A negative-pion capture process for several kinds of hydrocarbons was studied by the simultaneous observation of low-energy pionic x rays and ${\mathrm{\pi }}^0$ decays following the nuclear capture of pions by hydrogen. We measured the pionic x-ray intensity pattern and the pion-capture probabilities of constituent carbon and hydrogen. The x-ray intensity pattern depends on the chemical environment; a cascade calculation showed that this can be qualitatively attributed to the effects of a pion transfer from hydrogen. The pion-capture probabilities on hydrogen could be explained by a combined model involving a modified large mesomolecular model and an external transfer process. The external transfer parameter of the pion from a pionic hydrogen atom to carbon atoms was deduced to be ${\mathrm{\Lambda }}_{\mathrm{C}}$=1.7±0.2 based on the results for a series of alkanes in the condensed phase. The large difference between the pion-capture probabilities on hydrogen observed in alkanes and that in benzene can be well understood based on the proposed model. © 1996 The American Physical Society.

• Received 17 July 1995

DOI:https://doi.org/10.1103/PhysRevA.53.130