Stability of trapped electrons

The more incisive calculation of Springett, et al., (1968) allows the trapped electron wave function to penetrate into the liquid a little, which results in a somewhat modified criterion often quoted as 47r/)y/V02< 0.047 for the stability of the trapped electron. It should be noted that this criterion is also approximate. It predicts correctly the stability of quasi-free electrons in LRGs and the stability of trapped electrons in liquid 3He, 4He, H2, and D2, but not so correctly the stability of delocalized electrons in liquid hydrocarbons (Jortner, 1970). The computed cavity radii are 1.7 nm in 4He at 3 K, 1.1 nm in H2 at 19 K, and 0.75 nm in Ne at 25 K (Davis and Brown, 1975). The calculated cavity radius in liquid He agrees well with the experimental value obtained from mobility measurements using the Stokes equation p = eMriRr], with perfect slip condition, where TJ is liquid viscosity (see Jortner, 1970). Stokes equation is based on fluid dynamics. It predicts the constancy of the product Jit rj, which apparently holds for liquid He but is not expected to be true in general. 

Sulfates. The basic lattice of sulfate phosphors absorbs very short wavelength UV radiation. On excitation with X rays or radiation from radioactive elements, a large proportion of the energy is stored in deep traps. For this reason, CaS04 Mn is used in solid-state dosimeters. Of the glowpeaks which can be selected by thermoluminescence, more than 50 % fail to appear at room temperature because of a self selection of the shallow traps. Other activators, such as lead or rare-earth ions (Dy3 +, Tm3 +, Sm3+), stabilize the trapped electrons [5.399]—[5.401]. 

Photocatalytic reactions at the semiconductor surface can be described by the following six steps as shown in Fig. 5.3. (D Absorption of a unit of light associated with the formation of a conduction band electron and a valence band hole in the semiconductor. (2) Transfer of an electron and a hole to the surface. (D Recombination of electron-hole pairs during the reaction processes. Stabilization of an electron and a hole at the surface to form a trapped electron and a trapped hole, respectively. (D Reduction and oxidation of molecules at the surface. (6) Exchange of a product at the surface with a reactant at a medium. Among these reaction steps, the absorption of light in the bulk (step CD) and... 

Current theories of particle physics predict that, in a vacuum, the positron is a stable particle, and laboratory evidence in support of this comes from experiments in which a single positron has been trapped for periods of the order of three months (Van Dyck, Schwinberg and Dehmelt, 1987). If the CPT theorem is invoked then the intrinsic positron lifetime must be > 4 x 1023 yr, the experimental limit on the stability of the electron (Aharonov et al., 1995). 

The stability of solvated electrons in some polar solvents (liquid ammonia, HMPA, and amines) can be explained either by a solvent cavity in which the electron is trapped or by the presence of an expanded orbital occupied by the electron this orbital could... 

The neutral pure ice is a crystalline formation. Khodzhaev et ah (48) have established that under long y-irradiation stabilization of the electrons in ice will be observed. Figure 7 shows the absorption spectrum of ice subjected to y-irradiation at 77°K. Band with Amax = 280 n.m. (e 450M 1 cm.-1) is because of the radical OH e tr absorbs the light in the visible part of the spectrum (Amax is about 620 n.m.). The yield of e tr in this system is very low (—10-3 electrons/100 e.v.). The concentration of the trapped electrons is very low therefore, these electrons are not displayed in the EPR spectrum. It should be stressed that the value of Amax of the trapped electron practically coincides with the value of Amax for e tT in the hypothetic glassy ice. 

He also interpreted much of the data at low concentrations as an electrolyte solution while at high concentrations they were discussed as liquid metal. Much of the earlier studies of trapped electrons were dominated by the study of metal ammonia solutions, in part because of their exceptional stability. These studies were first collectively presented in the proceedings of Colloque Weyl I in 1963 (Lepoutre and Sienko, 1963). 

Contrary to path e, the other paths are based upon the stability of the electron-withdrawing substituted aromatic radical anions. Indeed such extensively delocalized species are often weak bases and nucleophiles and may be quite persistent (in the case of polycyanoaromatics, indefinitely persistent in solution under appropriate conditions). In this case, the following reaction depends on the formation of a reactive intermediate from the cation radical of the donor. Typical examples are as follows. A first possibility is fragmentation of the radical cation, yielding a radical that couples with the aromatic radical anion (path/). In a variation of this mechanism, the radical is first trapped by a radicophile and it is the radical adduct that couples with the radical anion (path g). A further possibility is addition of a nucleophile to the radical cation, and coupling of the resulting radical with the aromatic radical anion (path h). 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

Baird and Rehfeld express A ° in terms of the trap concentration and the chemical potentials of the empty trap and of the electron in the quasi-free and trapped states. Further, they indicate a statistical-mechanical procedure to calculate these chemical potentials. Although straightforward in principle, their actual evaluation is hampered by the paucity of experimental data. Nevertheless, Eq. (10.13) is of great importance in determining the relative stability of the quasi-free versus the trapped states of the electron if data on time-of-flight and Hall mobilities are available. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

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