PHYSICAL REVIEW B VOLUME 48, NUMBER 7 15 AUGUST 1993-I Structural determination of NH3 adsorbed on Ni(100) using angle-resolved photoemission extended fine-structure spectroscopy Yu Zheng Departments of Chemistry and Physics, Pennsylvania State Uniuersity, University Park, Pennsyluania 16802 and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 E. Moler and E. Hudson Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 Z. Hussain Accelerator and Fusion Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 D. A. Shirley Departments of Chemistry and Physics, Pennsylvania State Uniuersity, Uniuersity Park, Pennsylvania 16802 (Received 22 March 1993; revised manuscript received 17 May 1993) The structure of NH3 adsorbed on the Ni(100) surface was studied using the angle-resolved photoemis- sion extended fine-structure (ARPEFS) technique. The ARPEFS data were taken along the surface normal-emission direction at a sample temperature of 100 K. Multiple-scattering spherical-wave data analysis determined that NH3 adsorbs at the atop site of the Ni(100) surface with a N-Ni bond length of 0 2. 01 A and a 2. 8% expansion of the topmost Ni-Ni interlayer spacing. I. INTRODUCTION The atomic structures of ammonia (NH3) molecularly adsorbed on transition-metal surfaces are of interest in understanding both the surface chemical bond and the formation and decomposition of NH3. First, the adsorp- tion of NH3 on transition-metal surfaces represents a typ- ical surface chemical bonding based on the interaction between the lone pair of electrons in the molecules and the valence electrons of transition metals. Information about the structures of NH3 adsorbed on transition-metal surfaces is essential for understanding such surface chem- ical bonding. Second, when transition metals are used as catalysts for the formation and decomposition of NH3, they show very different activities. To understand the physical basis of these differences requires knowledge of the structures of NH3 adsorbed on transition-metal sur- faces. In the past few years, there have been several pub- lished reports of structural studies of NH3 adsorbed on the (111) surfaces of transition metals. ' s For example, the structure of NH3 adsorbed on the Ni(111) surface has been extensively investigated both experimentally and theoretically. ' lt has been found that NH3 adsorbs on the atop site of Ni(111), with its threefold rotational axis normal to the surface, three H atoms pointinp away from the surface, and a N-Ni bond length of 1. 97 A. However, there have been only a few structural studies of NH3 ad- sorbed on the (100) and (110) surfaces of transition met- 8 — 10 In this paper, we report an angle-resolved photoemis- sion extended fine-structure (ARPEFS) structural deter- mination of NH3 adsorbed on the Ni(100) surface. The ARPEFS method, which is based on the oscillatory varia- tion in the angle-resolved photoemission intensity from the core levels of adsorbates on substrate surfaces as a function of the photoelectron kinetic energy over a wide energy range, is a powerful technique for studying the atomic structure of adsorbed surfaces. " Unlike low- energy electron diffraction (LEED), it allows qualitative data analysis by Fourier transformation. This is similar to surface-extended x-ray-absorption fine structure (SEXAFS). However, Fourier transforms of ARPEFS are much more sensitive to the local geometry of ad- sorbed surfaces than those of SEXAFS, due to the high directional sensitivity of ARPEFS. This allows the deter- mination of the adsorption site from ARPEFS Fourier- transform analysis. In the ARPEFS study, the surface structure is quantitatively determined by fitting the ex- perimental data with the multiple-scattering spherical- wave (MSSW) calculations. In the past few years, ARPEFS has been successfully used to study both the lo- cal structure of adsorbed surfaces and the adsorbate- induced relaxation of the substrates. ' ' It has been shown that ARPEFS is capable of determining surface and near-surface structures of adsorbed surfaces, up to four or five substrate atomic layers. This strong depth sensitivity has recently been shown to arise largely from multiple-scattering effects. ' It has also been shown' that although atomic scattering phase shifts required for the MSSW calculations in the ARPEFS data analysis can be obtained only from theoretical calculations, the struc- ture of adsorbed surfaces can still be determined accu- rately by ARPEFS due to the weak k dependence of atomic forward-scattering factors. ' This paper is organized as follows. Section II gives the experimental details and the procedures for data collec- tion and reduction. Section III describes the details of 0163-1829/93/48(7)/4760(7)/$06. 00 48 4760 1993 The American Physical Society