Nobuya Miyoshi

Development of Ultra Small Shock Tube for High Energy Molecular Beam Source

A molecular beam source exploiting a small shock tube is described for potential generation of high energy beam in a range of 1–5 eV without any undesirable impurities. The performance of a non‐diaphragm type shock tube with an inner diameter of 2 mm was evaluated by measuring the acceleration and attenuation process of shock waves. With this shock tube installed in a molecular beam source, we measured the time‐of‐flight distributions of shock‐heated beams, which demonstrated the ability of controlling the beam energy with the initial pressure ratio of the shock tube.

Molecular beam study of the scattering behavior of water molecules from a graphite surface.

Gas flow in nanospaces is greatly affected by the scattering behavior of gas molecules on solid surfaces, resulting in unique mass transport properties. In this paper, the molecular beam scattering experiment of water molecules on a graphite surface was conducted to understand their scattering dynamics in an incident energy range that corresponds to their thermal velocity distribution at room temperature (35-130 meV). Because of the large adsorption energy (∼100 meV), the scattering behavior is quite sensitive to the incident energy even within this narrow energy range. For relatively large incident energies, the direct-inelastic and trapping-desorption channels have comparable contributions to the scattering process on the surface at 300 K. In contrast, when the incident energy decreases well below the adsorption energy on the surface, the trapping-desorption channel becomes dominant, changing the scattering pattern from directional to diffusive scattering. As a result, the tangential momentum accommodation coefficient (TMAC), which significantly impacts the mass transport in nanospaces, largely depends on the incident energy. A decrease in the incident energy from 130 to 35 meV doubles the TMAC (0.42 to 0.86). In addition to the incident energy, the TMAC shows a strong dependence on the surface temperature. With increasing the surface temperature from 300 to 500 K, the scattering becomes more directional because of the increasing contribution of the direct-inelastic channel, which reduces the TMAC for the incident beam energy of 35 meV to 0.48.

A non-diaphragm type small shock tube for application to a molecular beam source.

A non-diaphragm type small shock tube was developed for application to a molecular beam source, which can generate beams in the energy range from 1 to several electron volts and beams containing dissociated species such as atomic oxygen. Since repetitive high-frequency operation is indispensable for rapid signal acquisition in beam scattering experiments, the dimensions of the shock tube were miniaturized to reduce the evacuation time between shots. The designed shock tube is 2-4 mm in diameter and can operate at 0.5 Hz. Moreover, a high shock Mach number at the tube end is required for high-energy molecular beam generation. To reduce the shock attenuation caused by the wall boundary layer, which becomes significant in small-diameter tubes, we developed a high-speed response valve employing the current-loop mechanism. The response time of this mechanism is about 100 μs, which is shorter than the rupture time of conventional diaphragms. We show that the current-loop valve generates shock waves with shorter formation distances (about 200-300 mm) than those of conventional shock tubes. In addition, the converging geometry efficiently accelerates shock wave in the small-diameter tubes. The optimal geometry of the shock tube yields shock Mach number around 7, which indicates that the translation energy of molecular beams can exceed 1 eV even in the presence of the real gas effect.

Hyperthermal molecular beam source using a non-diaphragm-type small shock tube

We have developed a hyperthermal molecular beam source employing a non-diaphragm-type small shock tube for gas-surface interaction studies. Unlike conventional shock-heated beam sources, the capability of repetitive beam generation without the need for replacing a diaphragm makes our beam source suitable for scattering experiments, which require signal accumulation for a large number of beam pulses. The short duration of shock heating alleviates the usual temperature limit due to the nozzle material, enabling the generation of a molecular beam with higher translational energy or that containing dissociated species. The shock-heated beam is substantially free from surface-contaminating impurities that are pronounced in arc-heated beams. We characterize the properties of nitrogen and oxygen molecular beams using the time-of-flight method. When both the timing of beam extraction and the supply quantity of nitrogen gas are appropriately regulated, our beam source can generate a nitrogen molecular beam with translational energy of approximately 1 eV, which corresponds to the typical activation energy of surface reactions. Furthermore, our beam source can generate an oxygen molecular beam containing dissociated oxygen atoms, which can be a useful probe for surface oxidation. The dissociation fraction along with the translational energy can be adjusted through the supply quantity of oxygen gas.

High-Energy Molecular Beam Source Using a Non-diaphragm Type Small Shock Tube

The molecular beam technique [1] is one of the powerful tools to analyze gassurface interactions. Various methods have been developed to generate the molecular beams with the translational energy of 1-several eV, which corresponds to the typical activation energy of surface reactions. Although seeded beams combined with a heated nozzle are often used, the heatproof temperature of the nozzle limits the beam energy.Arc-heated beams [2] have the energy ofmore than 1 eV. The problem is, however, that the beams contain copper atoms due to electrode erosion and thus contaminate surfaces. Several researchers also investigated shock-heated beam sources [3]. The replacement of a diaphragm and the long evacuation time between each shot, however, make the conventional shock-heated beam sources impractical for the scattering experiments of gas molecules on surfaces, since the scattering experiments require signal accumulation for a large number of beam pulses. In order to overcome these shortcomings, we have been developing a beam source using a nondiaphragm type small shock tube [4]. Our objective is to develop a shock-heated beam source which can generate the beams with the translational energy of more than 1 eV with high operating frequency. It is noteworthy that the inner diameter of our shock tube is a few millimeters, far smaller than that of conventional shock tubes. The volume reduction leads to the shorter evacuation time, which enables generating molecular beams with high operating frequency. On the other hand, it should be noted that the boundary layer has significant effects on shock propagation in small diameter tubes [5]. In addition, we developed a high-speed valve employing a current-loop mechanism [4] as a substitute for a diaphragm to reduce shock formation distance, which determines the tube length.

Measurements of time-of-flight distributions of shock-heated molecular beams

Molecular beam source employing a shock tube is an attractive choice for the translational energy of around 1 eV, which corresponds to the typical activation energy of surface reactions. We developed a non-diaphragm type small shock tube as the molecular beam source generating high energy and dissociated molecular beams. The drastic reduction of the tube volume enables the repetitive operation of the shock tube at the frequency of 0.5 Hz. Installing the shock tube into the molecular beam setup, we measured the time-of-flight distributions of nitrogen and oxygen beams, and demonstrated the performance on controllability of the beam energy and the dissociation rate of molecules.

High‐Energy Molecular Beam Source Using a Small Shock Tube: Evaluation of Convergent Type Design

Molecular beam source using a small shock tube has the potential to frequently generate high energy molecular beam in a range of 1–5 eV without any undesirable impurities. We measured shock Mach numbers in 2 and 4‐mm‐diameter straight tubes to know about the propagation of shock wave in a very small shock tube. In addition, we measured shock Mach numbers in convergent shock tubes of which diameters linearly decrease from 4 mm to 2 mm, which demonstrated the possibility of a convergent shock tube to generate higher energy molecular beam than straight one.