The annihilation rate of positrons bound in $o$-Ps atoms and the fraction of positrons which form Ps have been measured for ${\mathrm{H}}_2$ gas at 293, 250, and 77 K at densities in the range 5.9-429 amagat, and for ${\mathrm{H}}_2$-Ne gas mixtures at 77 K having ${\mathrm{H}}_2$ partial densities in the range 1.9-286 amagat and with Ne concentrations in the range 0.02-0.98. Large deviations from the annihilation rates expected for free $o$-Ps atoms are observed and are attributed to the formation of localized cavity states. The results obtained with ${\mathrm{H}}_2$-Ne mixtures strongly support $o$-Ps-induced cavity formation. Complex density and temperature dependencies are observed for the Ps fraction in ${\mathrm{H}}_2$, and the addition of partial densities of Ne enhances positronium formation. The annihilation of slow positrons in ${\mathrm{H}}_2$ is discussed in paper I of this series.

• Received 18 July 1978

DOI:https://doi.org/10.1103/PhysRevA.20.357