16. GASES AND SOLIDS 703. Molecular beam scattering from single crystal ultrahigh vacuum conditions. (USA)
Studies of the angular distribution of scattered atomic and molecular beams from single crystal surfaces give detailed information about the gas-solid interaction. An ultrahigh vacuum system combining molecular beam scattering with in situ observation of the same crystal surface by low-energy electron diffraction is described. A rotating quadrupole mass spectrometer and lock-in detection of the modulated (150 Hz) incident beam provide density profiles of the scattered gases. The scattering of thermal energy helium and oxygen beams from the (100) face of a platinum single crystal is given to demonstrate the types of surface studies that may be carried out in such an apparatus. Data are presented showing the effects of CO adsorption and surface roughness on the scattered beam intensity, and the scattering of various molecular beams from clean and contaminated surfaces. L A West et al, J Vat Sci Technol, 8 (2), March/April
of 1 MeV He+ on SIC.
(Germany) The investigation of single crystals by backscattering of He+ ions in the MeV range provides valuable additional data about the crystal structure with a depth resolution down to about 100 A, at depths up to a few microns below the surface. The energy analysis of backscattered ions furthermore allows one to identify the mass of the target atom, which is important in ion implantation studies, and to select a certain depth in the crystal for analysis simply by evaluating corresponding portions of the energy spectrum. In this way it is possible to obtain information about the structure down to thicknesses of a few unit cells. Backscattering measurements of 1 MeV AHe+ ions on SIC were performed and compared to similar measurements on a diamond-type lattice of Si. M A Nicolet et al, Phys Staf Sol (a), 3 (4), Dee 1970, 1019- 1032. 705. Studies on radiative emission electron excitation. (Germany)
16 of ruby crystals under photon and
A study is made of the interaction of electrons and photons with ruby. The experimental apparatus for study of the evolution of luminescence by electron bombardment contains an electron gun, a high vacuum chamber with pressure below lo-’ torr, a crystal holder and a camera for analysis of the light emission. All the results show the important influence of the temperature and incident energy density on the light emission of ruby samples and lead to some possible hypotheses of the observed effects. (France) R Gauthier and P Pinard, Phys Sfaf Sol (a), 3 (4), Dee 1970, 995-1005. 766. Neon slow particle luminescence arising from bombardment carbon film with a fast neon ion beam. (USSR)
16 of a
Luminescence of slow neon particles arising from bombardment of a carbon film by a fast neon ion beam with a current density of 30 /LA/cm2 and an ion energy of 20 keV is investigated. The carbon film on a polycrystalline copper substrate is formed by dissociation of hydrocarbon molecules adsorbed on the target surface. V V Gritsyna Russian).
et al, Optika
29 (4), Or? 1970, 641-643
patterns based on low-energy ions.
(Germany) An autoradiographic method for recording the channelling patterns of low-energy ions is described. It is based on channel penetration of low-energy radioactive inert gas ions using a bombardment technique with a divergent ion beam. This bombardment technique enables channelling patterns to be resolved both in azimuthal and radial directions. The target chamber used for bombardment with a divergent ion beam is described. A parallel ion beam of radioactive inert gas ions is produced by a hot cathode Penning type ion source. The divergence angle of the ion beam in the target chamber can be varied from 0 to 42 grad. Using this technique, the chemically polished silicon wafers with surfaces parallel to (11 l), (110) and (100) planes were bombarded with 10 keV inert gas ions labelled with Kr 85 or Xe 133. Thereafter, a layer of the crystal material was removed using an amorphisation-dissolution technique. A good resolution of individual channelling directions and planes exhibiting enhanced penetration of low-energy ions was achieved. The potential of the inert
gas diffusion phenomena for revealing the channelling discussed. (Czechoslovakia) C Jech, Phys Stat Sol (a), 3 (3), Nov 1970, 579-583.
16 708. Voids in nickel after electron irradiation. (Germany) Nickel (001) oriented samples, made of pure material, annealed in high vacuum and electro-polished, were irradiated with electrons and observed in a 650 kV electron microscope. It is concluded that observed defects after electron irradiation are voids which may contain small amounts of gaseous impurities. K Urban,Phys Sfat Sol(a), 3 (3), Nov 1970, K167-K168. 16 709. Adsorption and desorption of atomic hydrogen on polycrystalline iron. (Germany) Using desorption spectrometry, the sorption properties of atomic hydrogen on polycrystalline iron were investigated. The glass experimental ultrahigh vacuum apparatus was evacuated by mercury diffusion pumps. An omegatron residual gas analyzer is used for measurement of the desorption rate. At the beginning of measurement, the pressure was always below 5 x lo-” torr. Atomic hydrogen is produced on a heated tungsten filament. The sample was made of rolled iron sheet containing 0.3 per cent of Mn, 0.08 per cent of Cu, 0.09 per cent of C and 0.02 per cent of other impurities as Si, P and S. This iron is currently used for the construction of vacuum systems. The sample was heated in ultrahigh vacuum before measurements. The sticking probability of hydrogen atom on polycrystalhne iron is about unity up to coverage of 5 x 1Ol3atoms/cm2. The decrease in the effective sticking probability observed at room temperature is explained by desorption phenomena. Desorption of this phase is of the second order with an activation energy of 0.51 eV and a preexponential factor of 2.1 x 1Oes cm*/s atom. It is found that at 250°K and 5 x lo-’ torr, the sojourn probability of hydrogen molecules on polycrystalline iron is smaller than 10e4. K D Brzoska, Exper TechPhys, 18 (l-2), 1970,3543 (in Germn). 710. Formation of positive ions by bombardment layers on metals by keV electrons. (Germany)
Positive ions with a yield of the order of 10v4 ions per electron are produced on bombardment of a nickel surface by electrons with energy in the range 5 to 25 keV. The yield is dependent on the residual gas pressure in the range of 10m9to 10e4 torr and on the degassing state of the target. It is found that ions are released from adsorbed gas layers. The yield of ion emission increases when the residual gas pressure rises above lo-’ torr. With degassed electrodes and pressures below lo-’ torr, the yield is not constant over a period of time, due to the bombardment phenomena. The experimental arrangement is described. A metal ultrahigh vacuum system evacuated by an iongetter pump is used. The pressure before measurement was maintained below 10vB torr. Ion formation by bombardment of keV electrons is important in field-electron emission and vacuum breakdown initiation. The role of gas coverage of the anode on the ion yield on the anode in the vacuum breakdown initiation mechanism is discussed. B Juttner, Exper Tech Phys, 18 (l/2), 1970, 4548 (in German). 16 711. Thermal desorption of carbon monoxide from metals. (Germany) The system Ni-SiO,/CO was investigated by thermal desorption analysis. It was possible to distinguish between a weakly bonded carbon monoxide species, desorbed between - 150 and lOO”C, and a more strongly bonded carbon monoxide species, which is desorbed between 100 and 300°C. There are also desorption effects at 460°C. The samples contain 80 per cent of silicon dioxide and 20 per cent of finely dispersed nickel. The desorption products were analyzed with a quadrupole mass spectrometer. The partial pressures during thermal desorption were measured with an omegatron. Experiments were performed in a glass ultrahigh vacuum system. It is found that CO which desorbs between 100 and 300°C originates in strongly bonded CO groups with linear or bridge grouping to nickel. Carbon monoxide which is desorbed between -150 and 100°C originates in weakly bonded CO groups. These groups are sorbed only in the case when all places with strongly bonded CO group are covered. It is probable that weakly bonded CO groups form a second layer on the strongly bonded first layer. It is supposed that CO which is desorbed between 400 and 500°C originates from the recombination of C and 0 atoms 351