Role of the electric field in surface electron dynamics above the vacuum level

Scanning tunneling spectroscopy is used to study the dynamics of hot electrons trapped on a Cu(100) surface in field-emission resonances (FERs) above the vacuum level. Differential conductance maps show isotropic electron interference wave patterns around defects whenever their energy lies within a surface-projected band gap. Their Fourier analysis reveals a broad wave-vector distribution, interpreted as due to the lateral acceleration of hot electrons in the inhomogeneous tip-induced potential. A line-shape analysis of the characteristic constant-current conductance spectra permits establishing the relation between apparent width of peaks and intrinsic linewidth of FERs, as well as the identification of the different broadening mechanisms.