Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Slow electrons thermalization

The subexcitation electrons lose their energy in small portions, which are spent on excitation of rovibrational states and in elastic collisions. In polar media there is an additional channel of energy losses, namely, the dipole relaxation of the medium. The rate with which the energy is lost in all these processes is several orders of magnitude smaller than the rate of ionizaton losses (see the estimates presented in Section II), so the thermalization of subexcitation electrons is a relatively slow process and lasts up to 10 13 s or more. By that time the fast chemical reactions, which may involve the slow electrons themselves (for example, the reactions with acceptors), are already in progress in the medium. For this reason, together with ions and excited molecules, the subexcitation electrons are active particles of the primary stage of radiolysis. [Pg.321]

A broad survey of studies of thermalization of slow electrons in gases is presented in Ref. 248. Recently, there has been published a series of papers by Koura249 devoted to simulation of electron thermalization in... [Pg.327]

The thermalization path length of subexcitation electrons has been the object of many discussions from the time the first track models appeared up to this day. The reason is that for quite a long time there were no direct methods of measuring the path lengths of slow electrons, while the corresponding theoretical analysis is very difficult owing to the need to take into account all the processes relevant to retardation of subexcitation electrons. [Pg.328]

Samuel and Magee250 were apparently the first to estimate the path length /th and time rth of thermalization of slow electrons. For this purpose they used the classical model of random walks of an electron in a Coulomb field of the parent ion. They assumed that the electron travels the same distance / between each two subsequent collisions and that in each of them it loses the same portion of energy A E. Under such assumptions, for electrons with energy 15 eV and for AE between 0.025 and 0.05 eV, they have obtained Tth 2.83 x 10 14 s and /th = 1.2-1.8 nm. At such short /th a subexcitation electron cannot escape the attraction of the parent ion and in about 10 13 s must be captured by the ion, which results in formation of a neutral molecule in a highly excited state, which later may experience dissociation. However, the experimental data on the yield of free ions indicated that a certain part of electrons nevertheless gets away from the ion far enough to escape recombination. [Pg.328]

So, as one can see, this method of determining the thermalization path length is not straightforward and involves many assumptions and suppositions. First, the value of r0 depends on the specific choice of the function F r) (see, e.g., Ref. 269) and is too uncertain to enable us to determine the processes that are responsible for retardation of subexcitation electrons in a medium (see the discussion in Ref. 271). However, by comparing the values of r0 for different substances we are able to determine some of the factors that affect the path length of slow electrons. Since the value of r0 depends on the density p of the liquid, it is reasonable to compare the products prQ rather than the values of r0 themselves. [Pg.334]

Thermalization Path Length of Slow Electrons in Water Electrolytic Solutions292... [Pg.338]

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). [Pg.8]

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). [Pg.96]

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. [Pg.763]


See other pages where Slow electrons thermalization is mentioned: [Pg.441]    [Pg.50]    [Pg.262]    [Pg.711]    [Pg.429]    [Pg.257]    [Pg.261]    [Pg.335]    [Pg.339]    [Pg.339]    [Pg.293]    [Pg.462]    [Pg.31]    [Pg.1691]    [Pg.350]    [Pg.241]    [Pg.137]    [Pg.104]    [Pg.147]    [Pg.99]    [Pg.82]    [Pg.918]    [Pg.269]    [Pg.147]    [Pg.114]    [Pg.241]    [Pg.592]    [Pg.1796]    [Pg.468]    [Pg.307]    [Pg.527]    [Pg.331]    [Pg.377]    [Pg.193]    [Pg.193]    [Pg.211]    [Pg.382]    [Pg.371]    [Pg.538]   
See also in sourсe #XX -- [ Pg.327 ]




SEARCH



Electrons thermalized

Slow electrons

© 2024 chempedia.info