Positron annihilation in gaseous hydrogen and hydrogen-neon mixtures. I. Low-energy positrons

Annihilation lifetime spectra have been measured for positrons in ${\mathrm{H}}_2$ gas at 293, 250, and 77 K at densities in the range 5.9-429 amagat, and in ${\mathrm{H}}_2$-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. The annihilation rate of slow positrons with ${\mathrm{H}}_2$ has a complex dependence on gas density and temperature. Particularly at low ${\mathrm{H}}_2$ densities, the presence of various partial densities of Ne can dramatically affect the positron-${\mathrm{H}}_2$ annihilation process. The slow-positron annihilation rates are discussed in terms of positron-${\mathrm{H}}_2$ collision complexes and the self-trapping of positrons within clusters of ${\mathrm{H}}_2$ molecules. The annihilation in ${\mathrm{H}}_2$ of positrons bound in Ps atoms is discussed in paper II of this series.