Negative and positive ion trapping by isotopic molecules in cryocrystals in case of solid parahydrogen containing electrons and H6+ radical cations

We performed electron spin resonance studies of trapped electrons and H6+ radical cations produced by radiolysis of solid parahydrogen (p-H2), p-H2-ortho-D2 (o-D2), and p-H2-HD mixtures. Yields of trapped electrons, H6+ radical cations, and its isotopic analogs H6-nDn+ (4≥n≥1) increased with increasing o-D2 and HD concentrations in solid p-H2. Electrons were found trapped near an o-D2 or an HD in solid p-H2 due to the long-range charge-induced dipole and quadrupole interactions between electrons and isotopic hydrogen molecules. H6+ radical cations diffuse in solid p-H2 by repetition of H6++H2→H2+H6+ and are trapped by ortho-D2 or HD to form H6-nDn+ (4≥n≥1) as isotope condensation reactions. Decay behaviors of these cations by the repetition, isotope condensation, and geminate recombination between electrons and H6-nDn+ (4≥n≥0) were reproduced by determining the corresponding reaction rate constants k1, k2, and k3. Values of 0.045 and 0.0015 L mol-1 min-1 were obtained for k1 (H6++D2→H2+H4D2+) and k2 (H4D2++D2→H2+H2D4+), respectively, and the value was quasinull for k3 (H2D4++D2→H2+D6+). These rate constants suggest that hole mobility drastically decreased in the repetition reaction when H6+ radical cations acting as hole carriers formed H4D2+ or H2D4+. HD and D2 molecules, therefore, act as electron and hole acceptors in irradiated solid p-H2-o-D2 and p-H2-HD mixtures.