Mechanisms for ion-induced plasmon excitation in metals

Transcription

1 Nuclear Instruments and Methods in Physics Research B 157 (1999) 110±115 Mechanisms for ion-induced plasmon excitation in metals R.A. Baragiola a, *, S.M. Ritzau a, R.C. Monreal b, C.A. Dukes a, P. Riccardi a,c a Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA b Dept. Fõsica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, C±V, Cantoblanco, Madrid, Spain c Dipartimento di Fisica, Universita degli Studi della Calabria, and INFM, Unita di Cosenza 87036, Arcavacata di Rende (CS), Italy Abstract We have studied the excitation of plasmons produced by 100 ev He,Ne and Ar and by 5±100 kev H and He projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of velocity. The e ect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions moving faster than a threshold velocity v th 1.3 v Fermi predicted by electron gas theories, kinetic plasmon excitation can occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that, contrary to theoretical expectations, plasmon excitation by H and He projectiles occurs below v th. With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: Gm; Mf; Nc; eF 1. Introduction It was recently discovered that neutralization accompanied by plasmon excitation is an important electron transfer process at surfaces of freeelectron metals, for ions carrying high potential energy [1,2]. This process, termed potential plasmon excitation, can occur at velocities lower than the threshold expected for kinetic plasmon excitation. In the kinetic mechanism, a major energy * Corresponding author. Tel.: ; fax: ; raul@virginia.edu loss process for fast charges penetrating condensed matter [3±5], plasmons can be excited because the valence electrons cannot respond instantaneously to screen the moving charge. The threshold velocity, v th, is determined theoretically from conservation of energy and momentum assuming direct Coulomb interactions of the fast charge with an electron gas. For heavy ions bombarding solids with low plasmon damping, v th 1.3 v Fermi. Several recent theoretical papers have been published on potential excitation of surface plasmons during surface neutralization [6±10]. This process competes with more studied neutralization processes, like Auger neutralization (AN) or res X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S X(99)

2 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110± onance neutralization followed by Auger de-excitation [11]. This latter process is very important in low work function surfaces, like those resulting from alkali adsorption. AN can occur if the maximum energy released upon neutralization of the incoming ion, E n ˆ I 0 /, is larger than /,the work function of the surface. Here I 0 is the neutralization energy of the ion I minus the image interaction (2 ev). Plasmons can be excited provided E n > E pl, the plasmon energy. With a work function of 4.3 ev for Al, slow He (I ˆ 24.6 ev) and Ne (I ˆ 21.6 ev) can excite the bulk plasmon of Al but Ar (I ˆ 15.8 ev) cannot, in agreement with observations [1] (E 0 plb ˆ 15.3 ev for k ˆ 0 and increases with momentum transfer k) [12]. On the other hand, excitation of surface plasmons in Al, and surface and bulk plasmons in Mg is allowed for the three ions (E 0 pls ˆ 10.6 ev) [1]. Plasmon excitations have been studied mainly theoretically since their detection in experiments is indirect, relying on the observation of the ejected electrons resulting from plasmon decay [3,4]. The characteristic energy distribution of electrons from this decay makes it possible to separate them from electrons originating from other processes such as AN, ionizations in ion±atom and ion±electron collisions, and Auger decay of inner-shell excitations [11]. Plasmon-assisted neutralization is also distinguishable from AN because it occurs later, after the plasmon lifetime [13]. We note that other, indirect, evidence of plasmon excitations appear in the energy loss, scattering and electron emission in the interaction of fast molecular ions with solids [14±17]. Kinetic excitation of plasmons has been described theoretically in recent papers [18±21] and observed in a few experiments on fast ion impact on metals at energies of tens and hundreds of kev [22±25] which have been limited to identifying the plasmon decay structures. Here we report new experimental results of studies designed to test the dependence of plasmon excitation on the state of the surface and the velocity of the ion. We analyze conditions for potential excitation of plasmons and provide evidence for kinetic plasmon excitation below the theoretically predicted threshold. 2. Results and discussion The measurements of the energy distributions of electrons ejected by ion impact were performed in two UHV systems. For energies <5 kev, the setup and methods have been described previously [1] with the addition of our ability to deposit cesium on the sample with sub-monolayer control. For energies >5 kev we use the second chamber, which is attached to a 100 kv ion accelerator. Ions are incident at 60 to the sample normal and electrons are detected in the direction perpendicular to the surface with a hemispherical energy analyzer. Samples in this second chamber are produced by in situ vapor deposition and the cleanliness is monitored by Auger electron spectroscopy. In our previous studies of potential plasmon excitation we have shown that for slow He ions on Mg, the electron structure due to plasmon decay is more important than that of AN [1]; Fig. 1 shows that the di erent groups of electrons are well-separated in energy. It is important to note that electron energies from AN depend on the potential energy of the ion whereas plasmon decay energies are intrinsic to the sample, i.e., independent of the type of ion, assuming the same momentum is transferred. The high-energy edge is indicative of Auger neutralization, which causes a dip in the derivative dn/de at I 0 2/ [11] or 11 ev for Ne and 14 ev for He, separated by the di erence of the ionization potential. This highenergy edge is broadened by the incomplete adiabaticity caused by the nite ion velocity normal to the surface [26]. In addition to the Auger neutralization edge, a prominent shoulder is observed. Its position, 7 ev, is not correlated with I; hence it is not due to AN involving structure in the density of valence states and is assigned to plasmon excitation. The high-energy edge of the shoulder is at E m ˆ E pl /, which corresponds to the case where the plasmon is absorbed by an electron at the Fermi level. The energy separation to the high-energy cuto of AN is I 0 / E pl and this is the reason why the two structures are more clearly separated in Mg than in Al. The plasmon edge is broadened by a constant value, given by the nite lifetime of the plasmon, and does not

3 112 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115 Fig. 1. Electron energy spectra N(E) for 106 ev ions and 1 kev electrons on Mg and Be, together with their derivatives dn(e)/de (bottom). The vertical scales in the Be spectra are in arbitrary units. Plasmon structure is seen in the Mg (shoulder at 7 ev) but not in the Be spectra. increase with the velocity of the ion, like the broadening of the AN edge. Fig. 1 shows the case of Be, where only Auger neutralization is seen (the structure in the derivative shifts with the potential of the ion), in contrast to Mg, where one can see plasmon structure that is xed in energy. The absence of a clear plasmon structure in Be is likely associated with the much larger width of the plasmon compared with that of Mg and Al [5], which lowers the value of the derivative. The visibility of plasmon decay thus becomes clearer in those materials that have a sharp plasmon resonance. Solids that should have clear plasmon decay structure are those that have sharp plasmon resonances. They include, besides Al and Mg, other alkaline earths, Si and alkalis. The absence of a clear structure does not mean that plasmons are not excited. In fact, we expect that plasmon excitation should dominate the neutralization behavior in all cases allowed by energy conservation. In addition, it is possible that plasmon excitation also accompanies Auger de-excitation of excited atoms at surfaces. In our previous paper [1], we noted that plasmon energies were close to but slightly lower than those of long-wavelength bulk plasmons, which are the ones more easily excited by fast charges. The excitation of bulk plasmons is unexpected, since neutralization occurs most likely when the ion is outside the surface. Current theories of

4 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110± plasmon excitations by external charges do not predict bulk plasmon excitation outside the solid. A possibility is that what we attribute to a bulk plasmon is in reality a surface plasmon of short wavelength, as suggested by Monreal [9]. To resolve this question we have produced slight alterations of the surface by sub-monolayer adsorption of Cs that, as veri ed by low energy electron energy loss spectroscopy, a ects the surface but not the bulk plasmon. In these experiments the Cs coverage was monitored by the change in work function, D/ (the maximum change, D/ ˆ 2.94 ev, occurs at a coverage of about half a monolayer). The sample was biased negative to allow the collection of all electrons. Fig. 2 shows that the plasmon structure disappears after a very small Cs coverage. This sensitivity to a change in the surface electronic structure suggests that the plasmons reported in our previous work [1] were surface plasmons, shifted by energy dispersion that occurs at large momentum transfers, in accordance with recent calculations [9]. Previously, we found that potential plasmon excitation was independent of ion velocity, up to cm/s [1]. Our new experiments for He on Al extend to cm/s v Fermi. The energy distributions for 20 kev He (Fig. 3) show that bulk plasmon decay structures are excited below the predicted threshold velocity for kinetic excitation. This is also the case for H, where potential excitation cannot occur at v ˆ 0 and should be weak even considering kinematic shifts of energy levels [27]. Fig. 4 shows the ratio of the intensity of the plasmon structure to the integral of the total energy distribution (the electron yield). The plasmon decay intensity dc p is computed for Fig. 2. Electron emission from an Al surface bombarded with 106 ev He ions for di erent Cs coverages characterized by a decrease in the work function, D/. The sample was biased by 5 V, and the vertical scales have been displaced for clarity. Notice the quick disappearance of the plasmon structure near 15 ev upon Cs adsorption. The structure near 10.5 ev is related to Cs. Fig. 3. Electron energy spectrum (top) and derivative (bottom) for Al excited by H and He at 20 kev, which travel with a velocity less than v Fermi.

5 114 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115 Fig. 4. Ratio of plasmon decay yields to total electron yields as a function of projectile velocity, for H and He on Al. The plasmon yield dc p is obtained in a window 2 ev wide, centered at the plasmon decay edge. The dashed lines are the calculated contributions of excitations by secondary electrons. The solid lines are to guide the eye through the experimental data, and have no other meaning. a 1 ev energy ÔwindowÕ around the plasmon edge after subtracting the tail of high-energy electrons. In this way we avoid uncertainties caused by uncertainties in the shape of the plasmon structure. There are several possible mechanisms to explain this unexpected result of excitation below v th. First, one needs to consider the nite plasmon width that results from plasmon damping. However, this can only introduce a small correction to v th, since, for these metals, the ratio of plasmon width to plasmon energy is very small. Then, one can consider that the constraints of energy and momentum conservation can be relaxed if one allows a target atom to absorb some of the momentum. However, this is not expected to be important, as judged from the similarity in the calculated energy loss of slow protons in an electron gas and in a crystal, which includes lattice e ects [28]. Another factor to consider is that the velocity distribution of valence electrons is displaced in the frame of the moving projectile. This can then allow the potential mechanism to occur, since it e ectively increases the potential energy available in the neutralization process by 'mvv Fermi, where m is the electron mass. An additional e ect that can contribute is the energy uncertainty caused by the nite ion velocity normal to the surface [29]. But electron capture accompanied by plasmon excitation is not limited to the surface; it can also occur inside solids, as has already been described theoretically [30]. Finally, plasmons can be excited by fast secondary electrons excited by the projectile. These fast electrons can result from several collision processes. Direct ionization may occur by binary ion-electron collisions or by electron promotion in the case of multi-electron projectiles. Also, fast electrons will result from Auger processes following the excitation of an inner-shell of the projectile or the target. To test excitation by fast electrons one can look for a correlation between the number of ejected electrons with su cient energy to excite a plasmon and the number of plasmon decays in a given energy spectrum of electrons. A better test requires modeling the energy distribution of fast electrons inside the solid from the observed distribution of ejected electrons. This can be done using electron-cascade models based on mean free paths for inelastic electron scattering. A fast electron produced at a depth z inside the metal will reach the surface without degrading its energy with probability exp( z/l)/2 and will lose energy by exciting a plasmon at a depth z 0 in a di erential dz 0 with probability exp[ (z z 0 )/L]dz 0 /l p, with L and l p being the total inelastic mean free path and the plasmon excitation mean free path, respectively. By considering all possible values of z and z 0 we can relate the number of excited plasmons to the number of energetic electrons ejected from the solid. The results are shown in Fig. 4. Details of the calculation were recently published [27]. It is seen that excitation by fast secondary electrons can account for plasmon excitation at velocities lower than the threshold for direct excitation.

6 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110± In conclusion, our new experiments have determined that a sharp plasmon resonance is necessary for the observation of plasmon decay structure in the energy spectra of secondary electrons. We expect that plasmon excitation should be important in neutralization whenever it is energetically allowed. This implies that most theories of Auger neutralization in ion-surface collisions need to be re-examined. Finally, we have found that kinetic plasmon excitation can occur for projectile velocities lower than the theoretical threshold, due to the e ect of energetic secondary electrons excited directly by the projectile. Acknowledgements This work was supported by NSF-DMR, SWRI, Iberdrola S.A., and the Spanish Comision Interministerial de Ciencia y Tecnologõa, contract PB References [1] R.A. Baragiola, C.A. Dukes, Phys. Rev. Lett. 76 (1996) [2] D. Niemann, M. Grether, M. Rosler, N. Stolterfoht, Phys. Rev. Lett. 80 (1998) [3] H. Raether, Excitation of plasmons and interband transitions by electrons, Springer, Berlin, [4] A. Liebsch, Electronic excitations at metal surfaces, Plenum Press, New York, [5] M. Rosler, Nucl. Instr. and Meth. B 69 (1992) 150. [6] F.A. Gutierrez, Surf. Sci. 370 (1997) 77. [7] R. Monreal, N. Lorente, Phys. Rev. B 52 (1995) [8] N. Lorente, R. Monreal, Surf. Sci. 370 (1997) 324. [9] R. Monreal, Surf. Sci. 388 (1997) 231. [10] M.A. Vicente Alvarez, V.H. Ponce, E.C. Goldberg, Phys. Rev. B 27 (1998) [11] R.A. Baragiola, in: J.W. Rabalais (Ed.), Low Energy Ion- Surface Interactions, ch. 4, Wiley, New York, [12] M. Rocca, Surface Sci. Repts. 22 (1995) 1. [13] R.A. Baragiola, Nucl. Instr. and Meth. B 78 (1993) 223. [14] D.S. Gemmell, J. Remillieux, J.C. Poizat, M.J. Guillard, R.E. Holland, Z. Vager, Phys. Rev. Lett. 34 (1975) [15] R.H. Ritchie, W. Brandt, P.M. Echenique, Phys. Rev. 14 (1976) [16] J. Eckardt, G. Lantschner, N. Arista, R.A. Baragiola, J. Phys. C 21 (1978) L851. [17] R.A. Baragiola, E. Alonso, O. Auciello, J. Ferron, G. Lantschner, A. Oliva Florio, Phys. Lett. 67A (1978) 211. [18] M. Rosler, Scanning Microsc. 8 (1994) 3. [19] M. Rosler, Appl. Phys. A 61 (1995) 595. [20] R. Zimmy, Surface Sci. 260 (1992) 347. [21] J.A. Gasspar, A.G. Eguiluz, D.L. Mills, Phys. Rev. B 51 (1995) [22] C. Benazeth, N. Benazeth, L. Viel, Surface Sci. 78 (1978) 625. [23] D. Hasselkamp, A. Scharmann, Surface Sci. 119 (1982) L388. [24] M.F. Burkhard, H. Rothard, K.-O.E. Groeneveld, Phys. Stat. Sol. (b) 147 (1988) 589. [25] N.J. Zheng, C. Rau, J. Vac. Sci. Technol. A 11 (1993) [26] H.D. Hagstrum, Phys. Rev. 139 (1965) A526. [27] S.M. Ritzau, R.A. Baragiola, R.C. Monreal, Phys. Rev. B 59 (1999) [28] I. Campillo, J.M. Pitarke, A.G. Eguiluz, Phys. Rev. B 58 (1998) [29] H.D. Hagstrum, Phys. Rev. 139 (1965) A526. [30] P.M. Echenique, F. Flores, R.H. Ritchie, Solid State Phys. 43 (1990) 230.