Initial stages of the reaction of oxygen with Si(1 0 0)

High resolution electron energy loss spectra and work function results are presented for the Si(1 0 0) surface held at 20 K during exposure to O₂ and subsequently annealed to 300 K. The loss spectra provide evidence for the presence at 20 K of physisorbed molecular and chemisorbed monatomic oxygen. When the oxygen covered surface is warmed, the physisorbed oxygen evaporates at thkap;35 K. Upon oxygen exposure at 20 K, the work function increases by ≈1.5 eV and decreases only when the surface is annealed above ≈90 K, indicating that certain metastable oxygen species besides the physisorbed molecular oxygen may exist on the surface at low temperatures.