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Threshold photoemission spectroscopy in solids
Journal of Electron Spectroscopy and Related Phenomena 154 (2007) 63–68
Threshold photoemission spectroscopy in solids F. Offi a,∗ , L. Avaldi b , R. Camilloni b , G. Dawber c , G.C. King c , G. Stefani a a
CNISM and Dipartimento di Fisica, Universit`a Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy b CNR-IMIP, CP10, I-00016 Monterotondo Scalo, Italy c Department of Physics and Astronomy, Manchester University, Manchester M13 9PL, UK Received 7 August 2006; received in revised form 24 November 2006; accepted 24 November 2006 Available online 30 November 2006
Abstract Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 32.80.Dz; 32.80.Fb; 78.70.Dm; 79.60.−i; 82.80.Pv Keywords: Threshold photoemission; Bulk electronic properties; X-ray absorption spectroscopy; TPES
1. Introduction Photoemission spectroscopy (PES) is one of the most direct tools to investigate the electronic properties of materials. Typical experiments involve the analysis of electrons with kinetic energy (Ek ) ranging from few eV up to 1.5 keV. Due to the short meanfree path of electrons with this energy, PES is a surface sensitive technique, as it was realized since the early days of its development [1]. Such a sensitivity to the topmost atomic layers may become a drawback for the study of samples of technological interest, which often are protected with capping layers, and for the investigation of solids, like for example strongly correlated electronic systems, whose surface properties are markedly different from the bulk ones [2]. A straightforward way to increase the depth sensitivity of PES experiments consists in the use of photoelectrons of higher kinetic energy (>5 keV). This so-called hard X-ray photoemission spectroscopy (HAXPES) has been demonstrated to provide an information depth of the order of ˚ at 6 keV, with a surface contribution around 2–7% 150–200 A of the total intensity [3]. A complementary approach to bulk sensitive photoemission involves the use of very low-energy electrons, exploiting the expected increase of the mean-free
∗
Corresponding author. Tel.: +39 06 55177229; fax: +39 06 5579303. E-mail address: [email protected] (F. Offi).
0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.043
path of electrons with kinetic energy below ∼5 eV [4]. The recently claimed bulk sensitivity in laser induced photoemission experiments (hν ∼ 6 − 7 eV, Ek < 3.5 eV) [5,6], has boostered a renewed interest on the subject. The large increase in electron mean-free path for low energy electrons is generally accepted as common wisdom. However, it has been the subject of some investigation over the years. The results, at least for some materials like for example rare earth elements, support the opposite hypothesis of a very short mean-free path for the low energy electrons [7]. The investigation of the depth sensitivity of low-energy electrons is often performed by using X-ray absorption spectroscopy (XAS) [8]. Collection of the total yield of secondary electrons (TEY) is by far the most used technique in XAS. Partial electron yield (PEY) collection is also currently used although many aspects of its interpretation are still under investigation [9]. Potentially PEY would be particularly well suited for studying surfaces and interfaces because the technique may be made sensitive to surface or bulk properties just selecting the kinetic energy of the collected electrons. In particular it is believed that PEY experiments where only low-energy secondary electrons (0–1 eV) are detected, provide information on bulk properties of real samples with no need of any surface treatement. A recent example is given by ultra violet (hν < 5 eV) photoemission electron microscope experiments, which showed a probing depth of about 16 nm for the magnetic signal in the Ag/Fe model system [10]. On the other hand, atoms and molecules have
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F. Offi et al. / Journal of Electron Spectroscopy and Related Phenomena 154 (2007) 63–68
been successfully studied by PEY experiments with low energy electrons over the years [11]. The technique, named threshold photoemission spectroscopy (TPES) uses monochromatic tunable photons to excite the sample right at threshold and detects almost zero kinetic energy electrons. The characteristics of this technique, namely high-energy resolution (better than 10 meV) and large efficiency (accepted solid angle close to 4π), have allowed accurate determination of the target electronic structure and the observation of resonant states embedded in the continuum for both valence and core orbitals [12,13]. The present work is the first attempt to apply threshold photoemission spectroscopy to solids. The aim is to verify the possible application of the technique to the investigation of bulk sensitive core level state phenomena in solids. Indeed the sample, a stainless steel needle, was supposed to develop already a rel˚ passive layer and was moreover coated atively thick (30–60 A) by graphite for the TPES experiments. Electrons with kinetic energy between 0 and 20 meV have been recorded as a function of the photon energy. In this way a narrow band of constant final states lying just above the spectrometer vacuum level is selected. The experiment should be extremely bulk sensitive because of the long mean-free path of threshold electrons (roughly of the order of hundreds of Angstrom for Ek ∼0–20 meV). By analogy with the TPES spectra measured in atoms and molecules in gas phase, we expect that the detected low-energy electrons might be produced via two different mechanisms: (i) by direct core photoemission; (ii) by decay of core excited states via lowenergy electron emission. The experiment is expected to clarify which is the dominant channel for production of the detected electrons, hence which information on solids TPES can provide. Should the first mechanism be dominant, TPES would be a bulk sensitive, highly efficient, constant final state photoemission spectroscopy, allowing for the investigation of core occupied states. On the contrary, should the second mechanism be the dominant one, then TPES would be a variety of PEY and would allow to measure the density of empty states involved in the photoabsorption process. To the purpose, the measured TPES spectrum has been compared with X-ray photoemission spectroscopy (XPS) measurements performed on the same sample and XAS measurements on Fe, Fe oxides and stainless steel available in literature [14–16]. 2. Experiment The threshold photoemission experiments have been performed at the Daresbury Laboratory Synchrotron Radiation Source (UK), where the high-resolution threshold spectrometer has been allied to the 5U.1 undulator beamline. The beamline, equipped with a plane grating monochromator, provides radiation in the energy range 70–1500 eV, with a resolving power and photon flux on the target of 1500 and 1011 photons/s, respectively, for energies close to the Fe L2,3 edges [17]. The photoelectron spectrometer has been described in detail elsewhere [18]. Briefly, it consists of a 127◦ electrostatic cylindrical deflector analyzer (CDA) and an input lens system, formed by two threeaperture asymmetric lenses. When the spectrometer is operated
in the threshold mode, a draw-out field is applied between the target and the entrance of the lens stack. The draw-out field collects very low-energy electrons with high efficiency and discriminates against high-energy electrons. The threshold energy resolution of the spectrometer has been measured to be better than 10 meV, while the calibration in terms of absolute energy scale and energy resolution was performed with TPES measurements in the region of the Ne 1s and Ar 2p thresholds [13,19]. The experiments presented in the next section were performed on a stainless steel sample coated with a macroscopic film of colloidal graphite (Aquadag). Threshold photoemission spectra were acquired during photon scans around the Fe L2,3 edges, with a photon resolution of about 0.5 eV and a residual pressure, in the scattering chamber, of about 4×10−4 Pa. With the incident photon flux of about 1011 photons/s, the typical count rate at the L3 maximum was about 4000 counts/s, with a signal to background ratio of approximately 1/6. XPS and Auger spectroscopy measurements, performed offline on the same sample, were done at the ESCA Service of the Area della Ricerca di Roma del Consiglio Nazionale delle Ricerche. The apparatus is supplied by a twin anode X-ray lamp (Al and Mg) and with a hemispherical analyzer with the stack of entrance lenses controlled by the program VG S5250 for automatic collection of data. The total energy resolution of these experiments was about 0.7 eV. 3. Results The TPES spectrum has been obtained without prior cleaning procedure of the sample, recording the threshold electron yield while scanning the photon energy. The result is reported in Fig. 1. Two sharp features are visible at photon energies of approximately 708 and 721 eV. These are roughly 1 eV higher than the Fe L3 and L2 thresholds, respectively. The XPS spectrum, measured off line and on the same sample at a later stage, is reported in Fig. 2(a), whereas in Fig. 2(b) is reported the spectrum of the same sample after 30 min of Argon ion sputtering. In the latter spectrum, the iron 2p spin–orbit split doublet is clearly recognized and the binding energies of 707 and 720 eV related to them are in good agreement with values reported in the literature for clean iron surfaces [20]. The XPS spectrum of the sample as used for TPES experiments [Fig. 2(a)] shows that a sizable contribution from oxidized iron is present, as evident from the broad peaks at roughly 710 and 723 eV in correspondence of the L3 and L2 edges, respectively. Typically in oxidized iron (FeO, Fe3 O4 , and Fe2 O3 ) the L orbital binding energy is chemically shifted by roughly 3–4 eV toward higher values [21,22], but from the available spectra is not possible to distinguish which oxide component is present at most. It is interesting to note that the spectra of Fig. 2 may be attribute both to iron and to stainless steel. The XPS spectra from stainless steel sample, clean and oxidized, are practically not distinguishable from the corresponding iron spectra [23]. In Fig. 2(a) the iron oxide contributions, even though not fully resolved from the metallic ones, are at least as intense as the metallic ones. This is a clear evidence for the fact that the sample as used for TPES was covered with an oxide layer whose thickness is a few photoelectron escape depth
F. Offi et al. / Journal of Electron Spectroscopy and Related Phenomena 154 (2007) 63–68
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˚ These findings are confirmed by the iron Auger (∼20–30 A). spectrum, and the oxygen XPS and Auger spectra (not shown here), measured on the same sample and at the same time. 4. Discussion In direct photoionization the position of the peaks satisfies the energy conservation law: hν = Eb + e Φsp + Ek
(1)
where hν is the photon energy, Eb the binding energy referred to the Fermi level and Φsp is the spectrometer work function [24]. In the TPES spectrum of Fig. 1 Ek is negligible with respect to the other two energies and the Eb values as derived from TPES should practically coincide with those obtained with XPS. Assuming for the electron spectrometer a work function of 4.5 eV (typical for a graphite-coated spectrometer), if the dominant contribution to the TPES spectrum was direct photoionization of Fe 2p, the structure corresponding to the 2p3/2 edge should have been found at 711.5 eV in Fig. 1 [see Eq. (1)]. This value is roughly 3.5 eV higher than the measured value. The amplitude of the observed discrepancy in binding energy is well beyond energy resolution and absolute energy calibration of the photon beam. On the other hand, the spin orbit splitting of the 2p3/2 and 2p1/2 TPES structures is in agreement with the one measured by XPS. Furthermore, besides the mismatch in the binding energy scale, a sharp disagreement is found when comparing the TPES and the clean iron XPS lineshapes
Fig. 2. XPS spectrum of the sample as measured for the TPES experiment (a) and after 30 min of sputtering (b). The binding energy scale was calibrated by measuring a Au reference sample.
Fig. 1. TPES spectrum in the region of the L2,3 edges of Fe.
(Fig. 3). In the figure a common abscissa axis is given, for the binding and the photon energy. Even if the overall energy resolution is similar for the two experiments (0.7 and 0.5 eV for XPS and TPES, respectively) it is evident that the TPES structure is broader than the iron XPS one. In particular, the TPES leading edge is about 0.6 eV broader than the XPS one. Direct photoemission, therefore, does not appear to be the main mechanism contributing to the TPES spectrum from solids. For a more quantitative evaluation of the possible contribution of direct photoemission we performed a fit of the TPES spectrum using a set of Voigt functions. The result of the fit is shown as thick solid line in Fig. 4(a), superimposed to the experimental data (open circles). Six asymmetric Voigt functions, shown in the figure as thin solid lines, and a linear background, dashed line in Fig. 4(a), were used to represent the spectrum. Labels 1 and 4 refer to the main peaks of the TPES spectrum, located at approximately 708 and 721 eV, respectively. The features due to direct photoemission, and located according to Eq. (1) at 711.5 and 724.5 eV, are labeled 3 and 6, respectively. The two further peaks, labeled 2 and 5, and located at approximately 2 eV from the main lines, were needed for a good representation of the experimental data, and will be discussed below. The used Voigt functions consist of a gaussian component, whose full width at half maximum
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Fig. 3. Comparison of the TPES spectrum in the 2p3/2 region with the XPS spectrum measured on the clean stainless steel sample.
(FWHM) has been set equal to the experimental resolution, and a lorentzian component accounting for the natural width of the peaks. The asymmetry parameter and the FWHM of the gaussian and the lorentzian components have been forced to be equal for all the peaks, while the energy positions were free parameters, except for the peaks 3 and 6, whose energy position was fixed according to Eq. (1). From Fig. 4(a) it is evident that peaks 3 and 6 are indeed vanishing small. In fact an acceptable fit could be obtained even excluding these two peaks, since the χ2 value is practically identical when performing the fit with or without peaks 3 and 6. More quantitatively we may state that the relative intensity of peaks 3 and 6, with respect to peaks 1 and 4, respectively, is less than 5%. Therefore, this is the estimated contribution of direct photoemission in the threshold spectrum. The present results are then compared with XAS measured by the total electron yield method from untreated stainless steel [14]. In Fig. 4(b) the two spectra are overlapped after subtraction of a linear background (note that in the TPES case the background was practically constant), using for the two spectra their original independent scales of photon energy. At the L3 edge the two spectra are quite similar as for to the energy position and the spectral shape. This may suggest that the same process is responsible for generation of TPES and XAS signals, and indirectly that the depth sensitivity of the two techniques may be similar. A comparison of the L2 edge in Fig. 4 reveals a different spectral shape for the two measurements: in particular the XAS spectrum presents an extra bump at higher photon energy.
Fig. 4. (a) TPES spectrum (open circles) fitted with Voigt profiles. The result of the fit is shown as thick solid line, while thin solid lines are the used Voigt components. Dashed line is the linear background. (b) Comparison of the TPES spectrum with the XAS spectrum measured on a stainless steel sample as taken from Ref. [14].
When comparing our TPES spectrum with XAS spectra of Fe and FeO [14] a better similarity is found in the region of the L2 edge as far as the spectral shape is concerned. In particular the XAS spectrum from pure Fe is quite similar to our TPES spectrum. It must be said that a precise investigation of the differences in XAS spectra between pure iron and its oxidation states is not easy to perform [16]. Fe and FeO display similar energy position for the L2 and the L3 edges, and similar spectral shape, but some differences at the L2 edge. Fe3 O4 and ␣-Fe2 O3 have an L3 peak position shifted at almost 2 eV higher energy than pure iron, with a characteristic double peak at both edges. This is particularly evident in the case of the L3 edge of ␣-Fe2 O3 . The results of the fit of Fig. 4(a) suggest that also in the TPES spectrum case we observe, at both edges, a component shifted of roughly 2 eV at higher photon energy than the main lines. However, its relative weight with respect to the main edge appear to be quite less than in the case of the complex oxides XAS spectra. All in all, from the comparison with the XAS results, our TPES spectrum may be attributed to Fe with some likely contribution of oxides
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components. From this point of view is therefore difficult at this stage to definitely establish if TPES has a higher depth sensitivity than XAS. However, it seems clear that oxidized state affect less the TPES spectrum than the corresponding XPS one. A clear difference between the two spectra [Fig. 4(b)] is the absence in TPES of the steps in correspondence of each edge, which characterize the XAS spectrum. These are typically attributed to transitions to unoccupied non-discrete states (s–p bands and continuum). It is interesting to note that Gao et al. find a flat background, similar to the one measured in TPES, in XAS from a Fe film when measuring the spectrum in PEY mode at a kinetic energy corresponding to zero binding energy of the electrons [25]. This was attributed to the influence of the Auger peaks, which, when measured at resonance, dominate the electron signal close to the Fermi level. In order to explain the peculiar behavior of the background in TPES, we have to discuss the mechanisms that may lead to the measured spectrum, considering the de-excitation of the system as an autoionization process [26]. To the purpose we will use the analogy with the interpretation of TPES spectra in gas phase. Two mechanisms are invoked to explain the production of threshold electrons following inner shell excitation. The first one involves the decay via a resonant Auger electron, with the excited electron as a spectator. If the intermediate state reached in this decay is embedded in the double continuum, then the system may suffer another decay, valence Auger or Koster–Cronig Auger decay. When this intermediate state is almost degenerate with a doubly charged ion state then a low-energy or threshold electron is emitted [27]. In the present case of TPES from a solid sample, this mechanism will involve the filling of the Fe 2p hole by an electron of the valence band with the simultaneous emission of another valence electron. The final state is therefore characterized by two holes in the valence band and an excited electron, and the process may be described as a a super Koster–Cronig process following a spectator core–valence–valence Auger decay. The energy balance is given the by width of the valence band and the minimum energy an electron must have to overcome the material work function Φ (typically about 4–5 eV). Even if differences exists in the width of the valence band for pure Fe [28], Fe oxides [29], and stainless steel [30], whatever are the details of the valence band structure of the present sample, the energy available to the system would be enough to promote a valence electron to the vacuum with a kinetic energy close to zero. The second mechanism invoked in atomic physics is the double shake-off [31]. After inner shell excitation two electrons are emitted and share the available energy in a continuous way. The energy distribution of the pair, peaks at the two extreme sharings and this explains the observation of low-energy electrons. The same process, that can be also regarded as a resonant double photoemission, may, of course, occurs also in the present case, where the energy available from the 2p excitation is well above the one needed to promote an electron pair into the continuum. Thus both the mechanisms proposed for isolated systems in gas phase may be active also in solids. The fact that no steps in the background of the TPES spectrum are observed does not tell that direct photoionization is not present. Indeed the photoionization peak might be hidden in the tail of the features observed in
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the TPES spectrum. On the other hand the observation of no background of threshold electrons above the L2,3 edges in the TPES spectrum clearly shows that the technique is not sensitive to low-energy electrons resulting from multiple scattering of energetic photoelectrons. 5. Conclusion TPES has been applied to measure quasi-zero kinetic energy electrons emitted after excitation of core level from a solid sample, resulting in a good combination of a very high collection efficiency and energy resolution. The TPES signal is mainly produced by two-electron decays of inner shell excited states, thus it provides a peculiar variety of PEY that focusses on many electron properties of the solid. We suggest that TPES on solids might be used to investigate electron correlation in the valence band and/or electron correlation between valence band and excited electron (or between electron below/above Fermi level). Even if the present results are not able to give a precise quantification of the information depth of the technique, they suggest that TPES has to be at least as bulk sensitive as XAS. Indeed among all the channels contributing to the XAS signal, in TPES only the ones, which result in the production of the lowest energy electrons and therefore the ones characterized by the longest mean-free paths, are selected. Further experiments on cleaner and simpler samples are needed to clarify these aspects in more details, exploiting the potentialities of the technique and disentangling the mechanisms generating the TPES signal. Acknowledgments Work partially supported by the European Community Large Scale Facilities Programme: SRS Daresbury Minor Grant 21/275. We thank S. Iacobucci, G. Panaccione, and P. Vilmercati for stimulating discussions. References [1] Y. Baer, P.F. Hed´en, J. Hedman, M. Klasson, C. Nordling, Solid State Commun. 8 (1970) 1479. [2] See for example, L. Braicovich, N.B. Brookes, C. Dallera, M. Salvietti, G.L. Olcese, Phys. Rev. B 56 (1997) 15047, and references therein. [3] M. Sacchi, F. Offi, P. Torelli, A. Fondacaro, C. Spezzani, M. Cautero, G. Cautero, S. Huotari, M. Grioni, R. Delaunay, M. Fabrizioli, G. Vank´o, G. Monaco, G. Paolicelli, G. Stefani, G. Panaccione, Phys. Rev. B 71 (2005) 155117. [4] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [5] T. Kiss, F. Kanetaka, T. Yokoya, T. Shimojima, K. Kanai, S. Shin, Y. Onuki, T. Togashi, C. Zhang, C.T. Chen, S. Watanabe, Phys. Rev. Lett. 94 (2005) 057001. [6] J.D. Koralek, J.F. Douglas, N.C. Plumb, Z. Sun, A.V. Fedorov, M.M. Murnane, H.C. Kapteyn, S.T. Cundiff, Y. Aiura, K. Oka, H. Eisaki, D.S. Dessau, Phys. Rev. Lett. 96 (2006) 017005. [7] See for example, J. Vogel, M. Sacchi, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181. [8] See for example, D.C. Koningsberger, R. Prins (Eds.), X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES Wiley, New York, 1987; J.C. Fuggle J. Inglesfield (Eds.), Unoccupied Electronic States, Springer-Verlag, Berlin, 1991.
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[9] R.Z. Bachrach, Synchrotron Radiation Research Advances in Surface and Interface Science: Techniques, Kluwer Academic Publishers, New York, 1992. [10] G.K.L. Marx, P.-O. Jubert, A. Bischof, R. Allenspach, Appl. Phys. Lett. 83 (2003) 2925. [11] G.C. King, L. Avaldi, J. Phys. B: Atom. Mol. Opt. Phys. 33 (2000) R215. [12] R.I. Hall, L. Avaldi, G. Dawber, P.M. Rutter, M.A. MacDonald, G.C. King, J. Phys. B: Atom. Mol. Opt. Phys. 22 (1989) 3205. [13] L. Avaldi, G. Dawber, R. Camilloni, G.C. King, M. Roper, M.R.F. Siggel, G. Stefani, M. Zitnik, J. Phys. B: Atom. Mol. Opt. Phys. 27 (1994) 3953. [14] F.J. P´erez, A. Gutierrez, M.F. L´opez, M.P. Hierro, F. Pedraza, Thin Solid Films 415 (2002) 258. [15] M.F. L´opez, A. Guti´errez, F.J. P´erez Trujillo, M.P. Hierro, F. Pedraza, J. Electron Spectrosc. Relat. Phenom. 114-116 (2001) 825. [16] T.J. Regan, H. Ohldag, C. Stamm, F. Nolting, J. L¨uning, J. St¨ohr, R.L. White, Phys. Rev. B 64 (2001) 214422. [17] C.S. Mythen, G. van der Laan, H.A. Padmore, Rev. Sci. Instrum. 63 (1992) 1313. [18] G.C. King, M. Zubek, P.M. Rutter, F.H. Read, J. Phys. E: Sci. Instrum. 20 (1987) 440. [19] L. Avaldi, G. Dawber, R. Camilloni, G.C. King, M. Roper, M.R.F. Siggel, G. Stefani, M. Zitnik, A. Lisini, P. Decleva, Phys. Rev. A 51 (1995) 5025.
[20] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Praire, 1992. [21] P. Mills, J.L. Sullivan, J. Phys. D: Appl. Phys. 16 (1983) 723. [22] C. Palacio, A. Arranz, J. Phys. Chem. B 105 (2001) 10805. [23] F. Martin, M.C. Lopez, P. Carrera, J.R. Ramos-Barrado, D. Leinen, Surf. Interf. Anal. 36 (2004) 8. [24] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, Wiley, New York, 1990. [25] X. Gao, H. Xu, T.S. Wee, W. Kuch, C. Tieg, S. Wang, J. Appl. Phys. 97 (2005) 103527. [26] See for example, P.A. Br¨uhwiler, O. Karis, N. M˚artensson, Rev. Mod. Phys. 74 (2002) 703. [27] T. Hayaishi, E. Murakami, A. Yagishita, F. Koike, Y. Morioka, J.E. Hansen, J. Phys. B: Atom. Mol. Opt. Phys. 21 (1988) 3203. [28] L. Ley, O.B. Dabbousi, S.P. Kowalczyk, F.R. McFeely, D.A. Shirley, Phys. Rev. B 16 (1977) 5372. [29] R.J. Lad, V.E. Henrich, Phys. Rev. B 39 (1989) 13478. [30] K.-J. Kim, T.-H. Kang, C.D. Park, B. Cho, S. Chung, B. Kim, J. Electron Spectrosc. Relat. Phenom. 101–103 (1999) 327. [31] P.A. Heimann, D.W. Lindle, T.A. Ferrett, S.H. Liu, L.J. Medhurst, M.N. Piancastelli, D.A. Shirley, U. Becker, H.G. Kerkhoff, B. Langer, D. Szostak, R. Wehlitz, J. Phys. B: Atom. Mol. Opt. Phys. 20 (1987) 5005.
Threshold photoemission spectroscopy in solids F. Offi a,∗ , L. Avaldi b , R. Camilloni b , G. Dawber c , G.C. King c , G. Stefani a a
CNISM and Dipartimento di Fisica, Universit`a Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy b CNR-IMIP, CP10, I-00016 Monterotondo Scalo, Italy c Department of Physics and Astronomy, Manchester University, Manchester M13 9PL, UK Received 7 August 2006; received in revised form 24 November 2006; accepted 24 November 2006 Available online 30 November 2006
Abstract Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 32.80.Dz; 32.80.Fb; 78.70.Dm; 79.60.−i; 82.80.Pv Keywords: Threshold photoemission; Bulk electronic properties; X-ray absorption spectroscopy; TPES
1. Introduction Photoemission spectroscopy (PES) is one of the most direct tools to investigate the electronic properties of materials. Typical experiments involve the analysis of electrons with kinetic energy (Ek ) ranging from few eV up to 1.5 keV. Due to the short meanfree path of electrons with this energy, PES is a surface sensitive technique, as it was realized since the early days of its development [1]. Such a sensitivity to the topmost atomic layers may become a drawback for the study of samples of technological interest, which often are protected with capping layers, and for the investigation of solids, like for example strongly correlated electronic systems, whose surface properties are markedly different from the bulk ones [2]. A straightforward way to increase the depth sensitivity of PES experiments consists in the use of photoelectrons of higher kinetic energy (>5 keV). This so-called hard X-ray photoemission spectroscopy (HAXPES) has been demonstrated to provide an information depth of the order of ˚ at 6 keV, with a surface contribution around 2–7% 150–200 A of the total intensity [3]. A complementary approach to bulk sensitive photoemission involves the use of very low-energy electrons, exploiting the expected increase of the mean-free
∗
Corresponding author. Tel.: +39 06 55177229; fax: +39 06 5579303. E-mail address: [email protected] (F. Offi).
0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.043
path of electrons with kinetic energy below ∼5 eV [4]. The recently claimed bulk sensitivity in laser induced photoemission experiments (hν ∼ 6 − 7 eV, Ek < 3.5 eV) [5,6], has boostered a renewed interest on the subject. The large increase in electron mean-free path for low energy electrons is generally accepted as common wisdom. However, it has been the subject of some investigation over the years. The results, at least for some materials like for example rare earth elements, support the opposite hypothesis of a very short mean-free path for the low energy electrons [7]. The investigation of the depth sensitivity of low-energy electrons is often performed by using X-ray absorption spectroscopy (XAS) [8]. Collection of the total yield of secondary electrons (TEY) is by far the most used technique in XAS. Partial electron yield (PEY) collection is also currently used although many aspects of its interpretation are still under investigation [9]. Potentially PEY would be particularly well suited for studying surfaces and interfaces because the technique may be made sensitive to surface or bulk properties just selecting the kinetic energy of the collected electrons. In particular it is believed that PEY experiments where only low-energy secondary electrons (0–1 eV) are detected, provide information on bulk properties of real samples with no need of any surface treatement. A recent example is given by ultra violet (hν < 5 eV) photoemission electron microscope experiments, which showed a probing depth of about 16 nm for the magnetic signal in the Ag/Fe model system [10]. On the other hand, atoms and molecules have
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been successfully studied by PEY experiments with low energy electrons over the years [11]. The technique, named threshold photoemission spectroscopy (TPES) uses monochromatic tunable photons to excite the sample right at threshold and detects almost zero kinetic energy electrons. The characteristics of this technique, namely high-energy resolution (better than 10 meV) and large efficiency (accepted solid angle close to 4π), have allowed accurate determination of the target electronic structure and the observation of resonant states embedded in the continuum for both valence and core orbitals [12,13]. The present work is the first attempt to apply threshold photoemission spectroscopy to solids. The aim is to verify the possible application of the technique to the investigation of bulk sensitive core level state phenomena in solids. Indeed the sample, a stainless steel needle, was supposed to develop already a rel˚ passive layer and was moreover coated atively thick (30–60 A) by graphite for the TPES experiments. Electrons with kinetic energy between 0 and 20 meV have been recorded as a function of the photon energy. In this way a narrow band of constant final states lying just above the spectrometer vacuum level is selected. The experiment should be extremely bulk sensitive because of the long mean-free path of threshold electrons (roughly of the order of hundreds of Angstrom for Ek ∼0–20 meV). By analogy with the TPES spectra measured in atoms and molecules in gas phase, we expect that the detected low-energy electrons might be produced via two different mechanisms: (i) by direct core photoemission; (ii) by decay of core excited states via lowenergy electron emission. The experiment is expected to clarify which is the dominant channel for production of the detected electrons, hence which information on solids TPES can provide. Should the first mechanism be dominant, TPES would be a bulk sensitive, highly efficient, constant final state photoemission spectroscopy, allowing for the investigation of core occupied states. On the contrary, should the second mechanism be the dominant one, then TPES would be a variety of PEY and would allow to measure the density of empty states involved in the photoabsorption process. To the purpose, the measured TPES spectrum has been compared with X-ray photoemission spectroscopy (XPS) measurements performed on the same sample and XAS measurements on Fe, Fe oxides and stainless steel available in literature [14–16]. 2. Experiment The threshold photoemission experiments have been performed at the Daresbury Laboratory Synchrotron Radiation Source (UK), where the high-resolution threshold spectrometer has been allied to the 5U.1 undulator beamline. The beamline, equipped with a plane grating monochromator, provides radiation in the energy range 70–1500 eV, with a resolving power and photon flux on the target of 1500 and 1011 photons/s, respectively, for energies close to the Fe L2,3 edges [17]. The photoelectron spectrometer has been described in detail elsewhere [18]. Briefly, it consists of a 127◦ electrostatic cylindrical deflector analyzer (CDA) and an input lens system, formed by two threeaperture asymmetric lenses. When the spectrometer is operated
in the threshold mode, a draw-out field is applied between the target and the entrance of the lens stack. The draw-out field collects very low-energy electrons with high efficiency and discriminates against high-energy electrons. The threshold energy resolution of the spectrometer has been measured to be better than 10 meV, while the calibration in terms of absolute energy scale and energy resolution was performed with TPES measurements in the region of the Ne 1s and Ar 2p thresholds [13,19]. The experiments presented in the next section were performed on a stainless steel sample coated with a macroscopic film of colloidal graphite (Aquadag). Threshold photoemission spectra were acquired during photon scans around the Fe L2,3 edges, with a photon resolution of about 0.5 eV and a residual pressure, in the scattering chamber, of about 4×10−4 Pa. With the incident photon flux of about 1011 photons/s, the typical count rate at the L3 maximum was about 4000 counts/s, with a signal to background ratio of approximately 1/6. XPS and Auger spectroscopy measurements, performed offline on the same sample, were done at the ESCA Service of the Area della Ricerca di Roma del Consiglio Nazionale delle Ricerche. The apparatus is supplied by a twin anode X-ray lamp (Al and Mg) and with a hemispherical analyzer with the stack of entrance lenses controlled by the program VG S5250 for automatic collection of data. The total energy resolution of these experiments was about 0.7 eV. 3. Results The TPES spectrum has been obtained without prior cleaning procedure of the sample, recording the threshold electron yield while scanning the photon energy. The result is reported in Fig. 1. Two sharp features are visible at photon energies of approximately 708 and 721 eV. These are roughly 1 eV higher than the Fe L3 and L2 thresholds, respectively. The XPS spectrum, measured off line and on the same sample at a later stage, is reported in Fig. 2(a), whereas in Fig. 2(b) is reported the spectrum of the same sample after 30 min of Argon ion sputtering. In the latter spectrum, the iron 2p spin–orbit split doublet is clearly recognized and the binding energies of 707 and 720 eV related to them are in good agreement with values reported in the literature for clean iron surfaces [20]. The XPS spectrum of the sample as used for TPES experiments [Fig. 2(a)] shows that a sizable contribution from oxidized iron is present, as evident from the broad peaks at roughly 710 and 723 eV in correspondence of the L3 and L2 edges, respectively. Typically in oxidized iron (FeO, Fe3 O4 , and Fe2 O3 ) the L orbital binding energy is chemically shifted by roughly 3–4 eV toward higher values [21,22], but from the available spectra is not possible to distinguish which oxide component is present at most. It is interesting to note that the spectra of Fig. 2 may be attribute both to iron and to stainless steel. The XPS spectra from stainless steel sample, clean and oxidized, are practically not distinguishable from the corresponding iron spectra [23]. In Fig. 2(a) the iron oxide contributions, even though not fully resolved from the metallic ones, are at least as intense as the metallic ones. This is a clear evidence for the fact that the sample as used for TPES was covered with an oxide layer whose thickness is a few photoelectron escape depth
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˚ These findings are confirmed by the iron Auger (∼20–30 A). spectrum, and the oxygen XPS and Auger spectra (not shown here), measured on the same sample and at the same time. 4. Discussion In direct photoionization the position of the peaks satisfies the energy conservation law: hν = Eb + e Φsp + Ek
(1)
where hν is the photon energy, Eb the binding energy referred to the Fermi level and Φsp is the spectrometer work function [24]. In the TPES spectrum of Fig. 1 Ek is negligible with respect to the other two energies and the Eb values as derived from TPES should practically coincide with those obtained with XPS. Assuming for the electron spectrometer a work function of 4.5 eV (typical for a graphite-coated spectrometer), if the dominant contribution to the TPES spectrum was direct photoionization of Fe 2p, the structure corresponding to the 2p3/2 edge should have been found at 711.5 eV in Fig. 1 [see Eq. (1)]. This value is roughly 3.5 eV higher than the measured value. The amplitude of the observed discrepancy in binding energy is well beyond energy resolution and absolute energy calibration of the photon beam. On the other hand, the spin orbit splitting of the 2p3/2 and 2p1/2 TPES structures is in agreement with the one measured by XPS. Furthermore, besides the mismatch in the binding energy scale, a sharp disagreement is found when comparing the TPES and the clean iron XPS lineshapes
Fig. 2. XPS spectrum of the sample as measured for the TPES experiment (a) and after 30 min of sputtering (b). The binding energy scale was calibrated by measuring a Au reference sample.
Fig. 1. TPES spectrum in the region of the L2,3 edges of Fe.
(Fig. 3). In the figure a common abscissa axis is given, for the binding and the photon energy. Even if the overall energy resolution is similar for the two experiments (0.7 and 0.5 eV for XPS and TPES, respectively) it is evident that the TPES structure is broader than the iron XPS one. In particular, the TPES leading edge is about 0.6 eV broader than the XPS one. Direct photoemission, therefore, does not appear to be the main mechanism contributing to the TPES spectrum from solids. For a more quantitative evaluation of the possible contribution of direct photoemission we performed a fit of the TPES spectrum using a set of Voigt functions. The result of the fit is shown as thick solid line in Fig. 4(a), superimposed to the experimental data (open circles). Six asymmetric Voigt functions, shown in the figure as thin solid lines, and a linear background, dashed line in Fig. 4(a), were used to represent the spectrum. Labels 1 and 4 refer to the main peaks of the TPES spectrum, located at approximately 708 and 721 eV, respectively. The features due to direct photoemission, and located according to Eq. (1) at 711.5 and 724.5 eV, are labeled 3 and 6, respectively. The two further peaks, labeled 2 and 5, and located at approximately 2 eV from the main lines, were needed for a good representation of the experimental data, and will be discussed below. The used Voigt functions consist of a gaussian component, whose full width at half maximum
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Fig. 3. Comparison of the TPES spectrum in the 2p3/2 region with the XPS spectrum measured on the clean stainless steel sample.
(FWHM) has been set equal to the experimental resolution, and a lorentzian component accounting for the natural width of the peaks. The asymmetry parameter and the FWHM of the gaussian and the lorentzian components have been forced to be equal for all the peaks, while the energy positions were free parameters, except for the peaks 3 and 6, whose energy position was fixed according to Eq. (1). From Fig. 4(a) it is evident that peaks 3 and 6 are indeed vanishing small. In fact an acceptable fit could be obtained even excluding these two peaks, since the χ2 value is practically identical when performing the fit with or without peaks 3 and 6. More quantitatively we may state that the relative intensity of peaks 3 and 6, with respect to peaks 1 and 4, respectively, is less than 5%. Therefore, this is the estimated contribution of direct photoemission in the threshold spectrum. The present results are then compared with XAS measured by the total electron yield method from untreated stainless steel [14]. In Fig. 4(b) the two spectra are overlapped after subtraction of a linear background (note that in the TPES case the background was practically constant), using for the two spectra their original independent scales of photon energy. At the L3 edge the two spectra are quite similar as for to the energy position and the spectral shape. This may suggest that the same process is responsible for generation of TPES and XAS signals, and indirectly that the depth sensitivity of the two techniques may be similar. A comparison of the L2 edge in Fig. 4 reveals a different spectral shape for the two measurements: in particular the XAS spectrum presents an extra bump at higher photon energy.
Fig. 4. (a) TPES spectrum (open circles) fitted with Voigt profiles. The result of the fit is shown as thick solid line, while thin solid lines are the used Voigt components. Dashed line is the linear background. (b) Comparison of the TPES spectrum with the XAS spectrum measured on a stainless steel sample as taken from Ref. [14].
When comparing our TPES spectrum with XAS spectra of Fe and FeO [14] a better similarity is found in the region of the L2 edge as far as the spectral shape is concerned. In particular the XAS spectrum from pure Fe is quite similar to our TPES spectrum. It must be said that a precise investigation of the differences in XAS spectra between pure iron and its oxidation states is not easy to perform [16]. Fe and FeO display similar energy position for the L2 and the L3 edges, and similar spectral shape, but some differences at the L2 edge. Fe3 O4 and ␣-Fe2 O3 have an L3 peak position shifted at almost 2 eV higher energy than pure iron, with a characteristic double peak at both edges. This is particularly evident in the case of the L3 edge of ␣-Fe2 O3 . The results of the fit of Fig. 4(a) suggest that also in the TPES spectrum case we observe, at both edges, a component shifted of roughly 2 eV at higher photon energy than the main lines. However, its relative weight with respect to the main edge appear to be quite less than in the case of the complex oxides XAS spectra. All in all, from the comparison with the XAS results, our TPES spectrum may be attributed to Fe with some likely contribution of oxides
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components. From this point of view is therefore difficult at this stage to definitely establish if TPES has a higher depth sensitivity than XAS. However, it seems clear that oxidized state affect less the TPES spectrum than the corresponding XPS one. A clear difference between the two spectra [Fig. 4(b)] is the absence in TPES of the steps in correspondence of each edge, which characterize the XAS spectrum. These are typically attributed to transitions to unoccupied non-discrete states (s–p bands and continuum). It is interesting to note that Gao et al. find a flat background, similar to the one measured in TPES, in XAS from a Fe film when measuring the spectrum in PEY mode at a kinetic energy corresponding to zero binding energy of the electrons [25]. This was attributed to the influence of the Auger peaks, which, when measured at resonance, dominate the electron signal close to the Fermi level. In order to explain the peculiar behavior of the background in TPES, we have to discuss the mechanisms that may lead to the measured spectrum, considering the de-excitation of the system as an autoionization process [26]. To the purpose we will use the analogy with the interpretation of TPES spectra in gas phase. Two mechanisms are invoked to explain the production of threshold electrons following inner shell excitation. The first one involves the decay via a resonant Auger electron, with the excited electron as a spectator. If the intermediate state reached in this decay is embedded in the double continuum, then the system may suffer another decay, valence Auger or Koster–Cronig Auger decay. When this intermediate state is almost degenerate with a doubly charged ion state then a low-energy or threshold electron is emitted [27]. In the present case of TPES from a solid sample, this mechanism will involve the filling of the Fe 2p hole by an electron of the valence band with the simultaneous emission of another valence electron. The final state is therefore characterized by two holes in the valence band and an excited electron, and the process may be described as a a super Koster–Cronig process following a spectator core–valence–valence Auger decay. The energy balance is given the by width of the valence band and the minimum energy an electron must have to overcome the material work function Φ (typically about 4–5 eV). Even if differences exists in the width of the valence band for pure Fe [28], Fe oxides [29], and stainless steel [30], whatever are the details of the valence band structure of the present sample, the energy available to the system would be enough to promote a valence electron to the vacuum with a kinetic energy close to zero. The second mechanism invoked in atomic physics is the double shake-off [31]. After inner shell excitation two electrons are emitted and share the available energy in a continuous way. The energy distribution of the pair, peaks at the two extreme sharings and this explains the observation of low-energy electrons. The same process, that can be also regarded as a resonant double photoemission, may, of course, occurs also in the present case, where the energy available from the 2p excitation is well above the one needed to promote an electron pair into the continuum. Thus both the mechanisms proposed for isolated systems in gas phase may be active also in solids. The fact that no steps in the background of the TPES spectrum are observed does not tell that direct photoionization is not present. Indeed the photoionization peak might be hidden in the tail of the features observed in
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the TPES spectrum. On the other hand the observation of no background of threshold electrons above the L2,3 edges in the TPES spectrum clearly shows that the technique is not sensitive to low-energy electrons resulting from multiple scattering of energetic photoelectrons. 5. Conclusion TPES has been applied to measure quasi-zero kinetic energy electrons emitted after excitation of core level from a solid sample, resulting in a good combination of a very high collection efficiency and energy resolution. The TPES signal is mainly produced by two-electron decays of inner shell excited states, thus it provides a peculiar variety of PEY that focusses on many electron properties of the solid. We suggest that TPES on solids might be used to investigate electron correlation in the valence band and/or electron correlation between valence band and excited electron (or between electron below/above Fermi level). Even if the present results are not able to give a precise quantification of the information depth of the technique, they suggest that TPES has to be at least as bulk sensitive as XAS. Indeed among all the channels contributing to the XAS signal, in TPES only the ones, which result in the production of the lowest energy electrons and therefore the ones characterized by the longest mean-free paths, are selected. Further experiments on cleaner and simpler samples are needed to clarify these aspects in more details, exploiting the potentialities of the technique and disentangling the mechanisms generating the TPES signal. Acknowledgments Work partially supported by the European Community Large Scale Facilities Programme: SRS Daresbury Minor Grant 21/275. We thank S. Iacobucci, G. Panaccione, and P. Vilmercati for stimulating discussions. References [1] Y. Baer, P.F. Hed´en, J. Hedman, M. Klasson, C. Nordling, Solid State Commun. 8 (1970) 1479. [2] See for example, L. Braicovich, N.B. Brookes, C. Dallera, M. Salvietti, G.L. Olcese, Phys. Rev. B 56 (1997) 15047, and references therein. [3] M. Sacchi, F. Offi, P. Torelli, A. Fondacaro, C. Spezzani, M. Cautero, G. Cautero, S. Huotari, M. Grioni, R. Delaunay, M. Fabrizioli, G. Vank´o, G. Monaco, G. Paolicelli, G. Stefani, G. Panaccione, Phys. Rev. B 71 (2005) 155117. [4] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [5] T. Kiss, F. Kanetaka, T. Yokoya, T. Shimojima, K. Kanai, S. Shin, Y. Onuki, T. Togashi, C. Zhang, C.T. Chen, S. Watanabe, Phys. Rev. Lett. 94 (2005) 057001. [6] J.D. Koralek, J.F. Douglas, N.C. Plumb, Z. Sun, A.V. Fedorov, M.M. Murnane, H.C. Kapteyn, S.T. Cundiff, Y. Aiura, K. Oka, H. Eisaki, D.S. Dessau, Phys. Rev. Lett. 96 (2006) 017005. [7] See for example, J. Vogel, M. Sacchi, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 181. [8] See for example, D.C. Koningsberger, R. Prins (Eds.), X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES Wiley, New York, 1987; J.C. Fuggle J. Inglesfield (Eds.), Unoccupied Electronic States, Springer-Verlag, Berlin, 1991.
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