Tunneling chemical exchange reaction D + HD → D2 + H in solid HD and D2 at temperatures below 1 K

High-Resolution Crossed-Beam Dynamics Studies of the D + Para-H2 → HD + H Reaction at 1.21 eV

Understanding kinetic isotope effects is important in the study of the reaction dynamics of elementary chemical reactions, particularly those involving hydrogen atoms and molecules. As one of the isotopic variants of the hydrogen exchange reaction, the D + para-H2 reaction has attracted much attention. However, experimental studies of this reaction have been limited primarily due to its strong experimental background noise. In this study, by using the velocity map ion imaging method and the near-threshold ionization technique, together with improvements on the vacuum condition in the vicinity of the collision zone, background noise was reduced significantly, and quantum state-resolved differential cross sections (DCSs) for the D + para-H2 reaction at a collision energy of 1.21 eV were acquired in a crossed molecular beams experiment. Interestingly, clear rotational state-dependent angular distributions were noticed in the quantum state-resolved DCSs. The most intense peak's positions for HD (v', j') products shift to different scattering directions as the product's ro-vibrational quantum number increases. Two different microscopic reaction mechanisms are found to be involved in this reaction for HD products in different vibrational states. The results show a direct correlation between the scattering angle and the product's rotational quantum number, revealing that the contributions of impact parameters are strongly influenced by the corresponding centrifugal barrier.

Purely Spatial Quantum Diffusion of H Atoms in Solid H_{2} at Temperatures below 1 K

We report on a direct measurement of the quantum diffusion of H atoms in solid molecular hydrogen films at T=0.7  K. We obtained a rate of pure spatial diffusion of H atoms in the H_{2} films, D^{d}=5(2)×10^{-17}  cm^{2} s^{-1}, which was 2 orders of magnitude faster than that obtained from H atom recombination, the quantity used in all previous work to characterize the mobility of H atoms in solid H_{2}. We also observed that the H-atom diffusion was significantly enhanced by injection of phonons. Our results provide the first measurement of the pure spatial diffusion rate for H atoms in solid H_{2}, the only solid state system beside ^{3}He-^{4}He mixtures, where atomic diffusion does not vanish even at temperatures below 1 K.

Formation of Nuclear-Polarized Phases of H Atoms Embedded in Solid H_{2} Films

We report on an experimental observation of two phases of hydrogen atoms in solid H_{2} films at temperatures of 0.1-0.8 K, characterized by a large enhancement of the nuclear spin polarization compared to that given by Boltzmann statistics (p=0.15 at T=0.15  K). The first phase with p=0.35(5) is formed spontaneously during sample storage in a high magnetic field (B=4.6  T). The second phase with an even higher nuclear polarization, p=0.75(7), can be achieved at T≤0.55  K by repeating sequences of dynamic nuclear polarization followed by a system relaxation. Upon warming through the range 0.55-0.65 K, the highly nuclear-polarized phase undergoes a phase transition to the spontaneously polarized phase which breaks down at T≃0.8  K, and the nuclear polarization gradually converges to the Boltzmann distribution. We discuss possible scenarios for explaining the nature of the observed phenomena.

ESR study of atomic hydrogen and tritium in solid T2 and T2:H2 matrices below 1 K

We report on the first ESR study of atomic hydrogen and tritium stabilized in solid T₂ and T₂:H₂ matrices down to 70 mK. The concentrations of T atoms in pure T₂ approached 2 × 10²⁰ cm^-3 (0.60%) and record-high concentrations of H atoms ∼1 × 10²⁰ cm^-3 (0.33%) were reached in T₂:H₂ solid mixtures where a fraction of T atoms became converted into H due to the isotopic exchange reaction T + H₂ → TH + H. The maximum concentrations of unpaired T and H atoms were limited by their recombination which becomes enhanced by efficient atomic diffusion due to the presence of a large number of vacancies and phonons generated in the matrices by β-particles. Recombination also appeared in an explosive manner, both being stimulated and spontaneously in thick films where sample cooling was insufficient. We suggest that the main mechanism for H and T migration is physical diffusion related to tunneling or hopping to vacant sites in contrast to tunneling chemical exchange reactions which govern diffusion of H and D atoms created in H₂ and D₂ matrices by other methods.