10912
J. Phys. Chem. B 2001, 105, 10912-10917
Electronic Band Formation at Organic-Metal Interfaces: Role of Adsorbate-Surface Interaction Gregory Dutton and X.-Y. Zhu* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: July 2, 2001
We report evidence for an excited molecular anionic resonance in a model system, bilayer C6F6 adsorbed on Cu(111). This resonance is dispersed parallel to the surface, with an effective electron mass of 1.5 me. Both the energetic position and the dispersion of this molecular resonance depend on the strength of moleculesurface interaction, which is weakened by the preadsorption of atomic H. With increasing coverage of preadsorbed H from θ ) 0 to 0.34 monolayer (ML), the position of the molecular resonance increases from 2.99 to 3.22 eV above the Fermi level. This is accompanied by a decrease in effective electron mass from 1.5 me on clean Cu(111) to 0.55 me on 0.34 ML H-covered Cu(111). We attribute this to the improved moleculemolecule interaction as the molecule-surface interaction weakens.
1. Introduction Electron transfer (ET) from a metal surface to a molecular anionic resonance is a central issue in many areas of chemistry and technology. In organic light emitting devices and field-effect transistors, electron injection from metallic electrodes into the molecular layer is a critical step.1,2 In molecular electronics, the formation of a molecular anionic resonance has been argued as the key factor responsible for the recent observation of negative differential resistance.3,4 In surface photochemistry on metals, photoinduced ET to molecular resonances is believed to be a dominant mechanism.5 Similar mechanistic issues are of importance in solar energy conversion and electrochemistry.6 From a theoretical perspective, it is a great challenge to treat the coupling of a localized molecular resonance to a delocalized metal band structure. Despite their significance, direct evidence for excited molecular anionic resonances on metal surfaces is scarce, and their properties are largely unknown. Previous experiments have relied mainly on inverse photoemission (IPE),7 electron stimulated desorption (ESD),8 and two-photon photoemission (2PPE).,9-11 IPE can probe the energetic position of unoccupied states; beyond energetics, its capability is limited. ESD and related experiments on surface photochemistry5 are indirect approaches. In contrast, 2PPE is capable of directly probing excited electronic states on metal surfaces in energy, time, and momentum spaces.12 Several groups have investigated adsorbate/ metal interfaces using 2PPE. In strong chemisorption systems, 2PPE studies have revealed adsorption-induced, unoccupied surface states, including the π* state of CO/Cu(111),9 the σ* state of Cs/Cu(111),10 and interface σ* states for thiolate selfassembled monolayers on Cu(111).11 In most systems involving weakly adsorbed molecules, the observed excited states have been attributed to modified image states.13-15 In the last example listed above, the influence of a molecular layer on the image state has often been described by the dielectric continuum model (DCM).13 Instead of considering the detailed electronic structure of adsorbates, the DCM uses a * To whom correspondence should be addressed. E-mail: zhu@ chem.umn.edu. Fax: (612) 626-7541.
dielectric constant to modify the Coulomb potential outside the metal surface; within the molecular layer, the potential is assumed to be shifted by the molecular electron affinity. In essence, the DCM is only physically meaningful when the image state is located within the molecular conduction band, the bottom of which is given by the electron affinity. The questions one may ask are, what is the nature of an anionic resonance when the molecular LUMO (lowest unoccupied molecular orbital) is located significantly below what is commonly observed for image states? Will such a molecular resonance show free electron like dispersion parallel to the surface as is observed for modified image states? Harris and co-workers recently carried out 2PPE studies of Ag(111) adsorbed with a series of aromatic molecules with increasing electron affinity: benzene, naphthalene, and anthracene.16 However, these experiments did not reveal any molecular resonance. Instead, the authors showed the sensitivity of image states to the presence of molecular layers, depending on the energetics of image states relative to molecular bands. In an attempt to answer the questions raised above, we explore a system where a true molecular anionic resonance may exist, perfluorobenzene (C6F6) on Cu(111). We investigate the influence of molecule-metal interaction strength on such a molecular resonance. We choose this system because C6F6 possesses high electron affinity17,18 and previous 2PPE measurements in our laboratory revealed a resonance (2.9 eV above the Fermi level) attributed to the σ* LUMO of C6F6.19 This system was reexamined in much detail by Gahl et al. who provided strong support for the molecular nature of the resonance and showed the inconsistency of the DCM in accounting for all experimental observations.20 With increasing coverage, the molecular resonance decreases in energy while the lifetime increases. A particularly interesting observation is that the molecular resonance becomes more delocalized parallel to the surface with increasing coverage beyond 1 monolayer (ML). This observation was not understood. Possible explanations for the observed dispersion may include band structure formation from intermolecular interaction, the coupling of molecular resonance to image states, and the coupling of the molecular resonance to surface or substrate band structures. In the present study, we use
10.1021/jp012485z CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001
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J. Phys. Chem. B, Vol. 105, No. 44, 2001 10913
preadsorbed atomic hydrogen to systematically modify the interaction between C6F6 molecules and the Cu(111) surface. These experiments suggest that the observed delocalization of the molecular resonance originates mainly from band formation due to intermolecular interaction. 2. Experimental Section All experiments were performed in an ultrahigh vacuum (UHV) chamber pumped by a turbo molecular pump, with a base pressure of 1 × 10-10 Torr. The system consisted of two levels. The upper level was equipped with a low-energy electron diffraction (LEED) and Auger electron spectrometer (AES) for surface analysis, an ion sputtering gun for surface cleaning, and a quadrupole mass spectrometer (QMS) for residual gas analysis (RGA) and thermal desorption spectroscopy (TDS). The lower level housed a hemispherical electron energy analyzer (Vacuum Generator 100AX) for two photon photoemission spectroscopy. The Cu(111) sample (o.d. 10 mm, polished to within 1° of the 〈111〉 direction) was mounted at the center of the chamber on a sample manipulator. It was cooled by liquid nitrogen and heated resistively through two tungsten wires mounted on the edge of the sample. Temperature was measured by a type K thermocouple inserted into a small hole on the edge of the Cu crystal. Cleaning was achieved by repeated cycles of Ar+ sputtering (1.0 keV) and annealing (750 K) to yield a sharp (1 × 1) LEED pattern. The clean surface was further confirmed by AES. TDS measurements were carried out with a linear sample heating rate of 1.5 K/s. H-covered Cu(111) was obtained by dosing H2 through a heated W filament at a sample temperature of 250 K. This leads to a saturation H coverage of ∼0.34 ML (H to surface Cu ratio), which corresponds to a surface H density of 6.0 × 1014/cm2.21 TDS from this surface showed a single H2 desorption peak at 300 K (data not shown), in agreement with the literature. A further increase in surface H coverage, achievable at lower surface temperature, is known to result in subsurface H,21 In the present study, we restricted the coverage to the region corresponding to only surface hydrogen. Hexafluorobenzene (Aldrich) was purified by freeze-pumpthaw cycles and its purity confirmed by RGA. Dosing was carried out through a positive shut-off valve (VTI) with calibrated leak rates, at a substrate temperature (Ts) of 130 K. This calibrated leak valve was connected to a 9 mm i.d. tube terminated at ∼5 mm from the sample surface. All coverages were determined on a relative scale with 1 ML corresponding to the saturation of the monolayer peak in the thermal desorption spectrum (see below). The setup for two-photon photoemission (2PPE) spectroscopy is shown in Figure 1. Laser light for 2PPE was from a modelocked Ti:Sapphire laser (Coherent MIRA) pumped by 532 nm continuous wave (CW) light from a solid-state laser (Coherent Verdi-8) with an output power of 8 W. The Ti:Sapphire operated at 76 MHz and was tunable from 690 to 1000 nm. A typical pulse width of ∼100 fs and typical power of 0.8-1 W were obtained. The visible output of the Ti:Sapphire laser was frequency doubled (SHG) or tripled (THG) in a femtosecond nonlinear harmonic generation system (Inrad). Typical power in the UV region was 100 mW at hν ) 4.15 eV (THG) and 120 mW at 3.49 eV (SHG). The laser light was compensated for group velocity dispersion (GVD) by two prism pairs before and after SHG/THG. The resulting UV light (spot size ∼4-6 mm) was focused by an f ) 50 cm lens onto the sample surface. The calculated beam waist was 60-70 µm. This gave an average power density of ∼800 W/cm2. There was no evidence of space-
Figure 1. Experimental setup for two-photon photoelectron spectroscopy.
charge effects at this power density. Only p-polarization (i.e., electric field vector in the plane of light incidence) was used in these experiments. Except for dispersion measurement, the sample was perpendicular to the electron energy analyzer, and the laser light was incident at 60° from surface normal. In dispersion measurements, the sample was rotated; thus both the angle of electron detection and the angle of light incidence varied. The electron energy analyzer was located inside a magnetic shield to minimize distortion by the Earth’s magnetic field. The Cu sample was inserted through a small hole at the top of the magnetic shield. A negative bias of 0.5-1.0 V was applied to the sample during some measurements to obtain the secondary electron threshold, which was used to establish the surface work function. Calibration using the well-established surface state showed no distortion in angle resolved measurements by this small bias voltage. 3. Results and Discussion 3.1. Weakening of C6F6 Interaction with Cu(111) by Preadsorbed H. C6F6 adsorbs and desorbs molecularly on both clean and H-covered Cu(111). As detailed before, C6F6 adsorbs in a stable bilayer structure on Cu(111) before multilayer formation.19 In the bilayer structure, the first layer is believed to adsorb with the molecular plane parallel to the surface, while the second layer likely adopts a perpendicular geometry. Figure 2 compares TDS of bilayer C6F6 from clean Cu(111) and H-covered surfaces at various H coverages. On clean Cu(111), TDS displays a monolayer desorption peak at 195 K with a shoulder feature at ∼185 K, and a peak at 165 K from the second layer. The adsorption of H on Cu(111) clearly weakens the chemical interaction between C6F6 and the Cu(111) surface, as evidenced by the up to 10 K decrease in the peak temperature for monolayer C6F6 desorption. An important observation is that the monolayer C6F6 desorption temperature decreases gradually with increasing H coverage and there is no broadening of this peak. Thus, preadsorbed H is ideal in controlling systematically the interaction between C6F6 and the Cu(111) surface, without the complication of substantial domain formation on different surface regions. Previous studies on H/Cu(111) have established that H adsorbs on bridge sites on the surface. This adsorption involves partial charge transfer of ∼0.05 e from Cu to the adsorbed H atom.22 As in other H/metal surface systems,23 there is a repulsive interaction between adsorbed H atoms; as a result, H atoms tend to form long range ordered structures on the surface. Ultraviolet photoemission by Greuter and Plummer showed that,
10914 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Figure 2. Thermal desorption spectra (TDS) of bilayer C6F6 from H-covered Cu(111) surfaces. The relative H coverage (0-0.34 ML) is indicated on each spectrum.
Figure 3. 2PPE spectra taken at two-photon energies: hν ) 4.15 eV (left) and 3.49 eV (right). Left panel from bottom to top: clean Cu(111), 1 ML C6F6/Cu(111), 2 ML C6F6/Cu(111), and 0.34 ML H/Cu(111); the dashed curve (topmost) is part of the spectrum from 2 ML C6F6 on H/Cu(111). Right panel from bottom to top: clean Cu(111), 2 ML C6F6/Cu(111), 0.30 ML H/Cu(111), and 2 ML C6F6 on 0.30 ML H/Cu(111). SS ) surface state; IS ) image state; MR ) molecular resonance.
for surface H prepared from atomic hydrogen adsorption on Cu(111) at ∼260 K, the surface state is removed (see below in 2PPE spectra); beyond this, its effect on substrate electronic structure is negligible.24 In fact, these authors called surface hydrogen “nearly invisible” in UPS. Since the size of an adsorbed H atom is very small (∼0.3 Å) as compared to surface Cu atoms, its effect on C6F6 adsorption is electronic in nature. 3.2. Molecular Resonance of C6F6 on Clean and H-CoVered Cu(111). The left panel in Figure 3 compares 2PPE spectra for clean Cu(111), 1 ML C6F6/Cu(111), 2 ML C6F6/Cu(111), 0.34 ML H/Cu(111), and 2 ML C6F6 on 0.34 ML H/Cu(111). The clean surface spectrum shows the intense surface state (SS) and
Dutton and Zhu
Figure 4. Dependence of the kinetic energy of photoelectrons from the molecular resonance as a function of photon energy for bilayer C6F6 on 0.34 ML H-covered Cu(111). The triangles were obtained from the set up in Figure 1. The circles were obtained on an electron timeof-flight system with nanosecond laser excitation, as detailed before.19
the n ) 1 image state (IS). The former originates from two photon photoionization of the occupied surface state while the latter involves one-photon excitation into the n ) 1 image state, followed by photoionization of this transient state.25 Note that image states for n > 1 are not accessible at this photon energy. The feature at 6 eV is due to photoionization of the Cu d-band. With the adsorption of 1 or 2 ML of C6F6 on the Cu(111) surface, both the surface state and the image state are attenuated, and a new peak (MR for molecular resonance) at ∼7 eV grows. Previous experiments showed that MR results from photoexcitation to an unoccupied state located at 2.9 eV above the Fermi level.19,20 This state was assigned to the σ* lowest unoccupied molecular orbital (LUMO) of C6F6. Note that in the presence of C6F6, the n ) 1 image state becomes only partially accessible at this photon energy. Details on the energetics of the molecular resonance and image states at different C6F6 coverages have been established in previous studies.19,20,26 The adsorption of 0.34 ML H on Cu(111) completely eliminates the surface state, in agreement with previous UPS studies.24 After the adsorption of bilayer C6F6 on this surface, a new peak at ∼7.4 eV appears. At a photon energy of 4.15 eV, we find that the adsorbate layer is easily damaged, particularly on the hydrogen terminated surface (presumably due to photodissociation or desorption). This problem is alleviated at lower photon energies. Thus, we focus on 2PPE measurements at lower photon energies for C6F6 on the hydrogen-covered surface. The right panel in Figure 3 shows 2PPE spectra taken at hν ) 3.49 eV (from bottom to top) for clean Cu(111), 2 ML C6F6/Cu(111), 0.30 ML H/Cu(111), and 2 ML C6F6 on 0.30 ML H/Cu(111). The spectrum of the clean surface is dominated by the surface state (SS); the image state is not accessible at this photon energy. Unlike that at hν ) 4.15 eV, the spectrum for 2 ML C6F6/Cu(111) shows the overlap of the molecule resonance (MR) with the surface state (SS). The adsorption of H on Cu(111) again eliminates the surface state and the further adsorption of bilayer C6F6 on H/Cu(111) leads to a well-defined resonance. The unoccupied nature of the resonance observed for 2 ML C6F6/H/Cu(111) is established by the dependence of 2PPE peak position on photon energy shown in Figure 4. The data can be well described by a straight line, with a slope of n ) 1.09 ( 0.10, which is essentially unity. Thus the observed resonance in the 2PPE spectrum of bilayer C6F6 on H-covered Cu(111) originates from photoexcitation to an unoccupied state located at 3.22 eV above the Fermi level, followed by photoionization
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J. Phys. Chem. B, Vol. 105, No. 44, 2001 10915
Figure 6. H-coverage dependences of surface work function for H/Cu(111) (triangles), surface work function for bilayer C6F6 on H/Cu(111) (open circles), and the position of the molecular resonance for bilayer C6F6 on H/Cu(111) (solid circles). Figure 5. 2PPE spectra (solid curves) of bilayer C6F6 on H/Cu(111) at the indicated surface H coverage. The corresponding spectra for H/Cu(111), without C6F6, are shown as dashed curves.
of this transient state. In view of results in the absence of H, we assign this unoccupied state on 2 ML C6F6/H/Cu(111) to the σ* LUMO of C6F6. Compared to the spectrum for bilayer C6F6 on Cu(111), the presence of monolayer H shifts the molecular resonance upward by 0.24 eV. As on clean Cu(111), the intensity of the molecular resonance for 1 ML C6F6 on H/Cu(111) is significantly weaker than that for 2 ML C6F6. In the following, we concentrate on results for the bilayer coverage and investigate the effect of surface hydrogen coverage on the C6F6 LUMO. 3.3. Dependence of the C6F6 Molecular Resonance on Surface H CoVerage. TDS results presented in section 3.1 establish that, with increasing coverage, preadsorbed H systematically weakens the interaction between C6F6 and the Cu(111) surface. We now investigate the effect of H coverage on the C6F6 molecular LUMO. Figure 5 shows a set of 2PPE spectra (solid curves) taken at hν ) 3.49 eV for bilayer C6F6 adsorbed on H/Cu(111) with the relative hydrogen coverage in the range of θH ) 0.0340.30 ML. Also shown are the corresponding 2PPE spectra (dashed curves) for H/Cu(111). Compared to the spectrum from clean Cu(111) (right panel in Figure 3), the adsorption of only 0.034 ML H almost eliminates the surface state. The surface state resonance is only visible as slightly increased background intensity. After the adsorption of bilayer C6F6 on the H-covered Cu(111) surfaces, each spectrum is dominated by the molecular resonance. Two effects are evident with increasing H coverage: (1) the intensity of the molecular resonance decreases, and (2) the peak position shifts upward. The first effect can be understood as an indication of the weakening of the electronic coupling between C6F6 molecules and the Cu(111) surface. Previous studies have shown that twophoton photoemission via an unoccupied state often involves two direct photoexcitation steps: the first step involves direct photoexcitation from an occupied metal state to the unoccupied state; the second step corresponds to photoexcitation from the transient state to the continuum.27 Indeed, measurement on the dependence of 2PPE intensity on light polarization verifies this direct photoexcitation mechanism for the molecular resonance in C6F6 on Cu(111).28 In such a direct photoexcitation process, the 2PPE intensity is proportional to the square of the transition dipole moment for each step. The transition dipole moment of the first step is proportional to the amplitude of the mixed
molecule-metal wave function inside the metal substrate; this amplitude is essentially a quantitative measure of the coupling of the molecular orbital with the substrate band structure. With increasing H coverage, or with decreasing strength of moleculesurface interaction, we expect such an amplitude to decrease. This can account for the observed decrease in 2PPE intensity in Figure 5. The second effect, i.e., the shift in energetic position of the molecular resonance with increasing surface H coverage, is shown quantitatively in the lower panel in Figure 6. Also shown in Figure 6 are the values of surface work function (φ) for H/Cu(111) (triangles) and for bilayer C6F6 on H/Cu(111) (open circles). The adsorption of H on Cu(111) increases the surface work function from 4.90 eV on a clean surface to 5.16 eV on the 0.34 ML H-covered surface. The change of φ with θH is not linear; the effect levels off at high surface H coverage due to dipole-dipole interaction. Adsorption of bilayer C6F6 on H/Cu(111) decreases the work function. The magnitude of work function change decreases with increasing coverage of preadsorbed H. This observation is consistent with the decreasing electronic interaction between C6F6 and the surface with increasing θH. The same effect accounts for the observed increase in the energetic position of the molecular resonance. For gas-phase C6F6, the vertical electron affinity (to the σ* LUMO) is near 0 eV.17 This anionic resonance is stabilized by 0.8 eV in the condensed state, due to polarization of the medium.18 Compared to the condensed phase, the molecular anionic resonance in the adsorbed state is further stabilized by 0.8-1.0 eV on clean and H-covered Cu(111). We can identify at least two contributions to the stabilization of a transient anionic resonance on a metal surface: (1) charge-image attraction (i.e., polarization of the substrate charge density) and (2) mixing of molecular wave function with substrate bands. The preadsorption of H on Cu(111) causes a small decrease in electron density near the surface region due to the partial electron transfer (∼5%) from Cu to H. This may result in a small decrease in the polarizability of the metal surface and, consequently, a decrease in stabilization of the anionic molecular resonance. Additional decrease in the stabilization of the anionic molecular resonance comes from the decrease in the wave function mixing between C6F6 and the Cu surface by preadsorbed H. Note that, unlike image states, the molecular resonance is not pinned to the vacuum level. This is evident in Figure 6. The binding energy of the molecular resonance in C6F6 increases from 1.60 eV on clean Cu(111) to 1.85 eV on 0.34 ML H/Cu-
10916 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Dutton and Zhu
Figure 8. Left panel: parallel dispersion curves (E vs k|) for the molecular resonance in bilayer C6F6 on H/Cu(111) at the indicated surface H coverages (0.0-0.34 ML). The solid curves are fits to the free electron like parabolic relationship in eq 2. The fits yield the effective electron mass (as a function of θH) in the right panel. Figure 7. 2PPE spectra for bilayer C6F6 on clean Cu(111) (left) and 0.34 ML H/Cu(111) (right) taken at the indicated angles (θ). The solid lines show positions of spectra at θ ) 0°. Photon energy: hν ) 4.15 eV (left) and 3.49 eV (right), respectively.
(111). This, along with dispersion results presented below, rules out the possibility of an image-like origin for the observed resonance. 3.4. EVidence for Band Formation and the Role of MoleculeSurface Interaction. We now present dispersion measurements for the molecular resonance. In this experiment, 2PPE spectra are recorded at different electron detection angle, θ, with respect to the surface normal. At each angle, the momentum vector parallel to the surface for the photoejected electron is given by
k| )
x
2meEk p2
sin θ
(1)
where me is the free electron mass and Ek is the kinetic energy (referenced to the vacuum level) of the photoejected electron. Ek is obtained from a Gaussian fit to the resonance in each spectrum. Figure 7 compares the angle-dependent 2PPE spectra for bilayer C6F6 on clean Cu(111) and 0.34 ML H-covered Cu(111). In both cases, the peak shifts upward in energy with increasing detection angle, indicating dispersion. The left panel in Figure 8 shows the peak position as a function of the electron momentum vector parallel to the surface, k| (from eq 1), for bilayer C6F6 with different coverages of preadsorbed H. Assuming free-electron-like behavior, the extent of dispersion or delocalization parallel to the surface in each case can be quantified as an effective electron mass, meff, based on fitting to
E k ) E0 +
p2k|2 2meff
(2)
where E0 is the electron kinetic energy at θ ) 0°. The solid lines are fits to eq 2. These fits yield the effective electron mass shown in the right panel. The effective electron mass decreases monotonically from meff ) 1.5me on clean Cu(111) to meff ) 0.55me on 0.34 ML H/Cu(111).
Image states on metal surfaces are known to possess freeelectron-like dispersion parallel to the surface. Previous 2PPE studies of adsorbate/metal interfaces have used dispersion as evidence for image-like states.15 This view needs to be revised in light of the results above. We believe the observed Hcoverage dependence of dispersion argues against an imagelike origin for the observed resonance in bilayer C6F6/H/Cu(111). Compared to C6F6/Cu(111), the presence of preadsorbed H in C6F6/H/Cu(111) is expected to only slightly modify the dielectric constant and the electrostatic potential for an electron outside the metal substrate. If we were to attribute the observed dispersion for C6F6/Cu(111) to an image-like state, we should expect similar dispersion for the C6F6/H/Cu(111) system. A previous 2PPE study by Gahl et al. also showed that image states and the dielectric continuum model (DCM) fail to explain experimental observations for multilayer C6F6 on clean Cu(111).20 Results from the present study clearly show that the observed molecular resonance is very sensitive to the interface between the adsorbate layer and the metal surface. All these lead us to assign the observed resonance in C6F6/Cu(111) or C6F6/H/Cu(111) to a molecular anionic resonance. The key question is: what is the origin of dispersion associated with the molecular resonance? We explore two contributions: (1) intermolecular electronic interaction, leading to molecular band formation and (2) mixing of molecular wave function with highly dispersed surface or substrate bands. We believe that the first contribution is mainly responsible for the observed dispersion in bilayer C6F6. Past studies of electronic band formation in molecular crystals have focused on π-conjugation molecules, such as polyacenes and phthalocyanines. These bands have been shown to possess reasonable widths of 0.1-0.2 eV.29 A recent study put the bandwidth of pentacence crystal at 0.5-0.6 eV.30 We are not aware of highquality band structure calculations for σ* systems. The LUMO in C6F6 is believed to be the σ* ring molecular orbital (MO).31 The diffused nature of this excited MO may well lead to significant bandwidth in the condensed state. The observed dispersion for bilayer C6F6 on the surface is not inconsistent with the expected bandwidth. The lattice constant of the Cu(111) surface is 2.54 Å. LEED showed that monolayer C6F6 formed a (3 × 3) structure on the Cu(111) surface.20 If we assume the same lattice spacing for bilayer C6F6 on clean and
Electronic Band Formation at Organic-Metal Interfaces H/Cu(111). The lattice constant in the adsorbate layer is 7.62 Å, the dispersion measurements presented above were carried out in the [112h] direction. The momentum vector, k|, at the boundary of the first Brillouin zone is 0.476 Å-1 for the adsorbate lattice. On the basis of the effective electron mass in Figure 8, we estimate that the bandwidth of the molecular anionic resonance is
