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Slow electron emission from slow hollow atoms produced near a clean metal surface
Nuclear Instruments and Methods in Physics Research B78 (1993) 9Y-105 North-Holland
NOMB
Beam Interactions with Materials & Atoms
Slow electron emission from slow hollow atoms produced near a clean metal surface F. Aumayr, H. Kurz, K. T6glhofer and HP. Winter Institut
fiir Allgemeine Physik, Technische Unicwsitiit Wien, Wledner Hauptstrasse R-IO, A-1040 Wien, Austria
Statistics of electron emission have been measured for bombardment of a &an polycrystalline gold surface by slow (impact velocities I-15 x 104 m/s) highly charged recoil ions N “+(q < 6), NeR-(q < 101, ArqA(q I 161, Kry-(q I 10) and I9 “(4 5 25) produced with the GSI UNILAC heavy ion accelerator, from which prccisc absolute total electron yields have been derived. As an example, with 1.50eV I B’ a total electron yield of 70 with about 1% probability for emission of z 85 electrons/ion was observed. A semi-classical over-barrier approach recently introduced by Burgdiirfer et al. 1991 has been further extended and successfully applied for modefling the impact-velocity dependences of measured total electron yields. In this way, new insights could be achieved on the ncutraliration and electron emission taking place during conversion of slow multicharged ions into hollow atoms and their subsequent rapid de-excitation in front of a metal surface.
1. Introduction Multicharged ions (“MCI”-Zq’ ) will capture eicctrons resonantly from states near the Fermi edge of a metal surface within a critical distance d, which is determined by the wavcfunctions overlap of the surface density-of-states (S-DOS) with empty projectile states. d, increases with higher projectile charge q and/or lower surface work function tl/m. Once within this critical distance, a slow MCI (impact velocity G’~K 1 a-u.1 will be rapidly further neutralized according to a charactcristic time t, related to the Fermi velocity ct.., which for metals is of the order of 1 a.u. [I]. The such dcvcloping multiply excited (“hollow”) atoms become subject to resonant ionization (RI) as well as autoionization (AI). which proccsscs together with continuing resonant neutralization (RN) will determine the projectile’s electronic population until it hits the surface [2,3]. As the result of AI, slow electrons should be emitted from the projectiles [4J the more efficiently the lower the impact velocity [SJ. Sufficiently slow particles may be reflected by the repulsive planar surface potcntial [3], whcrcas faster ones can travel well into the solid where they will undergo various de-excitation processes until their complete neutralization and stopping. Related scenarios have recently been studied in a number of ways, e.g. by analyzing the resulting totai efcctron emission yields [4-6], ejected electron energy distributions [3,6-Y], charge state composition of scattered projectiles [lo, 111 and soft X-ray emission [3,12,13]. The observed electron cncrgy distributions arc dominated by low energy continua (E, < 30 eV 0168-S83X/93/%06.0
[9,14]), onto which relatively small contributions from fast Auger electrons may be superimposed [3,7-9,141. The latter originate from transiently formed projectile inner shdl vacancies, which predominantly decay inside the solid [3,7J. Reviews on slow MCI-surface interaction [3,15] and rclatcd semiclassical [2] and classical [16J theories have recently been published. In addition, some quantum-mechanical calculations have been made for the relevant RN- [17J and Al processes [18J. The present paper deals with the statistics of electron cmission (“ES”) resulting from impact of slow MCI on a clean gold surface. Such ES, having been detcrmincd by means of a rcccntly developed method [19], cover the probabilities W, for emission of n = 0, I, 2,. . . electrons per impinging MCI and permit determination of reiatcd absolute total electron emission yields according to
(1)
2. Experimental method and presentation of typical measured results A recoil ion source [20] pumped by fast (3-11 MeV/amu) multicharged ions from the GSI UNKAC heavy ion accelerator delivered MCI fluxes of e.g. 100/s for Ar16+ or 104/s for Ar’*+ at the target surface. After extraction with several hundred volts and charge-to-mass separation in a 180” magnet, sclected recoil ions were guided toward the ES detector 121). Their rather well defined initial kinetic energy
8 IY93 - Elsevicr Scicncc Publishers B.V. All rights reserved
If. MULTIPLY CHARGED/IIEAVY
F. Aumayr et al. / Slow electron emission from slow hollow atoms
100
deceleration electrode (+400 v - LJ)
extraction electrode (+1.9 kV - U) QTzzzzJP
cage (+340 V - U)
focussing electrode (+400 v - U)
electron detector (+26.4 kV - U)
initial ion energy
final ion energy E=qUeV Fig. 1. Setup for measuring ion-induced ES and corresponding total electron yields for impact of slow ions (E t 2q eV) on clean gold. Indicated potentials refer to a nominal MCI impact energy of llq eV on target by decelerating a 4oOq cV primary ion beam.
permitted deceleration to nominal target impact energies E as low as (2 f 1)q eV by means of a four-cylin-
der lens in front of the target surface and appropriate biasing of the latter (cf. fig. 1). The final part of the MCI beam line and the ES detector assembly surrounding the target (polycrystalline gold surface prepared in atomically clean state by sputtering with a built-in Ar+ ion gun) was kept in UHV at a base pressure of typically 10 -s Pa during all measurements. ES have been determined for different MCI species, applying a fountain-type cylindrical electrode setup which collected all electrons ejected from the target with energies of E, 5 60 CV into the full 27~ solid angle and guided them toward a surface barrier dctector biased at +26 kV with respect to the target [21]. From the resulting pulse height spectra (figs. 2a and 2b show typical raw ES data for impact of 100 eV Ar9 -1 emission probabilities W, have been derived via a dcconvolution method described in more detail elsewhere [22]. Although in this way the probability W, for
0
IO
15
n
20
25
emission of no electron can principally not be detcrmined, it may safely be neglected for yields y 2 5. With the above described methods, measurements have been performed [23] for projectile species N4-’ (y I 6), NeY’ (q 5 lo), ArY+(q I 16), Kr”+(q < 10) and lq+(q s 25) over a wide range of impact energies. As typical examples, fig. 3 shows the total electron yields y (accuracy typically +3%‘c) vs impact velocity up for ArQ’ (q = 6-16) projectiles, and fig. 4 presents ES for Ar”* impact at three different energies. We point out that the present experimental technique offers a number of advanlages over the usual determination of MCI-induced total electron yields by measuring the currents of respectively primary ions and emitted clectrons [5,6], e.g. a much higher sensitivity (reducing the primary MCI current requirements dramatically), the rather precise absolute electron yields directly available from the essentially relative ES measurements and, most notably, the additional information hidden in the related electron emission multiplicity.
0
5
10
15
20
25
n
Fig. 2. (a) Comparison between measured (raw) ES for impact of 100 eV Ar 91- ions on clean Au (dots) and results of a fit (solid line). (b) Individual ES contributions to fitted data, considering electron backscattering from detector surface [22]. Contributions related to even numbered probabilities have been shaded for clarity.
101
F. Aumuyr et al. / Slow electron emission from slow hollow atoms
3. Modelling results
of experimental
data and discussion
of
-
First of all, from fig. 3 we observe that for all projectile charges with increasing impact velocity L’,, the electron yield gradually decreases and then levels off towards an apparently constant value, henceforth to be named y=. As already stated in [23], the vclocity-dcpcndent part of the total yield y (i.e. y - yJ prcsumably originates from AI of the hollow atoms-turned MCI during their way to the surface. Calculating d, according to [2], which gives practically equal results as a fully classical treatment [16], and taking W, = 5.1 cV for clean gold [l], we obtain (atomic units arc used unless otherwise stated)
(2) At our lowest impact energies eq. (2) delivers typical projectile flight times t,> 10 -I3 s inside d,. From the velocity-dependent parts of y as shown in fig. 3 we thus may obtain mean apparent AI rates of typically I lOI s- ‘. If subsequent electron emission events would involve constant AI rates, y-y= has obviously to scale like l/c,. However, fig. 3 clearly demonstrates that instead the relation y = const.t;;‘/*
+ yz
(3)
fits the measured total yields much better, which obscrvation has been tentatively related in ref. [23] to steadily increasing apparent mean lifetimes for subscqucnt AI processes (i.e. gradually decreasing apparent AI rates with shrinking distance of the hollow atoms from the
Arq+
-
Au
2
,O emission
multiplicity
n
Fig. 4. Electron emission statistics (ES) for impact of Ar”.’ at three different impact energies on clean Au. Dashed lines are fitted Gaussians. The solid line corresponds to a Poissonian with appropriate mean value y for the 500 eV measurements (also cf. text).
surface. We now explain this bchaviour of apparent AI rates in a more detailed way as follows. On their way toward the surface the hollow atoms become subject to both RI and AI, but only the latter processes give rise to the observable electron emission. It has already been pointed out that because of “screening dynamics” RN populates projectile Rydbcrg states with progrcssively lower principal quantum numbers, as the particles approach the surface [3]. Consequently, the AI processes should become less cfficicnt if taking place nearer to the surface, considering the decreasing chance for “down-electrons” to reach empty lower projectile states for enabling the “up-electrons” to make the respectively most probable AI transitions just into vacuum [18], and the fact that the competing Rl become gradually faster if taking place closer to the surface [2]. Fig. 3 shows, however, that at low impact velocity the yield deviates from its y vs v,, dependence as given by cq. (31, which we relate to a gain in projectile impact energy due to the corresponding image charge attraction. Before its first RN inside the distance d, (cf. eq. (211, an ion Z” ’ has already gained a kinetic energy A E,Ti, according to AE,,,,
Fig. 3. Total electron yields derived from measured ES vs. impact velocity, for impact of Ar”-. on clean polycrystalline gold (y = 6-16 [23]). The solid lines are fits according to eq. (31, cf. text. Deviation from fits at low t:p is being ascribed to image charge acceleration of the projectiles.
5ooev
Au
= y2/4d,
s 0.9~“~
(eV).
(4)
However, fitting a 4’‘/* dependence to experimental data for Ar 9+ (q I 6) scattered under grazing incidence from a metal surface, in which way the image charge acceleration could be directly determined [ll], results in AE+,
= 1.2q3’2
(cV).
(4a)
The discrepancy between both relations can be easily rcmovcd by considering in cq. (4) the further projectile II. MULTIPLY
CHARGED/HEAVY
F. Aumayr et al. / Slow electron emission from slow hollow atoms
102
acceleration inside d, during the ongoing MCI neutralization [2]. Turning now to the observed ES, they should correspond to Poissonian distributions if the slow electron emission were caused by a series of equally fast AI processes (H. Kurz et al., to be published). Fig. 4 shows ES for Ari*+ projectiles with three different impact energies. Whereas Gaussian distributions fit the ES rather well, Poissonians with appropriate mean values y are clearly too broad, as demonstrated for the 500 eV case. Apparently, the electron emission is less randomly distributed than to be expected for mutually independent ejection of individual electrons, which indicates electron emission due to other mechanisms than exclusively by AI. Coming back to the total electron yields, we first remark that earlier direct y measurements [4,&g] e.g. for Ar4’ (up to 4 = 8) have shown that y is practically proportional to the MCI’s total potential energy WR,wol. However, for 4 2 9 inner shell vacancy formation in the course of MCI neutralization was found to cause a lcvelling off from this bchaviour, as apparent also from our present measurements for Arq+ (cf. fig. 5 at q 2 9). A similar proportionally of y with W&,, holds for other MCI species in lower charge states as well, but breaks down if inner shell vacancies are produced in the course of neutralization (e.g. for Nq+/q = 6 or Neq-’ /q = 9 [5]). For initial MCI charge states where inner shell vacancy formation during neutralization will take place, WCnow instead observe a linearity of the total electron yield with projectile charge state 4 [23], as shown e.g. for A++ (q > 8) in fig. 6. A possible candidate process for such emission proportional to 4 could be “peclingoff’ of the 5 q electrons bound in highly excited states as the projectile reaches the surface [2]. However, the
35r--
1 q.16 15 0
i
14 13
12 11 0
l
l
1
0
Arq + 1500
Au
Fig. 5. Total electron yields y vs total potential energy W&,, (values taken from ref. [24]) for impact of A++ on clean gold. Note deviation from apparently linear dependence for 4 2 9, for further details cf. text.
”
4
8
12
16
! ? 20
charge state q Fig. 6. Dependences of total electron yields y (full and open circles for two different impact energies) and related values y, (diamonds) on primary ion charge state 4) for ArY+ impact on clean gold. From 4 = 8 on, straigth lines fit the data points
quite well.
assumedly impact velocity-independent contribution -y, (see above) cannot just result from such “peeling-off’, since for higher q-values y, clearly surpasses q (y_ = 12 for 4 = 12, but y, = 21 for q = 16, cf. fig. 6). We emphasize that kinetic electron emission (KE) cannot contribute to yg in an appreciable way, since in the here regarded impact velocity range (v, < 2 x lo5 m/s) the related KE yield cannot surpass about 0.5 electrons/ion [21). We therefore have proposed in [23] some “ultimate”, very fast Al- and/or Auger neutralization processes occurring rather close to the surface where the still populated highly excited projectile states will completely overlap with the occupied metal S-DOS. These processes should assumedly be sufficiently fast to remain independent of the MCI impact velocity within its here considered limits. However, to describe this scenario in a more quantitative way, we have now modelled the MCI neutralization near the metal surface, following recent work of Burgdorfer et al. 1991 [2]. Fig. 7 shows which transitions have been taken into account in our model calculations. Dynamical screening of the populated states and RN-, RI- and AI-transition rates from Burgdiirfer et al. 1991 have been taken into account, together with Auger transitions into unoccupied states in the conduction band and image charge acceleration of the projectiles. Furthermore, the “peeling off’ mechanism upon surface impact has been assumed to work for all Rydberg states with n > n,.+, populated in the “hollow atom”, corresponding to Bohr radii larger than the target metal screening distance given by cF/ope, where w,,~ is the metal electron plasma frequency [1,2]. To model our experimentally determined total electron yields in a satisfactory way, we had also to take into account an electron emission process not explicitly described in [2], i.e. “promotion” of captured electrons above the vacuum level due to the combined action of
F. Aumayr et al. /
Slow electron emission from slow h&w atoms
peeling off
approaching MCI
-
metal S-DOS
Fig. 7. States of a neutraI~jn~ MCI during its approach toward a metal surface. Electrons captured via RN can be emitted via AI, promotion into vacuum rcsultiny from screcning shift (SS) and image shift (IS), and “peeling off’? respectively. In addition, recapturing of electrons into the metal via RI takes place. For further detaik cf. text.
image charge shifting (IS) and binding energy reduction by screening of the particular state bccausc of RN into neighbor states (SS), cf. fig. 7. To give just one example for the quality of our modclling, fig. 8 shows the impact velocity dependence of the experimental yield for impact of Art*+ together with the calculated enc. Different ~on~rjbu~ions to the total electron cmission (i.e. from Al, “promotion” and “peeling off”, respectively) have been separately indicated by dashed lines. Cl corrcsponds to the sum of ail three processes, whereas for 62 it has been assumed that out of Cl only one half of ail Auger electrons emitted from the projectile due to AI have made it into vacuum while the other half will be absorbed by the metal. Finally, X1*0.7 indicates results obtained under the assump-
1, ***..
50
0
‘. -.
__r_-----__
I
l
2
4
_ _ _ gmionization ------_“ll__
“SStlS promotion” -_l_______-_-_-v”s~
1
!
6
I-.._&w...
8
10
12
14
---I 16
18
VP (104 m&f
Fig. 8. Comparison c&measured and modelled total electron yieIds vs impact velocity for neutralization of A.r’“* near a clean gold surface. For further e~~anations cf. text.
103
tion that 30% of all electrons emitted by the projectile were absorbed by the target metal. For the particular case considered here as well as for the e~~rjrn~ntal results obtained with a number of other MCI species a quite satisfactory agreement could be obtained with these model calculations, as will be dcmonst~ted in more detail elsewhere (H. Kun et al., to be pubiished). Our above presented scenario for electron emission is supported by results for the particular proje~tiIe species N”‘, NcB~.tO+ and I%*. For bombardment with the first three MCI species WC found a rather weak variation of the total electron yields with cP, which points to a strong domination of RI over clectron emitting processes during the projectiles approach toward the surface. ~ns~qu~ntly, the measured total electron yield seems to be almost exclusively due to the “peeling off’ process just upon surface impact. This rather interesting result strongly suggests a refinement of models developed both for the filiing of electronic states prior to the inner shell vacancy-related fast Auger electron emissittn [3,?-91 and the processes held responsible for the slow electron emission fig]. In the light of our above presented arguments, for hydrogenlike as well as fully stripped MCI the AI seems to become stronger supressed by RI than for equally charged more-electron projectiles, i.e. for 4 < Z - I. To support this claim, the case of 150 cV Iz5’ (y = 70) as an extreme example of pure potential emission is regarded, where more than n = 85 electrons can be ejected in a single MCI impact. In contrast to N”+ and Nc9*~tof, there electron emission during the projectiles approach toward the metal surface seems to be much tcss inhibited by RI, as a result of which the impact veloci~-dependent contribution y - y= = 40 electrons/ ion appears as a relatively much larger part of the totaf yield. We point out that RN of the Ni-like 125+ ions cannot produce strongly bound inner shell vacancies, which seems to favour the electron emission already during approach of the corresponding “hollow atom” to the surface. These results demonstrate that the above discussed breakdown of the linear y vs W,,,,t relationship for highly charged primary ions with a capability for inner shell vacancy formation during neut~li~tion can be well understood, since the potential energy stored in such inner shelf vacancies should actually not be taken into account for the respective total Wy,,t value (cf. fig. 5). In other words, the slow electron emission which makes up for the here observed total electron yields can only be brought forward by sufficiently rapid transitions among the higly excited Rydbcrg states which have been populated prior to surface impact. The large potential energy attached to the transiently produced inner shcli vacancies will almost comptetely be carried furtheron into the metal, where it becomes dissipated in such a way that no further important II. MULTIPLY CHARGED/HEAVY
104
I;. Aumayr et al. / Slow electron emksion from slow hollow atoms
contributions to the observable total electron yields arc produced. This also means that subtracting the potential energy stored in the transiently formed inner shell vacancies from the total projectile potential energy Wq,pO, will restore the linear y vs Wq,pC,, relationship otherwise only observable for small q-values, as can already be guessed from inspection of fig. 5 and will be shown quantitatively by Kurz ct al. (1992, to be publishcd).
4. Conclusions We have opened up a new way to investigate clectron emission caused by slow multicharged ion impact on a solid surface, by measuring the resulting electron emission statistics, from which then rather precise total electron yields can be obtained. By using this tcchniquc with a fast heavy ion-pumped recoil MCI source, very low MCI impact energies (only limited by the apparent image charge acceleration of projectile ions) could be reached, and rather small ion fluxes were sufficient to permit precise y measurements. Modelling the experimentally obtained total slow electron yields revealed that the electron emission is probably produced from three principally different sources. After first RN processes have taken place, during the projectile’s further flight toward the surface competition starts between AI and electron promotion into vacuum on the one side, and RI of the hollow atomsturned MCI near the surface on the other. The results of this competition will depend quite critically on the overlap of the surface density-of-states and the elcctronic state structure of the neutralizing projcctilcs. The slow electron yield produced during the projectiles flight generally increases with decreasing impact velocity, but under certain circumstances remains only a tiny fraction of the total electron yield. Just upon surface impact, a third, more or less impact velocity-indcpendent contribution to the total electron emission will be made, which can be ascribed to “peeling off’ of elcctrons having remained so far in high Rydberg states with n > ncri,. Due to this “peeling ofT’ the projectiles become highly recharged, but then probably screened rapidly by electrons captured resonantly into principal quantum states with rt sncrit. In this way, smaller hollow atoms are reproduced which then may also de-excite quite probably via fast Auger elccton- and/or X-ray emission [3,7]. Our measured total electron yields could bc successfully modclled for impact of various projectile species and -charge states, which adds quite satisfactorily to the understanding of the here discussed processes accompanying the approach of slow multicharged ions toward a metal surface. Our model calculations explain to what extent the considerable poten-
tial energy carried by slow highly charged ions can be utilized to extract a large number of electrons from the metal surface, and why for higher initial charge states only a fraction of the total potential MCI energy does actually contribute to the measured slow electron yield. The present results provide further insight in processes triggered by slow multicharged ion-metal surface interaction, which arc also of importance for the recently discussed fast Auger electron- and X-ray photon emission phenomena already observed in such collisions. Further, still more detailed understanding might bc expected from comparable work involving other MCI species as well as different, well defined target surfaces.
Acknowledgements The authors thank Dr. R. Mann (GSI Darmstadt) for his important role in the measurements, Prof. J. Burgdiirfer for useful discussions and communications, and Mr. C. Lemell for performing the present model calculations. This work has been supported by Austrian Fonds zur FGrderung dcr wissenschaftlichen Forschung under project no. P8315TEC, and by Kommission zur Koordination der Kernfusionsforschung at the Austrian Academy of Sciences.
References and N.D. Mcrmin, Solid State Physics (CHS Publ. Asia Ltd., Philadelphia, 1976).
III N.W. Ashcroft PI J. BurgdBrfer,
P. Lcrner and F.W. Meyer, Phys. Rev. A44 (1991) 5674. and Atomic Collisions, [31 H.J. Andrii et al., in Electronic eds. W.R. MacGillivray, I.E. McCarty and M.C. Standage (IOP Publ. Ltd./Adam Hilger, Bristol, 1992) p. 89. E.S. Mukhamadiev and [41 U.A. Arifov, L.M. Kishinevskii, E.S. Parilis, Zh. Tekh. Fix 43 (1973) 181 (Sov. Phys.-Tech. Phys. 18 (lY73) 1181. R. Geller, D. Hitz, P. Varga 141 M. Delaunay, M. Fehringer, and H.P. Winter, Phys. Rev. B3S (1987) 4232. [f-)11I.D. Hagstrum, Phys. Rev. 96 (1954) 325 and 336. C.C. Havener and F.W. 171 PA. Zcijlmans van Emmichovcn, Meyer, Phys. Rev. A43 (1991) 1404; F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven, J. Burgdiirfcr and D.M. Zchner, Phys. Rev. A44 (1991) 7214. 1987; 181 S.T. de Zwart, PhD Thesis, University tironingen, S.T. de &art, A.G. Drentje, A.L. Boers and R. Morgcnstern, Surf. Sci. 217 (1989) 298; L. Folkerts and R. Morgenstern, Europhys. Letters. 13 (1990) 377. M. Fehringer, R. Gcller, P. Varga and I’,1 M. Delaunay, 11.1’. Winter, Europhys. Lett. 4 (1987) 377. 1101S.T. de Zwart, T. Fried, U. Jellen, A.L. Boers and A.G. Drentje, J. Phys. B18 (1985) L623.
F. Aumayr et al. / Slow electron emission from slow hollow atoms
[ll] H. Winter, Europhys. Lett. 18 (1992) 207. [12] E.D. Donets, Nucl. Instr. and Meth. BY (1985) 522. [13] J.P. Briand, L. dc Billy, P. Charles, S. Essabaa, P. Briand, R. Geller. J.P. Desclaux, S. Bliman and C. Ristori, Phys. Rev. Lctt. 65 (1990) 159 and Phys. Rev. A43 (1991) 565; M. Schulz, C.L. Cocke, S. IIagmann, M. Stockli and II. Schmidt-Backing, Phys. Rev. A44 (1991) 1653. [14] J.W. McDonald, D. Schneider, M.W. Clark and D. Dewitt, Phys. Rev. Letters 68 (1992) 2297. [15] P. Varga, Comm. At. Mol. Phys. 23 (1989) I II; H.P. Winter, Z. Phys. D21 (1991) S129. [16] J.N. Bardsley and B.M. Penetrante, Comm. At. Mol. Phys. 27 (1991) 43. [17] U. Wille, Z. Phys. D21 (19Yl) S353 and Phys. Rev. A45 ( 1992) 3004.
105
[Ifi] N. Vaeck and J.E. Hansen. J. Phys. B24 (IYYI) L&Y. [19] G. Lakits, F. Aumayr and I1.P. Winter, Rev. Sci. Instr. 60 (1989) 3151. [20] R. Mann, Z. Phys. D3 (1986) 85. [21] G. Lakits, F. Aumayr, M. Heim and H.P. Winter, Phys. Rev. A42 (1990) 5780; H.P. Winter, F. Aumayr and G. Lakits, Nucl. Instr. and Meth. B58 (1991) 301. [22] F. Aumayr, G. Lakits and 1I.P. Winter, Appl. Surf. Sci. 47 (1991) 139. 1231 H. Kurq K. Tiiglhofer, H.P. Winter, F. Aumayr and R. Mann, Phys. Rev. Lett. 69 (1992) 1140. [24] R.L. Kelly and D.E. Harrison, At. Data 3 (1971) 177.
II. MULTIPLY CHARGED/HEAVY
NOMB
Beam Interactions with Materials & Atoms
Slow electron emission from slow hollow atoms produced near a clean metal surface F. Aumayr, H. Kurz, K. T6glhofer and HP. Winter Institut
fiir Allgemeine Physik, Technische Unicwsitiit Wien, Wledner Hauptstrasse R-IO, A-1040 Wien, Austria
Statistics of electron emission have been measured for bombardment of a &an polycrystalline gold surface by slow (impact velocities I-15 x 104 m/s) highly charged recoil ions N “+(q < 6), NeR-(q < 101, ArqA(q I 161, Kry-(q I 10) and I9 “(4 5 25) produced with the GSI UNILAC heavy ion accelerator, from which prccisc absolute total electron yields have been derived. As an example, with 1.50eV I B’ a total electron yield of 70 with about 1% probability for emission of z 85 electrons/ion was observed. A semi-classical over-barrier approach recently introduced by Burgdiirfer et al. 1991 has been further extended and successfully applied for modefling the impact-velocity dependences of measured total electron yields. In this way, new insights could be achieved on the ncutraliration and electron emission taking place during conversion of slow multicharged ions into hollow atoms and their subsequent rapid de-excitation in front of a metal surface.
1. Introduction Multicharged ions (“MCI”-Zq’ ) will capture eicctrons resonantly from states near the Fermi edge of a metal surface within a critical distance d, which is determined by the wavcfunctions overlap of the surface density-of-states (S-DOS) with empty projectile states. d, increases with higher projectile charge q and/or lower surface work function tl/m. Once within this critical distance, a slow MCI (impact velocity G’~K 1 a-u.1 will be rapidly further neutralized according to a charactcristic time t, related to the Fermi velocity ct.., which for metals is of the order of 1 a.u. [I]. The such dcvcloping multiply excited (“hollow”) atoms become subject to resonant ionization (RI) as well as autoionization (AI). which proccsscs together with continuing resonant neutralization (RN) will determine the projectile’s electronic population until it hits the surface [2,3]. As the result of AI, slow electrons should be emitted from the projectiles [4J the more efficiently the lower the impact velocity [SJ. Sufficiently slow particles may be reflected by the repulsive planar surface potcntial [3], whcrcas faster ones can travel well into the solid where they will undergo various de-excitation processes until their complete neutralization and stopping. Related scenarios have recently been studied in a number of ways, e.g. by analyzing the resulting totai efcctron emission yields [4-6], ejected electron energy distributions [3,6-Y], charge state composition of scattered projectiles [lo, 111 and soft X-ray emission [3,12,13]. The observed electron cncrgy distributions arc dominated by low energy continua (E, < 30 eV 0168-S83X/93/%06.0
[9,14]), onto which relatively small contributions from fast Auger electrons may be superimposed [3,7-9,141. The latter originate from transiently formed projectile inner shdl vacancies, which predominantly decay inside the solid [3,7J. Reviews on slow MCI-surface interaction [3,15] and rclatcd semiclassical [2] and classical [16J theories have recently been published. In addition, some quantum-mechanical calculations have been made for the relevant RN- [17J and Al processes [18J. The present paper deals with the statistics of electron cmission (“ES”) resulting from impact of slow MCI on a clean gold surface. Such ES, having been detcrmincd by means of a rcccntly developed method [19], cover the probabilities W, for emission of n = 0, I, 2,. . . electrons per impinging MCI and permit determination of reiatcd absolute total electron emission yields according to
(1)
2. Experimental method and presentation of typical measured results A recoil ion source [20] pumped by fast (3-11 MeV/amu) multicharged ions from the GSI UNKAC heavy ion accelerator delivered MCI fluxes of e.g. 100/s for Ar16+ or 104/s for Ar’*+ at the target surface. After extraction with several hundred volts and charge-to-mass separation in a 180” magnet, sclected recoil ions were guided toward the ES detector 121). Their rather well defined initial kinetic energy
8 IY93 - Elsevicr Scicncc Publishers B.V. All rights reserved
If. MULTIPLY CHARGED/IIEAVY
F. Aumayr et al. / Slow electron emission from slow hollow atoms
100
deceleration electrode (+400 v - LJ)
extraction electrode (+1.9 kV - U) QTzzzzJP
cage (+340 V - U)
focussing electrode (+400 v - U)
electron detector (+26.4 kV - U)
initial ion energy
final ion energy E=qUeV Fig. 1. Setup for measuring ion-induced ES and corresponding total electron yields for impact of slow ions (E t 2q eV) on clean gold. Indicated potentials refer to a nominal MCI impact energy of llq eV on target by decelerating a 4oOq cV primary ion beam.
permitted deceleration to nominal target impact energies E as low as (2 f 1)q eV by means of a four-cylin-
der lens in front of the target surface and appropriate biasing of the latter (cf. fig. 1). The final part of the MCI beam line and the ES detector assembly surrounding the target (polycrystalline gold surface prepared in atomically clean state by sputtering with a built-in Ar+ ion gun) was kept in UHV at a base pressure of typically 10 -s Pa during all measurements. ES have been determined for different MCI species, applying a fountain-type cylindrical electrode setup which collected all electrons ejected from the target with energies of E, 5 60 CV into the full 27~ solid angle and guided them toward a surface barrier dctector biased at +26 kV with respect to the target [21]. From the resulting pulse height spectra (figs. 2a and 2b show typical raw ES data for impact of 100 eV Ar9 -1 emission probabilities W, have been derived via a dcconvolution method described in more detail elsewhere [22]. Although in this way the probability W, for
0
IO
15
n
20
25
emission of no electron can principally not be detcrmined, it may safely be neglected for yields y 2 5. With the above described methods, measurements have been performed [23] for projectile species N4-’ (y I 6), NeY’ (q 5 lo), ArY+(q I 16), Kr”+(q < 10) and lq+(q s 25) over a wide range of impact energies. As typical examples, fig. 3 shows the total electron yields y (accuracy typically +3%‘c) vs impact velocity up for ArQ’ (q = 6-16) projectiles, and fig. 4 presents ES for Ar”* impact at three different energies. We point out that the present experimental technique offers a number of advanlages over the usual determination of MCI-induced total electron yields by measuring the currents of respectively primary ions and emitted clectrons [5,6], e.g. a much higher sensitivity (reducing the primary MCI current requirements dramatically), the rather precise absolute electron yields directly available from the essentially relative ES measurements and, most notably, the additional information hidden in the related electron emission multiplicity.
0
5
10
15
20
25
n
Fig. 2. (a) Comparison between measured (raw) ES for impact of 100 eV Ar 91- ions on clean Au (dots) and results of a fit (solid line). (b) Individual ES contributions to fitted data, considering electron backscattering from detector surface [22]. Contributions related to even numbered probabilities have been shaded for clarity.
101
F. Aumuyr et al. / Slow electron emission from slow hollow atoms
3. Modelling results
of experimental
data and discussion
of
-
First of all, from fig. 3 we observe that for all projectile charges with increasing impact velocity L’,, the electron yield gradually decreases and then levels off towards an apparently constant value, henceforth to be named y=. As already stated in [23], the vclocity-dcpcndent part of the total yield y (i.e. y - yJ prcsumably originates from AI of the hollow atoms-turned MCI during their way to the surface. Calculating d, according to [2], which gives practically equal results as a fully classical treatment [16], and taking W, = 5.1 cV for clean gold [l], we obtain (atomic units arc used unless otherwise stated)
(2) At our lowest impact energies eq. (2) delivers typical projectile flight times t,> 10 -I3 s inside d,. From the velocity-dependent parts of y as shown in fig. 3 we thus may obtain mean apparent AI rates of typically I lOI s- ‘. If subsequent electron emission events would involve constant AI rates, y-y= has obviously to scale like l/c,. However, fig. 3 clearly demonstrates that instead the relation y = const.t;;‘/*
+ yz
(3)
fits the measured total yields much better, which obscrvation has been tentatively related in ref. [23] to steadily increasing apparent mean lifetimes for subscqucnt AI processes (i.e. gradually decreasing apparent AI rates with shrinking distance of the hollow atoms from the
Arq+
-
Au
2
,O emission
multiplicity
n
Fig. 4. Electron emission statistics (ES) for impact of Ar”.’ at three different impact energies on clean Au. Dashed lines are fitted Gaussians. The solid line corresponds to a Poissonian with appropriate mean value y for the 500 eV measurements (also cf. text).
surface. We now explain this bchaviour of apparent AI rates in a more detailed way as follows. On their way toward the surface the hollow atoms become subject to both RI and AI, but only the latter processes give rise to the observable electron emission. It has already been pointed out that because of “screening dynamics” RN populates projectile Rydbcrg states with progrcssively lower principal quantum numbers, as the particles approach the surface [3]. Consequently, the AI processes should become less cfficicnt if taking place nearer to the surface, considering the decreasing chance for “down-electrons” to reach empty lower projectile states for enabling the “up-electrons” to make the respectively most probable AI transitions just into vacuum [18], and the fact that the competing Rl become gradually faster if taking place closer to the surface [2]. Fig. 3 shows, however, that at low impact velocity the yield deviates from its y vs v,, dependence as given by cq. (31, which we relate to a gain in projectile impact energy due to the corresponding image charge attraction. Before its first RN inside the distance d, (cf. eq. (211, an ion Z” ’ has already gained a kinetic energy A E,Ti, according to AE,,,,
Fig. 3. Total electron yields derived from measured ES vs. impact velocity, for impact of Ar”-. on clean polycrystalline gold (y = 6-16 [23]). The solid lines are fits according to eq. (31, cf. text. Deviation from fits at low t:p is being ascribed to image charge acceleration of the projectiles.
5ooev
Au
= y2/4d,
s 0.9~“~
(eV).
(4)
However, fitting a 4’‘/* dependence to experimental data for Ar 9+ (q I 6) scattered under grazing incidence from a metal surface, in which way the image charge acceleration could be directly determined [ll], results in AE+,
= 1.2q3’2
(cV).
(4a)
The discrepancy between both relations can be easily rcmovcd by considering in cq. (4) the further projectile II. MULTIPLY
CHARGED/HEAVY
F. Aumayr et al. / Slow electron emission from slow hollow atoms
102
acceleration inside d, during the ongoing MCI neutralization [2]. Turning now to the observed ES, they should correspond to Poissonian distributions if the slow electron emission were caused by a series of equally fast AI processes (H. Kurz et al., to be published). Fig. 4 shows ES for Ari*+ projectiles with three different impact energies. Whereas Gaussian distributions fit the ES rather well, Poissonians with appropriate mean values y are clearly too broad, as demonstrated for the 500 eV case. Apparently, the electron emission is less randomly distributed than to be expected for mutually independent ejection of individual electrons, which indicates electron emission due to other mechanisms than exclusively by AI. Coming back to the total electron yields, we first remark that earlier direct y measurements [4,&g] e.g. for Ar4’ (up to 4 = 8) have shown that y is practically proportional to the MCI’s total potential energy WR,wol. However, for 4 2 9 inner shell vacancy formation in the course of MCI neutralization was found to cause a lcvelling off from this bchaviour, as apparent also from our present measurements for Arq+ (cf. fig. 5 at q 2 9). A similar proportionally of y with W&,, holds for other MCI species in lower charge states as well, but breaks down if inner shell vacancies are produced in the course of neutralization (e.g. for Nq+/q = 6 or Neq-’ /q = 9 [5]). For initial MCI charge states where inner shell vacancy formation during neutralization will take place, WCnow instead observe a linearity of the total electron yield with projectile charge state 4 [23], as shown e.g. for A++ (q > 8) in fig. 6. A possible candidate process for such emission proportional to 4 could be “peclingoff’ of the 5 q electrons bound in highly excited states as the projectile reaches the surface [2]. However, the
35r--
1 q.16 15 0
i
14 13
12 11 0
l
l
1
0
Arq + 1500
Au
Fig. 5. Total electron yields y vs total potential energy W&,, (values taken from ref. [24]) for impact of A++ on clean gold. Note deviation from apparently linear dependence for 4 2 9, for further details cf. text.
”
4
8
12
16
! ? 20
charge state q Fig. 6. Dependences of total electron yields y (full and open circles for two different impact energies) and related values y, (diamonds) on primary ion charge state 4) for ArY+ impact on clean gold. From 4 = 8 on, straigth lines fit the data points
quite well.
assumedly impact velocity-independent contribution -y, (see above) cannot just result from such “peeling-off’, since for higher q-values y, clearly surpasses q (y_ = 12 for 4 = 12, but y, = 21 for q = 16, cf. fig. 6). We emphasize that kinetic electron emission (KE) cannot contribute to yg in an appreciable way, since in the here regarded impact velocity range (v, < 2 x lo5 m/s) the related KE yield cannot surpass about 0.5 electrons/ion [21). We therefore have proposed in [23] some “ultimate”, very fast Al- and/or Auger neutralization processes occurring rather close to the surface where the still populated highly excited projectile states will completely overlap with the occupied metal S-DOS. These processes should assumedly be sufficiently fast to remain independent of the MCI impact velocity within its here considered limits. However, to describe this scenario in a more quantitative way, we have now modelled the MCI neutralization near the metal surface, following recent work of Burgdorfer et al. 1991 [2]. Fig. 7 shows which transitions have been taken into account in our model calculations. Dynamical screening of the populated states and RN-, RI- and AI-transition rates from Burgdiirfer et al. 1991 have been taken into account, together with Auger transitions into unoccupied states in the conduction band and image charge acceleration of the projectiles. Furthermore, the “peeling off’ mechanism upon surface impact has been assumed to work for all Rydberg states with n > n,.+, populated in the “hollow atom”, corresponding to Bohr radii larger than the target metal screening distance given by cF/ope, where w,,~ is the metal electron plasma frequency [1,2]. To model our experimentally determined total electron yields in a satisfactory way, we had also to take into account an electron emission process not explicitly described in [2], i.e. “promotion” of captured electrons above the vacuum level due to the combined action of
F. Aumayr et al. /
Slow electron emission from slow h&w atoms
peeling off
approaching MCI
-
metal S-DOS
Fig. 7. States of a neutraI~jn~ MCI during its approach toward a metal surface. Electrons captured via RN can be emitted via AI, promotion into vacuum rcsultiny from screcning shift (SS) and image shift (IS), and “peeling off’? respectively. In addition, recapturing of electrons into the metal via RI takes place. For further detaik cf. text.
image charge shifting (IS) and binding energy reduction by screening of the particular state bccausc of RN into neighbor states (SS), cf. fig. 7. To give just one example for the quality of our modclling, fig. 8 shows the impact velocity dependence of the experimental yield for impact of Art*+ together with the calculated enc. Different ~on~rjbu~ions to the total electron cmission (i.e. from Al, “promotion” and “peeling off”, respectively) have been separately indicated by dashed lines. Cl corrcsponds to the sum of ail three processes, whereas for 62 it has been assumed that out of Cl only one half of ail Auger electrons emitted from the projectile due to AI have made it into vacuum while the other half will be absorbed by the metal. Finally, X1*0.7 indicates results obtained under the assump-
1, ***..
50
0
‘. -.
__r_-----__
I
l
2
4
_ _ _ gmionization ------_“ll__
“SStlS promotion” -_l_______-_-_-v”s~
1
!
6
I-.._&w...
8
10
12
14
---I 16
18
VP (104 m&f
Fig. 8. Comparison c&measured and modelled total electron yieIds vs impact velocity for neutralization of A.r’“* near a clean gold surface. For further e~~anations cf. text.
103
tion that 30% of all electrons emitted by the projectile were absorbed by the target metal. For the particular case considered here as well as for the e~~rjrn~ntal results obtained with a number of other MCI species a quite satisfactory agreement could be obtained with these model calculations, as will be dcmonst~ted in more detail elsewhere (H. Kun et al., to be pubiished). Our above presented scenario for electron emission is supported by results for the particular proje~tiIe species N”‘, NcB~.tO+ and I%*. For bombardment with the first three MCI species WC found a rather weak variation of the total electron yields with cP, which points to a strong domination of RI over clectron emitting processes during the projectiles approach toward the surface. ~ns~qu~ntly, the measured total electron yield seems to be almost exclusively due to the “peeling off’ process just upon surface impact. This rather interesting result strongly suggests a refinement of models developed both for the filiing of electronic states prior to the inner shell vacancy-related fast Auger electron emissittn [3,?-91 and the processes held responsible for the slow electron emission fig]. In the light of our above presented arguments, for hydrogenlike as well as fully stripped MCI the AI seems to become stronger supressed by RI than for equally charged more-electron projectiles, i.e. for 4 < Z - I. To support this claim, the case of 150 cV Iz5’ (y = 70) as an extreme example of pure potential emission is regarded, where more than n = 85 electrons can be ejected in a single MCI impact. In contrast to N”+ and Nc9*~tof, there electron emission during the projectiles approach toward the metal surface seems to be much tcss inhibited by RI, as a result of which the impact veloci~-dependent contribution y - y= = 40 electrons/ ion appears as a relatively much larger part of the totaf yield. We point out that RN of the Ni-like 125+ ions cannot produce strongly bound inner shell vacancies, which seems to favour the electron emission already during approach of the corresponding “hollow atom” to the surface. These results demonstrate that the above discussed breakdown of the linear y vs W,,,,t relationship for highly charged primary ions with a capability for inner shell vacancy formation during neut~li~tion can be well understood, since the potential energy stored in such inner shelf vacancies should actually not be taken into account for the respective total Wy,,t value (cf. fig. 5). In other words, the slow electron emission which makes up for the here observed total electron yields can only be brought forward by sufficiently rapid transitions among the higly excited Rydbcrg states which have been populated prior to surface impact. The large potential energy attached to the transiently produced inner shcli vacancies will almost comptetely be carried furtheron into the metal, where it becomes dissipated in such a way that no further important II. MULTIPLY CHARGED/HEAVY
104
I;. Aumayr et al. / Slow electron emksion from slow hollow atoms
contributions to the observable total electron yields arc produced. This also means that subtracting the potential energy stored in the transiently formed inner shell vacancies from the total projectile potential energy Wq,pO, will restore the linear y vs Wq,pC,, relationship otherwise only observable for small q-values, as can already be guessed from inspection of fig. 5 and will be shown quantitatively by Kurz ct al. (1992, to be publishcd).
4. Conclusions We have opened up a new way to investigate clectron emission caused by slow multicharged ion impact on a solid surface, by measuring the resulting electron emission statistics, from which then rather precise total electron yields can be obtained. By using this tcchniquc with a fast heavy ion-pumped recoil MCI source, very low MCI impact energies (only limited by the apparent image charge acceleration of projectile ions) could be reached, and rather small ion fluxes were sufficient to permit precise y measurements. Modelling the experimentally obtained total slow electron yields revealed that the electron emission is probably produced from three principally different sources. After first RN processes have taken place, during the projectile’s further flight toward the surface competition starts between AI and electron promotion into vacuum on the one side, and RI of the hollow atomsturned MCI near the surface on the other. The results of this competition will depend quite critically on the overlap of the surface density-of-states and the elcctronic state structure of the neutralizing projcctilcs. The slow electron yield produced during the projectiles flight generally increases with decreasing impact velocity, but under certain circumstances remains only a tiny fraction of the total electron yield. Just upon surface impact, a third, more or less impact velocity-indcpendent contribution to the total electron emission will be made, which can be ascribed to “peeling off’ of elcctrons having remained so far in high Rydberg states with n > ncri,. Due to this “peeling ofT’ the projectiles become highly recharged, but then probably screened rapidly by electrons captured resonantly into principal quantum states with rt sncrit. In this way, smaller hollow atoms are reproduced which then may also de-excite quite probably via fast Auger elccton- and/or X-ray emission [3,7]. Our measured total electron yields could bc successfully modclled for impact of various projectile species and -charge states, which adds quite satisfactorily to the understanding of the here discussed processes accompanying the approach of slow multicharged ions toward a metal surface. Our model calculations explain to what extent the considerable poten-
tial energy carried by slow highly charged ions can be utilized to extract a large number of electrons from the metal surface, and why for higher initial charge states only a fraction of the total potential MCI energy does actually contribute to the measured slow electron yield. The present results provide further insight in processes triggered by slow multicharged ion-metal surface interaction, which arc also of importance for the recently discussed fast Auger electron- and X-ray photon emission phenomena already observed in such collisions. Further, still more detailed understanding might bc expected from comparable work involving other MCI species as well as different, well defined target surfaces.
Acknowledgements The authors thank Dr. R. Mann (GSI Darmstadt) for his important role in the measurements, Prof. J. Burgdiirfer for useful discussions and communications, and Mr. C. Lemell for performing the present model calculations. This work has been supported by Austrian Fonds zur FGrderung dcr wissenschaftlichen Forschung under project no. P8315TEC, and by Kommission zur Koordination der Kernfusionsforschung at the Austrian Academy of Sciences.
References and N.D. Mcrmin, Solid State Physics (CHS Publ. Asia Ltd., Philadelphia, 1976).
III N.W. Ashcroft PI J. BurgdBrfer,
P. Lcrner and F.W. Meyer, Phys. Rev. A44 (1991) 5674. and Atomic Collisions, [31 H.J. Andrii et al., in Electronic eds. W.R. MacGillivray, I.E. McCarty and M.C. Standage (IOP Publ. Ltd./Adam Hilger, Bristol, 1992) p. 89. E.S. Mukhamadiev and [41 U.A. Arifov, L.M. Kishinevskii, E.S. Parilis, Zh. Tekh. Fix 43 (1973) 181 (Sov. Phys.-Tech. Phys. 18 (lY73) 1181. R. Geller, D. Hitz, P. Varga 141 M. Delaunay, M. Fehringer, and H.P. Winter, Phys. Rev. B3S (1987) 4232. [f-)11I.D. Hagstrum, Phys. Rev. 96 (1954) 325 and 336. C.C. Havener and F.W. 171 PA. Zcijlmans van Emmichovcn, Meyer, Phys. Rev. A43 (1991) 1404; F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven, J. Burgdiirfcr and D.M. Zchner, Phys. Rev. A44 (1991) 7214. 1987; 181 S.T. de Zwart, PhD Thesis, University tironingen, S.T. de &art, A.G. Drentje, A.L. Boers and R. Morgcnstern, Surf. Sci. 217 (1989) 298; L. Folkerts and R. Morgenstern, Europhys. Letters. 13 (1990) 377. M. Fehringer, R. Gcller, P. Varga and I’,1 M. Delaunay, 11.1’. Winter, Europhys. Lett. 4 (1987) 377. 1101S.T. de Zwart, T. Fried, U. Jellen, A.L. Boers and A.G. Drentje, J. Phys. B18 (1985) L623.
F. Aumayr et al. / Slow electron emission from slow hollow atoms
[ll] H. Winter, Europhys. Lett. 18 (1992) 207. [12] E.D. Donets, Nucl. Instr. and Meth. BY (1985) 522. [13] J.P. Briand, L. dc Billy, P. Charles, S. Essabaa, P. Briand, R. Geller. J.P. Desclaux, S. Bliman and C. Ristori, Phys. Rev. Lctt. 65 (1990) 159 and Phys. Rev. A43 (1991) 565; M. Schulz, C.L. Cocke, S. IIagmann, M. Stockli and II. Schmidt-Backing, Phys. Rev. A44 (1991) 1653. [14] J.W. McDonald, D. Schneider, M.W. Clark and D. Dewitt, Phys. Rev. Letters 68 (1992) 2297. [15] P. Varga, Comm. At. Mol. Phys. 23 (1989) I II; H.P. Winter, Z. Phys. D21 (1991) S129. [16] J.N. Bardsley and B.M. Penetrante, Comm. At. Mol. Phys. 27 (1991) 43. [17] U. Wille, Z. Phys. D21 (19Yl) S353 and Phys. Rev. A45 ( 1992) 3004.
105
[Ifi] N. Vaeck and J.E. Hansen. J. Phys. B24 (IYYI) L&Y. [19] G. Lakits, F. Aumayr and I1.P. Winter, Rev. Sci. Instr. 60 (1989) 3151. [20] R. Mann, Z. Phys. D3 (1986) 85. [21] G. Lakits, F. Aumayr, M. Heim and H.P. Winter, Phys. Rev. A42 (1990) 5780; H.P. Winter, F. Aumayr and G. Lakits, Nucl. Instr. and Meth. B58 (1991) 301. [22] F. Aumayr, G. Lakits and 1I.P. Winter, Appl. Surf. Sci. 47 (1991) 139. 1231 H. Kurq K. Tiiglhofer, H.P. Winter, F. Aumayr and R. Mann, Phys. Rev. Lett. 69 (1992) 1140. [24] R.L. Kelly and D.E. Harrison, At. Data 3 (1971) 177.
II. MULTIPLY CHARGED/HEAVY