The excitation of collective electronic modes in Al by slow single charged Ne ions

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Surface Science 480 2001) L420±L426 Surface Science Letters www.elsevier.nl/locate/susc The excitation of collective electronic modes in Al by slow single charged Ne ions P. Barone a, R.A. Baragiola b, A.

Xu a a Laboratorio IIS, Dipartimento di Fisica, Universita della Calabria and INFM ± Unita di Cosenza, 87036Arcavacata di Rende, Cosenza, Italy b Laboratory for Atomic and Surface Physics, University

1 Surface Science ) L420±L426 Surface Science Letters The excitation of collective electronic modes in Al by slow single charged Ne ions P. Barone a, R.A. Baragiola b, A. Bonanno a, M. Camarca a, A. Oliva a, P. Riccardi a,b, *,F.Xu a a Laboratorio IIS, Dipartimento di Fisica, Universita della Calabria and INFM ± Unita di Cosenza, 87036Arcavacata di Rende, Cosenza, Italy b Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Thornton Hall B-103, Charlottesville, VA, USA Received 15 November 2000; accepted for publication 1 March 2001 Abstract We have studied electron emission from decay of plasmons excited in the interactions of slow single charged Ne ions with Al surfaces as a function of incident energy ranging from 0.7 to 8 kev. Bulk plasmon excitation occurs above a threshold incident energy of 1 kev and initially coexists with potential excitation of surface modes. The excitation of bulk plasmons in Al is determined by fast electrons traveling inside the solid. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion±solid interactions; Secondary electron emission; Plasmons; Aluminum; Noble gases; Polycrystalline surfaces Electron emission is a fundamental process in slow ion±solid interactions. The phenomenon is generally attributed to two main excitation mechanisms [1]. In potential electron emission, electron excitation results when the potential energy brought by the incoming ion is released upon neutralization by electron capture from the surface. In kinetic electron emission, electron excitation results from the transfer of kinetic energy from the ion. Our basic understanding of both * Corresponding author. Address: Material Science and Engineering, University of Virginia, Thornton Hall B-103, Charlottesville, VA 22903, USA. Tel.: ; fax: addresses: pr4n@virginia.edu, riccardi@ s.unical.it P. Riccardi). kinetic and potential electron excitation mechanisms is currently undergoing a substantial development, after the recent observations of surface and bulk plasmon excitation in the interaction of slow ions with surfaces of free electron metals [2± 4]. The detection of plasmon excitation relies on the fact that the main plasmon decay channel is the excitation of valence electrons interband transitions) [5], that can result in electron emission with a characteristic energy distribution. The maximum energy of this structure, E m ˆ E pl / [5], where E pl is the plasmon energy and / is the metal work function, corresponds to the case where the plasmon energy is absorbed by an electron at the Fermi level. Energy and momentum conservation require a minimum threshold velocity vth for plasmon /01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S )

2 P. Barone et al. / Surface Science ) L420±L426 L421 excitation due to the motion of charges in metals. For ions much more massive than electrons), the threshold velocity for this ``kinetic'' plasmon excitation is vth 1:3 v F [6], where v F is the Fermi velocity. Direct kinetic excitation is thus unimportant for projectile velocities lower than vth and the observed plasmon excitation needs to be attributed to other mechanisms. For slow projectile ions that do not penetrate inside the bulk, plasmon excitation results from the conversion of the potential energy E n released upon neutralization of the incoming ions [2]. Potential plasmon excitation occur if E n E pl. It thus competes with the well known Auger neutralization process [7] and dominates the neutralization behavior whenever energetically allowed [2]. Theoretical calculations [8] suggest that this mechanism may excite monopole surface plasmons of high momentum q whose energy is thus shifted approaching that of the q ˆ 0 bulk plasmon. Results of angular studies of electron emission [3] showed that potential plasmon excitation occurs indeed at or above the surface and the excited plasmons can be either the predicted monopole or the multipole surface plasmons [9]. So far, this latter collective excitation, recently observed in electron energy loss experiments on Al surfaces [10], has not been taken into account in calculations. Bulk plasmon excitation has been recently reported in experiments of ion bombardment on Al and Mg surfaces in the kev energy range. Several excitation mechanisms have been considered to discuss the experimental results, such as neutralization inside the solid when multiply charged Ne ions interact with Al surfaces [4], excitation by fast electrons excited in the electronic collision cascade induced by projectile ions [11,12] and excitation of high momentum bulk plasmons accompanied with momentum transfer to the target atoms for kev proton impact on Al 1 1 1) at grazing incidence [13]. In a previous study [3] of Ne impact on Al surfaces, we found that a transition from surface to bulk plasmon excitation occurs as the energy of the ions is increased from 1 to 5 kev. A detailed study of this transition is needed to understand the possible mechanisms of these sub-threshold bulk plasmon excitations in ion±solid interactions. To this end we performed measurements of energy distributions of electrons emitted by Al surfaces under the impact of kev Ne ions as a function of incident energy. Bulk plasmons were found to be excited above a threshold incident energy that can be estimated to be about 1 kev. The increase of electron emission from bulk plasmon decay with incident energy indicates that plasmon excitation is related to kinetic electron excitation. The estimated threshold energy is close to the threshold energy for Al-2p Auger electron excitation by electron promotion [14] in violent atom±atom collisions, thus suggesting that fast Auger electrons are responsible for sub-threshold bulk plasmon excitation by singly charged noble gas ions. The experimental setup has been described elsewhere [3,12]. Brie y, beams of Neon ions were produced in a di erentially pumped Atomika ion source whose discharge voltage was set to 35 V to prevent the formation of double charged ions. The ion beams were collimated to a diameter of less than 1 mm and directed onto a sputter cleaned polycrystalline Al target. The emitted electrons were collected by an electrostatic energy analyzer with a semi-acceptance angle of 1:5 and operated at a constant pass energy mode DE ˆ 50 ev) and therefore at an approximately constant transmission over the measured energy range. The UHV chamber was shielded with l-metal to reduce the e ect of stray magnetic elds on electron trajectories. In Fig. 1 we report energy distributions of electrons emitted from Al surfaces by Ne ions as a function of incident energy. Ions impinged on the surface at an incidence angle H i ˆ 60 and the observation angle was H e ˆ 0 angles are measured with respect to surface normal). The spectra have been normalized so that their integrals equal known electron yields [15,16]. The reported spectra show characteristic features of kinetic electron emission: a low energy peak due to electrons produced in the electronic collision cascade inside the solid and the two discrete lines around 20±25 ev due to autoionizing decay of Ne excited by electron promotion in violent atom±atom collisions [17]. Consistently with previous experiments [2±4], the spectra show a prominent shoulder at 9±12 ev

3 L422 P. Barone et al. / Surface Science ) L420±L426 Fig. 1. Energy spectra N E of electrons emitted from the Al Surface by kev Ne impact. The spectra have been normalized so that their integrals equal known electron emission yields [15,16]. attributed to electron emission from plasmon decay. The visibility of this structure is usually enhanced by taking the derivative of the spectra. The shoulder in the spectra results in the minima in the derivative at energies E m ˆ E pl / [5] shown in Fig. 2. The spectrum acquired at the lowest incident energy of 0.7 kev used in these experiments shows a shoulder which results in the minimum at about 10.5 ev in the derivative, i.e. about 1 ev less than the energy of the q ˆ 0 Al bulk plasmon 15.5 ev minus / ˆ 4:3 ev for Al). This energy value suggests that the structure is more likely due to electron emission from decay of multipole surface plasmons excited at or above the surface [3] by potential energy transfer upon neutralization of the incoming ions. At the highest incident energy of 8 kev the observed plasmon structure appears at an energy closely corresponding to that of the q ˆ 0 Al bulk plasmon, consistently with previous observations [2,4]. The spectra acquired at intermediate energies show the transition from surface to bulk plasmon excitation. As the energy of the incident ions increases, we notice from Fig. 2 that the structure due to bulk plasmon decay grows on the high energy side of the surface plasmon decay structure. A similar behavior was already appearing in the data reported in Ref. [2] but not analyzed as such. Information about intensity of electron emission from plasmon decay can be obtained by analyzing the structure produced in the derivative of the spectra [18]. In panels a)± d) of Fig. 3 are reported examples of polynomial background subtraction from the derivative of the spectra. The uncertainty in the background subtraction is estimated to be 30% by varying the function representing the background and the regions on both side of the plasmon structure to which the tting procedure is applied. The corresponding negative peaks after subtraction are shown in panel e)± h) of Fig. 3. It is evident from panel e) of Fig. 3 that at 1 kev incident energy the structure is well reproduced by one gaussian curve centered at an energy below that corresponding to a bulk plasmon decay structure and assigned to decay of multipole

4 P. Barone et al. / Surface Science ) L420±L426 L423 Fig. 2. Derivative dn E =de of the spectra in Fig. 1 that evidentiate the transition from surface to bulk plasmon excitation. The dotted vertical lines are used to guide the eye and the derivatives are arbitrarily displaced in the vertical scale for clarity. surface plasmon. At incident energies J 5 kev panel h)) the structure is reproduced by a gaussian at the position of the bulk plasmon. The position and width of this gaussian remain nearly constant with increasing incident energy. At intermediate incident energies, as shown in panel f) and g) of Fig. 3, the structure is not reproduced by only one curve but by the superposition of two gaussians of constant width and position corresponding respectively to the surface and bulk plasmon features. Absolute yields for surface and bulk plasmon decay electrons can be calculated from the areas A SP, A BP of the corresponding gaussians by multiplying by a factor C which is independent on the excitation mechanisms since the plasmon is an intrinsic property of the target. The value C ˆ 2E F =3 ˆ 8 ev, where E F is the Fermi energy, has been recently estimated [18] by a simple model which includes electron attenuation e ects but not scattering and cascade of electrons in the solid. Electron cascade is taken approximately into account in the theory by Chung and Everhart [5], which produces derivative spectra in agreement with experiments for electron impact. From their results, we obtain a value C 40 ev. Given this uncertainty on the value of C, we just plot, in the upper panel of Fig. 4, the areas A SP and A BP of the gaussians corresponding respectively to the surface and bulk plasmon decay features. We observe that the growth of the bulk plasmon structure occurs for incident energies above a

5 L424 P. Barone et al. / Surface Science ) L420±L426 Fig. 3. a±d) Examples of polynomial background subtraction from the derivative dn E =de; e±h) Gaussian curve ts of the negative peaks obtained after background subtraction. threshold incident energy of about 1 kev and dominates the spectrum for incident energies above 2 kev. The comparable intensity of the two plasmon structures at incident energies right above the threshold is an important nding. At these energies, in fact, neutralization inside the solid is not expected to be an e cient mechanism for bulk plasmon production. This is because the majority of the Ne ions are neutralized to the ground state in the incoming trajectory before undergoing hard collisions with target atoms as shown by several experiments [19±26]. On the other hand, bulk plasmons can be e ciently excited by electrons traveling inside the solid with energies greater than a threshold value of about 35 ev [11]. This mechanism was indeed observed to contribute to bulk plasmon excitation in the case of multiply charged ion bombardment

6 P. Barone et al. / Surface Science ) L420±L426 L425 atom collisions, if the internuclear distance of the two colliding partner is smaller than a critical value Rth, i.e. if the incident energy is above a threshold value Eth. Threshold energies from Auger intensity measurements have been well studied in the past and the values reported in literature [28,29] are consistent with the estimated threshold for bulk plasmon excitation reported here. In conclusion our experiments point to a simple physical picture for plasmon excitation which is consistent with previous experiments of slow single charged Ne ions interacting with Al surfaces. At low incident energies, potential excitation of surface plasmons occurs above the surface if the potential energy E n released upon neutralization of the incoming ions exceeds the plasmon energy. Penetration inside the bulk of incoming particles results in an electronic collision cascade in which bulk plasmons are excited by excited electrons. The threshold behavior of bulk plasmon excitation can be determined by Al LVV Auger electrons excited by electron promotion in collisions between two target atoms. References Fig. 4. Top: Areas of the surface A SP ) and bulk plasmon A BP ) gaussians described in Fig. 3 versus Ne incident energy. The total electron emission yield c tot ) [15,16] and the yield c LMM ) [28,29] of Al-Auger electron emission are also reported after proper rescaling. The lines through data points are used to guide the eye; bottom: ratio R ˆ A BP =c tot versus incident Ne incident energy. on Al surfaces [27], along with the proposed mechanism of neutralization inside the solid. In the lower panel of Fig. 4 is shown, as a function of incident energy, the ratio R ˆ A BP =c tot, where c tot is the total electron emission yield [15,16]. After a sharp initial increase, the ratio R approaches a constant value as the incident energy increases. This behavior clearly indicates that bulk plasmon excitation is related to kinetic electron excitation induced by ion bombardment. At the threshold energy for bulk plasmon excitation, an important source of energetic electrons is the Auger decay of Al-2p excitation by electron promotion in atom± [1] R.A. Baragiola, in: J.W. Rabalais Ed.), Low Energy Ion- Surface Interaction, Wiley, New York, 1994 Chapter 4). [2] R.A. Baragiola, C.A. Dukes, Phys. Rev. Lett ) [3] P. Riccardi, P. Barone, A. Bonanno, A. Oliva, R.A. Baragiola, Phys. Rev. Lett ) 378. [4] D. Niemann, M. Grether, M. Rosler, N. Stolterfoht, Phys. Rev. Lett ) [5] M.S. Chung, T.E. Everhart, Phys. Rev. B ) [6] M. Rosler, Scanning Microsc ) 3. [7] 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. [8] R. Monreal, Surf. Sci ) 231. [9] K.D. Tsuei, E.W. Plummer, A. Liebsch, K. Kempa, A. Pelke, Phys. Rev. Lett ) 44. [10] G. Chiarello, V. Formoso, A. Santaniello, E. Colavita, L. Papagno. Phys. Rev. B ) [11] S.M. Ritzau, R.A. Baragiola, R. Monreal, Phys. Rev. B ) [12] P. Riccardi, P. Barone, M. Camarca, A. Oliva, R.A. Baragiola, Nucl. Instr. Meth. B 164± ) 886. [13] B. Van Someren, P.A.Z. van Emmichoven, I.F. Urazgil'din, A. Niehaus, Phys. Rev. A )

7 L426 P. Barone et al. / Surface Science ) L420±L426 [14] M. Barat, W. Lichten, Phys. Rev. A ) 211. [15] E.V. Alonso, R.A. Baragiola, J. Ferron, M.M. Jakas, A. Oliva-Florio, Phys. Rev. B ) 80. [16] K. Wittmack, Nucl. Instr. Meth. Phys. Res. B ) 288. [17] F. Xu, R.A. Baragiola, A. Bonanno, P. Zoccali, M. Camarca, A. Oliva, Phys. Rev. Lett ) [18] N. Stolterfoht, D. Niemann, V. Ho mann, M. Rosler, R.A. Baragiola, Phys. Rev. A ) [19] O. Grizzi, M. Shi, H. Bu, J.W. Rabalais, R.A. Baragiola, Phys. Rev. B ) [20] R. Souda, K. Yamamoto, W. Hayami, T. Aizawa, A. Ishizawa, Phys. Rev. Lett ) [21] R. Souda, K. Yamamoto, W. Hayami, T. Aizawa, A. Ishizawa, Surf. Sci ) 139. [22] G. Zampieri, F. Meier, R.A. Baragiola, Phys. Rev. A ) 116. [23] L. Guillemot, S. Lacombe, M. Maazouz, E. Sanchez, V.A. Esaulov, Surf. Sci ) 92. [24] L. Guillemot, S. Lacombe, V.N. Tuan, V.A. Esaulov, E. Sanchez, Y.A. Bandurin, A.I. Dashchenco, V.G. Drobnich, Surf. Sci ) 353. [25] F. Ascione, G. Manico, P. Alfano, A. Bonanno, N. Mandarino, A. Oliva, F. Xu, Nucl. Instr. Meth. B ) 401. [26] F. Xu, N. Mandarino, A. Oliva, P. Zoccali, M. Camarca, A. Bonanno, R.A. Baragiola, Phys. Rev. A ) [27] D. Niemann, M. Grether, M. Rosler, N. Stolterfoht, Nucl. Instr. Meth. 161± ) 90. [28] N. Mandarino, P. Zoccali, A. Oliva, M. Camarca, A. Bonanno, F. Xu, Phys. Rev. A ) [29] R.A. Baragiola, E.V. Alonso, H.J.L. Raiti, Phys. Rev. A ) 1969.

Bulk and surface plasmon excitation in the interaction of He þ with Mg surfaces

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