Width of particle beams desorbed in electron stimulated desorption: O+ and metastable CO from CO/Pt(111)

Abstract

The role of adsorbate vibrations parallel to the surface on the width of particle beams escaping from the surface in electron stimulated desorption (ESD) has been studied. Chemisorbed CO on Pt(111) was employed as a model system. The angular distribution of O+ and of the metastable a 3π–CO (CO*) species desorbed in ESD was measured. A cluster calculation was used to model the CO vibration on Pt(111), from which it was possible to calculate the CO* and O+ ESDIAD (electron stimulated desorption-ion angular distribution) beam widths. For CO* we were able to calculate the width of the ESDIAD beams for θCO=0.15 ML with an accuracy better than the error bar of the measurement in the temperature range of 100–300 K. This calculation shows that the CO* angular distribution is related to both the bond direction and the parallel momentum of the surface oscillator at the time of excitation. For θCO=0.5 ML, the width of the measured and calculated values are in good agreement in the temperature range of 100–230 K; above 230 K the experimental results show a positive deviation from the calculated values, indicating that an extra broadening mechanism occurs above ∼230 K, probably because of the onset of an order–disorder transition. For the O+ ESDIAD beam widths the experimental results show a somewhat sharper distribution than the calculated values, most likely because of the angularly anisotropic neutralization rate that was not quantitatively considered in the calculation.