Impurity Electron Localization in Early-Transition-Metal-Doped Gold

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Impurity Electron Localization in Early-Transition-Metal-Doped Gold Clusters Konstantin Hirsch, Vicente Zamudio-Bayer, Andreas Langenberg, Marlene Vogel, Jochen Rittmann, Silvia Forin, Thomas Moller, Bernd von Issendorff, and J. Tobias Lau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511927q • Publication Date (Web): 24 Jan 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Impurity Electron Localization in Early-Transition-Metal-Doped Gold Clusters K. Hirsch,∗,† V. Zamudio-Bayer,†,‡ A. Langenberg,†,§ M. Vogel,†,¶ J. Rittmann,†,∥ S. Forin,¶ T. Möller,¶ B. v. Issendorff,‡ and J. T. Lau∗,† Institut f¨ ur Methoden und Instrumentierung der Forschung mit Synchrotronstrahlung, Helmholtz-Zentrum Berlin, Albert-Einstein-Straße 15, 12489 Berlin, Germany, Physikalisches Institut, Universität Freiburg, Stefan-Meier-Straße 21, 79104 Freiburg, Germany, and Institut f¨ ur Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany E-mail: [email protected]; [email protected]

∗

To whom correspondence should be addressed Helmholtz-Zentrum Berlin ‡ Universität Freiburg ¶ Technische Universit¨ at Berlin § Current address: Max Planck Institut f¨ ur Plasmaphysik, Wendelsteinstraße 1, 17491 Greifswald, Germany ∥ Current address: Faculté des sciences de base, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland †

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Abstract We have performed x-ray absorption spectroscopy of size selected, free, cationic, transition-metal-doped (Sc, Ti, V, Cr) gold clusters in a size range n = 1 − 9. The electronic configuration of the impurity atom was determined by modeling the x-ray absorption spectrum in a charge-transfer-multiplet framework which makes it possible to quantify the amount of localization of the impurity 3d states. Depending on the dopant element and the host cluster size we find a wide variety in the behavior of local electronic structures. ScAu+ n clusters show strong hybridization of the scandium 3d states with the host electronic states except for ScAu+ 1 where we find a completely localized 3d electron. In TiAu+ n clusters a pronounced odd-even alternation is present in the local electronic structure of the impurity atom. The 3d occupation number of the titanium dopant is approximately 2 and 1.6 electrons in odd and even numbered clusters, respectively. In CrAu+ n clusters the electronic structure of the dopant is governed by shell closure of the gold host which leads to almost unperturbed 3d states + in CrAu+ n , n = 2, 6, 8 and hybridization of the 3d-states in CrAun , n = 1, 5, 7. Contrary

to the other systems investigated the 3d occupation of 3.3 electrons in VAu+ n clusters is independent of the cluster size. Only in special cases we find an integer number of localized 3d impurity electrons. Furthermore, in all cases the local electronic structure of the dopant does not strongly depend on the exact coordination of the dopant atom. This finding allows for a better understanding of the bonding beyond a simple shell model approach with its ad hoc assumption of integer numbers of delocalized impurity electrons.

Keywords X-ray absorption spectroscopy, Ion trap, Binary clusters, Free-electron gas, Synchrotron radiation, Gold, Transition metals


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Introduction How the bond is mediated in very small metal clusters is a central question to cluster chemistry. Starting with the discovery of highly abundant species in the mass spectrum of simple metals, 1 the appearance of these ‘magic numbers’ was explained within a phenomenological shell model 1,2 of the electronic structure by assuming that fully delocalized valence electrons populate free-electron gas states. The same model was also applied to more complex metals such as gold. 3 Despite the modifications of cluster geometries by the directional bond formed by the 5d electrons the model worked surprisingly well to describe the bonding in small gold clusters. 3 In contrast, 3d transition metal clusters do not form good free-electron gases since their 3d states tend to be on the brink of localization that results in finite magnetic moments, 4–10 and delocalization that mediates bonding and magnetic ordering. The amount of delocalization of the 3d states of a transition metal impurity that is introduced into a free-electron gas host cluster was highly debated in the literature, 11–22 since localization not only affects the nature of bonding but also the magnetic properties of the system. So far, the amount of electron localization in doped clusters could only be probed indirectly, e.g., in photofragmentation studies combined with a phenomenological shell model 11–17 or by valence band photoelectron spectroscopy which probes the total density of states. 18–20 Here, we apply the local and element specific method of x-ray absorption spectroscopy in combination with Hartree-Fock and charge transfer multiplet calculations to transitionmetal-doped gold clusters in order to unambiguously determine the amount of 3d-electron localization. The local electronic structure is studied systematically by exchanging the dopant atom and by varying the cluster size. This was done in order to vary at will the number of 4s and 3d electrons of the impurity as well as the number of delocalized electrons in the host material. We show that the local electronic structure strongly depends on the nature of the dopant atom. For example, the 3d state of the early transition metal scandium strongly hybridizes with the host states. In titanium doped gold clusters, we find a strong odd-even effect in the number of localized dopant electrons. In contrast, the electronic structure of 3 ACS Paragon Plus Environment


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the impurity is almost independent of the number of gold atoms in vanadium doped gold clusters. In chromium doped gold clusters the impurity electronic structure is determined by shell closure in the gold host. These very different types of localization can be understood in terms of a joint effect of 3dk 4s(0,1) → 3dk−1 4s(1,2) promotion energy, dopant’s cationic ground state, mean 3d radius and shell closure in the free electron gas.

Experimental and Computational Details A detailed description of the experimental setup can be found elsewhere. 23–26 Only a brief introduction will be given here. A beam of transition-metal-doped gold clusters is produced in a magnetron sputtering source by co-sputtering of gold and transition metal targets. A single cluster size is selected in a quadrupole mass filter. Subsequently, the size-selected cluster ion beam is stored in a linear quadrupole radio-frequency Paul trap, where the clusters are cooled to liquid nitrogen temperature by collisions with a helium buffer gas. A monochromatic x-ray beam delivered by beamline U49/2-PGM-1 of the synchrotron radiation facility BESSY II is coupled in co-axially to the ion trap axis. Upon absorption of x-ray photons at the L2,3 edges of the transition metals, the clusters relax via Auger cascades and undergo fragmentation. The dominant product ions are tabulated in the Supporting Information. These product ions are detected via time-of-flight mass spectrometry. Recording the ion yield as a function of photon energy gives the x-ray absorption spectrum, which is acquired with a typical photon energy resolution of 125 meV. This enables us to resolve the multiplet structure present in the x-ray absorption spectrum of all investigated clusters. The multiplet structure is very sensitive to the symmetry of the environment and to the local occupation of the 3d states or, more precisely, to the s- and d-projected unoccupied valence states, as well as to their hybridization due to bonding. This sensitivity makes x-ray absorption spectroscopy a fingerprint method. 27,28 Although the energy resolution in principle would allow us us to resolve individual transitions, the high density of states at the Fermi


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level and the multiplet structure causes overlap of the life-time broadened lines. 29 In order to extract the electronic ground state configuration we performed atomic HartreeFock 30 as well as charge transfer multiplet calculations. 31 In the latter case, an Anderson impurity model Hamiltonian, comprising several parameters, namely the charge transfer energy, Coulomb and hopping integrals as well as a Hubbard U , is solved in a configuration interaction approach. By tuning these parameters, we were able to simulate the experimental x-ray absorption spectra, and to extract the dopant’s ground state configuration. The parameters of the calculation of all modeled spectra presented in the paper are given in the Supporting Information. Additional density-functional-theory calculations were performed for all clusters investigated experimentally. Established ground state structures 21 were re-optimized in the plane wave code quantum espresso 5.0 (Ref. 32) employing the PBE approximation to the exchangecorrelation functional. 33 Details on the calculations, the obtained structural parameters and the complete valence density of states of all clusters investigated can be found in the Supporting Information.

Local Electronic Structure of Scandium Doped Gold Clusters In Fig. 1 the x-ray absorption spectra of mass selected ScAu+ n clusters at the scandium L2,3 edges are presented in a size range of n = 1−6. The spectral signatures vary drastically with the cluster size, reflecting the different occupation of the impurity 3d states and impurity + 34 environments. The x-ray absorption spectra of ScAu+ additionally shown 1 and atomic Sc ,

in Fig. 1, show almost identical spectral features that indicate a very similar local electronic structure. The [Ar]3d1 4s1 ground state electronic configuration of Sc+ features a single 3d as well as 4s valence electron. The participation of the scandium 3d electron in bonding would drastically alter the x-ray absorption signature in comparison to the isolated Sc+ cation, 28 5 ACS Paragon Plus Environment


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Figure 1: X-ray absorption spectra of scandium doped gold clusters at the L2,3 edges of scandium in a size range of n = 1−6. The spectral signature changes drastically as a function of the cluster size. Additionally shown are the experimental x-ray absorption spectrum of isolated Sc+ (Ref. 34), a charge transfer model calculation (dashed line) and the re-optimized ground state structures. 21 which is not the case in Fig. 1. Hence, we deduce that the 3d electron of the scandium dopant is not or only very weakly involved in the bonding to the gold atom and therefore remains essentially atomically localized, which is also reflected in the density of states, shown in Fig. 2. In these calculations the electronic state close to the Fermi energy, which is highlighted by the arrow, has almost pure scandium d character. Accordingly, theory and experiment show that the bonding in ScAu+ 1 is purely mediated by the s electrons of scandium and gold. This can also be rationalized by shell closure of two electrons in a jellium view of the molecular orbitals. In case of ScAu+ 2 the spectral signature becomes quite narrow and exhibits two prominent 6 ACS Paragon Plus Environment

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Figure 2: Spin resolved total (black dotted line) and scandium d-projected (red solid line) density of states shown for ScAu+ n , n = 1 − 6. Positive and negative values of the density of states represent spin-up and spin-down states, respectively. main lines comparable to the x-ray absorption spectrum of the 3d0 4s1 calcium cation, 34 not shown here. This similarity might seem to hint at a formal 3d0 configuration of the scandium dopant in ScAu+ 2 . However, since we are able to model the x-ray absorption spectrum of 31 ScAu+ (cf. Fig. 1) we can determine the ground state of the 2 in a charge transfer scheme

dopant atom from the experimental spectrum. Here, the ground state can be described as a configuration interaction of the two configurations [Ar]3d0 and [Ar]3d1 with almost equal relative contributions of 48% and 52%, respectively. We also find this strong hybridization of the dopant 3d electron state in the density of states where the scandium 3d state contributes



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to valence states that are spread over several electron volts, cf. Fig. 2. The experimentally determined scandium 3d occupation of 0.5 electrons shows the strong limitations of ad hoc assumed integer numbers of delocalized electrons to discuss the bonding mechanism within a phenomenological shell model approach. The finding of hybridized 3d electrons also applies to larger ScAu+ n clusters. Although we were not able to model the x-ray absorption signal within the charge transfer multiplet framework for n > 2 when mixing only two configurations, the deviation from the atomic spectral signature shows that the 3d state of the scandium dopant hybridizes strongly with the gold electronic states. This can also be seen from the comparison of the total and scandium d-projected density of states depicted in Fig. 2: The amount of pure scandium d character of the highest occupied molecular orbital in ScAu+ n (n > 1) is strongly decreased in contrast to ScAu+ 1 . The stronger hybridization originates from increased coordination as compared to ScAu+ 1 , as can be seen from the structures shown in Fig. 1. It is astonishing that the 3d electron is strongly localized in ScAu+ 1 although the mean radius of the 3d orbital (1.6 ˚ A) is comparable to the internuclear distance (2.5 ˚ A) suggesting at least some hybridization as is the case for larger ScAu+ n . However, delocalization of the gold 6s and scandium 4s electrons only, results in a shell closure in a two dimensional free-electron gas substantially stabilizing the bond. This in turn inhibits the participation of the scandium 3d electron in bonding and therefore leads to a complete localization of the dopant’s 3d electron. Shell closure could also be expected for the two-dimensional ScAu+ 6 cluster, again resulting in a localized scandium 3d state. However, localization of the 3d state can only be observed in ScAu+ 1 , presumably due to the large increase in coordination of the scandium dopant in ScAu+ 6 that leads to larger overlap or hybridization of the gold valence and scandium 3d states. Electron localization becomes more pronounced in titanium doped gold clusters, since the mean radius of the 3d orbitals decreases along the transition metal series. 35


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Odd-Even Effects in the Electronic Structure of Titanium Doped Gold Clusters

Figure 3: X-ray absorption spectra of titanium doped gold clusters at the L2,3 edges of titanium. Additionally shown are charge transfer model calculations (dashed line), experimental x-ray absorption spectrum of Ti+ shifted by −0.83 eV (Ref. 34) and the ground state structures, taken from Ref. 21. 9 ACS Paragon Plus Environment


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Figure 3 shows the x-ray absorption spectra of titanium-doped gold clusters at the L2,3 edges of titanium in a size range of n = 1 − 9. There is a strong odd-even effect visible in the x-ray absorption spectrum of TiAun+ clusters, which is most prominent in the oscillating excitation energy of the main feature of the L3 edge, indicated by the dashed line in Fig. 3. Furthermore, planar TiAu+ n clusters with odd n up to n = 5 exhibit almost identical spectral signatures. The x-ray absorption signal of these clusters can be modeled in a charge transfer scheme that reveals an almost pure [Ar]3d2 configuration (95 %) with only a small admixture of an [Ar]3d1 configuration (5 %) leading to 1.95 localized 3d electrons at the titanium impurity and to the strong similarity with the x-ray absorption spectrum of isolated Ti+ in Fig. 3. Hence, both 3d electrons in odd-numbered TiAu+ n clusters remain strongly localized at the titanium site but do not participate in bonding, which is then mediated by the titanium 4s electron. In even-numbered TiAu+ n clusters very similar spectral signatures among each other can be found as well. Additionally, a slightly increasing broadening of the spectral features with increasing cluster size can be observed most probably caused by an increasing coordination of the titanium dopant as a function of cluster size, cf. cluster geometries depicted in Fig. 3. As already mentioned, x-ray absorption probes the local electronic structure. Therefore changes in the symmetry or the unoccupied density of states due to bonding or fractional electron transfer to or away from 3d states would result in a deviation from the atomic spectral shape. We can exclude strong effects due to changes of the local symmetry since the spectral shape does not strongly change among the even-numbered TiAu+ n clusters although the coordination increases linearly with the cluster size. Again, by modeling the x-ray absorption spectra within the charge transfer model approach, cf. Fig. 3, we are also able to extract the ground state configuration of the titanium dopant 2 1 in even-numbered TiAu+ n clusters. We find a mixture of [Ar]3d and [Ar]3d configurations

with relative contributions of 59% and 41% respectively, yielding a formal 3d occupation of


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Figure 4: Total 3d occupation and difference of majority and minority spin 3d state occupation derived from a Löwdin (this work, DFT) and a Mulliken (Ref. 21, DFT) population analysis and from fit to the experimental data (CTM model calculations) of TiAu+ n clusters. 1.6 electrons. In support of this, a small odd-even variation of the 3d occupancy can also be seen in the Löwdin population analysis 36,37 of the DFT results presented in Fig. 4. However, the population analysis overestimates the number of local 3d electrons and simultaneously underestimates the magnitude of the odd-even effect in the titanium 3d occupation by about a factor of four. We want to shed some more light on the odd-even aspect by analyzing the density of states. In Fig. 5 the total density of states and the titanium d-projected density of states is displayed for the whole series of TiAu+ n clusters, n = 1 − 9. Again, the occupied density of states at the Fermi level is very similar among odd and even numbered TiAu+ n clusters, respectively. This is especially true for the titanium 3d states to which x-ray absorption at the L2,3 edges is sensitive. While in even numbered TiAu+ n clusters only one non-hybridized 3d electronic state is present, in odd numbered TiAu+ n clusters two almost non-hybridized 3d states can be found. In contrast to these well localized states near the Fermi level, highlighted by the arrows, in even numbered TiAu+ n clusters the second 3d electron hybridizes with the sd states of gold well below the Fermi level, cf. Fig. 5. The finding of two atomic-like 3d electrons in odd-numbered TiAu+ n clusters that do not participate in bonding, and of hybridization of one of the titanium 3d states in even-numbered TiAu+ n clusters is in very good agreement with the experimental data and charge transfer multiplet



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calculations yielding a 3d occupation of 1.95 and 1.6 electrons, respectively. The stronger localization of the 3d electrons in odd numbered TiAu+ n clusters is also sup-

Figure 5: Total density of states (black dotted line) and 3d-projected titanium density of states (red solid line) of TiAu+ n clusters. Electronic states of mainly titanium 3d character are highlighted by an arrow. Positive and negative values of the density of states represent spin-up and spin-down states, respectively. ported by the size dependent x-ray absorption onset shown in Fig. 6. Here, odd-numbered TiAu+ n clusters with two localized 3d electrons exhibit an absorption onset at lower excitation energy than even-numbered TiAu+ n clusters with one localized and one hybridized and therefore more delocalized 3d electron state. As was shown for pure transition metal clusters, a shift of the absorption onset towards lower excitation energy indicates a stronger localization of the 3d electrons. 24,38 12 ACS Paragon Plus Environment

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Figure 6: The absorption onset of TiAu+ n clusters as function of size suggests stronger 3d electron localization for odd numbered TiAu+ n clusters compared to even numbered ones. Furthermore, the degree of 3d localization also correlates with the local spin magnetic moments which are shown in Fig. 4. These were obtained from a Löwdin 36,37 (this work) and a Mulliken 39 population analysis (Ref. 21) of the DFT results. The difference of about 0.2 electrons between the Löwdin and the Mulliken population analysis is due to the strong basis set dependence of both methods. 40 The size dependent trend, however, is similar, apart from the case of TiAu+ 7 that will be discussed below. As the population analysis will not give the correct number of electrons it also fails to predict the correct local spin magnetic moments, but is still able to reproduce the general trend of the odd-even alternation of the local spin magnetic moments extracted from the charge-transfer multiplet calculations. From charge-transfer multiplet calculations, we find full spin polarization of the local titanium spin magnetic moment of 1.95 µB and 1.6 µB for odd- and even-numbered TiAu+ n clusters, respectively, also shown in Fig. 4. Although the same geometrical structure of TiAu+ 7 is found in this work and the work of Torres et al., a different local spin moment was reported. 21 The deviation from the oddeven pattern in TiAu+ 7 is probably caused by a geometrical 2D → 3D transition, increasing the mean coordination of the titanium atom and therefore maximizing the titanium-gold interaction. This in turn results in a reduction of the local spin moment as found in our pop-



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ulation analysis. This hypothesis is additionally supported by the size dependent absorption onset shown in Fig. 4. Although the absorption onset in TiAu+ 7 is reduced compared to the neighboring cluster sizes, the absolute value is comparable to even numbered TiAu+ n clusters and significantly enhanced with respect to the value for small odd numbered TiAu+ n clusters. Therefore a similar delocalization of the 3d electrons as in even numbered TiAu+ n clusters can be expected, which is consistent with our density-functional-theory calculations. This is strong evidence that the electronic structure of TiAu+ 7 presented here is indeed the one present in our experiment. The titanium dopant does not strongly disturb the overall gold cluster geometry. All the ground state structures of TiAu+ n (n = 1 − 9) clusters posses identical symmetry as the + ground state structures of neutral gold clusters, 41–43 except for TiAu+ 7 and TiAu9 .

The origin of the strong odd-even effect in TiAu+ n clusters might have its origin in electronpairing effects in the free-electron gas formed in the cluster, as observed in pure gold clusters. 3,44,45 To elucidate this, we neglect the 3d electrons of titanium for now and consider the dopants 4s electrons as fully delocalized. There are then n + 1 delocalized s−derived electrons in the TiAu+ n system, where n electrons stem from the 6s orbitals of gold while one electron is contributed by the 4s orbital of titanium. Hence, there is an odd number of delocalized electrons present in case of even numbered TiAu+ n clusters and vice versa. Now we additionally consider the impurity 3d electrons. In even numbered TiAu+ n clusters we found a partial delocalization of the 3d electrons. The energy cost to delocalize a titanium 3d electron can, in a zero order approximation, be estimated by the 3d2 4s1 → 3d1 4s2 promotion energy, 13 since the more extended 4s orbital is far more likely to hybridize with the gold sd valence states than the compact titanium 3d orbitals. The large titanium 3d2 4s1 → 3d1 4s2 promotion energy of about 3 eV (Ref. 46) could explain why we find only a partial delocalization of the titanium 3d electrons: The transfer of 0.41 3d electrons to a 4s state implies that a promotion energy of about 1.2 eV is counterbalanced by the gain from kinetic energy reduction and reduction of intra-atomic Coulomb interaction due to delocalization of a frac-


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tional 3d electron. Hence, the large promotion energy inhibits a complete delocalization of the titanium 3d electron. The situation is different for odd-numbered TiAu+ n clusters: Here an even number of delocalized gold 6s and titanium 4s electrons leads to a double occupation of free-electron gas states. Hence, in order to delocalize a titanium 3d electron not only the promotion energy has to be counterbalanced by the energy gain of delocalization, but also the energy difference to the next unoccupied free-electron gas state. This is contrary to the case of even-numbered TiAu+ n clusters where the highest free-electron gas state in energy is only singly occupied. The resulting high energy barrier eventually stabilizes the localization + of both 3d electrons in odd-numbered TiAu+ n clusters. Thus, titanium in TiAun clusters

serves as an electron donor. But due to the high promotion energy, titanium cannot provide the exact number of electrons to form doubly occupied states in the free-electron gas for even-numbered TiAu+ n clusters, leading to a partial occupation of the titanium 3d-states. Moreover, there is a second effect beneath the strong odd-even effect. From the density of states presented in Fig. 5 it can be seen that the free-electron-gas states, mainly all states which do not exhibit titanium 3d character, are more strongly bound in case TiAu+ 1 and TiAu+ 5 as compared to the other cluster sizes. This may originate from shell closure in a two dimensional potential well with two and six delocalized electrons, 16 respectively. All these results suggest that one always finds fully localized 3d electrons and an even number of delocalized 4s/6s electrons in odd numbered TiAu+ n clusters. This was already suggested from the odd-even staggering in the photofragmentation of titanium doped gold clusters. 13 However, in case of TiAu+ 5 cluster it could not be conclusively decided from the photofragmentation whether the titanium impurity delocalizes all of its 3d and 4s valence electrons in a three dimensional structure or just one 4s electron in a two dimensional potential well. 17 By providing element specific information via x-ray absorption spectroscopy we now show that both 3d electrons stay localized which is consistent with a delocalization of the titanium 4s electron in its two dimensional structure.



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Independence of the Local Electronic Structure on Impurity Coordination: Vanadium Doped Gold Clusters

Figure 7: X-ray absorption spectra of vanadium doped gold clusters at the L2,3 edges of vanadium. The spectral signature is almost identical for all cluster sizes, although dramatic structural changes are present, as can be seen from the ground state structures. 21 Additionally shown is a charge transfer model calculation (71% [Ar]3d3 and 29% [Ar]3d4 , dashed line). In Fig. 7 the x-ray absorption spectra of vanadium doped gold clusters at the vanadium L2,3 edges together with their ground state geometries 21 are presented in the size range of n = 1 − 7. As can be seen clusters with n = 1 − 5 exhibit almost identical spectral shapes. Because of the increased life-time broadening at the L2 compared to the L3 edge and the high density 16 ACS Paragon Plus Environment

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of states the L2 edge is not as well resolved as the L3 edge. The similarity of the mentioned spectral shapes is therefore especially striking at the L3 edge where we are able to resolve characteristic features. There is, however, a splitting of the main line at the L3 edge in + VAu+ 2 which is absent in VAun , n = 1, 3 − 5. This could be induced by a crystal field

splitting which is supposedly enhanced in this more compact structure as compared to the + open structures of, e.g. ScAu+ 2 and TiAu2 . As the small deviation in the spectral signature + in VAu+ 2 is presumably caused by the geometric effects, the similar spectral shapes of VAun ,

n = 1 − 5, indicate that these clusters have the same local electronic structure. As in TiAu+ n clusters the coordination of the vanadium impurity generally has no strong influence on the local electronic structure, although the coordination increases linearly with the cluster size. From the electronegativities of gold and the third-row transition metals it can be expected that the charge is always located at the impurity atom. 47 Since the vanadium cation with its [Ar]3d4 4s0 ground state configuration 48 offers no 4s electron for bonding, in contrast to scandium and titanium, at least some of the vanadium 3d states can be expected to hybridize with the host materials electronic states in order to form a bond. However, the promotion of a 3d electron into the far more extended 4s orbital which then forms a molecular orbital with the host electronic states is also conceivable. This can be assumed to be energetically favorable since the 3d4 4s0 → 3d3 4s1 promotion energy of 320 meV (Ref. 48) is rather small. The similar x-ray absorption spectra for all VAu+ n clusters, n = 1−5, suggest that the number of 3d electrons is cluster size independent. From charge transfer multiplet calculations, also shown in Fig. 7, we extract the occupation number of the 3d states to be 3.3 resulting from a configuration interaction of [Ar]3d3 and [Ar]3d4 configurations with relative contributions of 71% and 29%, respectively, which is in excellent agreement with a Löwdin population analysis of the DFT results yielding a constant number of 3d electrons of 3.5 for all cluster sizes. Hence, vanadium-doped gold clusters are qualitatively different from TiAu+ n clusters since they are missing the pronounced odd-even effect in their electronic structure. In order to feature an even number of delocalized electrons in even numbered VAu+ n clusters, the



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vanadium atom would have to delocalize a second 3d electron. Again this involves an energy cost which can be estimated by the promotion energy. Since this is larger than 5 eV (Ref. 48), delocalization of a second 3d electron seems to be very improbable. Therefore localization of 3.3 electrons in the 3d state is stabilized. Contrary to the absence of a size dependence of the electronic structure of the impurity for the smaller clusters, the sudden change in the spectral shape from cluster size five to six is somehow puzzling. A possible explanation might be that the structure depicted in Fig. 7 is not the one experimentally present. However, it is unlikely that the clusters produced in the cluster source are not the ground state species, because of the very mild cooling conditions present in the source, which was shown to produce preferably the geometrical ground state structures. 26 As the potential energy surface of metal clusters is very complex even for small cluster sizes, it seems more likely that the theoretically predicted structure 21 does not + + represent the ground state geometry of VAu+ 6 . Furthermore, the spectra of VAu6 and VAu7

are very similar which suggests that also VAu+ 6 has rather a three- than a two-dimensional structure.

Local Electronic Structure of Chromium Doped Gold Clusters In Fig. 8 x-ray absorption spectra of chromium doped gold clusters at the L2,3 edges of chromium are presented in a size range of n = 1 − 8. From Fig. 8 it can be seen that the spectral signature changes drastically with the host cluster size, again indicating changes in occupation of the 3d states or the symmetry of the environment. The x-ray absorption spectra of three CrAu+ n clusters (n = 2, 6, 8) attract particular attention. Here a strong resemblance with the spectra of atomic Cr+ can be found, 34 also evident from a comparison to an atomic Hartree-Fock calculation of Cr+ , 30,49 cf. dotted lines in Fig. 8. Hence, in these cases the 3d electrons of chromium seem to be mainly undisturbed by the 18 ACS Paragon Plus Environment

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Figure 8: X-ray absorption spectra of chromium doped gold clusters at the L2,3 edges of chromium. The spectral signature changes drastically as a function of the cluster size. Additionally shown are an atomic Hartree-Fock calculation of Cr+ (dotted line), the x-ray absorption spectrum of Cr+ (Ref. 34), a charge transfer multiplet calculation (dashed line) and the reoptimized ground state structures from Ref. 21. gold host matrix or, put differently, do not participate in bonding. The strong localization of the 3d electrons is also evidenced by a shift of the absorption onset towards lower photon energy 38 as compared to neighboring cluster sizes, cf. Fig. 9. The interaction of the host with the chromium impurity in these particular systems is strongly suppressed by shell closure in the free-electron gas of the gold host cluster. Shell 19 ACS Paragon Plus Environment


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Figure 9: Size dependent absorption onset in the x-ray absorption of CrAu+ n . Strong localization of the 3d electrons in CrAu+ , n = 2, 6, 8, is reflected in a shift towards lower energy n compared to the neighboring cluster sizes. closure is found for two and six electrons in a two-dimensional and for eight electrons in a three-dimensional potential well. Indeed, the gold hosts in CrAu+ n , n = 2, 6, 8, feature suit+ able geometries, i.e. two dimensional structures in CrAu+ 2 , CrAu6 and a three dimensional

structure in CrAu+ 8 , as can be seen from the structures depicted in Fig. 8. Therefore, in CrAu+ n , n = 2, 6, 8, the chromium impurity is less strongly bound to the host as compared to the other cluster sizes, as was already shown in another publication. 22 However, there is also a striking similarity among the x-ray absorption spectra of CrAu+ n, n = 1, 5, 7, cf. Fig. 8. This can also be understood in terms of shell closure in a free electron gas. In order to reach shell closure in these clusters, chromium would have to delocalize one 3d electron, due to its cationic ground state configuration 3d5 4s0 (Ref. 46) missing a 4s electron. This in turn leads to the deviation of the spectral signature from a local [Ar]3d5 configuration. However, the 3d5 4s0 → 3d4 4s1 promotion energy is about 1.5 eV (Ref. 46) preventing the complete delocalization of one 3d electron and resulting in only a partial delocalization as extracted from charge transfer multiplet calculations. We find these clusters to exhibit a mixed ground state configuration of 57% [Ar]3d4 and 43% [Ar]3d5 , i.e., a 3d occupation of 4.4 electrons. Therefore we can estimate that the energy gain by delocalizing one electron in order to form shell closure in a free electron gas with two, six and eight


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electrons, respectively, is smaller than 1.5 eV in CrAu+ n , n = 1, 5, 7. The above discussion shows that the bonding in CrAu+ n clusters is dominated by shell closure effects in the gold host cluster, preventing hybridization with the chromium 3d states in some cases.

Conclusion Albeit the very different size dependence in the x-ray absorption spectra of MAu+ n (M = Sc, Ti, V, Cr), a common ground in the bonding mechanism of the impurity to the host can be deduced, which can be traced back to an interplay of the promotion energy, the dopant’s cationic ground state, the mean 3d radius and shell closure in the free electron gas. This interplay makes the size dependence of the electronic structure of transition metal (Sc, Ti, V, Cr) doped gold clusters so rich. We could show that the impurity atom in general acts as an electron donor if hybridization of the 3d impurity and host electronic states is present. Whenever possible the MAu+ n systems tend to form a closed shell in the free-electron gas and preferably exhibit spin-pairing in these states. Whether this is possible depends on an interplay of the number of host atoms and on the nature of impurity. While the gold host cluster contributes n electrons to the free-electron gas, the number of electrons contributed by the impurity does not only depend on its cationic ground state configuration, e.g., the presence of a 4s electron, but also on the 3dk 4sl → 3dk−1 4sl+1 promotion energy and the mean 3d radius. The resulting hybridization generally leads to non-integer impurity 3d-state occupation, as shown experimentally here. Hence, an ad hoc assumption of an integer number of delocalized electrons cannot generally be made and is only possible in very special cases. In the future it would be worthwhile to perform Stern-Gerlach experiments on transitionmetal-doped gold clusters to directly study their magnetic properties. Furthermore, x-ray magnetic circular dichroism spectroscopy exploiting the element-specificity of the method



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could even provide information on the inter-cluster spin coupling of the impurity local moment and spin of the free-electron gas of the gold host.

Acknowledgement Beam time for this project was granted at BESSY II beamline U49/2-PGM-1, operated by Helmholtz-Zentrum Berlin. Technical assistance by HZB staff members O. Schwarzkopf, R. Follath, G. Reichardt, and H. Pfau during beam time is gratefully acknowledged. BvI acknowledges travel support by HZB.

Supporting Information Available Parameters of the density functional theory, charge transfer multiplet, and Hartree Fock calculations.

This material is available free of charge via the Internet at http://pubs.

acs.org/.

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Impurity Electron Localization in Early-Transition-Metal-Doped Gold

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