Chemical reactions of positronium

In the case of quenching the reduction (shortening of the o-Ps lifetime) is explained by the chemical reaction of positronium and the quenching species (which in many cases are additives present in the polymer matrix). The chemical rate constant for the reaction between the Ps and quenching species can be expressed in terms of the concentration of the quenching species [M], and it is found that it can be described by pseudo-first order kinetics as [42, 80] ... 

AMOC allows time-dependent observations of the occupations and transitions of different positron states tagged by their characteristic Doppler broadening. Chemical reactions of positronium have been studied by beam-based AMOC as well as bound states between positrons (e+) and halide ions (cf. Sect. 2). 

Positronium reactions of a chemical nature form the third, and from the chemical aspect the most important, group of interactions. (It must be noted that certain types of the ortho-para conversion reactions are also of a chemical nature, e.g., the free radical reactions.) The main types of chemical reactions of positronium are illustrated by the following examples ... 

One of the reasons why the study of the chemical reactions of positronium is extremely interesting is that, in a certain sense, we may speak here of the extreme limit of microchemistry, when the reaction times of the individual reacting atoms can be directly followed and quantitatively measured. In these processes the positronium atom at the same time also features as a labelled atom, giving an indication of its own fate by means of the annihilation. 

We have used the AMOC method so far to study chemical reactions of positrons, oxidation reactions of positronium, as well as the spin conversion of positronium in the presence of organic radicals. In all cases, the method was well suited for the time-domain observation of the reactions. In this contribution, we will confine ourselves to the reactions of positrons, as they take place in liquid solutions of halides. 

A positron in an electronic media can pick up an electron and form a neutral atom called Positronium (Ps) [9], The existence of Ps and its chemical reaction with molecules was detected from annihilation photons in 1951 [10], Ps is formed in most molecular systems. Due to the different combinations of positron and electron, there are two states of Ps the para-Ps (p-Ps) from the anti-parallel spin, and the ortho-Ps (o-Ps) from the parallel spin combination. The lifetime and the annihilation events for p-Ps and o-Ps are very different from each other, as given by electromagnetic theory. Figure 1.1 shows basic physical properties of Ps and compares them with the H atom, although it should not be considered an isotope of H (see problems 1.5 and 1.6 and answers at the end of this chapter). 

When inhibition of positronium formation occurs, it is seen as a decrease in the o-Ps intensity, while quenching of o-Ps, as a result of chemical reactions, will shorten the o-Ps lifetime (x3). While quenching and inhibition of positronuim have been extensively studied in solution (see Chapter 5) it is a factor that has often been overlooked in polymer studies. There are numerous examples of inhibition and quenching effects in polymers [42, 80-82]... 

The lifetimes given above relate to positronium in vacuum. In the medium a new possibility of destruction appears the positron bound in Ps can annihilate with one of strange electrons having appropriate (opposite) spin orientation. The process is called pick-off and leads to two quantum annihilation. If the medium is paramagnetic, another process shortening the o-Ps lifetime is possible the interaction with magnetic moments can transform o-Ps into p-Ps, which decays almost immediately (conversion process). Both e and Ps can participate in chemical reactions with molecules of the medium changing the Ps formation probability... 

Shantarovich, V. P., On the role of free volume in pick-off annihilation and positronium chemical reactions chemistry, J. Radioanal. Nucl. Chem., 210, 357-369 (1996). 

The mean lifetime of the free para-Ps in vacuo is Ts = 1.25 x 10 s, and that of ortho-Ps is 1.4 x 10" s. These lifetimes appear to be very short, but if they are compared with the shortest time intervals experimentally measurable, or, for example, with the period of intramolecular vibration of atoms (10 s), it can be seen that they are sufficiently long for the positronium atom to take part in chemical reactions or to enter into other interactions with the particles of the medium, and to allow the following of these processes in time. This is particularly the case as regards ort/jo-positronium. 

The most interesting information from the chemical point of view may be obtained by measurement of the distribution of the lifetimes of the positrons or positronium atoms. In addition to the lifetime, this method also gives a measure of the relative probability of ort/io-positronium formation as a separable parameter. Hence, from the lifetime distribution, information is obtained not only on the rates of the chemical reactions and other interactions of the positronium, but also on the probability of positronium formation, e.g., the extents of the possible inhibitory processes. The only deficiency of the method is that it does not provide a possibility for simple differentiation between ortho-para conversion processes and the chemical reactions, since both processes cause a decrease in the lifetime. 

The mechanism of the formation of the positronium atom, which is the hound state of a positron and an electron, as Well as its subsequent reactions are highly dependent on the physical and chemical microstructure of the environment in which they occur. 

The statistical weights of the above reactions vary in a wide range in different materials. Positronium formation can be as significant as 30%. A positronium atom, being similar to hydrogen atoms, is a reactant particle. Thus, the list of spur reactions should be elongated by its reactions. On the other hand, most of the chemical applications of positrons are based on the use of positronium atoms, consequently these reactions are treated separately in Sect. 27.2.3. 

As discussed in the previous section, all the possible interactions of o-Ps decrease its lifetime. The degree of this lifetime decrease is another parameter that can be related to physical and chemical properties of a system. The connection is quite obvious in the case of chemical reactions and ortho-para conversion. In the case of pick-off annihilation, the effects of the substance on positronium lifetime are expressed indirectly through the overlap integral in 0 Eq. (27.7). Any change in t/ v (e.g., changes of electron orbits) or in the overlap integral (e.g., free-volume changes) is reflected in the lifetime of o-Ps. 

Usually not just positronium atoms but also positrons are sensitive to changes in the composition or the structure of materials. If the wave function of positronium is replaced with that of the positron inO Eqs. (27.7) and (27.13), one obtains the theoretical foundation of the above statement. Positrons even participate in spur reactions. Consequently, positrons are sensitive to the changes of free volume, structural changes of materials, and chemical composition. Thus, there are usually several distinct positron states in a material, aU of them being sensitive to any change in electron density. The pieces of information provided by the annihilation characteristics of these states complete each other. 

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