Solid Hydrogen Films Research Articles

Nuclear-Polarized Phases of H Atoms Embedded in Solid Molecular Hydrogen Films

Nuclear-Polarized Phases of H Atoms Embedded in Solid Molecular Hydrogen Films

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

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 1K.

Time evolution calculation of muon catalysed fusion: Emission of recycling muons from a two-layer hydrogen film

Recycling muon having approximately 10 keV kinetic energy produced in a muon catalysed fusion (μCF) cycle is expected to be a source of negative muon beam with high-spatial/time coherence. We report a time evolution calculation of μCF cycle with a particular focus on the emission of recycling muons from a hydrogen film target. Rate equations for the μCF reactions are solved using the 4th-order Runge-Kutta method. Considering a two-layer solid hydrogen film target composed of 1 mm hydrogen-deuterium layer and 2 µm deuterium-tritium layer, we have numerically simulated the time evolution of the muonic states and their populations in these layers. The muon atomic and molecular processes in the layers are solved simultaneously by introducing a probability of muonic deuterium atoms traveling between the layers via the Ramsauer-Townsend effect. The use of T2 amplifies the recycling muon emission. A weak dependence of the emission fraction of the recycling muons on the T2 concentration is predicted, which can be utilized to minimize the usage of tritium.

A generator of thin solid hydrogen films as targets for nuclear research

Shell-free solid hydrogen films with a thickness of up to 25 μm were for the first time manufactured as targets for nuclear research. Films with a width of 3–10 mm are pushed away from a target generator with a constant velocity of 5–20 mm/s to the open space of the vacuum chamber, where they are irradiated by heavy ion beams.

NONLINEAR FREQUENCY CONVERSION BY RAMAN COHERENCE PREPARED IN SOLID HYDROGEN FILM

We investigated the Raman coherence characteristics in the solid hydrogen film deposited on a sapphire substrate. By using Raman coherence prepared with two single-frequency pulsed lasers, we generated the multiorder coherent Raman sidebands in solid hydrogen film. High-order Raman sidebands were obtained under strong pumping conditions (≥230 MW/cm2). The generated anti-Stokes(AS)–Raman sidebands extended from ultraviolet (292 nm for AS5 band) to visible (565 nm for AS1 band) region. The multiorder Raman sideband generation is thought to be due to the parametric coupling of pump and coupling lasers. The frequency conversion efficiency shows the maximum (14%) at the pumping intensity of 360 MW/cm2. From the experiment that makes the multimode probe beam beat with the prepared Raman coherence, we found that the prepared Raman coherence replicates the probe beam to its Raman sidebands.

Stimulated Raman Scattering in Vapor-Deposited Solid Hydrogen Film

The solid para-hydrogen film was prepared by depositing the para-hydrogen vapor onto a sapphire substrate. From the stimulated Raman scattering experiment using two ns-lasers whose laser-frequency difference is equal to the fundamental vibrational transition of the solid para-hydrogen, we observed the collinear generation of Raman sidebands. The high-order Raman sidebands were obtained under the strong pumping intensity (≥230 MW/cm2). The generated five anti-Stokes(AS)-Raman sidebands extend from ultraviolet (292 nm for AS 5 band) to visible (565 nm for AS 1 band) region. Also, we found that the first AS-Raman signal shows weak rotational components as well as a strong vibrational line. From the experiment that makes the multimode probe beam beat with the prepared Raman coherence, we found that the prepared Raman coherence replicates the probe beam to its Raman sidebands. The frequency conversion efficiency from the probe beam to the first AS-Raman sideband rises to the maximum (4.7%) at 284 MW/cm2, and then falls with the increase of the pumping intensity. This is thought to be due to the fact that some molecules in the first vibrational level incline to go to the higher vibrational level under the higher pumping intensity. For the strong pumping intensity (290 MW/cm2), four anti-Stokes and one Stokes Raman sidebands of the probe beam were generated.

Generation and investigation of radioactive muonic atoms in solid hydrogen films

At RIKEN-RAL muon facility, a concept project is under way to extend the standard technique of muonic atom spectroscopy to the use of radioactive isotope beams. This new method would allow studies of the nuclear properties and nuclear sizes of very exotic nuclei at facilities where both intense negative muon and radioactive nuclear beams would be available. The present status of a feasibility study is being reported.

The effect of roughness on the T3-dewetting of molecular hydrogen

Triple-point dewetting is a well-known behaviour of molecular hydrogen and other van der Waals systems like noble gases on a solid substrate. Recent theoretical and experimental investigations (Phys. Rev. Lett. 88 (2002) 55702) suggest that it is caused primarily by the roughness of the substrate. Strain induced due to the mismatch of the lattice constant of the substrate and the growing layers of the adsorbed materials is increased by the micro-roughness of the substrate which eventually leads to the growth of only a thin solid film of the adsorbate. The dominating role of the substrate roughness is demonstrated, e.g., by ellipsometric measurements on smooth Si surfaces (rms 0.15 nm ), where a thicker solid hydrogen film than the 3 monolayers on “usual” substrates is observed. We present a way to modify and improve the surface quality of substrates for such wetting studies of solid van der Waals films.

Experimental setup for X-ray spectroscopy of muonic atoms formed from implanted ions in solid hydrogen

An experimental setup comprising of a cryogenic target and an ion implantation system has been constructed to perform muonic X-ray spectroscopy with muonic atoms formed from implanted ions in solid hydrogen films. Gaseous mixtures of hydrogen (H 2) and deuterium (D 2) have been frozen onto a thin 60-mm diameter silver foil, and then irradiated with charged particles of energies up to 33 keV/q . Already, solid films of H 2 and D 2 mixtures with implanted helium ions have been successfully used in muon catalyzed fusion related experiments at RIKEN-RAL Muon Facility. This new method allows studies of the nuclear charge distribution of unstable atoms by means of the muonic X-ray method at facilities where both negative muon and radioactive nuclear beams would be available.

Wetting behaviour of solid and liquid hydrogen films

Adsorbates on strong binding substrates like H 2 on Au show triple-point dewetting. To study this phenomenon in detail, we have reduced the van der Waals interaction of the H 2 films with the Au surface, by preplating the substrate with Ne, Ar or CH 4. From a theoretical model, which considers the deformation of the adsorbate by the attractive van der Waals potential and the lattice mismatch due to different lattice constants of adsorbate and substrate, complete wetting is expected when preplating the substrate. Our experiments show, however, that for all combinations of preplated material the equilibrium H 2 film thickness is – apart from small differences – the same as without preplating. This indicates that the present picture of the triple-point wetting of H 2 is incomplete.

Proposal for X-ray spectroscopy of muonic atoms formed from implanted ions in solid hydrogen films

A new method is proposed to extend muonic atom X-ray spectroscopy to the study of nuclear beams, including radioactive beams, by stopping both muon and nuclear beams in a solid hydrogen film. The muon transfer reaction to higher Z nuclei is used then to form muonic atoms. This method would allow studies of the nuclear charge distribution of unstable atoms.

Time-of-flight studies of emission of μt from frozen hydrogen films

In recent TRIUMF experiments, a μ- beam is stopped in a solid hydrogen film with a small fraction of T2. The Ramsauer-Townsend (RT) mechanism allows μt to escape into vacuum with a few eV of energy. To study the emission process, an imaging system was used to determine the position of muon decays. Experimental histograms are in good agreement with a Monte Carlo simulation.

Non diffusive mobility of solid hydrogen films

We have used a “hole-burning” technique to study the mobility of thin solid films of H2 and D2 for temperatures between 1.6 K and 5 K. Even at low temperatures where transport through the vapor is negligible, the solid films remain mobile. The transport is thermally activated with an activation energy of 19 K for H2 and 38 K for D2. The time dependence of the regrowth shows that surface transport is not due to simple diffusion.

Wetting behavior of solid hydrogen films

We have studied the dewetting of quench-condensed solid hydrogen films on bare and Ne plated Ag substrates. Information about the morphology of the films during the dewetting process is obtained from measurements of photoelectron tunneling through the films and complementary surface plasmon resonance data. Plating the Ag substrate with Ne films of various thickness allows to tune the strength of the van der Waals interaction with the hydrogen and thus to change the H2 films from a state of incomplete to complete wetting.

Solid hydrogen films in a nonequilibrium state

Molecular hydrogen can be prepared as a metastable quantum solid by quench-condensation onto a substrate at sufficiently low temperature. Using surface plasmon resonance and light scattering techniques we have investigated such H 2 films with thicknesses up to several 100A˚, condensed onto a silver surface at 1.5 K. Since solid H 2 does not wet silver these films are in an extreme nonequilibrium state. At a temperature of 2.3 K we observe a dramatic rearrangement of the film overstructure on scales of ∼1 μm, which implies an extremely high mobility of the H 2 molecules at a temperature far below the desorption threshold (3.8 K).

Possible correlation effects of surface state electrons on a solid hydrogen film

We have investigated the transport properties of surface state electrons on thin quench-condensed hydrogen films for various electron densities. The surface state electron mobility showed a continuous dependence on the plasma parameter Γ in the range from 20 to 130, indicating a strong influence of correlation effects within the electron system for all measured values of Γ.

Surface state electrons on solid hydrogen films

We investigated the surface of quench condensed hydrogen films on a glass substrate by measuring the conductivity of surface state electrons (SSE) between 1.5 and 4.2 K. Initially the films were very rough (no conductivity signal). After thermal annealing the SSE conductivity increased drastically, behaving with temperature like a thermally activated process. Saturated helium films on the hydrogen led to a large increase in the SSE conductivity. Shortly before helium film completion a sharp dip in the conductivity is observed.

28. Effect of peppermint on theheart

Recycling muon having approximately 10 keV kinetic energy produced in a muon catalysed fusion (μCF) cycle is expected to be a source of negative muon beam with high-spatial/time coherence. We report a time evolution calculation of μCF cycle with a particular focus on the emission of recycling muons from a hydrogen film target. Rate equations for the μCF reactions are solved using the 4th-order Runge-Kutta method. Considering a two-layer solid hydrogen film target composed of 1 mm hydrogen-deuterium layer and 2 µm deuterium-tritium layer, we have numerically simulated the time evolution of the muonic states and their populations in these layers. The muon atomic and molecular processes in the layers are solved simultaneously by introducing a probability of muonic deuterium atoms traveling between the layers via the Ramsauer-Townsend effect. The use of T2 amplifies the recycling muon emission. A weak dependence of the emission fraction of the recycling muons on the T2 concentration is predicted, which can be utilized to minimize the usage of tritium.