ELECTRON-SPIN-RESONANCE (ESR) STUDIES OF ADSORBATE DYNAMICS ON SINGLE-CRYSTAL SURFACES - POSSIBILITIES AND LIMITATIONS

ESR is not one of the standard surface science tools because it is not inherently surface sensitive but rather probes the entire sample. However, ESR can be made into a surface science tool if we use paramagnetic adsorbates on thin nonmagnetic transition metal substrates or transition metal oxide films interacting with paramagnetic or nonmagnetic adsorbates. In contrast to NMR, ESR exhibits a sensitivity which allows one to detect monolayer and submonolayer coverages of spins (> 10(12) spins). The experiments can be carried out either in ultrahigh vacuum (UHV) or under an ambient gas atmosphere. - ESR provides us with the unique opportunity to study adsorbate dynamics by studying line shape changes as a function of temperature. ESR is also sensitive to molecular orientation so that the orientation distribution of the molecular planes may be derived experimentally. Thus ESR allows us to derive information not easily accessible with other techniques. We describe the set up of a UHV/ESR experiment in which the ESR measurement may be combined with LEED/Auger and TDS experiments. Several types of results are discussed: 1. ESR of chemisorbed and physisorbed molecules on metal surfaces including angle dependent measurements. 2. ESR of molecules on oxide surfaces at submonolayers coverages. 3. We will touch on possible future experiments involving pulsed ESR techniques to study dynamics in the adsorbate for a wide variety of processes including rotation and diffusion.