- Email: [email protected]
Observation of excitonic satellites in the photoelectron spectra of Ne and Ar clusters
10 January 2002
Chemical Physics Letters 351 (2002) 235–241 www.elsevier.com/locate/cplett
Observation of excitonic satellites in the photoelectron spectra of Ne and Ar clusters U. Hergenhahn a
a,b,* ,
A. Kolmakov c,1, M. Riedler c, A.R.B. de Castro d, €fken c, T. Mo €ller c O. Lo
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany b Max-Planck-Institut f€ur Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany c Hasylab am DESY, Notkestr. 85, 22603 Hamburg, Germany d LNLS and IFGN UNICAMP, CP 6192, 13084-971 Campinas, Brazil Received 9 August 2001
Abstract We have recorded synchrotron radiation excited photoelectron spectra of free Ne and Ar cluster beams in the valence and inner valence region. Varying the cluster sizes from a few up to some hundred atoms, the development of inelastic energy losses of the outgoing photoelectrons is clearly visible. The first few energy loss features can be related to creation of excitons and interband transitions within the cluster. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Our knowledge of the electronically excited states of rare gas clusters has been obtained mostly from observations of their radiative decay after VUV excitation [1–4] or photoionisation [5–7]. Other, more straightforward investigations of the excited state electronic structure of these species, for example with electron energy loss spectroscopy (EELS), were hampered by the low target densities in a free cluster beam and the high excitation en*
Corresponding author: Fax: +49-30-84135-603. E-mail address: [email protected] (U. Hergenhahn). 1 Present address: Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA
ergies that are needed [8]. In a recent series of investigations on the rare gas solids synchrotron radiation was used as the excitation source and electron time-of-flight (tof) spectroscopy as the detection technique [9,10]. This combination turned out to be very useful for the investigation of the energies and dynamics of electronically excited states in condensed rare gas layers. In this Letter, we will report on the application of the same technique to beams of free Ne and Ar clusters. For Ne clusters, we have recorded electron spectra with photon energies ranging from the 2p to well above the 2s ionisation threshold. The complete electron spectra were recorded by the parallel data acquisition capability inherent to the tof method. In addition to the 2p and 2s photoelectron lines, the spectra reveal characteristic energy losses of outgoing photoelectrons in
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 3 9 4 - X
236
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
the clusters, as well as a bulk of low kinetic energy electrons characteristic for emission of secondary electrons. We will interpret the characteristic lines of energy loss in terms of quasi-particle production within the clusters and will present their dependence on incident photon energy. For argon clusters we have observed similar features.
2. Experimental A beam of free clusters was produced by expanding Ne or Ar gas through a cryogenically cooled nozzle of 200 lm diameter and 4° half opening angle. In this arrangement under suitable conditions, discussed e.g. by Hagena [11,12], clusters are formed. Their size varies with stagnation pressure and nozzle temperature. Pressures of a few times 10 mbar, along with liquid He cooling of the nozzle, were used in our experiments. Exact settings of the parameters are discussed further below. More details of the cluster beam apparatus and the vacuum chamber can be found in [13]. An electron tof spectrometer was mounted in this set-up under an angle of 60° with respect to the electric field vector of the synchrotron radiation, which was as close to the ‘magic’ angle (54.7°) as possible with the vacuum chamber used. The input orifice of the spectrometer was at ca. 10 mm from the cluster beam ionisation point. The spectrometer consisted mainly of a conical drift tube held at ground potential and a set of Ø 34 mm microchannel plates (MCPs). The MCPs are 180 mm away from the interaction region, and the solid angle for electron detection achieved such as around 19 mSr. The XUV undulator BW3 at the Hamburger Synchrotronstrahlungslabor (HASYLAB, Hamburg, Germany) was used as a photon source [14]. To provide for an adequate time difference between electron creation and its arrival at the MCP detector the use of the dual bunch mode of the storage ring, with a bunch period of 480 ns, was essential. Data processing included correcting the data for pile-up effects and signal loss due to our singlehit electronics [15] and subtraction of a small constant background consisting of MCP noise.
After that the tof spectra had to be converted to an energy axis. This can be done by knowledge of the tof-length and of the position of the peak caused by scattered light hitting the MCP, pertaining to 0 ns flight time. In practice, we have treated these two quantities and a correction polynomial for the photon energies as slightly adjustable. The parameters were then determined in such a way that for a series of gas phase Ne spectra the correct conversion to binding energy ðEb ð2pÞ ¼ 21:60 eV, Eb ð2sÞ ¼ 48:48 eVÞ was obtained. A jacobian factor was multiplied with the data points to convert to spectra of equal area [16]. The transmission function of our spectrometer strongly decreases for electron kinetic energies below 5 eV, probably due to lack of shielding for residual magnetic fields. No correction for that was attempted.
3. Results Photoelectron spectra (PES) from gaseous Ne and from a free Ne cluster beam are compared in Fig. 1. The occurrence of an additional feature superimposed on a step-like background in the cluster PES in-between the Ne 2p and 2s photolines, and of a much higher electron intensity at low kinetic energy, is immediately apparent. Also, a shift of the main lines of the cluster spectrum of about 0.5 eV towards lower binding energies can be seen. For the Ne 2s line, from analysing all our spectra we get a binding energy of 48.05(10) eV. This can be compared with values of Kass€ uhlke [17] for thick Ne layers. This author gives a binding energy of 47.90 eV for emission of surface and 48.90 eV for emission of bulk sites. The literature value for the gas phase 2s binding energy is 48.475 eV. Since our clusters contain at most 300 atoms (see below) true bulk sites do not play a strong role in their photoemission characteristics. The similarity of our value for the binding energy with the one for Ne surfaces is therefore plausible. We now turn towards a more detailed discussion of the feature at 39 eV binding energy, or at 17.6(2) eV energy loss relative to the 2p main line. Its identification is straightforward, when our
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
Fig. 1. Photoelectron spectra of gaseous Ne (bottom) and Ne clusters (top panel) taken under identical experimental conditions. A satellite peak of the 2p line pertaining to exciton production with an energy loss of 17.6 eV can easily be identified in the cluster spectrum. The gas phase ionisation energies of the Ne 2p1 and 2s1 states taken from the literature are marked by vertical dotted lines. A shift of the main lines in the cluster spectra towards about 0.5 eV smaller binding energy is consistently observed. The satellite structure observed at binding energies of 50–60 eV is well known in gaseous Ne [18]. Satellites in this binding energy range pertain to excitation into Neþ 2p4 nl states, with n ¼ 3; 4 and l ¼ s, p. Interestingly, for the satellite lines a binding energy shift with respect to the gas phase is not observed. The hatched area in the top panel depicts the scaled gas phase spectrum for comparison.
spectrum is compared to energy loss spectra of electron beams scattered off a solid Ne surface. These experiments have been performed with low beam energies (42 eV, [19]) and energies in the keV range [20,21]. It was shown that the lowest energy loss feature that could be observed occurs at 17.2(5) [19] or 17.74 eV [20], respectively. This feature was readily assigned as creation of the lowest exciton – a metastable but delocalised electron–hole pair – in solid Ne by inelastic electron scattering. Correspondingly, our feature can be explained as a satellite line resulting from en-
237
ergy loss of the outgoing Ne 2p photoelectrons due to exciton creation inside the cluster. This assignment is corroborated by inspection of a series of Ne photoelectron spectra recorded at different photon energies, displayed in Fig. 2. The constant energy offset between the Ne 2p line and the excitonic feature can clearly be seen. The apparent broadening of the excitonic loss peak towards higher energies are mostly due to the decreasing resolution of our tof spectrometer for faster electrons. If we compare the FWHM of this peak and of the 2p photoline, derived from a fit of a Gaussian profile to the high kinetic energy flank, they are very similar. At 45 eV, the smallest photon energy where both peaks are clearly discerned, we find a total width (FWHM) of 1.1(1) eV for the 2p and the excitonic peak. The apparatus broadening can be approximated by DE=Ekin 1=30. This would give an apparatus contribution of about 0.78 eV to the 2p peak width. However, this cannot simply be subtracted since the line will contain several unresolved components pertaining to emission from different cluster sites. The width of the excitonic peak reflects that line structure. Its inherent broadening in the electron scattering study of Daniels and Kr€ uger [20] was found to be only 90 meV. Above the main excitonic feature, the appearance of a step-like background manifests an onset of interband transitions caused by the photoelectron. In some spectra excitation of a second discrete excitonic state at 19.8(3) eV energy loss is also visible. This value is clearly below the energy found for the n ¼ 2 exciton in solid Ne (20.36 eV, [20]). In an atomic language, we can assign our first excited state to a Neð2p1 3sÞNeð2p1 ÞNeN 2 configuration of an N atom rare gas cluster. The second state would then pertain to a Neð2p1 3pÞ excitation, which is dipole forbidden and can therefore only take place at low electron impact energies. This is indeed observed in Fig. 2. The same effect was found for solid Ne by Kass€ uhlke [17]. From the series of spectra shown in Fig. 2 it can be seen that the intensity ratio of the excitonic peak and the 2p line is not constant. This ratio is displayed in Fig. 3. The spectra shown in Fig. 2 were taken consecutively, counting each spectrum
238
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
Fig. 2. Photoelectron spectra of a beam of Ne clusters taken with photon energies from 34.3 to 78.4 eV. The apparent broadening of the excitonic satellite line is mostly due to the non-constant energy resolution of our tof spectrometer. Production of a huge amount of low kinetic energy electrons can clearly be seen.
Fig. 3. Ratio of the area underneath the first excitonic satellite line to the area of the Ne 2p line in the photoelectron spectra from a free Ne cluster beam. Stagnation pressure and nozzle temperature for the different data sets were: crosses (27 mbar/30 K), filled circles (42 mbar/32 K), triangles (27 mbar/32 K) (two data sets), open circles (20 mbar/32 K).
for 200 s, and their exciton/2p ratio is displayed in Fig. 3 by crosses. The general behaviour of the ratio is consistent with the energy dependence for the electron mean free path in rare gas solids, which rapidly decreases within the first five eV above threshold [22].
The amount of inelastically scattered electrons should be a function of cluster size at least for average sizes hN i up to some hundred atoms. We have therefore varied the parameters of our gas expansion to produce cluster beams differing in 2:35 mean size. hN i was calculated as 33ðC =1000Þ ,
where C is the scaling parameter defined in [12] (see also [23]). A trend towards more inelastic scattering for larger clusters can be seen from our beams with smallest and largest mean sizes (Fig. 3). The experimental uncertainty of the values for the exciton/2p ratio can be assumed similar to the deviation of the Ne 2s/2p ratio in our gas phase spectra from the literature values. We thus arrive at an uncertainty of 0:008. Another point that may influence the exciton/2p ratio is the presence of gaseous Ne in our beam, which will diminish all measured values by a certain amount compared to an ideal cluster beam. This admixture probably varied somewhat between different series of data, and can be estimated to be up to 20%. This variation most probably explains the irregularities between the hN i ¼ 70 and hN i ¼ 150 data sets. We have also investigated Ar clusters. Their photoelectron spectrum has been measured before
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
with HeI radiation by Carnovale et al. [24]. A comparison of a gas phase and a cluster spectrum from our work is shown in Fig. 4. This essentially shows the same features as we have discussed for Ne. The mean cluster size here is hN i ¼ 200. We find a binding energy for the Ar cluster 3p photoelectron line of 15.0(2) eV, compared to a gas phase literature value of 15.878 eV. This energy difference lies between the shift found for the bulk and surface components of Ar cluster 2p photoelectrons by Bj€ orneholm et al. [25]. The onset of electron inelastic energy losses occurs here at 12.2(2) eV. This figure is in good agreement with threshold photoelectron–photoion co-incidence (TPEPICO) measurements further discussed below [26].
Fig. 4. Photoelectron spectra of gaseous Ar (bottom) and Ar clusters (top panel) taken under identical experimental conditions. A satellite peak of the 3p line pertaining to exciton production with an energy loss of 12.2 eV, as well as a shift in binding energy of about )0.8 eV can be identified in the cluster spectrum. The gas phase ionisation energies of the Ar 3p1 and 3s1 states are marked by vertical dotted lines. We attribute the bulk of electrons in the binding energy region 30–40 eV of the cluster spectrum to secondary electron production. The distinct lines in this energy region pertain to satellite excitations [27].
239
4. Discussion and conclusion So far, we have tacitly assumed that the energy loss process observed in our photoelectron spectra can be seen as the photoionisation of an np electron, and its subsequent energy loss due to inelastic scattering off a second site in the cluster. In contrast to that, creation of a quasi-particle at the site of the original ionisation would lead to a different energy loss compared to the electron beam studies. The spatial extent of the excitons in rare gas clusters have been discussed e.g. in [1]. We just quote that the ðn ¼ 1Þ excitons of interest here have an extent comparable to the atom–atom separation, while spatially more extended exciton states can occur for higher n. The processes revealed in our study certainly have an impact on the modelling of cluster fragmentation. The currently accepted picture (see e.g. [24,28]) is that around 1012 s after vacancy creation the positive charge localises and forms a dimer (or oligomer) unit within the cluster, which results in a loss of binding energy. The excess energy is thermalised and leads to fragmentation of the cluster. Our study directly determines the fraction of photoelectrons created inside a cluster, which may not leave the unit without giving energy back into the cluster in the form of internal excitation or creation of secondary electrons, as in the solid state. The secondary quasi-particle (exciton and secondary hole) creation provides an additional important stimulant for cluster fragmentation or Coulomb explosion. This can occur at the stage of quasi-particle self-trapping or after its radiative decay. Radiative exciton decays were shown to lead to photodesorption when they occur near the surface of solid Ne [29]. Desorption of metastables was also seen in [10]. Cluster fragmentation by electron ejection plus internal excitation at a different site was also studied by mass spectrometry after electron and photon impact. In the electron impact excitation studies, for Ne and Ar the existence of metastable subunits with sizes of 2–10 atoms were observed [30,31]. These subunits were tentatively identified as excited oligomers hosting a trapped exciton, which was created by scattering of one of the two outgoing electrons. This channel opens up at ap-
240
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
pearance energies 16.4 and 11.9 eV above cluster single ionisation, which is somewhat lower but comparable to our values for the energy loss. In the TPEPICO studies after photon impact [26], again the decay of Arþ 4 metastable units was identified. This was explained as either due to creation of an exciton by the outgoing photoelectron, or as a simultaneous two-electron transition at different centres. While energetically we cannot rule out the second process, the photon energy dependence of the exciton creation probability observed in our measurements at least for Ne clearly favours the first of these explanations. These authors also resolved a number of processes with higher loss energies that lead to the features visible in our spectra between 28 and 30 eV binding energy. Intercluster photoelectron scattering processes have been discussed also in the context of zero kinetic energy electron (ZEKE) yield measurements by Knop et al. [32]. These authors demonstrated that intercluster scattering, as opposed to scattering of free electrons on other clusters in the beam, is the main source of the ZEKE signal. This corroborates our interpretation of the hump of slow electrons in our photoelectron spectra. Extending future studies of the excitonic losses towards very small cluster sizes might provide an interesting comparison to molecular photoionisation. Here, attempts to explain creation of photoelectron satellites by scattering of the outgoing photoelectron on the molecular core have been made [33], but so far it is not clear whether this model is more powerful than molecular orbital based methods. In a number of recent theoretical studies a new type of radiationless decay, termed Interatomic Coulombic Decay, was predicted for van-derWaals clusters [34,35]. Briefly, due to the possibility of creating doubly charged states with charges localised on different constituents of the cluster, single-hole states, which would be stable in the monomer, may show radiationless decays. One may expect this type of decay for 2s vacancies in a Ne cluster [35]. We have carefully examined our spectra to find direct evidence for ICD. However, probably due to the bad transmission of our electron spectrometer in the kinetic energy region
where these electrons can be expected (1–3 eV), this was to no avail. Future searches for this effect would certainly benefit from studying clusters with as small as possible sizes, so that electrons from ICD will not have to compete with the broad continuum of secondary electron emission. In conclusion, we have directly observed the extent to which photoelectrons are subject to energy loss due to inelastic electron impact excitation of the cluster. Creation of excitons at 17.6 and 19.8 eV excitation energy as well as interband transitions were clearly seen. For cluster sizes of some hundred atoms the behaviour was quite similar to the rare gas solids.
Acknowledgements Financial support from the Deutsche Forschungsgemeinschaft (DFG) under grant no. Mo 719/1-2 is kindly acknowledged.
References [1] J. W€ ormer, M. Joppien, G. Zimmerer, T. M€ oller, Phys. Rev. Lett. 67 (1991) 2053. [2] M. Joppien, R. M€ uller, J. W€ ormer, T. M€ oller, Phys. Rev. B 47 (1993) 12984. [3] M. Joppien, R. Karnbach, T. M€ oller, Z. Phys. D 26 (1993) 175. [4] R. von Pietrowsky, M. Rutzen, K. von Haeften, S. Kakar, T. M€ oller, Z. Phys. D 40 (1997) 22. [5] E. R€ uhl, C. Heinzel, A.P. Hitchcock, H. Baumg€artel, J. Chem. Phys. 98 (1993) 2653. [6] F. Federmann, L. Bj€ orneholm, A. Beutler, T. M€ oller, Phys. Rev. Lett. 73 (1994) 1549. [7] A. Knop, B. Wassermann, E. R€ uhl, Phys. Rev. Lett. 80 (1998) 2302. [8] A. Burose, C. Becker, A. Ding, Z. Phys. D 20 (1991) 35. [9] P. Wiethoff, H.-U. Ehrke, B. Kass€ uhlke, C. Keller, W. Wurth, D. Menzel, P. Feulner, Phys. Rev. B 55 (1997) 9387. [10] P. Feulner, P. Averkamp, B. Kass€ uhlke, Appl. Phys. A 67 (1998) 657. [11] O.F. Hagena, Surf. Sci. 106 (1981) 101. [12] O.F. Hagena, Z. Phys. D 4 (1987) 291. [13] R. Karnbach, M. Joppien, J. Stapelfeldt, J. W€ ormer, T. M€ oller, Rev. Sci. Instrum. 64 (1993) 2838. [14] O. Bj€ orneholm, F. Federmann, C. Larsson, U. Hahn, A. Rieck, S. Kakar, T. M€ oller, A. Beutler, F. F€ ossing, Rev. Sci. Instrum. 66 (1995) 1732.
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241 [15] T. Luhmann, Rev. Sci. Instrum. 68 (1997) 2347. [16] O. Hemmers, S.B. Whitfield, P. Glans, H. Wang, D.W. Lindle, R. Wehlitz, I.A. Sellin, Rev. Sci. Instrum. 69 (1998) 3809. [17] B. Kass€ uhlke, Ph.D. Thesis, Technische Universit€at M€ unchen, Germany, 1999; B. Kass€ uhlke, Elektronische Struktur von Adsorbaten und Kondensaten untersucht mit zweidimensionaler Elektronen-Flugzeitspektroskopie, Herbert Utz Verlag, M€ unchen, Germany, 1998. [18] G. Kutluk, T. Takaku, M. Kanno, T. Nagata, E. Shigemasa, A. Yagishita, F. Koike, J. Phys. B 27 (1994) 5637, and references therein. [19] H.H. Farrell, M. Strongin, Phys. Rev. B 6 (1972) 4711. [20] J. Daniels, P. Kr€ uger, Phys. Status Solidi B 43 (1971) 659. [21] L. Schmidt, Phys. Lett. A 36 (1971) 87. [22] N. Schwentner, Phys. Rev. B 14 (1976) 5490. [23] U. Buck, R. Krohne, J. Chem. Phys. 105 (1996) 5408. [24] F. Carnovale, J.B. Peel, R.G. Rothwell, J. Valldorf, P.J. Kuntz, J. Chem. Phys. 90 (1989) 1452.
241
[25] O. Bj€ orneholm, F. Federmann, F. F€ ossing, T. M€ oller, Phys. Rev. Lett. 74 (1995) 3017. [26] J. de Vries, B. Kamke, H. Steger, B. Weisser, M. Honka, W. Kamke, J. Chem. Phys. 101 (1994) 2372, and references therein. [27] B. Langer, J. Viefhaus, O. Hemmers, A. Menzel, R. Wehlitz, U. Becker, Phys. Rev. A 51 (1995) R882. [28] H. Haberland, Surf. Sci. 156 (1985) 305. [29] T. Kloiber, W. Laasch, G. Zimmerer, F. Coletti, J.M. Debever, Europhys. Lett. 7 (1988) 77. [30] M. Foltin, G. Walder, A.W. Castleman, T.D. M€ark, J. Chem. Phys. 94 (1991) 810. [31] M. Foltin, T.D. M€ark, Chem. Phys. Lett. 180 (1991) 317. [32] A. Knop, H.W. Jochims, A.L.D. Kilcoyne, A.P. Hitchcock, E. R€ uhl, Chem. Phys. Lett. 223 (1994) 553. [33] A. Pavlychev, J. Phys. B 32 (1999) 2077. [34] L.S. Cederbaum, J. Zobeley, F. Tarantelli, Phys. Rev. Lett. 79 (1997) 4778. [35] R. Santra, J. Zobeley, L.S. Cederbaum, N. Moiseyev, Phys. Rev. Lett. 85 (2000) 4490.
Chemical Physics Letters 351 (2002) 235–241 www.elsevier.com/locate/cplett
Observation of excitonic satellites in the photoelectron spectra of Ne and Ar clusters U. Hergenhahn a
a,b,* ,
A. Kolmakov c,1, M. Riedler c, A.R.B. de Castro d, €fken c, T. Mo €ller c O. Lo
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany b Max-Planck-Institut f€ur Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany c Hasylab am DESY, Notkestr. 85, 22603 Hamburg, Germany d LNLS and IFGN UNICAMP, CP 6192, 13084-971 Campinas, Brazil Received 9 August 2001
Abstract We have recorded synchrotron radiation excited photoelectron spectra of free Ne and Ar cluster beams in the valence and inner valence region. Varying the cluster sizes from a few up to some hundred atoms, the development of inelastic energy losses of the outgoing photoelectrons is clearly visible. The first few energy loss features can be related to creation of excitons and interband transitions within the cluster. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Our knowledge of the electronically excited states of rare gas clusters has been obtained mostly from observations of their radiative decay after VUV excitation [1–4] or photoionisation [5–7]. Other, more straightforward investigations of the excited state electronic structure of these species, for example with electron energy loss spectroscopy (EELS), were hampered by the low target densities in a free cluster beam and the high excitation en*
Corresponding author: Fax: +49-30-84135-603. E-mail address: [email protected] (U. Hergenhahn). 1 Present address: Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA
ergies that are needed [8]. In a recent series of investigations on the rare gas solids synchrotron radiation was used as the excitation source and electron time-of-flight (tof) spectroscopy as the detection technique [9,10]. This combination turned out to be very useful for the investigation of the energies and dynamics of electronically excited states in condensed rare gas layers. In this Letter, we will report on the application of the same technique to beams of free Ne and Ar clusters. For Ne clusters, we have recorded electron spectra with photon energies ranging from the 2p to well above the 2s ionisation threshold. The complete electron spectra were recorded by the parallel data acquisition capability inherent to the tof method. In addition to the 2p and 2s photoelectron lines, the spectra reveal characteristic energy losses of outgoing photoelectrons in
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 3 9 4 - X
236
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
the clusters, as well as a bulk of low kinetic energy electrons characteristic for emission of secondary electrons. We will interpret the characteristic lines of energy loss in terms of quasi-particle production within the clusters and will present their dependence on incident photon energy. For argon clusters we have observed similar features.
2. Experimental A beam of free clusters was produced by expanding Ne or Ar gas through a cryogenically cooled nozzle of 200 lm diameter and 4° half opening angle. In this arrangement under suitable conditions, discussed e.g. by Hagena [11,12], clusters are formed. Their size varies with stagnation pressure and nozzle temperature. Pressures of a few times 10 mbar, along with liquid He cooling of the nozzle, were used in our experiments. Exact settings of the parameters are discussed further below. More details of the cluster beam apparatus and the vacuum chamber can be found in [13]. An electron tof spectrometer was mounted in this set-up under an angle of 60° with respect to the electric field vector of the synchrotron radiation, which was as close to the ‘magic’ angle (54.7°) as possible with the vacuum chamber used. The input orifice of the spectrometer was at ca. 10 mm from the cluster beam ionisation point. The spectrometer consisted mainly of a conical drift tube held at ground potential and a set of Ø 34 mm microchannel plates (MCPs). The MCPs are 180 mm away from the interaction region, and the solid angle for electron detection achieved such as around 19 mSr. The XUV undulator BW3 at the Hamburger Synchrotronstrahlungslabor (HASYLAB, Hamburg, Germany) was used as a photon source [14]. To provide for an adequate time difference between electron creation and its arrival at the MCP detector the use of the dual bunch mode of the storage ring, with a bunch period of 480 ns, was essential. Data processing included correcting the data for pile-up effects and signal loss due to our singlehit electronics [15] and subtraction of a small constant background consisting of MCP noise.
After that the tof spectra had to be converted to an energy axis. This can be done by knowledge of the tof-length and of the position of the peak caused by scattered light hitting the MCP, pertaining to 0 ns flight time. In practice, we have treated these two quantities and a correction polynomial for the photon energies as slightly adjustable. The parameters were then determined in such a way that for a series of gas phase Ne spectra the correct conversion to binding energy ðEb ð2pÞ ¼ 21:60 eV, Eb ð2sÞ ¼ 48:48 eVÞ was obtained. A jacobian factor was multiplied with the data points to convert to spectra of equal area [16]. The transmission function of our spectrometer strongly decreases for electron kinetic energies below 5 eV, probably due to lack of shielding for residual magnetic fields. No correction for that was attempted.
3. Results Photoelectron spectra (PES) from gaseous Ne and from a free Ne cluster beam are compared in Fig. 1. The occurrence of an additional feature superimposed on a step-like background in the cluster PES in-between the Ne 2p and 2s photolines, and of a much higher electron intensity at low kinetic energy, is immediately apparent. Also, a shift of the main lines of the cluster spectrum of about 0.5 eV towards lower binding energies can be seen. For the Ne 2s line, from analysing all our spectra we get a binding energy of 48.05(10) eV. This can be compared with values of Kass€ uhlke [17] for thick Ne layers. This author gives a binding energy of 47.90 eV for emission of surface and 48.90 eV for emission of bulk sites. The literature value for the gas phase 2s binding energy is 48.475 eV. Since our clusters contain at most 300 atoms (see below) true bulk sites do not play a strong role in their photoemission characteristics. The similarity of our value for the binding energy with the one for Ne surfaces is therefore plausible. We now turn towards a more detailed discussion of the feature at 39 eV binding energy, or at 17.6(2) eV energy loss relative to the 2p main line. Its identification is straightforward, when our
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
Fig. 1. Photoelectron spectra of gaseous Ne (bottom) and Ne clusters (top panel) taken under identical experimental conditions. A satellite peak of the 2p line pertaining to exciton production with an energy loss of 17.6 eV can easily be identified in the cluster spectrum. The gas phase ionisation energies of the Ne 2p1 and 2s1 states taken from the literature are marked by vertical dotted lines. A shift of the main lines in the cluster spectra towards about 0.5 eV smaller binding energy is consistently observed. The satellite structure observed at binding energies of 50–60 eV is well known in gaseous Ne [18]. Satellites in this binding energy range pertain to excitation into Neþ 2p4 nl states, with n ¼ 3; 4 and l ¼ s, p. Interestingly, for the satellite lines a binding energy shift with respect to the gas phase is not observed. The hatched area in the top panel depicts the scaled gas phase spectrum for comparison.
spectrum is compared to energy loss spectra of electron beams scattered off a solid Ne surface. These experiments have been performed with low beam energies (42 eV, [19]) and energies in the keV range [20,21]. It was shown that the lowest energy loss feature that could be observed occurs at 17.2(5) [19] or 17.74 eV [20], respectively. This feature was readily assigned as creation of the lowest exciton – a metastable but delocalised electron–hole pair – in solid Ne by inelastic electron scattering. Correspondingly, our feature can be explained as a satellite line resulting from en-
237
ergy loss of the outgoing Ne 2p photoelectrons due to exciton creation inside the cluster. This assignment is corroborated by inspection of a series of Ne photoelectron spectra recorded at different photon energies, displayed in Fig. 2. The constant energy offset between the Ne 2p line and the excitonic feature can clearly be seen. The apparent broadening of the excitonic loss peak towards higher energies are mostly due to the decreasing resolution of our tof spectrometer for faster electrons. If we compare the FWHM of this peak and of the 2p photoline, derived from a fit of a Gaussian profile to the high kinetic energy flank, they are very similar. At 45 eV, the smallest photon energy where both peaks are clearly discerned, we find a total width (FWHM) of 1.1(1) eV for the 2p and the excitonic peak. The apparatus broadening can be approximated by DE=Ekin 1=30. This would give an apparatus contribution of about 0.78 eV to the 2p peak width. However, this cannot simply be subtracted since the line will contain several unresolved components pertaining to emission from different cluster sites. The width of the excitonic peak reflects that line structure. Its inherent broadening in the electron scattering study of Daniels and Kr€ uger [20] was found to be only 90 meV. Above the main excitonic feature, the appearance of a step-like background manifests an onset of interband transitions caused by the photoelectron. In some spectra excitation of a second discrete excitonic state at 19.8(3) eV energy loss is also visible. This value is clearly below the energy found for the n ¼ 2 exciton in solid Ne (20.36 eV, [20]). In an atomic language, we can assign our first excited state to a Neð2p1 3sÞNeð2p1 ÞNeN 2 configuration of an N atom rare gas cluster. The second state would then pertain to a Neð2p1 3pÞ excitation, which is dipole forbidden and can therefore only take place at low electron impact energies. This is indeed observed in Fig. 2. The same effect was found for solid Ne by Kass€ uhlke [17]. From the series of spectra shown in Fig. 2 it can be seen that the intensity ratio of the excitonic peak and the 2p line is not constant. This ratio is displayed in Fig. 3. The spectra shown in Fig. 2 were taken consecutively, counting each spectrum
238
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
Fig. 2. Photoelectron spectra of a beam of Ne clusters taken with photon energies from 34.3 to 78.4 eV. The apparent broadening of the excitonic satellite line is mostly due to the non-constant energy resolution of our tof spectrometer. Production of a huge amount of low kinetic energy electrons can clearly be seen.
Fig. 3. Ratio of the area underneath the first excitonic satellite line to the area of the Ne 2p line in the photoelectron spectra from a free Ne cluster beam. Stagnation pressure and nozzle temperature for the different data sets were: crosses (27 mbar/30 K), filled circles (42 mbar/32 K), triangles (27 mbar/32 K) (two data sets), open circles (20 mbar/32 K).
for 200 s, and their exciton/2p ratio is displayed in Fig. 3 by crosses. The general behaviour of the ratio is consistent with the energy dependence for the electron mean free path in rare gas solids, which rapidly decreases within the first five eV above threshold [22].
The amount of inelastically scattered electrons should be a function of cluster size at least for average sizes hN i up to some hundred atoms. We have therefore varied the parameters of our gas expansion to produce cluster beams differing in 2:35 mean size. hN i was calculated as 33ðC =1000Þ ,
where C is the scaling parameter defined in [12] (see also [23]). A trend towards more inelastic scattering for larger clusters can be seen from our beams with smallest and largest mean sizes (Fig. 3). The experimental uncertainty of the values for the exciton/2p ratio can be assumed similar to the deviation of the Ne 2s/2p ratio in our gas phase spectra from the literature values. We thus arrive at an uncertainty of 0:008. Another point that may influence the exciton/2p ratio is the presence of gaseous Ne in our beam, which will diminish all measured values by a certain amount compared to an ideal cluster beam. This admixture probably varied somewhat between different series of data, and can be estimated to be up to 20%. This variation most probably explains the irregularities between the hN i ¼ 70 and hN i ¼ 150 data sets. We have also investigated Ar clusters. Their photoelectron spectrum has been measured before
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
with HeI radiation by Carnovale et al. [24]. A comparison of a gas phase and a cluster spectrum from our work is shown in Fig. 4. This essentially shows the same features as we have discussed for Ne. The mean cluster size here is hN i ¼ 200. We find a binding energy for the Ar cluster 3p photoelectron line of 15.0(2) eV, compared to a gas phase literature value of 15.878 eV. This energy difference lies between the shift found for the bulk and surface components of Ar cluster 2p photoelectrons by Bj€ orneholm et al. [25]. The onset of electron inelastic energy losses occurs here at 12.2(2) eV. This figure is in good agreement with threshold photoelectron–photoion co-incidence (TPEPICO) measurements further discussed below [26].
Fig. 4. Photoelectron spectra of gaseous Ar (bottom) and Ar clusters (top panel) taken under identical experimental conditions. A satellite peak of the 3p line pertaining to exciton production with an energy loss of 12.2 eV, as well as a shift in binding energy of about )0.8 eV can be identified in the cluster spectrum. The gas phase ionisation energies of the Ar 3p1 and 3s1 states are marked by vertical dotted lines. We attribute the bulk of electrons in the binding energy region 30–40 eV of the cluster spectrum to secondary electron production. The distinct lines in this energy region pertain to satellite excitations [27].
239
4. Discussion and conclusion So far, we have tacitly assumed that the energy loss process observed in our photoelectron spectra can be seen as the photoionisation of an np electron, and its subsequent energy loss due to inelastic scattering off a second site in the cluster. In contrast to that, creation of a quasi-particle at the site of the original ionisation would lead to a different energy loss compared to the electron beam studies. The spatial extent of the excitons in rare gas clusters have been discussed e.g. in [1]. We just quote that the ðn ¼ 1Þ excitons of interest here have an extent comparable to the atom–atom separation, while spatially more extended exciton states can occur for higher n. The processes revealed in our study certainly have an impact on the modelling of cluster fragmentation. The currently accepted picture (see e.g. [24,28]) is that around 1012 s after vacancy creation the positive charge localises and forms a dimer (or oligomer) unit within the cluster, which results in a loss of binding energy. The excess energy is thermalised and leads to fragmentation of the cluster. Our study directly determines the fraction of photoelectrons created inside a cluster, which may not leave the unit without giving energy back into the cluster in the form of internal excitation or creation of secondary electrons, as in the solid state. The secondary quasi-particle (exciton and secondary hole) creation provides an additional important stimulant for cluster fragmentation or Coulomb explosion. This can occur at the stage of quasi-particle self-trapping or after its radiative decay. Radiative exciton decays were shown to lead to photodesorption when they occur near the surface of solid Ne [29]. Desorption of metastables was also seen in [10]. Cluster fragmentation by electron ejection plus internal excitation at a different site was also studied by mass spectrometry after electron and photon impact. In the electron impact excitation studies, for Ne and Ar the existence of metastable subunits with sizes of 2–10 atoms were observed [30,31]. These subunits were tentatively identified as excited oligomers hosting a trapped exciton, which was created by scattering of one of the two outgoing electrons. This channel opens up at ap-
240
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241
pearance energies 16.4 and 11.9 eV above cluster single ionisation, which is somewhat lower but comparable to our values for the energy loss. In the TPEPICO studies after photon impact [26], again the decay of Arþ 4 metastable units was identified. This was explained as either due to creation of an exciton by the outgoing photoelectron, or as a simultaneous two-electron transition at different centres. While energetically we cannot rule out the second process, the photon energy dependence of the exciton creation probability observed in our measurements at least for Ne clearly favours the first of these explanations. These authors also resolved a number of processes with higher loss energies that lead to the features visible in our spectra between 28 and 30 eV binding energy. Intercluster photoelectron scattering processes have been discussed also in the context of zero kinetic energy electron (ZEKE) yield measurements by Knop et al. [32]. These authors demonstrated that intercluster scattering, as opposed to scattering of free electrons on other clusters in the beam, is the main source of the ZEKE signal. This corroborates our interpretation of the hump of slow electrons in our photoelectron spectra. Extending future studies of the excitonic losses towards very small cluster sizes might provide an interesting comparison to molecular photoionisation. Here, attempts to explain creation of photoelectron satellites by scattering of the outgoing photoelectron on the molecular core have been made [33], but so far it is not clear whether this model is more powerful than molecular orbital based methods. In a number of recent theoretical studies a new type of radiationless decay, termed Interatomic Coulombic Decay, was predicted for van-derWaals clusters [34,35]. Briefly, due to the possibility of creating doubly charged states with charges localised on different constituents of the cluster, single-hole states, which would be stable in the monomer, may show radiationless decays. One may expect this type of decay for 2s vacancies in a Ne cluster [35]. We have carefully examined our spectra to find direct evidence for ICD. However, probably due to the bad transmission of our electron spectrometer in the kinetic energy region
where these electrons can be expected (1–3 eV), this was to no avail. Future searches for this effect would certainly benefit from studying clusters with as small as possible sizes, so that electrons from ICD will not have to compete with the broad continuum of secondary electron emission. In conclusion, we have directly observed the extent to which photoelectrons are subject to energy loss due to inelastic electron impact excitation of the cluster. Creation of excitons at 17.6 and 19.8 eV excitation energy as well as interband transitions were clearly seen. For cluster sizes of some hundred atoms the behaviour was quite similar to the rare gas solids.
Acknowledgements Financial support from the Deutsche Forschungsgemeinschaft (DFG) under grant no. Mo 719/1-2 is kindly acknowledged.
References [1] J. W€ ormer, M. Joppien, G. Zimmerer, T. M€ oller, Phys. Rev. Lett. 67 (1991) 2053. [2] M. Joppien, R. M€ uller, J. W€ ormer, T. M€ oller, Phys. Rev. B 47 (1993) 12984. [3] M. Joppien, R. Karnbach, T. M€ oller, Z. Phys. D 26 (1993) 175. [4] R. von Pietrowsky, M. Rutzen, K. von Haeften, S. Kakar, T. M€ oller, Z. Phys. D 40 (1997) 22. [5] E. R€ uhl, C. Heinzel, A.P. Hitchcock, H. Baumg€artel, J. Chem. Phys. 98 (1993) 2653. [6] F. Federmann, L. Bj€ orneholm, A. Beutler, T. M€ oller, Phys. Rev. Lett. 73 (1994) 1549. [7] A. Knop, B. Wassermann, E. R€ uhl, Phys. Rev. Lett. 80 (1998) 2302. [8] A. Burose, C. Becker, A. Ding, Z. Phys. D 20 (1991) 35. [9] P. Wiethoff, H.-U. Ehrke, B. Kass€ uhlke, C. Keller, W. Wurth, D. Menzel, P. Feulner, Phys. Rev. B 55 (1997) 9387. [10] P. Feulner, P. Averkamp, B. Kass€ uhlke, Appl. Phys. A 67 (1998) 657. [11] O.F. Hagena, Surf. Sci. 106 (1981) 101. [12] O.F. Hagena, Z. Phys. D 4 (1987) 291. [13] R. Karnbach, M. Joppien, J. Stapelfeldt, J. W€ ormer, T. M€ oller, Rev. Sci. Instrum. 64 (1993) 2838. [14] O. Bj€ orneholm, F. Federmann, C. Larsson, U. Hahn, A. Rieck, S. Kakar, T. M€ oller, A. Beutler, F. F€ ossing, Rev. Sci. Instrum. 66 (1995) 1732.
U. Hergenhahn et al. / Chemical Physics Letters 351 (2002) 235–241 [15] T. Luhmann, Rev. Sci. Instrum. 68 (1997) 2347. [16] O. Hemmers, S.B. Whitfield, P. Glans, H. Wang, D.W. Lindle, R. Wehlitz, I.A. Sellin, Rev. Sci. Instrum. 69 (1998) 3809. [17] B. Kass€ uhlke, Ph.D. Thesis, Technische Universit€at M€ unchen, Germany, 1999; B. Kass€ uhlke, Elektronische Struktur von Adsorbaten und Kondensaten untersucht mit zweidimensionaler Elektronen-Flugzeitspektroskopie, Herbert Utz Verlag, M€ unchen, Germany, 1998. [18] G. Kutluk, T. Takaku, M. Kanno, T. Nagata, E. Shigemasa, A. Yagishita, F. Koike, J. Phys. B 27 (1994) 5637, and references therein. [19] H.H. Farrell, M. Strongin, Phys. Rev. B 6 (1972) 4711. [20] J. Daniels, P. Kr€ uger, Phys. Status Solidi B 43 (1971) 659. [21] L. Schmidt, Phys. Lett. A 36 (1971) 87. [22] N. Schwentner, Phys. Rev. B 14 (1976) 5490. [23] U. Buck, R. Krohne, J. Chem. Phys. 105 (1996) 5408. [24] F. Carnovale, J.B. Peel, R.G. Rothwell, J. Valldorf, P.J. Kuntz, J. Chem. Phys. 90 (1989) 1452.
241
[25] O. Bj€ orneholm, F. Federmann, F. F€ ossing, T. M€ oller, Phys. Rev. Lett. 74 (1995) 3017. [26] J. de Vries, B. Kamke, H. Steger, B. Weisser, M. Honka, W. Kamke, J. Chem. Phys. 101 (1994) 2372, and references therein. [27] B. Langer, J. Viefhaus, O. Hemmers, A. Menzel, R. Wehlitz, U. Becker, Phys. Rev. A 51 (1995) R882. [28] H. Haberland, Surf. Sci. 156 (1985) 305. [29] T. Kloiber, W. Laasch, G. Zimmerer, F. Coletti, J.M. Debever, Europhys. Lett. 7 (1988) 77. [30] M. Foltin, G. Walder, A.W. Castleman, T.D. M€ark, J. Chem. Phys. 94 (1991) 810. [31] M. Foltin, T.D. M€ark, Chem. Phys. Lett. 180 (1991) 317. [32] A. Knop, H.W. Jochims, A.L.D. Kilcoyne, A.P. Hitchcock, E. R€ uhl, Chem. Phys. Lett. 223 (1994) 553. [33] A. Pavlychev, J. Phys. B 32 (1999) 2077. [34] L.S. Cederbaum, J. Zobeley, F. Tarantelli, Phys. Rev. Lett. 79 (1997) 4778. [35] R. Santra, J. Zobeley, L.S. Cederbaum, N. Moiseyev, Phys. Rev. Lett. 85 (2000) 4490.