Microscopic behavior of positively charged particles in solid hydrogen by the muon spin rotation and relaxation method

We have investigated the microscopic behavior of the positive muon (μ⁺) in solid H₂ and D₂ down to 0.6 K using pulsed-beam muon spin rotation/relaxation (μ⁺SR) methods. By studying muon spin relaxation phenomena in solid para-H₂ and ortho-D₂, we found that the positive muon exists in three different states: H₂μ⁺ or D₂μ⁺ (20–25%), muonium (15–20%), and “bare μ⁺” (diamagnetic μ⁺, ∼60%). In the H₂μ⁺ molecule in solid para-H₂, the muon spin is depolarized and the muon feels a local magnetic field B_loc of 1–2 G as deduced from longitudinal magnetic field μ⁺SR measurements. The magnitude of B_loc is inconsistent with the magnetic dipolar field (∼20 G) expected from the nuclear magnetic moments of the two protons in the H₂μ⁺ molecule. This fact suggests that the H₂μ⁺ molecule is in a rotationally excited state. In solid ortho-D₂, muonium and bare μ⁺ diffuse with a characteristic time between jumps τ_c of ∼5×10^-8 sec for muonium and ∼2×10^-6 sec for bare μ⁺ at 6.8 K. The diffusion rate of muonium in ortho-D₂ is two orders of magnitude higher than that of the bare muon and independent of temperature below 10 K.