Electron-induced excitation

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

The X-ray emission process followii the excitation is the same in all three cases, as it is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. 

Electron-transfer-induced FQ is the most practical and efficient mechanism of signal transduction for the detection of explosives. This is because explosives, especially 2,4,6-trinitrotoluene (TNT), are often highly electron-deficient molecules that readily accept electrons from excited fluorophores. In addition, explosive devices that contain TNT also usually contain a synthetic by-product, 2,4-dinitrotoluene (DNT), which is also highly electron deficient. A basic frontier molecular orbital-based mechanism for electron transfer FQ is illustrated in Figure 3. 

Excitation Eunctions of O2 and 02-Doped Ar Eilms. Resonances can be best identified by the structures they produce in excitation functions of a particular energy-loss process (i.e., the incident-electron energy dependence of the loss). Fig. 7 is reproduced from a recent study [118] of the electron-induced vibrational and electronic excitation of multilayer films of O2 condensed on the Pt(lll) surface and shows the incident electron energy dependence of major losses at the indicated film thickness and scattering angles. Also shown in this figure is the scattered electron intensity of the inelastic background... 

The cross sections for ESD processes on most surfaces are usually much smaller than cross sections for comparable gas phase processes involving electron-induced dissociation and dissociative ionization . This may be a consequence of the fact that many fragments remain adsorbed on the surface and/or that non-radiative processes such as those described in Sect. 2.1.1 cause the molecule to de-excite before it dissociates. For 100 eV electrons, typical cross sections for gas-phase dissociation are 10 cm (see Ref. 150). For most adsorbates, cross sections lie in range of 10 to 10 cm. A few examples of higher cross sections for adsorbed layers are known, and many examples of smaller cross sections exist. 

Figure 3.8 illustrates the photon-induced excitation of NO chemisorbed on Pt(lll) based on the scenario of substrate mediated excitation by hot electrons (see Section 4.8.1.1). Hot electrons (and hot holes) are created in the optical skin depth... 

Fig. 13.13 Typical selective field ionization data for laser excited Na 50d atoms (a) data with electron beam gated off ( ), data following collisions with 25 eV electrons (+) corrected for electron-induced background signals (b) net signal due to electron impact. The horizontal bars indicate the range of field strengths over which n = 50 atoms are expected to ionize adiabatically and diabatically (from ref. 36).

Such a comparative study has been made by Byakov and his collaborators.29 255 They have shown that in the case of water the main contribution to the loss rate given by formula (6.3) comes from excitation of intramolecular vibrations rather than from dipole relaxation. This is all the more so in nonpolar media where the main channel of continuous losses is not the relaxation of constant dipole moments (which are zero) but the polarization losses due to the electron-inducing dipole moments in molecules. The possible exceptions are the media consisting of molecules with a high degree of symmetry, such as methane and neopentane, which have no active vibrations in the IR region. 

The fast desorption of CO in CO/Cu(OOf) has been measured [33] and also calculated. [30,31] The collision induced vibrational excitation and following relaxation of CO on Cu(001) has also been experimentally explored using time-of-flight techniques, and has been analyzed in experiments [34] and theory. [23,32] Our previous treatment of instantaneous electronic de-excitation of CO/Cu(001) after photoexcitation is extended here to include delayed vibrational relaxation of CO/Cu(001) in its ground electronic state. We show results for the density matrix, from calculations with the described numerical procedure for the integrodifferential equations. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

A nontrivial result with respect to the well-known formula of Migdal (1941) for the probability of the -decay-induced excitation of an atom is the justification in the above derivation for taking the electron wave functions at the equilibrium nuclear configuration of the parent molecule, as well as the... 

The greater the number of functions 4 J, belonging to the orthonormal set, the more completely and in more detail the spectrum of the /(-decay-induced excitations of a molecule can be calculated. Consequently, the method for calculating the wave functions of the daughter ion must be such that at a reasonable volume of calculation we would be able to construct a sufficiently large number of multielectron wave functions of excited states. The Hartree Fock method allows one to construct the wave functions of excited states as the combinations of determinants symmetrized in a certain way. Within this method the excitation is considered to be a transition of an electron from an occupied Hartree-Fock molecular orbital into a vacant one. 

In order to make allowance for the influence of electron correlation on the probability of the / -decay-induced excitation of a molecule, let us use the configuration interaction method. We will consider the configurations that take account only of the single and double electron excitations into the virtual excited orbitals. For the latter we will use the orbitals obtained by the Huzinaga-Arnau method (see above). The wave function of the ground state of the parent molecule is... 

In view of the fact that the probability of the molecular electron shell restructuring is due solely to the atom nearest to the radioactive one, in Fig. 3 we show the dependence of the probability (of remaining in the ground state) on the ionization potential /f for the shell of the atom nearest to He which is most distorted by the transition T - He+. As follows from Fig. 3, for metal hydrides there is a linear dependence on the potential For nonmetals (C, O, or F) the probability is independent of Apparently, the obtained dependence on /, is an illustration of the degree of ionicity of the chemical bond. As a whole, the problem of the influence of properties of an immediate neighbor of the radioactive atom on the /1-decay-induced excitations of the molecule requires further study. 

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