Nuclear Instruments and Methods in Physics Research B 164±165 (2000) 886±890 www.elsevier.nl/locate/nimb Plasmon excitation in Al by keV Ne and Ar ions P. Riccardi a a,* , P. Barone a, M. Camarca a, A. Oliva a, R.A. Baragiola b Laboratorio IIS, Dipartimento di Fisica, Universit a della Calabria, and INFM Unit a di Cosenza 87036 Arcavacata di Rende, Cosenza, Italy b Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA Abstract We present experimental studies of plasmon excitation by keV Ne and Ar single charged ions on Al surfaces. Plasmons are identi®ed through the characteristic energy spectrum of electrons from plasmon decay. At 1 keV incident energy, surface plasmons are excited by Ne but not by Ar . For higher energies we observe bulk plasmon excitation resulting, most likely, from excitation by fast electrons, such as Al-2p Auger electrons, traveling inside the solid. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 71.45.Gm; 73.20.Mf; 34.70.+e 1. Introduction Plasmon excitation in ion-solid interactions is attracting renewed interest since the experimental discovery of the process of potential plasmon excitation [1]. For projectile velocities far below the threshold for direct kinetic excitation [2], potential plasmon excitation occurs at the expense of the potential energy released upon neutralization of the incoming ion by electron capture from the solid. This mechanism thus competes with the well-known Auger Neutralization (AN) process [3±5]. AN can occur if En I 0 ÿ U > U, where En is the maximum energy released in the neutralization event, U is the metal work function and I 0 is * Corresponding author. E-mail address: riccardi@fermi.®s.unical.it (P. Riccardi). the ionization potential of the parent atom shifted by the image interaction D [3]. Ion neutralization can be accompanied by plasmon excitation provided that En > Epl , where Epl is the plasmon energy. The plasmons then decay by excitation of valence electrons [6,7] that may result in electron emission, which is revealed by a characteristic structure produced in the electron energy distribution [8]. This structure has a maximum energy, Em Epl ÿ U, corresponding to the absorption of the plasmon energy by an electron at the Fermi level. Analysis of the energy distributions of electrons emitted by metal surfaces under slow noble gas ions impact [1] reveals clear plasmon structures for those materials that have sharp plasmon resonances, like the free electron metals Al and Mg. In the case of Mg surfaces, the plasmon structure induced by He is well separated from the highenergy edge of AN, the energy separation being 0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 1 2 7 - 1 P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890 I 0 ÿ U ÿ Epl . Moreover, the structure due to electron emission from plasmon decay is more important than that of AN, thus leading to the conclusion that potential plasmon excitation should dominate the neutralization behavior whenever energetically allowed. Potential excitation of surface plasmons has been the subject of several theoretical studies [9,10]. Current theories do not predict bulk plasmon excitation by low energy ions that do not penetrate inside the solid. However, the energy distributions of electrons emitted by Al surfaces under slow He and Ne impact [1] showed structures whose energies were closely corresponding to that expected from bulk plasmon decay. Theoretical analysis by Monreal [11] showed that neutralization may produce surface plasmons of high momentum q, whose energies are shifted approaching those of the q 0 bulk plasmon. However, experimental studies of surface plasmon dispersion in Al have not shown such high energies [12]. Theories have not considered the eect of the hole produced in the solid during neutralization. Here we report energy distributions of electrons emitted by Al surfaces under the impact of keV, Ne and Ar single charged ions. It is shown that potential excitation of surface plasmons occurs for low energy Ne ions but not for Ar ions. At higher energies, we observe bulk plasmon excitation most likely resulting from fast electrons traveling through the solid. 2. Experiments The experiments were performed in an ultrahigh vacuum chamber with a base pressure of 3 10ÿ10 Torr Ne and Ar ions were produced in a dierentially pumped Atomika ion source by electron impact. The discharge voltage was set at 35 eV for Ne and 30 eV for Ar in order to ensure absence of double charged ions contamination in the ion beam. The ion current was of the order of 10ÿ9 A and had a Gaussian spatial distribution in both horizontal and vertical directions with a FWHM less than 1 mm, as measured by a movable Faraday cup at the target position. 887 A polycrystalline Al sample (purity 99.999%) was mounted on a manipulator that allowed variation of the ion incidence angle hi relative to surface normal. The sample was sputter cleaned by 5 keV Ar and Ne ions, cleanliness was assured by the absence of oxygen, and carbon signals in Auger spectra induced by 2 keV electrons. The emitted electrons were collected by a hemispherical electrostatic energy analyzer mounted on a goniometer. This analyzer, lying in the incidence plane, has semi-acceptance angle of 1.5° and was operated at a constant pass energy mode (DE 50 eV) and therefore an approximately constant transmission over the measured energy range. Electron energies are referenced to the vacuum level of the sample with an accuracy of 0.1 eV by comparing the energy of electrons from autoionization lines of Ne 2p4 3s2 , with published value [14]. To measure accurately low energy electrons the chamber was shielded with l-metal to reduce the eect of stray magnetic ®elds in the neighborhood of the sample and analyzer. 3. Results and discussion Fig. 1 shows representative electron energy spectra of the electrons emitted from the Al surface by incident 1 and 5 keV Ne ions and 2 keV electrons. The electron induced spectrum shows two structure commonly attributed to electron emission from decay of q 0 surface and bulk plasmon excited by fast electrons. The derivative of the spectrum make these structures evident [8] by revealing two minima at energies Em Epl ÿ U. The minima occur at 6.5 and 11.2 eV for the surface and bulk plasmon, respectively. Ion induced spectra compare well with previous results [1,13] and show features due to kinetic electron emission [19]: a low energy peak of cascade electrons and the two Ne** autoionization lines around 20±25 eV [14]. The spectrum for 5 keV Ne impact on Al shows a broad feature whose derivative has a minimum closely corresponding in energy to that of the bulk plasmon in the electron induced spectrum. This structure has already been observed for 4.5 keV Ne [13] and attributed to bulk plasmon decay. A similar 888 P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890 Fig. 1. Top: energy spectra of electrons emitted from an Al surface by the impact of 1 keV Ne , 5 keV Ne , and 2 keV electrons using the geometry shown in the inset. Bottom: derivative dN(E)/dE that enhances the structure due to plasmon decay. structure appears in the spectrum induced by 1 keV Ne . This structure was also attributed to decay of bulk plasmons excited upon neutralization of the incoming ions [1]. However, in agreement with previous observations [1], we notice that the minimum in the derivative spectrum for 1 keV Ne impact is shifted by about 1 eV to lower energy from that observed in the electron and 5 keV Ne spectra. In previous research [15], we studied the variations with angle of incidence of the energy dis- tributions of the electrons emitted from Al surfaces under impact by 1±5 keV Ne ions. We observed that the energy and the intensity of the plasmon structure for 1 keV Ne impact is largely independent on the incidence angle. This ®nding suggested that plasmons are excited at or above the surface and is consistent with the idea of excitation by the shake up due to the sudden disappearance of the dipole formed by the incoming ion and the image charge [1]. The observation that the energy of the plasmon excited by 1 keV Ne ions is lower than that of even the q 0 bulk plasmon, along with the observation that excitations occur at or above the surface, supports the interpretation of surface plasmons of high momenta, postulated by Monreal [11]. This would imply that surface plasmons can exist with energies higher than previously reported. At higher impact energies a transition from surface plasmon excitation to bulk plasmon excitation occurs. Angular measurements for 5 keV Ne on Al have shown that the plasmon energy is close to that of the q 0 bulk plasmon and that plasmon intensities increase as the incidence angle deviates from the surface normal. This is consistent with the idea that plasmon excitation occurs mainly in the bulk of the solid and that the intensity of the emission from plasmon decay depends on the depth were excitation occurred. In Fig. 2, we report energy distributions of electrons emitted from Al surfaces by Ar ions as a function of incident energy. Ions were impinging on the surface at an incidence angle hi 60° and the observation angle was he 0°. The spectra are shown normalized so that their integrals equal known electron yields [16]. The main feature of the spectrum induced by 1 keV Ar on Al is the broad distribution that results in the minimum at about 5 eV in the derivative spectrum. We have shown [15] that Ar neutralization at Al surface is not mediated by plasmon excitation and the minimum at 5 eV in the derivative correspond to the high energy edge of Auger neutralization. As the ion energy increases, the spectra show the evolution from the potential electron emission to the kinetic electron emission regime. The dip in the derivative at 5 eV decreases and a broad structure appears. This structure results in the minimum in the P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890 889 Fig. 3. Plasmon yields cp versus incident energy for Ar impact on Al. cp has been evaluated following the procedure outlined in [16]. Fig. 2. Top: energy spectra of electrons emitted from the Al surface by keV Ar impact. The parameter is the ion energy in keV. The spectra have been normalized so that their integrals equal known electron emission yields. Bottom: derivative dN(E)/dE. derivative observed around 11 eV. Comparison with Fig. 1 shows that the energy of this structure corresponds to that of the structure appearing in the spectrum of the electrons emitted by an Al surface under electron and 5 keV Ne irradiation. Therefore, it is assigned to electron emission from bulk plasmon decay. Bulk plasmon excitation in experiments of Ar ions bombardment on Al have been observed in the past [17] but at much higher incident energies than those reported here. A dip in the derivatives close to 6.5 eV due to a q 0 surface plasmons is apparent in the spectra, but cannot be clearly resolved. The results of Fig. 2 indicate that bulk plasmon excitation in the case of Ar ions on Al is related to kinetic electron excitation. This idea is also supported by the behavior of the intensity of the plas- mon structure as a function of ion energy, plotted in Fig. 3. Plasmon intensities have been evaluated following the procedure outlined in [18]. The energy dependence suggests that there is a threshold incident energy for bulk plasmon excitation. A precise evaluation of this threshold is made dicult because electron emission from bulk plasmon decay is masked by the background of secondary electrons. To understand the origin of this kinetic plasmon excitation we note that so-called plasmon loss satellites have been observed in the past in the Al-2p Auger spectra excited by electron impact as well as by electron promotion in violent atom±atom collisions in our energy range [20,21]. Our data are consistent with the idea that, at the threshold, plasmon may be excited by fast Auger electrons traveling inside the solid, consistent with the explanation for sub-threshold plasmon excitation by keV protons recently published [22]. In conclusion, we have studied the excitation of Al plasmons by keV Ne and Ar ions. In the case of low energy Ne on Al, data are consistent with theoretical prediction of potential excitation of surface plasmons of short wavelength. At higher incident energies, bulk plasmon excitation occurs and the reported data show that plasmons can be eciently excited by fast electrons traveling inside the solid. 890 P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890 Acknowledgements This research has been supported by Istituto Nazionale per la Fisica della Materia, INFM and the National Science Foundation. References [1] R.A. Baragiola, C.A. Dukes, Phys. Rev. Lett. 76 (1996) 14. [2] M. R osler, Scanning Microsc. 8 (1994) 3. [3] H.D. Hagstrum, in: N.H. Tolk, J.C. Tully, W. Heiland, C.W. White (Eds.), Inelastic Ion±Surface Collisions, Academic Press, New York, 1977, p. 1. [4] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [5] H.D. Hagstrum, Y. Takeishi, D.D. Pretzer, Phys. Rev. 139 (1965) 526. [6] N.B. Gornyi, Sov. Phys. Sol. State 8 (1966) 1523. [7] N.B. Gornyi, Sov. Phys. J. 10 (1967) 15. [8] M.S. Chung, T.E. Everhart, Phys. Rev. B 15 (1977) 4699. [9] R. Monreal, N. Lorente, Phys. Rev. B 52 (1995) 4760. [10] N. Lorente, R. Monreal, Surf. Sci. 370 (1997) 324. [11] R. Monreal, Surf. Sci. 388 (1997) 231. [12] K.D. Tsuei, E.W. Plummer, A. Liebsch, E. Pelke, K. Kempa, P. Bakshi, Surf. Sci. 247 (1991) 302. [13] D. Niemann, M. Grether, M. R osler, N. Stolterfoht, Phys. Rev. Lett. 80 (1998) 15. [14] F. Xu, R.A. Baragiola, A. Bonanno, P. Zoccali, M. Camarca, A. Oliva, Phys. Rev. Lett. 72 (1994) 4041. [15] P. Riccardi, P. Barone, A. Bonanno, A. Oliva, R.A. Baragiola, Phys. Rev. Lett. 84 (2000) 378. [16] E.V. Alonso, R.A. Baragiola, J. Ferron, M.M. Jakas, A. Oliva Florio, Phys. Rev. B 22 (1980) 80. [17] D. Hasselkamp, A. Scharmann, Surf. Sci. 119 (1982) L388. [18] N. Stolterfoht et al., Nucl. Instr. and Meth. B 164±165 (2000) 511. [19] R.A. Baragiola, in: J.W. Rabalais (Ed.), Low Energy Ion±Surface Interaction, Wiley, New York, 1994, Chapter 4. [20] R.A. Baragiola, E. Alonso, H. Raiti, Phys. Rev. A 25 (1982) 1969. [21] N. Mandarino, P. Zoccali, A. Oliva, M. Camarca, A. Bonanno, F. Xu, Phys. Rev. A 48 (1993) 2828. [22] S.M. Ritzau, R.A. Baragiola, R.C. Monreal, Phys. Rev. B 59 (1999) 15506.
© Copyright 2026 Paperzz