Ps states in condensed matter the contact density parameter

Fitting the variations of either t4(B), T3av(B) or R(B) with B enables one to derive the value of p. Fig 4.7 shows the variations of R(B) for a nonpolar solvent, isooctane (r = 0.88) [73] and a polymer, polyethylene terephthalate in the latter case there is a rapid decrease of R(B), denoting a low value of r (0.23). 

In all liquids studied until now, except some amides, Ps is found to have a spherical non-excited wave-function, with r) varying between about 0.6 and 0.9, going from the most to the least polar solvents, as shown in Table 

Attempts have been made to correlate r with some macroscopic properties of the liquids, without great success [73, 74]. 

In amides, the variations of R(B) with B are peculiar, showing a rapid initial decrease followed by a much slower one [75]. These data are fitted on the assumption that two Ps states are present the first state is abundant (68 % in formamide and about 80 % in methylformamides) and normal , 

As seen by applying a magnetic field, the Ps states are more varied in solids than in liquids [44, 76, 77], Positronium appears to be in a normal state, with r = T], or there are two states, either normal or anomalous (Table 4.3). No clear correlation has been found yet between these various possibilities and properties of the solids. The existence of two different Ps states in a solid would correspond to the presence of two different sites, or classes of sites, where Ps annihilates. However, complementary experiments should be carried out to substantiate this hypothesis. 

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