emit from the n,n triplet, while those which are unreactive emit from some other excited state. 

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