Atomic beam diffraction and scattering from A (3±5) W (110)–carbide surface

Abstract

Atomic beams of helium, neon, and argon generated from a nozzle source have been scattered from a tungsten surface which x-ray, LEED, and Auger spectroscopic measurements show to be a (3×5) W (110)–carbide structure superimposed upon the (110) plane of tungsten and rotated 35° from the principal axes. Helium scattering from this surface at 298 K produces a prominent bimodal structure characteristic of the classical surface rainbow maxima described by McClure. Superimposed upon this structure are at least five distinct peaks or shoulders corresponding to the (2̄,0), (1̄,0), (0,0), (1,0), and (2,0) diffraction directions along the 5-spacing of the superstructure. Neon scattering is observed to produce a broad, bimodal rainbow pattern but no features that can clearly be identified as surface diffraction. As the incident neon-beam energy is increased by seeding the beam with helium, the rainbow scattering becomes more prominent and some evidence of diffraction is seen. This result suggests that the strength of the periodic interaction felt by the incident gaseous atom is responsible for the presence or absence of rainbow and diffractive scattering. Argon spatial distributions exhibit no structure and are trapping dominated. The variation of scattered intensities with surface temperature suggests that for helium and neon a significant fraction of the incident beam is preferentially scattered in directions corresponding to out-of-plane diffraction maxima. In general, the results corroborate the observation of helium diffraction from the (3×5) W (110)–carbide surface previously reported by Weinberg and Merrill. However, several significant differences between their observations and those obtained in the present work are noted and discussed.