High-resolution photoemission from a tunable quantum well: Cu(111)/Na

Abstract

High-resolution photoemission (⌬Eϭ5 meV) at a low photon energy ͑3.82 eV͒ is used to probe discrete quantum-well-type subband states near, below, and above E F for Cu͑111͒ covered with 2 ML or less of Na. A subband characteristic of the monolayer range shifts gradually to lower energy as the coverage is increased, extending below the Fermi level for coverages above 0.85 ML. Combined with previous observations of shifts for filled and empty states the present results show that the Na monolayer has continuously tunable quantumwell state energies. Beyond the monolayer range quantum-well states characteristic of both one and two atomic layers are observed, indicating growth of the second layer via monolayer high islands. A small downshift, by 25 meV, with increasing coverage in the second layer is ascribed to an increase of the island size. Lorentzian photoemission line shapes are observed for well-ordered samples. The linewidth varies linearly with temperature in the probed range ͑130-295 K͒ and this is ascribed to the phonon contribution to the width. Structural disorder leads to an asymmetric line, which is Lorentzian on the steeper, low-kinetic-energy side.

Decay of electronic excitations at metal surfaces

“…Intriguingly, the binding energies of the QWS are strongly affected by the Na coverage [151]. For example, for coverages of 1 and 0.95 monolayers, binding energies of À127 and À42 meV with respect to the Fermi level were observed with scanning tunneling spectroscopy [181] confirming previous data from photoelectron spectroscopy [151] which yielded values of approximately À100 and À75 meV. For the lanthanides Lu, Ho and Gd, Bauer et al observed nearly symmetric spectral lines (rather than bands with a sharp onset), presumably due to the large effective mass of the surface states on these surfaces [195].…”

“…2 For Na overlayers on Cu(1 1 1) STS measurements have been performed for energies below as well as above E F [181]. Intriguingly, the binding energies of the QWS are strongly affected by the Na coverage [151]. For example, for coverages of 1 and 0.95 monolayers, binding energies of À127 and À42 meV with respect to the Fermi level were observed with scanning tunneling spectroscopy [181] confirming previous data from photoelectron spectroscopy [151] which yielded values of approximately À100 and À75 meV.…”

“…These states can appear in systems formed by ultrathin metal films adsorbed on metal surfaces [151][152][153]. If the substrate has a local band gap in the direction normal to the surface the electron can be trapped in quantum well states.…”

“…The electron-electron contribution, G ee , to G has been calculated using the model potential proposed in Ref. [57] and the electron-phonon contribution G ep has been calculated Table 11 Calculated bulk, vacuum and mixed terms to the decay rate G ee of n ¼ 1 and 2 image-potential states, for 0-2 ML of Ar/ Cu(1 0 0) by applying the Debye and Einstein model combined with the experimental value l ¼ 0:24 determined by Carlsson et al [151] for 0.95 ML Na/Cu (1 1 1). The value of the Debye energy, ho D ¼ 18 meV, has been deduced from helium-atom scattering for 1 ML of Na on Cu(1 0 0) [319] and corresponds to the lower value obtained in a first-principles calculation [320].…”

“…Various techniques have been used to measure binding energies and dispersion of QWS. Angle-resolved photoemission spectroscopy was used to study QWS in alkali-metal overlayers on different metals [151,156,157], in silver overlayers on Fe(1 0 0) [152,158,159] and on V(1 0 0) [153,160,161]. Unoccupied QWS in overlayers have been studied with inverse photoemission [162][163][164][165][166][167][168][169][170][171] and with 2PPE spectroscopies [4,[172][173][174][175].…”