The UHV chamber had a base pressure of 5 ×10 -10 mbar and is equipped with a cylindrical mirror analyzer based Auger electron spectrometer (AES), a quadrupole mass spectrometer (QMS) for incident beam flux measurements and residual gas analysis. The Cu(110) surface can be heated and the surface temperature was monitored using a K-type thermocouple attached to the side of the crystal. For routine measurements, the Cu(110) surface was cleaned by ion bombardment (Ar ion at 3 keV, ion current 0.6 µA, 30 min) followed by annealing at 800 K (30 min). Surface cleanliness was checked using AES measurements.