Resonance of Chemisorbed Alkali Atoms on Noble Metals

A.G. Borisov,* V. Sametoglu, A. Winkelmann, A. Kubo, N. Pontius, J. Zhao, V.M. Silkin, J. P. Gauyacq, E. V. Chulkov, P.M. Echenique, and H. Petek Laboratoire des Collisions Atomiques et Moléculaires, UMR CNRS-Université Paris-Sud 8625, Bâtiment 351, Université Paris-Sud, 91405 Orsay CEDEX, France Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan BESSY mbh, Albert-Einstein-Strasse 15, 12489 Berlin, Germany Donostia International Physics Center DIPC, P. Manuel de Lardizabal 4, San Sebastián 20018, Spain Centro de Fı́sica de Materiales CSIC-UPV/EHU and Departamento de Fı́sica de Materiales UPV/EHU, Facultad de Quı́mica, Aptd. 1072, San Sebastián E-20080, Spain (Received 14 July 2008; published 22 December 2008)

For atoms and molecules in front of the metal surfaces, electronic states of the joint system reflect the bound state-continuum interaction between the localized molecular orbitals and the propagating states of the substrate.Resonances appear, with widths (inverse of the lifetimes) corresponding to the rates of the electron (hole) escape from the atom or molecule into the metal.A detailed understanding of this phenomenon and the ability to predict the energies and lifetimes of the quasistationary states are of paramount importance for various processes from projectile-surface charge transfer [1] and catalysis [2] to molecular electronics [3] and electron transport in tunneling junctions [4].Moreover, electron and photon induced reactions at surfaces can be promoted when the lifetimes of adsorbate-localized resonances are sufficient for the energy transfer between electrons and nuclei [5,6].
Until now the only known unoccupied spectroscopic feature of the alkali chemisorption interface has been the alkali-localized resonance appearing at $2 eV below the vacuum level in the low coverage limit [12][13][14][15][16]18,19,21,22].This resonance is oriented perpendicular to the surface, and it is formed essentially by the ns and np z valence orbitals of the alkali atom with (m ¼ 0) symmetry [16,[23][24][25].Here m is the projection of the angular momentum on the surface normal (z axis).On noble metal surfaces with a projected band gap the lifetime of the resonance can attain tens of femtoseconds [12][13][14]16,23].Thus, transient occupation of the resonance can initiate surface femtochemistry [15].It is noteworthy that an excited sp z resonance has been also reported in theoretical and scanning tunneling microscopy studies of Au and Cu adatoms on metal surfaces [26], as well as in core excited noble gas adatoms [27,28].
In this Letter we show that, in agreement with recent predictions [16], alkali atom adsorption is associated with resonant states of several symmetries and not only the one.We report a joint experimental and theoretical study of the angle-and time-resolved photoemission from alkali atom covered noble metal surfaces [Na, K and Cs on Cu (111) and on Ag(111)].In addition to the resonance we find a new alkali induced feature located at 0:3-0:7 eV higher energy.Comparison between calculated and measured data allows its unambiguous assignment to an adsorbate-localized state oriented parallel to the surface and formed by the mixing between the p and d valence orbitals of the alkali atom with (m ¼ AE1) symmetry.Long lifetimes of 20-30 fs are obtained from interferometric two-pulse correlations (I2PCs) of 2PP [29], in good agreement with theory and consistent with the resonance stabilization by the projected band gap of the substrate.
2PP measurements for alkali atom covered noble metal surfaces were performed as described previously [25].
Alkali atoms were deposited at background pressures of <4 Â 10 À10 bar from SAES getter sources onto cleaned and annealed Cu(111) or Ag(111) surfaces.The coverages where kept below 0.1 ML to insure large adsorbateadsorbate spacing and the absence of ordered structure [12,14,17,27].2PP signal was excited by $16 fs, 3.1 eV pulses from the second harmonic of a Ti:sapphire laser oscillator.The p-polarized excitation light was incident at 45 with respect to the lens axis of a hemispherical electron energy analyzer.Angle-resolved photoemission was obtained by rotating the sample with respect to the fixed laser and analyzer axes.The analyzer lens had an angular acceptance of AE1 .I2PC scans for specific photoelectron energies and angles were measured [29].
Figure 1 shows representative angle-resolved 2PP spectra.The corresponding surface-projected band structures of Cu(111) and Ag(111) as well as the energy diagram for 2PP is sketched in Fig. 2. For finite alkali coverage, the 2PP signal is dominated by resonant excitation out of occupied dispersive Shockley surface state and bulk lower sp band of the substrate to the unoccupied resonances localized on the adatom.The latter do not disperse with the electron momentum parallel to the surface.The resonance appears as a prominent feature with associated angle-resolved spectra dominated by emission perpendicular to the surface [24].
In addition, at 0.3-0.7 eV higher energy, for the K and Cs covered surfaces we observe a new nondispersive, i.e., adsorbate-localized feature.It is characterized by a distinctly different angular dependence of the electron emission, pointing at the different symmetry of the underlying state.Namely, the 2PP intensity of this new feature on Cu (111) has a minimum in the direction perpendicular to the surface whereas pronounced maxima appear at emission angles of AE17 AE 2 with respect to the surface normal.For Ag(111), the same angular dependence is observed, but the emission maxima are distorted by the participation of the adsorbate resonance as the intermediate state in the lower to upper sp-band (L sp !U sp ) two-photon transition at k k ¼ AE0:46 # A À1 .Based on the comparison with theoretical calculations presented below, we assign the new resonances to the m ¼ AE1 states with symmetry.They are formed by the hybridization of the np and ðn À 1Þd valence orbitals of chemisorbed K and Cs atoms.The resonance does not appear for Na=Agð111Þ.Its high excitation energy (as calculated here, see Table I) makes this state unobservable at this coverage with 3.1 eV photons.
In order to explain the experimental data we have performed a wave packet propagation (WPP) study of the energies, decay rates (widths) via resonant one-electron transfer into the metal, and wave functions of the quasistationary states of the alkali-adsorbate on metal-surface system.The method and its particular application for the alkali adsorbates including finite coverage effect is detailed in Refs.[23,32].
Figure 3 shows the calculated energy and width of the lowest lying resonances of and symmetries for a single K atom in front of the Cu(111) surface.Including finite but small coverage results in the shift of the resonance energy towards the Fermi level, while the width is mainly unaffected (see also Ref. [32] and below).Similar data are obtained for all alkali atoms studied here on both surfaces.The results are shown from large alkali-surface separations down to chemisorption distances.Thus, we identify the  resonance as originating from the 4s parent orbital of the free-standing alkali atom, and two degenerate resonances originating from the 4pðm ¼ AE1Þ parent orbitals oriented parallel to the surface.As the K atom approaches the surface the energies of the alkali-localized resonances are upshifted by the interaction with the image of the alkali ion core [16,20,32].Remarkably, excited states originating from the np Rydberg orbitals, have been also reported for the core excited noble gas adatoms [33,34].
The widths of the resonances first increase exponentially for decreasing atom-surface distance because of the increasing coupling with the metal.When K moves into the chemisorption distance range, however, the widths stabilize and even strongly decrease because of the combination of two effects [16,23,32]: (i) Close to the surface the 4s-4p z (-state) and 4p-3d (-state) hybridization pushes the electronic density of the hybrid states away from the surface, and the coupling with metal is reduced; (ii) as illustrated in Fig. 2, because of the projected band gap the alkali-localized states are in energy resonance with electronic states of the substrate only for finite values of k k .The resonant decay of the adsorbate-localized states via electron transfer into the substrate along the ''easiest'' direction of the surface normal is thus not possible.
A representative example of the wave function of the alkali-localized resonance is shown in Fig. 4(a) for the single Cs adsorbate on Cu(111).The resonance wave function is pushed away from the surface by the 6p-5d hybridization of the Cs valence orbitals.The node in the direction of the surface normal results from the m ¼ 1 symmetry of the state.The electron density inside the metal corresponds to the outgoing flux because of the decay of the quasistationary state.In Fig. 4(b) we show the calculated angular distribution of the electrons photo emitted from the resonant state by the ''probe'' pulse of 3.1 eV polarized along the surface normal.We have explicitly checked that results are robust with respect to inclusion of the Fresnel factors and the finite (small) coverage.In agreement with experimental data, the distribution is characterized by a maximum at 21 and no emission in the direction of the surface normal because of the symmetry of the resonance.
Theoretical results are summarized in Table I.For all studied systems, alkali atom adsorption induces a long-lived adsorbate-localized excited electronic state of -symmetry oriented parallel to the surface.This state coexists with the well-documented unoccupied resonance of symmetry.The latter shows a nearly adsorbateindependent energy of the order of À2 eV with respect to the vacuum level as calculated here (see also [16,25]).Therefore, calculations predict the resonance to be located typically 0.8-1.1 eV above the resonance so that, e.g., for the Cs=Cuð111Þ case the 6s=6p energy separation of 1.46 eV for the free atom is reduced to ÁE = ¼ 0:8 eV   at the surface.The calculated ÁE = is larger than experimentally observed values of 0.3-0.7 eV.We attribute this difference in part to the effect of the finite adsorbate coverage used in the experiments.The adsorbate dipole layer induces a shift of the resonance energy towards the Fermi level [22,25,32].In particular, this enables the observation of both and resonances for K and Cs with present 3.1 eV photons.The energy shift appears different for and states.For Cs=Cuð111Þ, using model approach of Ref. [32], we calculate ÁE = ¼ 0:66 eV at the coverage corresponding to the work function reduction by 1.6 eV.The corresponding experimental value is 0:43 AE 0:03 eV.
Besides the electrostatic effect, interaction between orbitals located at neighboring adsorbates cannot be excluded.
The lifetime of the resonance has been evaluated as: Here, À 1 " e is the present WPP one-electron resonant decay rate.À ee is the multielectron decay rate resulting from the inelastic interactions between adsorbate-localized and substrate electrons.Based on the previous studies for the state it has been estimated at %20 meV [23].The obtained lifetime of the resonance is in the 10-30 fs range in agreement with the present experimental measurements.Thus, the resonance has a lifetime comparable to that of the resonance [12][13][14][15][16].
In conclusion, combining experiment and theory, we could observe and describe a new alkali-adsorbate induced feature in the unoccupied part of the electronic density of states at chemisorption interface.For alkali atom adsorption on (111) faces of Cu and Ag, along with the well studied resonance, an alkali-localized state of symmetry has been found.Results reported here should be of general nature for the substrates presenting a projected band gap and adsorbates with strong electron donation to the surface.Finally, the discovery of a new type of longlived resonances oriented parallel to the surface and thus responding differently to an external excitation, as compared to the previously known resonance, extends the possibilities to induce and control various processes involving excited electrons.
This work was supported by the U.S. Department of Energy Grant No. DE-FG02-03ER15434, Petroleum Research Fund (44158-ACS), Spanish MEC grant, the European FP6-NoE grant Nanoquanta.H. P. and V. M. S. acknowledge support from the IKERBASQUE foundation.

FIG. 1 (
FIG. 1 (color online).Waterfall plots of 2PP spectra forNa=Agð111Þ, K=Agð111Þ, K=Cuð111Þ, and Cs=Cuð111Þ plotted against emission wave vector parallel to the surface k k .Spectroscopic features highlighted by the colored lines and labels correspond to the two-photon absorption from the lower sp band (L sp ) to the upper sp band (U sp ), two-photon absorption from the surface state (SS), and transitions through alkalilocalized m ¼ 0 () and m ¼ AE1 () intermediate states.

FIG. 2 (
FIG. 2 (color online).Energy diagram of the states and the 2PP excitation pathways (arrows).Alkali coverage corresponds to Fig. 1, where the associated work function decrease is 1.6 eV for Cu(111) [1.2 eV for Ag(111)].Energy with respect to Fermi level E F is shown as a function of the electron momentum parallel to the surface k k .E vac is the vacuum level energy.The projected band gap extends from L sp to U sp ; SS stands for the surface state.For the energetics of the clean surfaces, see Ref. [31].

FIG. 3 (
FIG. 3 (color online).(a)Energies and (b) widths of the alkalilocalized states of and symmetry for a single K atom in front of Cu(111).The energy is given with respect to the vacuum level.Results are shown as a function of alkali distance from the image plane[30].The labels reflect the symmetry of the state and the parent orbital of the free-standing alkali atom.

FIG. 4 (
FIG. 4 (color online).(a) Logarithm of the electron density of the quasistationary state for the Cs=Cuð111Þ system.Single adsorbate case.Origin of cylindrical coordinates (, z) is on the adsorbate.z axis is perpendicular to the surface.The black horizontal line shows the image plane.(b) Calculated angular distribution of the electrons photo emitted from the quasistationary state shown in panel (a).Arrow labeled E shows the polarization of the laser pulse.

TABLE I .
[23,25,30] theoretical results for the resonance.Vanishing coverage limit.The adsorption distance Z ads (atomic units) is given with respect to the image plane[23,25,30].E (eV), is the resonance energy with respect to the vacuum level.À 1 " e (meV) is the one-electron decay rate.(fs) is the lifetime obtained with the many-body decay taken into account (see the text).