Angle-Resolved Photoemission as a Sensitive Probe of Surface Quality: Sputtered versus Annealed Cu(001)
Angle-resolved photoemission spectroscopy (ARPES) requires a well-ordered surface, whose quality is routinely judged from low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). We compare LEED, XPS, and ARPES of a Cu(001) single crystal measured in situ at four stages of surface preparation: as-prepared, after aging for 24 h at 6 K, after Ar$^+$ sputtering, and after annealing. The sputtered and annealed surfaces show nearly identical $(1\times1)$ LEED patterns and Cu 2$p$ core levels, but the sputtered surface yields a momentum-independent ARPES spectrum, and the band dispersion recovers after annealing. We attribute this contrast to how each probe responds to sputtering-induced defects: LEED reflects the average lattice, which remains periodic, whereas ARPES reflects the electronic states, whose crystal momentum is no longer well defined. Our results indicate that a sharp LEED pattern and clean core-level spectra do not guarantee a surface suitable for ARPES.