Article pubs.acs.org/JPCC
Photoionization Yields, Appearance Energies, and Densities of States of Copper Clusters Avik Halder, Chuanfu Huang, and Vitaly V. Kresin* Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089-0484, United States ABSTRACT: We present the results and analysis of photoionization measurements of neutral Cun=24−98 clusters, thermalized to T = 60 and 215 K. The photoion yield curves were used (1) to determine the cluster ionization appearance energies via a fit to the Fowler law of surface photoemission and (2) to explore the suitability of these curves for accessing information about electron states lying below the ionization threshold. The appearance energies follow a standard shell pattern up to n ≈ 40, but a subsequent structure suggests that the electronic shells become perturbed and split by crystal field effects. Nevertheless, it is apparent that the spectra are determined essentially by the number of delocalized electrons, i.e., by the sequential filling of electronic shell sublevels. The average shift of the appearance energies with cluster temperature was found to be in good agreement with a theoretical prediction for bulk copper. An important finding is that the derivatives of the ion yield curves overlap well with the photoelectron spectra of isoelectronic cluster anions. This means that accurate photoionization yield curves of cold clusters can be used to trace their subthreshold electronic densities of states.
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follows the criteria for metal-cluster shell ordering.2,10−13 On the other hand, the presence of its d shell makes it distinct from an alkali metal, affecting the cohesive energies,14 the atomic pseudopotential,15 and the efficiency of light absorption.16,17 Following a description of the experimental setup, we discuss the mass spectra obtained at different wavelengths, the ionization appearance energies and their temperature dependence, and the correspondence between photoionization yield curves, photoelectron spectra, and the electronic state densities.
INTRODUCTION Photoionization of free metal clusters, used in tandem with mass spectrometry, has been a rich source of information on the electronic structure of these systems and its evolution with cluster size (see, e.g., the reviews in refs 1−5). Most experiments monitoring the photoion yield from neutral clusters have focused on the determination of their ionization thresholds (also referred to as appearance energies, ionization energies, or ionization potentials). These thresholds, together with abundance distributions, are sensitive gauges of the electronic stability of nanoclusters. Furthermore, they can be influenced by the particles’ internal temperature which affects both the electronic and the vibrational degrees of freedom. Thus, it is productive to monitor how the photoionization behavior varies with size and temperature for different cluster families. Exploration of deeper-lying states, on the other hand, has been primarily the domain of photoelectron spectroscopy (see, e.g., the reviews in refs 6−9). Such work commonly uses sizeselected cluster anions because they have lower electron detachment energies than neutrals. However, the post-threshold part of a neutral cluster’s photoion yield curve (which is of course the same as the photoelectron yield curve) also often displays a structure which derives from subthreshold levels. It is therefore useful to explore the relation between photoelectron spectra and photoionization yield curves. In this paper we describe the results of laser ionization experiments on neutral copper clusters ranging in size from 24 to 98 atoms, acquired for cluster temperatures of 60 and 215 K. Copper, as a coinage metal, is an interesting cluster material. On one hand it is an excellent monovalent conductor which © XXXX American Chemical Society
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EXPERIMENT Cun clusters are produced inside a magnetron sputtering/ condensation source.18,19 DC argon ion sputtering of a 1 in. metal target at 200−250 V and 10−20 W produces a cloud of metal vapor. A continuous flow of argon and helium gas at ≈80 and 180 sccm, respectively, is maintained inside the liquid nitrogen-cooled condensation tube. This creates a pressure of ≈0.75 mbar inside the tube, and the supersaturated metal vapor coalesces to form the metal clusters. The source produces both neutral and ionized clusters; in this work we are interested in the former. At the end of the aggregation chamber (7.6 cm diameter, 10 cm aggregation length) the clusters enter a specially designed thermalizing tube of 12 cm length, 1.6 cm inner diameter, and Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: December 1, 2014 Revised: January 9, 2015
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The Journal of Physical Chemistry C with a 6 mm exit hole.20 It is maintained at ≈0.6 mbar pressure in order to allow the clusters to undergo ∼105 collisions with the gas molecules and equilibrate to within at most a few kelvin of the tube wall temperature.20,21 To obtain the temperature of 215 K the tube is cooled by direct contact with the aggregation chamber and counterheated by an electric rope heater. To reach the low temperature of 60 K the tube is wrapped by copper braids secured to the first stage of a closed-cycle helium refrigerator (CTI Cryo-Torr). The thermalizing tube temperature is monitored by several RTD or silicon diode sensors embedded along its length. By carefully optimizing the thermal contacts and operating conditions and by wrapping the tube with multiple layers of superinsulation, it can be equilibrated to within ±1 K along its entire length. The two temperatures are the highest and lowest achievable with the present thermalizing tube design. Clusters exiting the thermalizing tube proceed toward a conical skimmer with a 2 mm aperture positioned 2 cm from the exit hole. In the subsequent chambers, maintained at a pressure of ∼10−7 Torr, the neutral clusters are ionized using 5 ns pulses from a tunable Nd:YAG/OPO laser system (EKSPLA NT342/3/UV). The ionization takes place within the homebuilt extraction region of a linear Wiley−McLaren time-of-flight mass spectrometer. Laser pulse energy is carefully monitored immediately past the ionization region, and the ion yield curves are corrected for the intensity drifts of both light and cluster beams. The latter is taken into account by normalizing all measured ion rates to reference spectra taken at 216 nm after each collection interval. For the present experiments the wavelength is varied within the range of 210−250 nm using a fluence of ≈500 μJ/cm2 in order to ensure single photon ionization. The linearity of the ion yield Y(E) is illustrated in Figure 1.
In order to obtain the ion yield curves for individual clusters, the mass spectra were deconvoluted as follows. Analysis showed that within the investigated size regime each time-offlight peak can overlap with at most its second nearest neighbors; hence, for each cluster size Cux the intensity was found by fitting five Gaussians to the mass spectrum points spanning the range from x − 2 to x + 2 and then integrating the strength of the central peak, x.
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RESULTS: MASS SPECTRA AND PHOTOION YIELD CURVES Mass spectra obtained at T = 60 K for two representative wavelengths are shown in Figure 2. At 240 nm (5.17 eV), the
Figure 2. Time-of-flight mass spectra obtained at 240 nm and (portion shown in the inset) 225 nm for clusters thermalized to 60 K.
photon energy falls within the range of cluster ionization energy variations (cf. Figure 4). The intensity maxima and groupings are similar to the pattern observed for copper clusters at the same wavelength in ref 22 using a 77 K laser vaporization source. As in that work, one can observe abundance maxima at sizes which both match spherical or spheroidal shell ordering expectations23 (e.g., Cu+21,41) as well as deviate from them (e.g., the Cu+49,61,92 peaks which suggest especially low appearance energies relative to their neighbors). At 225 nm (5.51 eV), the photon energy exceeds the ionization energies of all the clusters in the studied size range, and the abundance pattern changes. Now all the cluster sizes are clearly visible, and there is a clear odd−even intensity alteration, especially prominent for sizes below Cu42 (see the inset in Figure 2). This change has also been observed in ref 22. Photoion (i.e., photoelectron) yield curves were derived from mass spectra obtained at different laser wavelengths for two different cluster temperatures. They serve as the “parent” curves for further analysis. Examples are plotted in Figure 3.
Figure 1. Cluster ion yield (integrated yield in the range from Cu40 to Cu48) as a function of laser fluence at λ = 216 nm. The linearity of the plot confirms that the ion signals derive from single-photon ionization. Photoionization data discussed in this paper were acquired at a fluence of ≈500 μJ/cm2.
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IONIZATION THRESHOLDS Deriving appearance energies In from cluster ion yield plots Y(E) is not a trivial procedure due to the lack of a general theory of cluster photoionization. A number of more or less ad hoc threshold fit algorithms have been applied in the literature, a common one being simple linear extrapolation of Y(E). On the other hand, an excellent description of nanoparticle work functions is provided by the Fowler law of photoemission24,25 which was originally developed for metal surfaces, and we have demonstrated that it works very well for smaller metal clusters as well.26,27 Since the temperature of the copper clusters in the
At the end of the 1.3 m long flight path the cluster ions are detected using a channeltron detector (DeTech Inc.) custom engineered to permit high-voltage conversion dynode operation, here 14 kV, which plays a vital role in the efficient detection of heavy ions. Time-of-flight mass spectra are collected using an ORTEC MCS-pci multichannel scaler. The data were acquired in wavelength steps of 1 nm with a total collection time of ≈30 h for each temperature. B
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possibly due to the promotion of more directional bonding by the presence of the d-band; see also the discussion in the next section. An additional piece of information that can be extracted from the data is the change of the ionization threshold with temperature. As can be seen in Figure 4, the temperature shift from 60 to 215 K is very small and, especially for the larger clusters, lies practically at the limit of the experimental resolution. Therefore, at the present stage it is not justified to attempt a size-by-size listing of the temperature coefficients. Instead, we averaged over all the cluster sizes in the data and found ⟨ΔI/ΔT⟩ ≈ −4.9 × 10−5 eV/K. This observed shift should be an electronic effect. The clusters’ melting point can be estimated31,32 to lie at approximately 0.3−0.4 of the bulk melting point (1358 K), i.e., higher than the present temperature range, and indeed, in the density of states distributions (see below) almost no differences could be observed. Therefore, it is interesting to compare the above ionization threshold shift with the expected thermal shift of the work function Φ of solid bulk metals. Reference 33 considers the effect of thermal expansion on the electron density and Fermi level parameters of many metals and deduces their expected dΦ/dT behavior as a function of temperature. For the T intermediate between 60 and 215 K, the tabulated coefficients for Cu predict dΦ/dT ≈ −5 × 10−5 eV/ K, in quite remarkable agreement with the average value deduced above. This attests, as demonstrated previously using alkali nanoparticles,25 that free clusters are a convenient system for exploring such subtle work function shifts because they are not plagued by surface contamination effects which are a concern for experiments on bulk surfaces.
Figure 3. Selection of representative photoion yield curves Y(E) for the product range Cu+48−63. The blue and red curves denote cluster temperatures of 60 and 215 K, respectively. The data were acquired with photon energy steps of 1 nm (corresponding to energy steps of 20−25 meV). The abrupt shifts of onset thresholds to lower energies at n = 49 and 61−63 correspond to the appearance of energy minima in Figure 4 and to the enhanced ion intensities in Figure 3.
present study is low, the Fowler formula can be approximated by the quadratic rise Y (E) ∝ (E − In)2
(1)
The appearance energy can therefore be determined from a straight line intercept for [Y(E)]1/2 near the threshold, and we found that this provides a good fit to the data curves (indeed, better than a straight line fit to Y(E) itself). Such a procedure also was used previously for the determination of work functions of cold free alkali nanoparticles.28 Figure 4 shows our results for Cu24−Cu98.
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PHOTOELECTRON YIELD CURVES AND ELECTRONIC DENSITIES OF STATES As mentioned in the Introduction, the post-threshold part of a cluster’s photoelectron yield curve Y(E) contains contributions from deeper-lying electron states. It is a convolution of electronic transitions out of all the accessible states.34 That is, one can write ∞
Y (E ) =
∫−E M(ε)f (ε)D(ε)dε
(2)
where ε is the electron energy; M is the dipole transition matrix element for one-photon absorption; f is the Fermi−Dirac distribution function; and D is the density of states function of the cluster electrons. The vacuum level energy is set to ε = 0. Thus, the derivative dY/dE is proportional to D(ε), and if the matrix element is a smooth function of energy (as is the distribution function), then the dominant structure of the derivative will map out the shape of the electronic state density. In principle, then, there is a direct relation between a photoelectron yield curve derivative and a photoelectron spectrum. Of course, the latter is a more direct, clean, and precise measurement which produces a direct view of the electronic states. However, it is more demanding technically and, moreover, is typically restricted to cluster anions in view of the photon energies available outside of an accelerator setting. Therefore, it is worthwhile to check whether a photoionization experiment on a neutral cluster can yield sensible information about electronic states lying below the ionization level. The above assumes that the main contribution comes from single-electron transitions. In principle, collective effects in
Figure 4. Solid dots: ionization energies of neutral copper clusters extracted from the yield curves in Figure 3 using the threshold fit in eq 1, for two values of cluster internal temperature. Open circles: an earlier measurement of Cun ionization energies from ref 22 obtained by bracketing the appearance intervals of cluster peaks in time-of-flight mass spectra. Close values for n ≤ 41 were also reported in ref 29.
The data show significant odd−even oscillations for n ≤ 42 and several noticeable minima, features which have also been observed in ref 22. The “plunges” in the appearance energies for n = 49, 61, 92 are not trivially explainable within the elementary cluster shell structure framework, as already mentioned above (although there is a coincidence for n = 49, as for this size the topmost 1g shell is exactly half filled). Interestingly, the ionization thresholds of Ag49 and Ag61 clusters are also strong minima,30 while Cu48 is especially nonreactive toward oxygen.12 These effects hint at the perturbation of electronic shells by the ionic (pseudo)potential framework, C
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the difference to electrostatic image charge forces, one finds42 δE ≈ e2/R, where R ≈ rsa0n1/3 is the cluster radius (a0 is the Bohr radius, and rs is the Wigner−Seitz electron density parameter, 2.7 for copper). For the cluster sizes shown in Figure 5 this predicts a shift ranging from ≈2.9 eV for Cu41 to ≈2.5 eV for Cu63. This is valuable for guidance; however, the precise shift is sensitive to the cluster shape and higher-level nonelectrostatic corrections. Therefore, for the comparison in Figure 5 we adjusted δE so as to optimize peak overlap. The values of shifts used for the individual clusters are listed in the caption, and they fluctuate but are agreeably close to the above estimate. The photoelectron spectra supplied in ref 39 correspond to cluster internal temperatures of 300 K. However, it is pointed out that for the coinage metal clusters no variation, except beam narrowing, was observed as the temperature was reduced to 100 K and then even lower. This is ascribed to the high melting point of these materials and is consistent with the fact that our photoionization data also do not show any substantial change from 215 to 60 K, except for statistical variations. The match between the dY(E)/dE curves derived from our photoionization yield spectra and the densities of states D(ε) derived from the photoelectron spectra39 is overall highly satisfactory. This observation is one of the main results of this paper: a carefully measured photoionization curve of sufficiently cold clusters can reveal information about the subthreshold structure of the electronic density of states. The fact that the spectra of isoelectronic Cun and Cu−n−1 overlap illustrates how, even when the influence of ions is evident, it is the sequential filling of electronic shells that governs the spectral sequence. The effect of perturbation caused by ionic pseudopotentials arranged in a certain geometric structure is to lower the symmetry and give rise to splitting of the shell levels and to additional spectral gaps and subshell closings. This general framework was first considered in ref 43 and subsequently analyzed for a number of specific cluster families, e.g., in ref 40, for noble-metal clusters. For example, in the range of n ≈ 51−58 the geometry of copper and silver clusters is close to the icosahedron.40,44,45 The appearance of a gap opening at Cu49 and Cu61−63, evidenced by the ionization energies in Figure 4 and directly visible in the density-of-state plots in Figure 5, is also possibly caused by subshell closing induced by crystal field splitting. The precise form of the crystal field, the essential physics of its evolution with size, the influence of the d band (which is absent in the alkalis but present in copper and silver), and the overlay between crystal field effects and Jahn−Teller cluster shape deformations represent a set of interesting conceptual questions for future theoretical and experimental studies.
photoabsorption should be kept in mind, as the decay of collective oscillations (“plasmons”) may lead to photoelectron emission35,36 and modify the yield profile and its derivative. The latter would be especially affected by a sharp overlapping collective resonance. The present data, however, should be mostly free of such interference. Indeed, the band structure of copper does not lead to a sharp surface resonance in nanoparticles and clusters but instead to a gradually increasing cross section above 2.5 eV (see, e.g., ref 17), showing no features in the energy range of Figure 3 that could give rise to any peaks in dY/dE. In small metal clusters optical absorption can also excite red-shifted volume-plasmon-like resonances;37 however, they are broadened and have relatively little strength, whereas the bulk plasmon in metallic copper is already quite diffuse and weak.38 Thus, while collective and d-band contribution to photoionization dynamics is an interesting question and might be illuminated by a measurement of absolute oscillator strengths (unfortunately not currently viable for neutral cluster beam ionization), it is not expected to significantly influence the present analysis. In Figure 5 we plot the derivatives of some of the Cun photoionization yield curves, such as those in Figure 3. The
Figure 5. Blue and red solid lines: selected dY(E)/dE curves derived from the neutral Cun cluster photoionization spectra in Figure 3 for clusters at 60 and 215 K, respectively. Green dashed lines: photoelectron spectra for isoelectronic Cu−n−1 cluster anions from ref 39. The photoelectron spectra were normalized by an overall vertical factor and shifted so as to align the binding energy scale with the photoionization threshold; the energy axes were not scaled in any other way. The magnitude of the alignment shift in eV was δE = (2.74, 2.42, 2.68, 2.34, 2.45, 2.35, 2.41, 2.33, 2.45, 2.5) for n = (41, 49, 50, 51, 52, 58, 59, 61, 62, 63), close to the magnitude expected for the difference between the ionization energy of the neutral cluster and the electron affinity of the anion (see text for details).
results at 60 and 215 K match rather closely, as mentioned in the preceding section. Conveniently, in ref 39 (see also refs 13, 40, and 41) one finds a set of photoelectron spectra of cold Cun− anions. They are in essence direct images of the clusters’ densities of states and can be compared with the derivative plots. This is done in Figure 5 by superimposing the binding energy plots from ref 39 for isoelectronic clusters; i.e., the ionization yield data for Cun are compared with the photoelectron spectra for Cu−n−1. An item to note at this point is that for a proper comparison one needs to line up the threshold of the photoionization yield curve with the binding energy scale of the photoelectron spectrum. The two are offset due to the difference δE between the ionization energy of the neutral cluster and the electron affinity of the anion. In the first approximation, which assigns
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CONCLUSIONS
A beam of neutral copper clusters was produced by a magnetron sputtering/condensation source equipped with a newly designed thermalization tube. Accurate photoelectron yield curves for clusters with internal temperatures of 60 and 215 K were measured using tunable laser single-photon ionization. The following is a summary of the main results. −Ionization thresholds were found to be well fitted by the low-temperature form of the Fowler photoemission law (quadratic dependence of the yield on the photon energy above the threshold, eq 1). This form has a well-defined D
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(5) Alonso, J. A. Structure and Properties of Atomic Nanoclusters; Imperial College Press: London, 2005. (6) Mason, M. G. Photoemission Studies of Supported Metal Clusters, the Early Years. In Cluster Models for Surface and Bulk Phenomena; Nato Science Series Bl Pacchioni, G., Bagus, P. S., Parmigiani, F., Eds.; Plenum: New York, 1992; Vol. 283. (7) von Issendorff, B.; Cheshnovsky, O. Metal to Insulator Transitions in Clusters. Annu. Rev. Phys. Chem. 2005, 56, 549−80. (8) von Issendorff, B. The Electronic Structure of Alkali and Noble Metal Clusters. In Handbook of Nanophysics: Clusters and Fullerenes; Sattler, K. D., Ed.; CRC Press: Boca Raton, 2011. (9) Tchaplyguine, M.; Ö hrwall, G.; Björneholm, O. Photoelectron Spectroscopy of Free Clusters. In Handbook of Nanophysics: Clusters and Fullerenes; Sattler, K. D., Ed.; CRC Press: Boca Raton, 2011. (10) Katakuse, I.; Ichihara, T.; Fujita, Y.; Matsuo, T.; Sakurai, T.; Matsuda, H. Mass Distributions of Copper, Silver and Gold Clusters and Electronic Shell Structure. Int. J. Mass Spectrom. 1985, 67, 229− 236. (11) Katakuse, I.; Ichihara, T.; Fujita, Y.; Matsuo, T.; Sakurai, T.; Matsuda, H. Mass Distributions of Negative Cluster Ions of Copper, Silver and Gold. Int. J. Mass Spectrom. 1986, 74, 33−41. (12) Winter, B. J.; Parks, E. K.; Riley, S. J. Copper Clusters: The Interplay Between Electronic and Geometrical Structure. J. Chem. Phys. 1991, 94, 8618−8621. (13) Taylor, K. J.; Pettiette-Hall, C. L.; Cheshnovsky, O.; Smalley, R. E. Ultraviolet Photoelectron Spectra of Coinage Metal Clusters. J. Chem. Phys. 1992, 96, 3319−3329. (14) Harrison, W. A. Theoretical Alchemy: Modeling Matter; World Scientific: Singapore, 2010. (15) Harrison, W. A. Solid State Theory; Dover: New York, 1980. (16) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (17) Gomez, L. F.; Loginov, E.; Halder, A.; Kresin, V. V.; Vilesov, A. F. Formation of Unusual Copper Clusters in Helium Nanodroplets. Int. J. Nanosci. 2013, 12, 1350014. (18) Haberland, H.; Karrais, M.; Mall, M.; Thurner, Y. Thin Films From Energetic Cluster Impact: A Feasibility Study. J. Vac. Sci. Technol. A 1992, 10, 3266−3271. (19) Haberland, H.; Mall, M.; Moseler, M.; Qiang, Y.; Reiners, T.; Thurner, Y. Filling of Micron-Sized Contact Holes With Copper by Energetic Cluster Impact. J. Vac. Sci. Technol. A 1994, 12, 2925−2930. (20) Halder, A. Temperature-Dependent Photoionization and Electron Pairing in Metal Nanoclusters. Ph. D. dissertation, University of Southern California, Los Angeles, 2015. (21) Hock, C.; Schmidt, M.; von Issendorff, B. Low-Temperature Caloric Behavior of a Free Sodium Nanoparticle. Phys. Rev. B 2011, 84, 113401. (22) Knickelbein, M. B. Electronic Shell Structure in the Ionization Potentials of Copper Clusters. Chem. Phys. Lett. 1992, 192, 129−134. (23) Clemenger, K. Ellipsoidal Shell Structure in Free-Electron Metal Clusters. Phys. Rev. B 1985, 32, 1359−1362. (24) Burtscher, H.; Siegmann, H. C. Aerosols, Large Clusters in Gas Suspensions. In Clusters of Atoms and Molecules II: Solvation and Chemistry of Free Clusters, and Embedded, Supported and Compressed Clusters; Haberland, H., Ed.; Springer: Berlin, 1994. (25) Wong, K.; Tikhonov, G.; Kresin, V. V. Temperature-Dependent Work Functions of Free Alkali-Metal Nanoparticles. Phys. Rev. B 2002, 66, 125401. (26) Wong, K.; Kresin, V. V. Photoionization Threshold Shapes of Metal Clusters. J. Chem. Phys. 2003, 118, 7141−7143. (27) Prem, A.; Kresin, V. V. Photoionization Profiles of Metal Clusters and the Fowler Formula. Phys. Rev. A 2012, 85, 025201. (28) Wong, K.; Kasperovich, V.; Tikhonov, G.; Kresin, V. V. PhotoIonization Efficiency Curves of Alkali Nanoclusters in a Beam and Determination of Metal Work Functions. Appl. Phys. B: Laser Opt. 2001, 73, 407−410. (29) Persson, J. L.; Andersson, M.; Holrngren, L.; Åklint, T.; Rosén, A. Ionization Potentials of Oxidized Copper Clusters. Chem. Phys. Lett. 1997, 271, 61−66.
physical basis and is convenient for extracting appearance energies. −The average shift of the cluster ionization energy with temperature was found to be in excellent agreement with a theoretical prediction for bulk copper. The use of nanocluster beams is a useful contamination-free route toward exploring subtle shifts in metal work functions. −The behavior of ionization energy minima and oscillations matches a basic shell structure pattern up to approximately Cu40 but then transitions to a more complicated sequence. The sharp minimum of ionization energies at sizes such as Cu49,61,92 is also evidenced by the appearance of an energy gap in these clusters’ electronic density of states. −Nevertheless, even in the latter case the electronic density of states curves are found to follow the number of cluster electrons rather than atoms, hence the spectra continue to be governed by the filling of electronic shell sublevels. Evidently the effect of the crystal field is to split the shell orbitals and introduce additional subshell gaps and closings. It is important to seek a conceptual understanding of the underlying geometric structures, their transformations, and possible connection to s− d band interaction effects peculiar to noble-metal clusters. −The photoelectron yield curves contain information not only about electronic states at the threshold but also about the deeper-lying ones. A comparison between yield curve derivatives and photoelectron spectra of isolectronic cluster anions shows a convincingly good overlap of the spectral features. This result validates the use of accurate photoionization curves of cold clusters as a source of information about the electronic density of states of their subthreshold levels. In particular, this enables tracing the density of states structure for neutral clusters. (In a recent application, this allowed us to interpret an unusual transition in the photoionization curve of aluminum clusters as evidence of novel superconducting high-Tc pairing.46)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1-213-740-0868. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciate a useful discussion with Prof. Bernd von Issendorff and Dr. Oleg Kostko and the contribution of Dr. A. Liang at the initial stages of experiment design. We also would like to thank the staff of the USC Viterbi/Dornsife Machine Shop for their skillful technical help. This work was supported by the U.S. National Science Foundation under Grant No. DMR−1206334.
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REFERENCES
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