Observation of a molecule–metal interface charge transfer related feature by resonant photoelectron spectroscopy

We report the discovery of a charge transfer (CT) related low binding energy feature at a molecule–metal interface by the application of resonant photoelectron spectroscopy (RPES). This interface feature is neither present for molecular bulk samples nor for the clean substrate. A detailed analysis of the spectroscopic signature of the low binding energy feature shows characteristics of electronic interaction not found in other electron spectroscopic techniques. Within a cluster model description this feature is assigned to a particular eigenstate of the photoionized system that is invisible in direct photoelectron spectroscopy but revealed in RPES through a relative resonant enhancement. Interpretations based on considering only the predominant character of the eigenstates explain the low binding energy feature by an occupied lowest unoccupied molecular orbital, which is either realized through CT in the ground or in the intermediate state. This reveals that molecule–metal CT is responsible for this feature. Consequently, our study demonstrates the sensitivity of RPES to electronic interactions and constitutes a new way to investigate CT at molecule–metal interfaces.


Introduction
One of the crucial questions for the performance of organic electronics is dynamical charge transfer (CT) across the metal-organic interface. The technique of resonant photoelectron spectroscopy (RPES) is able to investigate this phenomenon within the core hole clock technique [1]. For small adsorbates like noble gases [2], sulphur atoms [3,4] and small molecules [5] a quantitative extraction of CT times has been successfully performed in the fs-and as-regime. Also for large π-conjugated molecules a considerable body of literature with a quantitative analysis of CT times exists [6][7][8][9][10]. However, the complicated electronic structure of these molecules and possible strong interactions with the substrate pose a tremendous challenge to the quantitative extraction of intensities from the RPES data. In order to permit quantitative understanding for these systems first the interaction at the interface and second its consequence for RPES need to be understood in more detail.
In RPES certain signals in the photoelectron spectroscopy (PES) spectrum can get enhanced due to additional autoionization channels which open up in the resonance case. For example the 6 eV satellite in Ni metal [11] was assigned to a two hole final state due to the intensity enhancement and energy dispersion observed while tuning the photon energy ( ν h ) through a resonance [12,13]. Furthermore, similar satellites for Cr and Fe metal could only be discovered due to resonant intensity enhancement [14]. In compounds ν h can be chosen to selectively excite one of its constituents resonantly in order to enhance signals of this particular species [15][16][17]. Moreover the surface sensitivity of RPES allows to apply the concept of selective resonant enhancement to quasi two-dimensional systems like surface alloys [18]. However, which signal belonging to the selected species gets enhanced is a matter of localization of the resonantly excited electron. For the excitation into a delocalized d-band of a metal for example it is the incoherent Auger process that mainly gains in intensity and the enhanced signal disperses with a constant kinetic energy (E K ) [12][13][14]19]. Exciting resonantly into a localized f-orbital on the other hand leads to a coherent and energy conserving process in which the enhanced signal stays at constant binding energy ( both the coherent and the incoherent signal will be visible. Directly on resonance the continuous PES channel and the discrete Auger channel with the same E K of the emitted electron interfere [23,24] and cannot be distinguished, but above the resonance the dispersion of the Auger signal makes this signal move away from the coherent signal at constant E B . Consequently the coherent and the incoherent part can often be separated for simple systems [2]. For an adsorbate at a surface the latter can be caused by CT across the adsorbate substrate interface after excitation which in principle allows a determination of the CT time with the core-hole clock technique [1]. So if the intensities of the signals corresponding to both channels can be extracted from the RPES data a quantitative value for the CT time can be determined [3,4]. In this work we apply RPES to a model system of a large π-conjugated molecule adsorbed on a metal surface, namely coronene adsorbed on Ag(111). First we present the observation of a low E B feature in a PES map recorded in the energetic region of the largest resonant enhancement of the highest occupied molecular orbital (HOMO). This low E B feature is found to originate from the interface and its spectroscopic signature shows characteristics of strongly coupled molecule-metal interfaces. Hence RPES reveals an electronic interaction between coronene and Ag(111), a system that shows no evidence for strong coupling in other electron spectroscopic techniques. We then explain the appearance of the low E B feature within a cluster model that treats the molecule-metal interface as a system of localized molecular states coupled to the metal substrate. The cluster model has been applied particularly to transition metal oxides, where the correlated localized 3d states are hybridized with the states of the surrounding ligands. In the present case we transfer this concept to the localized, possibly correlated states 2 of a π-conjugated molecule, which are interacting through hybridization with a bath of conduction electrons, which are considered as (discrete) metal states. In this model the low E B feature is identified as a resonantly enhanced satellite of the HOMO signal which explains why this feature cannot be detected in direct PES. The thereby employed eigenstates, both the ground and the photoemission final state, are quantum mechanical superpositions of two basis states of which one takes CT between molecule and metal into account. Based on simplifications of the cluster model alternative explanations are discussed. Considering only the main character of the final state assigned to the low E B feature suggests resonantly enhanced PES from the lowest unoccupied molecular orbital (LUMO) as the mechanism responsible for this feature. Consequently, CT in the ground state is concluded from this interpretation. Reducing the ground state to its predominant character, on the other hand, leads to the conclusion that dynamical CT in the intermediate state needs to be involved in the generation of the low E B feature. Hence independent of the interpretation the low E B feature is related to CT between molecule and metal.

Experimental
Measurements were performed at BESSY II at the undulator beamline UE52-PGM ( Δ > E E 14 000 at ν = h 400 eV, with = c 10 ff and 20 μm exit slit [27]) in a UHV chamber with a pressure below 5×10 −10 mbar. All PES maps were recorded with p-polarized light and 70°angle of incidence with respect to the surface normal, a beamline exit slit of 40 μm, and a c ff value of 10. This results in a Δ ν h better than 40 meV at ν = h 290 eV. Photoelectron intensities were detected with a Scienta R4000 electron analyzer with Δ = E 75 meV for the PES detail map (figure 2(a)). ν h was calibrated with the Fermi energy (E F ) resulting in an accuracy better than 50 meV (for E B and ν h ). PES intensities were normalized with the ring current and the beamline flux curve which was recorded separately by measuring the clean surface [28]. For the PES detail map (figure 2(a)) the 2nd order C1s signal was subtracted with a reference spectrum of the same sample previous to the normalization with the beamline flux curve. The Ag(111) substrate was cleaned by several sputter and annealing cycles and its cleanness was confirmed by PES. Coronene molecules were purified by sublimation and evaporated from a Knudsen cell at a pressure below 10 −8 mbar and at room temperature. Film thickness was determined by corelevel intensities of the adsorbate and the substrate, using the effective electron attenuation lengths given in [29]. eV. Below the onset of the first absorption peak of coronene at approximately 284 eV these are the only significant contribution to the spectra. At larger ν h (but still below the direct photoionization into vacuum) the situation changes dramatically and signals originating from coronene get strongly enhanced. Integrating over a constant E K from 272.0 to 274.5 eV (denoted by the blue parallelogram in figure 1(a)) results in the spectrum displayed in figure 1(b). This 1D spectrum is equal (within the chosen ν h increment) to the partial electron yield near edge x-ray absorption fine structure spectroscopy (NEXAFS) spectrum and can be used for the identification of the NEXAFS resonances. Since the measurement of a constant E K window is referred to as constant final state (CFS) spectroscopy we call the obtained 1D spectrum CFS NEXAFS in the following (for further details see [30]). In the ν h region of the two most intense NEXAFS resonances A and B we further observe a substantial intensity enhancement of the HOMO signal which is located at slightly lower E B than the rising edge of the Ag4d bands. These NEXAFS resonances A and B can be assigned to an excitation into the LUMO [31]. Resonances C-E, which are not in the focus of the following analysis, are due to transitions into higher unoccupied orbitals [31]. For closer inspection the area within the cyan box in figure 1(a) is recorded in a subsequent measurement with lower energy increments and higher resolution. Figure 2(a) displays this PES detail map. Here not only the intensity enhancement of the HOMO signal but also its line-shape variation as a function of ν h becomes obvious. This effect is due to a difference in the vibronic progression of the HOMO signal which is a consequence of the particular vibronic excitations within the photon absorption [32] and hence a function of ν h (for a detailed discussion see [30]). Here we focus on the additional feature centered at ≈ E 0.5 B eV which becomes clearly visible after the intensity of the PES detail map is multiplied by a factor of 10. Integrating over the constant E B windows marked by the hatched areas in figure 2(a) results in the filled symbols displayed in figure 2(b). Figure 2(b) reveals that the intensity of the low E B feature amounts to about 5% of the intensity of the HOMO signal. Moreover, the low E B feature and the HOMO signal obviously exhibit a very similar intensity variation as a function of ν h . Interestingly, a comparison to the corresponding data of the clean Ag(111) substrate (open symbols in figure 2(b)) demonstrates the absence of the low E B feature for the bare Ag(111) surface. Such a feature is furthermore not observed in the corresponding PES data of a coronene multilayer film [30] 3 . This leads to the conclusion that the low E B feature in figure 2(a) originates from the metal-organic interface and hence a particluar interaction of the Ag(111) substrate with the coronene adsorbate film must be present. This conclusion is corroborated by the broad line-shape of the low E B feature which is presented in figure 3. Here energy distribution curves (EDC) from the PES detail map ( figure 2(a)) are compared to corresponding EDC from clean Ag(111). The broad and smeared out resonantly enhanced intensity of the 0.8 ML coronene/ Ag(111) film is similar to the signals observed for strongly coupled molecule-metal interfaces [33], which are characterized by an occupied LUMO [34]. Interestingly, in direct PES (with ν = h 40.8 eV) a possible LUMO signal is found to be below the detection limit of approximately 1% with respect to the intensity of the HOMO signal at any point in the probed k-space [35]. Moreover, the strong changes of the interfacial PES core level 4 and NEXAFS spectra with respect to the multilayer spectra, which are generally observed for strongly coupled molecule-metal systems, cannot be found for coronene. From these experiments one would conclude that electronic interaction in the present system is weaker compared to the cases discussed in [33,34]. Thus the finding of signatures of electronic interaction at the here investigated molecule-metal interface demonstrates the sensitivity of RPES to such interactions. We would like to note that any metallic interface states cannot explain this observation of an additional signal at the coronene/Ag(111) interface since it does not appear in direct PES [36,37] thus demanding for an alternative explanation.

Results and discussion
The similar ν h dependent intensity variation of the low E B feature and the HOMO signal suggests that the former feature is a satellite excitation of the latter. The fact that the low E B feature originates from the metalorganic interface further requires an involvement of a metal state in the responsible excitation process. An established theoretical description which is capable of treating such excitations in a simplified way is the cluster model [38][39][40][41]. In order to apply this theory to the coronene/Ag(111) interface the coronene molecule has to be seen as an impurity with localized states (molecular orbitals) coupled to metallic states (Ag(111) s-p bands) similar to the single impurity Anderson model [42]. The cluster model emerges from the Gunnarsson-Schönhammer theory [43,44] by setting the bandwidth of the metallic band to zero. Consequently, the molecule-metal interface is treated as a 'cluster' of a molecule coupled to several metal ligand atoms with Hereby C stands for the C1s core level of one chemical species. All states except the single particle states C, H, L, and M of the system are omitted. The superscript denotes the occupation of the particular level ( ∈  n ) i.e., 1 an additional electron is found in the LUMO and one is missing in the metal. The latter basis state thus is the CT state. Hence the eigenstate described in equation (1)  note that all hopping matrix elements are in the latter evaluation set equal, independent of the core level occupation) and Coulomb interaction parameters that take into account the mutual repulsion of electrons in molecular orbitals and the attraction of electrons in molecular orbitals by a core hole (see [38][39][40]). Furthermore, the eigenenergies of the given eigenstates are also functions of these cluster model parameters.
The quantities extracted from the valence PES and RPES experiments which can be calculated within the cluster model are the energetic separation and the intensity ratio of the main line and its satellite. In case of PES 1 in a different way. This can be achieved by CT from M to L during the time scale of the core hole life time. In this interpretation the low E B feature is a signature of dynamical interface CT. Consequently, the interpretation of the low E B feature depends on the initial choice of the picture in which the process that leads to this feature is described. In the cluster model explanation CT is included in one of the basis states with which the eigenstates are constructed as a quantum mechanical superposition. In the simplified picture of states with one predominant basis state, CT is either considered in the ground state or in the intermediate state. Thus independent of the applied picture the low E B feature is an indicator for a significant CT between molecule and metal.

Conclusion
In conclusion we find an interface induced low binding energy (E B ) feature in the RPES data of the moleculemetal interface system coronene/Ag(111) which is absent in the direct PES spectra. A detailed analysis of the line-shape of this feature provides evidence for electronic interaction not observed by other electron spectroscopic techniques. Its emergence is explained within a cluster model which identifies this feature as a resonantly enhanced satellite of the HOMO. Interpretations of this feature are based on considering only the predominant character of the employed eigenstates. In the cluster model these eigenstates are quantum mechanical superpositions of the chosen basis states of which one includes transfer of an electron between molecule and metal. Reducing the final eigenstate assigned to the low E B feature to its main character identifies resonantly enhanced PES from the LUMO as the responsible mechanism. On the other hand the assumption of a pure initial state without CT requires the involvement of dynamical CT in the process which generates the low E B feature. Hence our finding and interpretation of the low E B feature constitutes a new way to investigate CT at molecule-metal interfaces.