Mesic molecular effects in the capture of negative pions stopped in gaseous hydrogen isotopes

The influence of molecular structure on the nuclear capture probability of stopped negative pions has been observed by comparing the ${\pi }^0$ gamma-ray spectrum from ${\pi }^{-}$ mesons stopped in HD gas to that from a mixture of equal amounts of ${\mathrm{H}}_2$ plus ${\mathrm{D}}_2$. The fraction of stopped pions that are captured by a proton in the ${\mathrm{H}}_2$+${\mathrm{D}}_2$ mixture is $f_{{\mathrm{H}}_2{\mathrm{D}}_2}=0.417\mathrm{}\pm 0.004$, while for HD it is $f_{\mathrm{HD}}=0.338\mathrm{}\pm 0.008$, independent of the gas pressure between 6 and 90 atm. The ratio, $\frac{f_{{\mathrm{H}}_2{\mathrm{D}}_2}}{f_{\mathrm{HD}}}$, of the fractions is 1.23±0.03.