Lobular scattering of heavy molecules from low mass surface layers; Br 2 from a graphite covered surface

Abstract

The scattering of Br 2 molecules from thin graphite layers on platinum foils has been studied during UHV conditions with a room temperature, almost thermal Br 2 molecular beam. The angular variation of Br + 2 and Br + signals at surface temperatures of 1300–1600 K has been measured with a mass spectrometer. The scattering pattern for Br + 2 shows a main lobe at angles 30–40° from the normal, when the specular direction is at 60–70°. The best but not satisfactory fit of a hard cube model is found with a surface mass equal to the mass of four to five C atoms. This result is not easily interpreted. Since multiple collisions with several C atoms in the surface are expected, “cube” models are really not applicable in their usual form, which is only valid for single collisions with the surface. To circumvent the problem, calculations have been made with a novel phonon transfer model. Good agreement with experiments is found, assuming transfer of one single perpendicular phonon in the graphite layer to translation of the Br 2 molecule in the normal direction. The maximum phonon energy in graphite corresponds to 1900 K. This is much larger than the Debye temperature in most metals (θ D = 310 K for W), which makes it understandable that one-phonon processes can dominate in the scattering.