Hydrogen mean free path

Persson M, Wiizen L and Andersson S 1990 Mean free path of a trapped physisorbed hydrogen moiecuie Phys. Rev. B 42 5331... 

FIGt 22-48 Transport mechanisms for separation membranes a) Viscous flow, used in UF and MF. No separation achieved in RO, NF, ED, GAS, or PY (h) Knudsen flow used in some gas membranes. Pore diameter < mean free path, (c) Ultramicroporoiis membrane—precise pore diameter used in gas separation, (d) Solution-diffusion used in gas, RO, PY Molecule dissolves in the membrane and diffuses through. Not shown Electro-dialysis membranes and metallic membranes for hydrogen. 

The rate of dissociation increases rapidly above 2000°C. It also increases with decreasing pressure.P" ] The rate of recombination (i.e., the formation of the molecule) is rapid since the mean-free-path dependent half-life of atomic hydrogen is only 0.3 sec. 

Device quality a-Si H made by HWCVD (as they termed it) was first reported by Mahan et al. [19, 527], They obtained n-Si H with hydrogen concentrations as low as 1%. Deposition rates as high as 5 nm/s [528] and 7 nm/s [529] have been achieved for n-Si H of high quality. In order to obtain device quality material it was shown by Doyle et al. [525] that the radicals that are generated at the filament (atomic Si and atomic H) must react in the gas phase to yield a precursor with high surface mobility. Hence, the mean free path of silane molecules should be smaller than the distance between filament and substrate, d(s- Too many reactions between radicals and silane molecules, however, result in worse material. In fact, optimal film properties are found for values of pdf of about 0.06 mbar-cm [530, 531]. 

The thermosphere is the thin outer layer of our atmosphere extending from the mesopause near 80 km. altitude out to the exosphere, some several thousand kin. altitude, where the mean free path is sufficiently long to allow escape of atomic hydrogen and helium and atmospheric capture of coronal gas constituents. In the lower thermosphere, heated by solar ultraviolet and x-radiation, the temperature increases rapidly with altitude, with temperatures above 400 km. varying between about 700° and 2100°K., depending on solar activity. 

A) Cross-sections for scattering of photoelectrons by a hydrogen molecule according to Hoffman et al. (1982) (B) mean free path of photoelectrons in hydrogen at 300 K, pressure = 1 mbar, calculated by using the cross-sections of panel (A). 

Comment Under conditions of extremely low pressure prevailing in the intergalatic space, the mean free paths of atoms are tremendously large so that it takes 1,38,000 years for a hydrogen atom to collide with another hydrogen atom ... 

Compute the mean free path of a hydrogen molecule in hydrogen at 0°C, using simple theory and then using a maxwellian velocity distribution. 

To model experimentally the formation and properties of solid particles that form in the outflows of stars under realistic conditions appropriate to a circumstellar outflow, we would need a system large enough that the walls of the chamber are unimportant and the chamber itself needs to be pumped down to pressures that are several orders of magnitude less than the partial pressure of the least abundant reactant. In a typical stellar outflow SiO is present at 10-6 the abundance of hydrogen, while hydrogen is present at 1010 particles per cm3. In practical terms, this means that the experimental system must be capable of achieving a vacuum better than 10-15 atm and operate at about 10-9 atm or less. These conditions are barely achievable in terrestrial laboratories. However, for wall effects to be unimportant, the chamber radius must be several times the mean free path of the gas at 10-9 atm the mean free path is 100m. 

This equation shows that the tunnel effect is possible. In addition, the emitted carrier has the energy of the order of e E(IX, where X is the mean free path. This energy is a little less than 1 eV, and the positive hole with this energy can react with water to yield H+, and evolve oxygen, after Eq. 6. Two protons react with the two trapped electrons at the micro cracks to evolve hydrogen. 

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