Auger and electron energy loss spectroscopy of oxygen chemisorption on tin

Abstract

Studies of the room temperature chemisorption of O 2 on Sn surfaces using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) are reported. AES and EELS spectra are presented for clean Sn, SnO, SnO 2 and Sn with exposure to O 2. Previously unreported fine structure in the MNN Auger spectrum of Sn are assigned to plasmon loss peaks or associated with multiplet structure of the final two-hole 4d 8 state. The MNN Auger lines in tin oxide are chemically shifted 5.5 eV down in energy with respect to those of Sn and contained no fine structure. The Auger spectrum of Sn with exposure to O 2 at 5 × 10 −6 Torr appears to be a composite of emission from clean Sn and tin oxide. Because the Auger spectra from SnO and SnO 2 were found to be nearly identical, the Auger data from Sn + O 2 could not be used to determine which oxide - SnO or SnO 2 - was present following chemisorption. This difficulty was overcome by comparing instead EELS data obtained from Sn + O 2 with EELS spectra characteristic of SnO and SnO 2. Analysis of the present EELS and AES data and consideration of available photoemission data on the Sn/O system lead to the following as being the most likely conclusion: under the conditions of O 2 chemisorption used in the present experiment, both SnO and SnO 2 are present at the surface with SnO being most probably located over the SnO 2. The present data also lend support to the possibility, suggested by earlier photoemission measurements, that a metal-rich surface is retained during these chemisorption experiments. The exact distribution of metallic Sn in the surface region remains an open question.