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5268. Free-electron Čerenkov laser
Classified abstracts 5262-5271 small clusters were measured at 7 K. On copper clusters as well as on nickel films we have found a surface N2 species which has experiericed a rigid shift of about I eV to smaller binding energies and which weinterpret in essence as physisorbed Nz. From the relative intensities olhhe 4¢r, In and 5~r levels and from a weakly indicated split of the In level into two components separated by ~<0.5eV, tentative conclusion of a non-random, probably side-on alignment of the N2 molecules has been drawn. (West Germany) D SchmeilSer et al, Vacuum, 31 (10-12), 1981, 439-442. 11 5262. Surface electronic structure calculations for Na and CI adsorbates on Ca. (GB) The experimentally well-studied adsorption systems of alkali and halogen overlayers on copper provide examples to demonstrate that ab initio calculations in an atomic-orbital basis can produce a remarkably accurate representation of surface electronic structure. The results for clean Cu are consistent with the bulk Cu band structure and the known properties of the (111) and (001) surfaces (including the position and effective mass of prominent surface states). The extremely narrow and intense photoemission peak observed at Er for adsorbed Na on Cu(l 11) is reproduced in the calculations. Calculations of the electronic structure of CI/Cu(001) can be used to discriminate between contending adsorption sites from the electron distribution curves in normal photoemission previously reported by Westphal and Goldmann. We compare the experimental results with theoretical predictions for four possible adsorption sites in the same (x/2 x x/2) R45" overlayer lattice: (i) directly above surface Cu atoms, (ii) two-fold-coordinated bridge positions, (iii) four-fold-coordinated sites above the surface, and (iv) alternate Cu sites in a reconstructed surface layer. Only sites (iii) or (iv) would be consistent with the normal-emission data; angle-resolved data for off-normal emission strongly favour the four-fold sites above the surface. D Bullet, Vacuum, 31 (10-12), 1981, 445--448. 11 5263. The adsorption of thiophene on a C a ( I l l ) surface. (GB) The interaction of thiophene with a Cu (111 ) surface, at room temperatu re, is investigated by angle-resolved photoemission. Adsorbate induced features appear only after relatively large exposures, ca 10 -2 torr s and appear to correspond to associatively adsorbed thiophene. From the energies of the adsorbate induced features and their dependence on photon incidence angle and polar electron emission angle, it is suggested that thiophene adsorbs on this face of copper with the aromatic ring parallel to the metal surface. N V Tichardson and J C Campozano, Vacuum, 31 (10-12), 198 I, 449-451. 11 5264. Tilted CO chemisorbed on Pt {110}. (GB) A variety of techniques, including LEED, AES, thermal desorption (for surface characterization and comparison with previous work), high resolution electron energy loss spectroscopy (HREELS), and angleresolved ultraviolet photoelectron spectroscopy (ARUPS) (for local structure determination) have been used to study CO on Pt {110}. Both the Pt{ll0} (1 x I)-CO and the Pt{ll0}(2 x 1 plgl-CO structures were examined. The HREELS stucty revealed absorption bands for both structures at 476 cm - t (Pt-CO stretch) and at 2097 cm -1 (C-O) stretch), assigned to chemisorbed bonded end-on to a single Pt atom. ARUPS results show an unusually clear distinction between 4¢7, In and 5~-d orbitals in the chemisorbed complex, and polar plots obtained with the analyser motion in the orthogonal plane to the incidence plane reveal that the C - O axis is strongly tilted away from the surface normal. S R Bare et al, Vacuum,31 (10-12), 1981, 463--465.
12. GASEOUS ELECTRONICS 12 5265. Voltage-current relationship for pulsed arc discharges. (USA) A theoretical treatment of the electrical behaviour of pulsed discharges in gas-filled flashlamps is presented. The theory is based on a simple model of the plasma and a balance of electrical power input and radiation output. It is assumed that the plasma radiates as a greybody with an emmissivity that depends on the electron density. The principal result is an analytic expression for the arc voltage as a function of current, with initial voltage, arc length and diameter, and atom density as parameters. The expression describes the entire voltage pulse, including the initial fall, the following minimum and the peak. The predicted voltages substantially agree with measurements, including dependences on lamp length and diameter and 760
intitial voltage. The theory is an improvement on the empirical V-I relation commonly used in flashlamp circuit analyses. P J Walsh et al, J Appl Phys, 52 (9), 1981, 5476-5482. 12 5266. Role of the anode area in the behavior of magnetic multipole discharges. (USA) A theoretical model and experimental measurements are presented for a hydrogen discharge in a magnetic multipole (bucket) plasma source where the plasma potential is negative with respect to the anode. This plasma potential depends critically on the anode cusp field, and the measurements show clearly that electrostatic ion confinement is obtained when the anode field is strong (1400 G), which leads to a high electrical efficiency for useful ion production. However, when the anode field is reduced, the plasma potential reverts to its normal positive value with respect to the anode with a consequent reduction in electrical efficiency. (GB) A J T Holmes, Ray Sci lnstrum, 52 (12), 1981, 1814-1823. 12 5267. Vacuum ultraviolet H 2 laser excited by a traveling-wave discharge. (USA) A vacuum ultraviolet molecular hydrogen laser was pumped by a travelling-wave discharge in a novel slotted Blumlcin transmission line. Reported herein is the design of the laser system and the results of a measurement of the speed of the travelling-wave discharge in a VUV H 2 laser, (25.2 + 1.8 cm/ns). Also described is the experimental technique used to measure the speed of the breakdown propagation. J Kirkland et al, Rev Sci lnstrum, 52 (9), 1981, 1338-1341. 12 5268. Free-electron Cerenkov laser. (USA) Through the S matrix approach, a model of the free-electron Cerenkov laser is formulated, here the classical electron current in the field of the wiggler magnet interacts with a quantized radiation field in a refractive medium of finite extent. The S matrix yields the radiated multiphoton distribution function where initially an arbitrary number of photons is present in the interaction volume. From the distribution function, one readily obtains the mean photon multiplicity, which in turn allows us to evaluate the gain at a given frequency. The imposition of the quantum recoil on the calculations affects the gain inasmuch as it now can be evaluated in principle for small and large photon densities. The presence of the refractive medium can enhance the calculated gain in most of the radiation frequencies as compared with a vacuum. Josip ,~alon, J Appl Phys, 52 (11), 1981, 6882-6887. 12 5269. Response of the electric field in a positive column in helium to a current perturbation. (USA) The response of the electric field in a positive column of a hot cathode discharge in helium to a current perturbation has been measured over a wide pressure range. The relaxation of the electric field, electron density, and drift velocity proceed via a two time-constant process which can be interpreted as being due to direct electron impact ionization of helium atoms in the ground state and the production and subsequent ionization of metastable helium atoms in the 23S state. S Ault et al, J Appl Phys, 52 (9), 1981, 5483-5488.
II. Vacuum apparatus and auxiliaries 21. PUMPS AND P U M P FLUIDS 21 5270. A simple automatic power-supply for titanium subfimatiou pump. (GB) A simple automatic power-supply using a bidirectional thyristor and a phase-control circuit was developed to operate a titanium sublimation pump (TSP) in a long-term performance without any troublesome adjustment. It can operate TSP either at constant voltage or at constant current to the filament. To avoid the fracture of Ti filament due to the thermal stress or the structural change of Ti, it can also keep the filament at any elevated temperature during the off-cycle of TSP. (Japan) Tetsuo Horiguchi and Nolmo Fukoka, J Phys E: Sci Instrum, 15 (1), 1982, 71-73. 22. GAUGES AND MEASUREMENT OF LOW PRESSURES 22 5271. New method for the calibration of vacuum gauges. (USA) A method for determining vacuum ionization gauge coefficients is described. It uses a molecular beam collision technique. (France)
12. GASEOUS ELECTRONICS 12 5265. Voltage-current relationship for pulsed arc discharges. (USA) A theoretical treatment of the electrical behaviour of pulsed discharges in gas-filled flashlamps is presented. The theory is based on a simple model of the plasma and a balance of electrical power input and radiation output. It is assumed that the plasma radiates as a greybody with an emmissivity that depends on the electron density. The principal result is an analytic expression for the arc voltage as a function of current, with initial voltage, arc length and diameter, and atom density as parameters. The expression describes the entire voltage pulse, including the initial fall, the following minimum and the peak. The predicted voltages substantially agree with measurements, including dependences on lamp length and diameter and 760
intitial voltage. The theory is an improvement on the empirical V-I relation commonly used in flashlamp circuit analyses. P J Walsh et al, J Appl Phys, 52 (9), 1981, 5476-5482. 12 5266. Role of the anode area in the behavior of magnetic multipole discharges. (USA) A theoretical model and experimental measurements are presented for a hydrogen discharge in a magnetic multipole (bucket) plasma source where the plasma potential is negative with respect to the anode. This plasma potential depends critically on the anode cusp field, and the measurements show clearly that electrostatic ion confinement is obtained when the anode field is strong (1400 G), which leads to a high electrical efficiency for useful ion production. However, when the anode field is reduced, the plasma potential reverts to its normal positive value with respect to the anode with a consequent reduction in electrical efficiency. (GB) A J T Holmes, Ray Sci lnstrum, 52 (12), 1981, 1814-1823. 12 5267. Vacuum ultraviolet H 2 laser excited by a traveling-wave discharge. (USA) A vacuum ultraviolet molecular hydrogen laser was pumped by a travelling-wave discharge in a novel slotted Blumlcin transmission line. Reported herein is the design of the laser system and the results of a measurement of the speed of the travelling-wave discharge in a VUV H 2 laser, (25.2 + 1.8 cm/ns). Also described is the experimental technique used to measure the speed of the breakdown propagation. J Kirkland et al, Rev Sci lnstrum, 52 (9), 1981, 1338-1341. 12 5268. Free-electron Cerenkov laser. (USA) Through the S matrix approach, a model of the free-electron Cerenkov laser is formulated, here the classical electron current in the field of the wiggler magnet interacts with a quantized radiation field in a refractive medium of finite extent. The S matrix yields the radiated multiphoton distribution function where initially an arbitrary number of photons is present in the interaction volume. From the distribution function, one readily obtains the mean photon multiplicity, which in turn allows us to evaluate the gain at a given frequency. The imposition of the quantum recoil on the calculations affects the gain inasmuch as it now can be evaluated in principle for small and large photon densities. The presence of the refractive medium can enhance the calculated gain in most of the radiation frequencies as compared with a vacuum. Josip ,~alon, J Appl Phys, 52 (11), 1981, 6882-6887. 12 5269. Response of the electric field in a positive column in helium to a current perturbation. (USA) The response of the electric field in a positive column of a hot cathode discharge in helium to a current perturbation has been measured over a wide pressure range. The relaxation of the electric field, electron density, and drift velocity proceed via a two time-constant process which can be interpreted as being due to direct electron impact ionization of helium atoms in the ground state and the production and subsequent ionization of metastable helium atoms in the 23S state. S Ault et al, J Appl Phys, 52 (9), 1981, 5483-5488.
II. Vacuum apparatus and auxiliaries 21. PUMPS AND P U M P FLUIDS 21 5270. A simple automatic power-supply for titanium subfimatiou pump. (GB) A simple automatic power-supply using a bidirectional thyristor and a phase-control circuit was developed to operate a titanium sublimation pump (TSP) in a long-term performance without any troublesome adjustment. It can operate TSP either at constant voltage or at constant current to the filament. To avoid the fracture of Ti filament due to the thermal stress or the structural change of Ti, it can also keep the filament at any elevated temperature during the off-cycle of TSP. (Japan) Tetsuo Horiguchi and Nolmo Fukoka, J Phys E: Sci Instrum, 15 (1), 1982, 71-73. 22. GAUGES AND MEASUREMENT OF LOW PRESSURES 22 5271. New method for the calibration of vacuum gauges. (USA) A method for determining vacuum ionization gauge coefficients is described. It uses a molecular beam collision technique. (France)