JOWM~ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Electron Spectnurcopy and Related Phenomena, 29 (1983) 265-272 Elsewer Sclentlfic Pubhshing Company, Amsterdam - Printed m The Netherlands 265 zyxwvutsr SPATIAL INTENSITY DISTRIBUTIONS FROM ELECTRON IMPACT SCATTERING MODES. W(lOO)(l x l)H SIMON R. BARE, PETER HOFMANN, MARK SURMAN AND DAVID A. KING The Donnan Laboratories,University of Liverpool, Liverpool L69 3BX, England. ABSTRACT The detailed has been examined with the detector mechanism of electron energy loss vibrational spectroscopy in the present study by recording a large data base of spectrq moved both in and out of the incidence plane, for saturation _.___ coverage of atomic H adsorbed on WI1001 at 300 K. In agreement with earlier work we find only a single electron loss (130 meV) in the specular direction, while away from the specular dlrectlon additional losses at 80 meV and 160 meV appear. These lossescorrespondto the symmetric stretching, the wagging and the asymnetric stretching modes respectivelyof H atoms occupying a C2V bridge site. Large intensity variations of all these modes are found in backscatter- ing directions and out of the incidence plane, in general agreement with recent theoretical predictions. The usefulness of these spatial intensity variations in adsorbate structure determlnatlonwill be discussed. The signal enhancement with electron collection in the plane orthogonal to the incidence plane may allow the observation of otherwise weak impact modes. The great maJor1t.y of high resolution electron energy loss studies reported to date make only partial use of this technique as a surface science tool by only recording the loss current in the specular direction. It is believed that the inelastic scattering mechanism close to the specular direction is dominated by a long range Coulomblc interactionbetween the incident electrons and the vibrating dipoles on the surface. As a consequence only those vibrational modes with a dynamic dipole moment perpendicularto the surface are observable (ref. 1). This 'surface normal dipole selection rule' obviously restricts an unambiguous identificationof the actual bonding sites and characterisationof the adsorbed species. In this respect it is of great importance to be able to observe all vibrational modes. - Away from the specular direction the inelastic electron scattering mechan- lsm IS not only due to the long range dipole component, but IS also due to impact scattering via short range interactionwith the ion-cores of the adsor- bate lattice. The short range impact scattering potential produces scattering over a wide angular range and as a consequence the incidence electron is able to penetrate into the surface region and undergoes multiple scattering between 036%2048/83/0000+J000/$03.00 0 1983 Elsevler Sclentlflc Pubhshmg Company