Diffusion of hydrogen fluoride in solid parahydrogen

Abstract

We studied diffusion of hydrogen fluoride (HF) in solid parahydrogen (pH2) around 4 K. Diffusion rates were determined from time dependence of FT-IR spectra of HF monomers. The absorption of HF monomers shows temporal decay due to dimerization reaction via diffusion. It was found that the rates are affected by the sample temperature, the initial HF concentration, and annealing of samples. The observed non-Arrhenius-type temperature dependence suggests that the diffusion is dominated by a quantum tunneling process, that is, "quantum diffusion." Deceleration of the diffusion in condensed samples and acceleration in annealed samples were also observed. These results can be attributed to the fact that lower periodicity of samples due to impurities or defects suppresses the quantum tunneling. It seems to be difficult to explain the observed dependences by three possible diffusion mechanisms, exchange of chemical bonds, direct cyclic exchange, and exchange with mobile vacancy. Therefore, we propose a hypothetical mechanism by exchange of vacancies originating from quantum effect.

Diffusion of Water Molecules in Quantum Crystals

“…The obtained kernel values are, for example, 6.1(3) × 10 −31 and 2.7(2) × 10 −31 m 3 s −1 for (H 2 O) 3 in the sample with initial H 2 O concentrations of 600 and 1200 ppm, respectively. These values are about the same order of magnitude as the diffusion rate of HF in solid parahydrogen reported by Ooe et al 26 In Figure 3, the aggregation kernels are shown for samples that were kept at either 4.0 or 4.2 K and for both continuous (open markers) and minimized MIR light exposures (filled markers). The kernel for the sample with 600 ppm under continuous MIR exposure was significantly larger than the one for minimized MIR exposure.…”

“…It has been predicted that the defects in quantum crystals will become delocalized and move through the crystal in a coherent manner. 24,25 A non-Arrhenius-type temperature dependence on the diffusion rate of HF molecules in solid parahydrogen 26 suggests that the diffusion is caused by tunneling assisted by vacancies. On the other hand, Br atoms 27 and Li atoms 28 in solid parahydrogen diffuse only at an elevated temperature of 4.3−4.4 K, which is interpreted as thermal diffusion.…”

“…The diffusion observed here was clearly affected by the phonon excitation of solid parahydrogen by the MIR light source as in the case of HF diffusion. 26 The mutual aggregation rate constant β i,k is proportional to the diffusion coefficient, D, of water molecules. What we observe in Figure 3 is that the diffusion coefficient is approximately inversely proportional to the concentration of water molecules.…”

“…In quantum crystals such as solid hydrogen, there is a significant overlap of the wavefunctions of molecules on neighboring lattice sites, which allows defects such as vacancies or impurities to exchange positions with nearest-neighbor molecules. It has been predicted that the defects in quantum crystals will become delocalized and move through the crystal in a coherent manner. , A non-Arrhenius-type temperature dependence on the diffusion rate of HF molecules in solid parahydrogen suggests that the diffusion is caused by tunneling assisted by vacancies. On the other hand, Br atoms and Li atoms in solid parahydrogen diffuse only at an elevated temperature of 4.3–4.4 K, which is interpreted as thermal diffusion.…”

Infrared absorption spectra of SiF4 and its clusters in solid parahydrogen

“…The pH 2 crystals were prepared in a way similar to that described in our previous papers [19,20]. Briefly, normal hydrogen (nH 2 ) was converted into pH 2 with a purity better than 99.95% by passing nH 2 through a magnetic catalyst Fe(OH)O kept at about 14 K. SiF 4 was prepared from the reaction with HF gas with glass.…”