PROBING 3 DISTINCT IODINE MONOLAYER STRUCTURES AT PT(111) BY MEANS OF ANGULAR-DISTRIBUTION AUGER MICROSCOPY - RESULTS AGREE WITH SCANNING TUNNELING MICROSCOPY

We report angular distributions of Auger electrons (507 and 518 eV) emitted from three distinct monolayer structures of iodine atoms on single-crystal platinum: Pt(111)(square-root 3 x square-root 3)R30-degrees-I, Pt(111)(square-root 7 x square-root 7)R19.1-degrees-I, and Pt(111)(3 x 3)-I. Each of these monolayers has been characterized by means of LEED, STM, AES, XPS, TDMS, and electrochemistry. Accordingly, these monolayers provide an opportunity to explore the nature of Auger electron angular distributions for surface atomic layers of known structure and to correlate the results with what is already known regarding these structures. Evident near grazing angles of emission in the measured angular distributions are intensity minima along trajectories corresponding to the I-I internuclear directions and intensity maxima along trajectories corresponding to the gaps between neighboring iodine atoms. Visible near the surface normal are features due to electrons emanating from the Pt(111) substrate. Simulations of the measured angular distributions in which iodine atoms are treated as point emitters and spherical scatterers of Auger electrons are in qualitative agreement with experiment. These results suggest that iodine atoms act predominantly to block Auger electrons in this energy range, producing minima in the angular distributions. Accordingly, the locations of the minima are a direct consequence of the relative positions of atoms, making measurements of Auger electron angular distributions useful for real-space probing of surface atomic and monolayer structure.