Positronium solvation sites in liquid amides revealed by magnetic field effects: influence of the chemical structure of the molecules

The conformation of positronium (Ps) is examined in pure liquids through the effects of an external magnetic field ( B) in positron annihilation lifetime spectroscopy experiments. In continuity with previous work predicting a low value of the contact density parameter (η) in liquids of high static dielectric constant, three amides are studied: formamide, N-methylformamide (NMFA) and N,N-dimethylformamide (DMF). By contrast with what has been found in all pure liquids previously studied, the variations with B of parameter R, the normalized integral of counts in a time window set for the long-lived Ps triplet state, are “anomalous” and cannot be described using a single fitting parameter (η). Of various attempts to recover the data, only the hypothesis of two distinct Ps states leads to acceptable standard deviations. The first state appears “normal”, with a single fitting parameter of rather low value denoting a well expanded Ps wave function as compared to vacuum: η n=0.35 in NMFA and DMF and 0.51 in formamide. The second state is “anomalous”, requiring the use of two different contact density parameters affecting the hyperfine splitting (η a) and the intrinsic decay rate constant(η′ a) respectively: both parameters are only poorly defined. The existence of these two states of Ps is correlated with the presence of two potential Ps solvation sites on the planar amide molecules, located either at the free doublet of the nitrogen atom, or at the two doublets of the oxygen atom. The relative amount of the anomalous state decreases from formamide (≈32%) to NMFA and DMF (≈18%), in proportion with the accessibility to the nitrogen doublet, which is therefore identified as the site promoting this state. A general classification of the interactions between Ps and molecules in increasing order of strength is proposed, along with the Ps states expected in each case, according to the local symmetries and charge densities.