On the preparation of clean tungsten single crystals

The cleaning procedure consists of two-step-flashing: (i) cycles of low power flashes T similar to 1200 K) at an oxygen partial pressure of P(o2) = 6 x 10(-8) mbar, to remove the carbon from the surface, and (ii) a single high power flash (T similar to 2200 K), to remove the oxide layer. The removal of carbon from the surface through the chemical reaction with oxygen during low power flash cycles is monitored by thermal desorption spectroscopy. The exposure to O(2) leads to the oxidation of the W surface. Using a high power flash, the volatile W-oxides and the atomic oxygen are desorbed, leaving a clean crystal surface at the end of procedure. The method may also be used for cleaning other refractory metals like Mo, Re and It. (C) 2009 Elsevier B.V. All rights reserved.

Near threshold vibrational excitation of molecules by positron impact: A projection operator approach

We report vibrational excitation (v(i) = 0 -> v(f) = 1) cross-sections for positron scattering by H(2) and model calculations for the (v(i) = 0 -> v(f) = 1) excitation of the C-C symmetric stretch mode of C(2)H(2). The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near threshold behavior of H(2) and C(2)H(2) significantly differ in the sense that no low lying singularity (either virtual or bound state) was found for the former, while a e(+)-acetylene virtual state was found at the equilibrium geometry (this virtual state becomes a bound state upon stretching the molecule). For C(2)H(2), we also performed model calculations comparing excitation cross-sections arising from virtual (-i kappa(0)) and bound (+i kappa(0)) states symmetrically located around the origin of the complex momentum plane (i.e. having the same kappa(0)). The virtual state is seen to significantly couple to vibrations, and similar cross-sections were obtained for shallow bound and virtual states. (c) 2007 Elsevier B.V. All rights reserved.