De-excitation Mechanisms

Depending on the electronic structure of the surface the de-excitation of metastable helium atoms occurs either by resonance ionization (RI) with a subsequent Auger neutralization (AN), or by Auger de-excitation (AD). These mechanisms are schematically shown in Fig. 2.5 and explained below in more detail. 

Every kinetic energy therefore corresponds to two electrons with binding energies being symmetrically to Eb Thus, the energy distribution cannot directly 

Additionally, the comparison with photoemission spectra allows to determine the effective energy of the li state to be  

If the metastable atoms get close to the surface without R1 taking place, then AD can occur. This is the dominant de-excitation process if R1 is suppressed, which occurs if the level of the excited helium atom lies below the Fermi level (/eff with /eff being the ionization potential, see Fig. 2.7) or if there is an insufficient overlap with empty states due to an adsorbate layer. In this case the li hole is hlled by an electron from the solid or the adsorbate layer with a simultaneous ejection of the excited 2s atomic electron. 

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One of the major paradoxes of the current time is that no investigator has isolated and measured the visual spectrums of the chromophores of vision in the laboratory. The above figure illustrates why. The chromophores of vision only exhibit their unique spectral characteristics when in the liquid crystalline state. Furthermore, in the absence of an appropriate de-excitation mechanism, the molecules are excited rapidly (bleach) due to ambient laboratory light and remain in the excited state indefinitely. To achieve a steady-state condition allowing repetitive spectral absorption measurements, a de-excitation mechanism must be provided. 

This section will discuss the enhanced overall spectrum without regard to the de-excitation environment for the materials under test. It is critically important that this environment be addressed in any in-vitro experiments where the de-excitation mechanism provided by the photoreceptor cells is not present (See Chapter 12). 

The Rhodonine family of chromophores, when in the liquid crystalline state and with an adequate de-excitation mechanism consists of four distinct chromophores based on retinol and four based on retinol2. These are the chromophores of animal vision and their application is pervasive. The presence of all four chromophores in chordate... 

The relaxation mode of the chromophores is strictly by energy transfer to the dendrites of the photoreceptor cell when in-situ. The transfer is controlled by the unified Photoexcitation/De-excitation mechanism. 

The Rhodonines, as defined in this work, are the actual chromophores of vision when in the liquid crystalline state and supported by an auxiliary de-excitation mechanism. They can be placed in the chemical hierarchy in a number of ways. Some of these are addressed by Zollinger207 from the perspective of a dye. Others are addressed by Kennard, et. al from a crystallographers perspective208. At the current time, it is most appropriate to define the Rhodonines in terms of the broad retinoid family. This family also includes the retinenes which the Rhodonines resemble. The retinenes are chromogens of the Rhodonines. It is the Rhodonines which are the actual chromophores of vision. Within this family, the unique characteristic of the Rhodonines is their carboxylic-ion system. 

In contrast to the CH3CN situation, the spectra of interstellar ammonia give considerable insight into excitation and de-excitation mechanisms. From the observed intensities of the interstellar ammonia lines it has been derived that the excitation temperature 7 12, determined from the relative intensities of the (1,1) and the (2,2) lines, is notably lower than the excitation temperature r13 determined from the intensities of the (1,1) and (3,3) lines. Thus the (3,3) level shows an excess population over the (1,1), (2,2) levels. In other words, ortho-ammonia is not in equilibrium with para-ammonia. However, a more detailed study of the two para-ammonia levels (1,1) and (2,2) also reveals that their relative populations are not given by a simple Boltzmann factor for each of them. The (1,1) level has population in excess over the Boltzmann distribu-... 

Fig. 2.5 De-excitation mechanisms resonance ionization RI with a subsequent Auger neutralization AN left Auger de-excitation AD right...

INS is highly surface sensitive. Interpretation is difficult due to the presence of more than one de-excitation mechanism. 

For an oxidized surface, the value of y is 10" - 1.7-10 and it decreases with increasing the experimental temperature. In this case the activation energy of a change in yis 2.1 kcal/mole. From these data it can be inferred that the heterogeneous de-excitation of singlet oxygen proceeds in terms of the physical adsorption mechanism similar to that described for glass. 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

From the above-made review of literature, one may infer that the interaction of metastable atoms of rare gases with a surface of semiconductors and dielectrics is studied, but little. The study of the mechanism of transferring energy of electron-excited particles to a solid body during the processes under discussion is urgent. The method of sensor detection of rare gas metastable atoms makes it possible to obtain new information about the heterogeneous de-excitation of metastable atoms inasmuch as it combines high sensitivity with the possibility to conduct measurements under different conditions. 

The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole of the molecule. In classical terms, the interaction can be viewed as a perturbation of the molecule s electric field. In quantum mechanics the scattering is described as an excitation to a virtual state lower in energy than a real electronic transition with nearly coincident de-excitation and a change in vibrational energy. The scattering event occurs in 10 14 seconds or less. The virtual state description of scattering is shown in Figure 1. 

ICP is an emission technique, which means that it does not use a light source. The light measured is the light emitted by the atoms and monoatomic ions in the atomizer. The ICP atomizer is an extremely hot plasma, which is a high-temperature ionized gas composed of electrons and positive ions confined by a magnetic held. The extremely high temperature means that the atoms and monoatomic ions undergo sufficient excitation (and de-excitation) such that relatively intense emission spectra result. The sample is drawn in with a vacuum mechanism that will be described. The intensity of an emission line is measured and related to concentration. 

Luminescence is, in some ways, the inverse process to absorption. We have seen in the previous section how a simple two-level atomic system shifts to the excited state after photons of appropriate frequency are absorbed. This atomic system can return to the ground state by spontaneous emission of photons. This de-excitation process is called luminescence. However, the absorption of light is only one of the multiple mechanisms by which a system can be excited. In a general sense, luminescence is the emission of light from a system that is excited by some form of energy. Table 1.2 lists the most important types of luminescence according to the excitation mechanism. 

Once a center has been excited we know that, in addition to luminescence, there is the possibility of nonradiative de-excitation that is, a process in which the center can reach its ground state by a mechanism other than the emission of photons. We will now discuss the main processes that compete with direct radiative de-excitation from an excited energy level. 

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