Positronium in high temperature phases of long-chain even n-alkanes

Positron lifetime spectra were measured for a set of long-chain alkanes CnH2n+2. Phase transitions in solid are usually visible as a change of ortho-positronium (ortho-Ps) lifetime (and Ps formation probability). In the case of n-alkane molecules containing more than 25 carbon atoms in the chain, up to the longest in which the rotator phase exists (n = 38), unexpectedly there is no change of ortho-Ps lifetime at the transition from the rotator to liquid. In the phase M appearing when the chains are still longer (n ⩾ 36), the ortho-Ps lifetime is shorter and the step of lifetime at melting is again well visible. It indicates better degree of molecular order in M phase than in rotator one. At low temperatures the lifetime in the samples with n ⩾ 38 obtained by crystallization is shorter than in molten ones.