Eur. Phys. J. D 9, 551–556 (1999) THE EUROPEANPHYSICAL JOURNAL D

EDP Sciences©c Societa Italiana di Fisica

Springer-Verlag 1999

Low energy acetone dimer ion/surface collisions studied withhigh energy resolutionC. Mair1, T. Fiegele1, F. Biasioli1, and T.D. Mark1,2

1 Institut fur Ionenphysik, Leopold Franzens Universitat, Technikerstr. 25, A-6020 Innsbruck, Austria2 Dept. Plasmaphysics, Univerzita Komenskeho, Mlynska dolina, SK-842 15 Bratislava 4, Slovensko

Received: 2 September 1998 / Received in final form: 14 January 1999

Abstract. We have recently constructed the tandem mass spectrometer set-up BESTOF (consisting ofa B-sector field combined with an E-sector field, a Surface and a Time Of Flight mass spectrometer) whichallows the investigation of ion/surface reactions with high primary mass and energy resolution. Using BE-STOF we have extended previous investigations from molecular projectile ions to cluster ions. In particular,we have studied here surface induced dissociation (SID) and surface induced reactions (SIR) of acetonedimer ions upon impact on a hydrocarbon covered stainless steel surface as a function of incident collisionenergy up to about 80 eV.

PACS. 34.90.+q Other topics in atomic and molecular collision processes and interactions – 82.65.Fr Filmand membrane processes: ion exchange, dialyse, osmosis, electroosmosis – 82.30.Fi Ion–molecule, ion–ion,and charge transfer reactions

1 Introduction

Ion surface (reactive) collisions is a research area which israpidly growing in an effort to identify and explore newmethods for both characterizing gaseous ions and the na-ture of the surface [1, 2]. Besides physical and chemicalsputtering the following processes have been identified andinvestigated in the past years for collisions in the rangeof tens of eV laboratory energy: (1) reflection, (2) surfaceinduced dissociation (SID), (3) charge exchange reactions(CER) and (4) surface induced reactions (SIR).

In addition of being of fundamental importance, poly-atomic ion-surface reactions are also relevant to techno-logical applications [2] encompassing such diverse fields as(i) secondary ion mass spectrometry, (ii) reactive scat-tering for surface analysis, (iii) surface-induced dissoci-ation for structural analysis, (iv) surface modificationsfor the preparation of new electronic materials (includ-ing the large area of plasma processing) and, quite im-portantly, (v) plasma-wall interactions in electrical dis-charges and fusion plasmas [3]. For example, dielectricetching using fluorocarbon plasmas [4] is a major tool inthe integrated circuit (IC) manufacturing industry. Modelsof low-temperature, nonequilibrium plasmas, in particu-lar for the description of the physical phenomena, havedeveloped rapidly [5]. However, lack of fundamental datafor the most important species is the single largest factorlimiting the successful application of such models to emerg-ing problems of industrial interest [5]. A similar situationexists for data concerning plasma wall interactions of rele-vance to the plasma edge of fusion tokamaks [3, 6].

A particular interesting and exciting sub-field in thearea of ion surface collisions is the interaction of clusterswith surfaces [7]. Whereas cluster science has developed inthe past two decades to a well established field in physicsand chemistry (see for instance an early review in 1984summarizing the pioneering years [8] as compared to a re-cent textbook on the same subject [9]), recently a novel andpotentially very useful area has emerged, i.e., the interac-tion of neutral or ionized clusters with surfaces [7] includ-ing a large variety of atomically clean or adsorbate covered,polycrystalline or single crystal surfaces. Depending on theinitial kinetic energy of the impinging cluster projectile theinteraction with a surface leads either to cluster deposition,to cluster fragmentation or even to cluster ionization or toemission of electrons (see [7] and references given in [10]).Experimental studies concerning cluster-ion/surface “re-actions” have been mainly restricted to SID reactions (seereferences in [11] and some early examples [12]), neverthe-less including in the case of singly-charged fullerene ionsalso the observation of surface pick-up reactions [13]. Re-cently, Cleveland and Landman [14] investigated the struc-ture, energetics and dynamics of shock conditions gen-erated in a nanocluster upon impact on a sodium chlo-ride crystalline surface using molecular dynamics simula-tions for a 561 atom argon cluster incident with a velocityof 3 km/s (energy of ≈ 1.9 eV per atom). Their findingsdemonstrated that the impact of this cluster on the sur-face results in a piling-up phenomenon leading to high-energy collision cascades and the development of a newtransient medium in the cluster environment characterizedby extreme density, pressure and temperature conditions,

552 The European Physical Journal D

propagating in the cluster in a shock wave-like manneron a time scale of about 1 ps. In their conclusions theysuggested that in the presence of reactants embedded inthis colliding cluster, such collisions could catalyze chem-ical reactions between cluster constituents as well as be-tween cluster constituents and the surface material. More-over, Landman and co-workers [14] argued that becauseof the highly nonequilibrium nature of this new chemicalmedium, such reactions may evolve in dynamic and kineticpathways quite different from those known in equilibriumthermodynamic conditions. In a follow-up paper, Levineand co-workers suggested – based on molecular dynamicscalculations – the possible existence of intracluster reac-tions such as “burning air” in mixed nitrogen/oxygen clus-ters [15] which definitively would constitute an extension ofthe processes (1) to (4) summarized above.

Typically, polyatomic ion-surface reactions are studiedwith a tandem mass spectrometer set-up consisting ofa combination of two or more mass analyzers such asa magnetic sector field analyzer (B), a quadrupole massanalyzer (Q) or a time of flight (TOF) analyzer [1, 2, 16].The first mass analyzer is used to select the primary ionand the second mass analyzer (sometimes complementedby an energy analyzer) is employed to record the secondarymass spectrum as a function of the collision energy (andsometimes as a function of energy and angle). In order toallow a quantitative investigation of SID and SIR processes(for instance to determine activation energies [17]) it is ofutmost importance to control and determine accuratelythe collision energy and to achieve energy spreads as smallas possible. So far the best energy resolution achieved forthe study of polyatomic ions was a FWHM distribution ofabout 2–4 eV [17], whereas for cluster ions distributions ofseveral tens of eV FWHM have been the state of the art forthe past few years [10, 13, 18].

In a recent effort to improve this situation we have con-structed the tandem mass spectrometer set-up BESTOF(consisting of a B-sector field combined with an E-sectorfield, a Surface and a Time Of Flight mass spectrom-eter) [11, 19–25] which allows the investigation of ion/sur-face reactions with high primary mass and energy reso-lution, i.e., energy spreads of as low as 80 meV FWHM havebeen achieved. Using BESTOF we have extended previ-ous investigations in several respects [11, 19–25], e.g., wehave investigated surface induced reactions (using stain-less steel and gold surfaces) of fullerene ions as a func-tion of cluster size n and cluster charge state z (up toz = 5) thereby extending the previous measurements ofKappes and co-workers [13] for singly-charged fullereneions to multiply-charged fullerene ions [25]. The resultsobtained corroborate measurements carried out in collab-oration with Winter and co-workers [26–28] on the elec-tron number statistics (from which total electron yieldshave been derived) for the impact of slow multiply-chargedfullerene ions on an atomically clean polycrystalline goldsurface showing as the most surprising result a completesuppression of potential electron emission. In this con-tribution we will first describe shortly the characteris-tics of this newly constructed tandem mass spectrom-eter system BESTOF. We will then discuss results ob-

Cluster source

Ion source

B-sector

E-sector

Surface

TOF-MS

Fig. 1. Schematicviewof theexperimentalapparatusBESTOF.

tained with this machine using the acetone dimer ion asa projectile ion.

2 Experimental

The experimental apparatus BESTOF (see Fig. 1) con-structed recently in Innsbruck consists of a double focusingtwo sector field mass spectrometer (reversed geometry) incombination with a linear time-of flight mass spectrometer.Neutral van der Waals clusters are produced by super-sonic expansion through a 20 µm nozzle in a continuous,cooled or heated cluster source. The pressure of the ex-panding gas can be as high as 2 bar and the temperaturein the expansion vessel can be controlled in the range from−180 to 120 ◦C. Alternatively, neutral or ionized metalclusters can be produced by a PACIS (pulsed arc clus-ter ion source) cluster source [29]. Moreover, a neutral C60

beam may be produced by evaporating pure C60 powderin a temperature-controlled Knudsen type oven operatedtypically at around 900 K. After passing through a skim-mer the neutral cluster beam or C60 beam enters trans-versely into a Nier-type electron impact ion source. Theneutral clusters are ionized by impacting them with elec-trons whose energy can be varied from below the ionizationenergy up to about 500 eV.

The ions produced are extracted from the ion source re-gion and accelerated to about 3 keV for mass- and energy-analysis by the double-focusing two-sector-field mass spec-trometer. The nominal mass resolution of this two-sectorfield mass spectrometer exceeds at 3 keV a value of 10 000and thus allows easily the selection of isotopically pureprimary ions. After passing the exit slit of the mass spec-trometer, ions are refocused by an Einzel lens and the de-celeration optics positioned in front of the stainless steelsurface. Field penetration effects are minimized by shield-ing the surface with conical shield plates. The incident im-pact angle δ of the primary ions at the surface is usuallykept at 45◦ and the scattering angle is fixed at 90◦.

The collision energy of ions impacting on the surface isdefined by the potential difference between the ion sourceand the surface. The potential difference (hence, the colli-sion energy) can be varied from zero to about 2 keV witha typical resolution better than 200 meV. We have deter-mined the energy and energy spread of the primary ion

C. Mair et al.: Low energy acetone dimer ion/surface collisions studied with high energy resolution 553

Fig. 2. Total reflected ion current for the surface impact ofthe propane ion C3H+

8 as a function of the nominal retard-ing potential in the vicinity of the ion acceleration potentialof approximately 2929 V (upper panel). The derivative of theleading edge of this total reflected ion shown in the lower panelindicates a fwhm for the primary ion beam spread of about90 meV.

beam by using the surface as a retarding potential andmeasuring the (reflected) total ion signal as a function ofthe surface potential. The energy spread is then given bythe FWHM of the first derivative of the total ion signal.Figure 2 shows as an example in the upper panel the totalreflected ion current for the impact of the propane ionC3H+

8 as a function of the nominal retarding potential inthe vicinity of the ion acceleration potential of approxi-mately 2929 V. If the retarding potential is above the ac-celeration potential no ions will hit the surface and thusno ions will be detected, if the retarding potential is low-ered primary ions will start to be able to hit the surface andthe reflected ion current will strongly increase to its peakvalue. The ensuing decrease of the reflected ion currentwith decreasing retarding potential (amounting to increas-ing collision energy) is due to the loss of primary ions atthe surface via neutralization processes. The derivative ofthe leading edge of this total reflected ion current shown inthe lower panel of Fig. 2 indicates in this case a FWHM forthe primary ion beam spread of about 90 meV. This energyspread is caused by the ion production process and the ionextraction from the ion source and can be minimized in thebest case to about 80 meV.

A fraction of the secondary ions formed at the sur-face exits the shielded chamber through a 1 mm diameterorifice. These ions are then subjected to the pulsed ex-traction and acceleration field which initiates time-of-flight

analysis of these ions. This second mass analyzer is a lin-ear time-of-flight mass selector with a flight tube of about80 cm in length. The mass selected ions are detected bya double stage multi-channelplate which is connected toa fast scaler (with a time resolution of 5 ns per channel)and a laboratory computer. Mass resolution has been im-proved steadily the past two years and is to date approxi-mately 100. The present experiments have been carried outusing a stainless steel surface under ultra high vacuumconditions (10−10 Torr) maintained in our bakeable turbo-pumped surface collision chamber. However, even theseconditions do not exclude the production of monolayers ofhydrocarbon contaminants (pump oil etc.) on the surfacewhenever the valve between the mass spectrometers andthe surface collision chamber is opened and the pressure inthe target region is rising to the 10−9 Torr range.

3 Results

The positive ion mass spectrum after ionization of theacetone molecule by 70 eV electrons shows a very sim-ple fragmentation pattern: the most abundant peaks arethe fragment ion peak at m/z = 43 (CH3CO+) and theparent peak at m/z = 58 (CH3COCH+

3 ). All other frag-ment ions are much less abundant (accounting each to lessthan 7% of the peak at m/z = 43) and are due to CH+

i

(mass 12, 13, 15), C2H+i (mass 24–29), C2H2O+ (mass

42) and C3HiO+ (mass 53, 55, 57, 58). In contrast to the

monomer, the positive ion mass spectrum after ioniza-tion of an acetone cluster beam by 70 eV electrons showsa more complicated fragmentation pattern: besides themonomer acetone parent ion peak and the various frag-ment ions (mass 43 and smaller mentioned above), addi-tional series of cluster ions are present such as the stoi-chiometric cluster ions (CH3COCH3)+

n , the protonatedcluster ions (CH3COCH3)nH+ and the acetylated ion se-ries (CH3COCH3)n CH3CO+. The mass spectrum is domi-nated by the stoichiometric cluster ion series, but the abun-dance of the non-stoichoimetric ions – which are readilyidentified as ion-molecule reaction products in the acetonesystem – is of the same order of magnitude

It is interesting to note that the ratio of the abundance ofthe two non-stoichiometric cluster ions (CH3COCH3)H+/(CH3COCH3) CH3CO+ is 1.5, similar to the ratio of theanalogous products formed in the gas phase via reactionsof the acetone parent cation with the neutral acetone mole-cule [30–32]. Thus these ions are formed, in analogy to thegas-phase, by exothermic intra-cluster ion molecule reac-tions of acetone molecular ions with neighbouring acetonemolecules in the cluster, the ions being initially generatedby electron impact ionization of neutral clusters. Becauseof subsequent stabilizing monomer evaporations the en-suing final cluster ion size is usually considerably smallerthan that of the initial neutral cluster hit by the elec-tron [33]. Nevertheless, in the present investigation we willonly concentrate on the interaction of the stoichiometricdimer ion with the hydrocarbon covered stainless steel sur-

554 The European Physical Journal D

Fig. 3. Secondary mass spectra for the impact of the acetonedimer ion (CH3COCH3)+

2 for different collision energies.

face. For further results see forthcoming papers from ourlaboratory [23, 34].

Figure 3 illustrates the surface collision reaction prod-ucts for the impact of mass selected dimer ions(CH3COCH3)+

2 at the collision energy of 6, 15, 20 and30 eV. In line with the results for other polyatomic ions [1,2, 11, 19–22], interaction of the acetone dimer ion witha surface at very low collision energies only leads to the re-flection of the projectile dimer ion (see Fig. 3, top panel),the absolute abundance drastically decreasing with in-creasing collision energy due to neutralisation reactions.Above a certain collision energy of about 7 eV the totalion current is increasing due to the increased productionof product ions. At around 20 eV the major secondaryions are (i) the protonated monomer ion (CH3COCH3)H+,which was already noted to be a major intra-cluster ion-molecule reaction product in the case of electron impactionization of neutral acetone clusters, and (ii) the monomerion (CH3COCH3)+ very likely arising from the simple col-lision induced decomposition of the dimer projectile ion.Both types of product ions are observed at collision ener-gies as low as 10 eV. The abundance of the acetyl cation,which is a minor fragment secondary ion at 20 eV, increasesmonotonically with collision energy. The acetyl ion is themajor collision-induced decomposition product for the ace-tone molecular ion in the gas phase [35]; it is also a majordissociation product formed in electron impact ionization(see above) and low energy surface-induced reaction uponimpact of the acetone molecular ion on a surface [19]. Con-sequently, we identify the acetyl cation with the breakup

of molecular acetone ions after surface impact of the dimerion. In addition to the acetyl ion several fragment ions,CH+

3 and CH2OH+ (and to a minor extent C2OH+), areobserved in appreciable amounts at higher collision en-ergies. CH+

3 is known to be a fragment ion of the ace-tone monomer ion (similar to the acetyl case, see above),whereas the CH2OH+ ion is known [19] to be a secondarydissociation product of protonated acetone cations (pro-duced by surface pick-up reactions of the acetone mono-mer ion in surface collisions) when the collision energy isincreased. As both, the acetone monomer ion and the pro-tonated acetone ion are present in the present secondarymass spectrum and both are decreasing in relative abun-dance as the other fragment ions , i.e., CH+

3 , CH2OH+ andCH3CO+, are increasing in relative abundance with in-creasing collision energy, the production pathways of thesefragment ions appear to be straightforward. As will be dis-cussed below, the production route(s) of the protonatedacetone monomer ion, however, is the real interesting pointin this study.

As discussed above, surface reactions and decompos-ition of the acetone monomer cation gives as a major prod-uct ion the protonated acetone ion formed exclusively byH-abstraction from hydrocarbons chemisorbed on the sur-face [19]. As shown in Fig. 3, this protonated acetone ion isalso the most prominent reaction product from the surfacereaction of acetone dimer ions (and also for higher clusteranalogues discussed in [23, 34]). Thus in line with the afore-mentioned it is conceivable that in principle this ion maybe produced by two competing reactions by the impactingdimer ion, i.e.,

(CH3COCH3)+2 + S−→ (CH3COCH3)+∗

2 (1a)

−→ (CH3COCH3)H+ + CH2COCH3

(CH3COCH3)+2 + S/RH (1b)

−→ (CH3COCH3)H+ + CH3COCH3 + R

where reaction (1a) represents a surface collisional acti-vation of the impinging dimer ion followed by an intra-cluster ion molecule reaction between the acetone ion andits neighbouring neutral molecule (discussed already in thecontext of electron impact ionization of neutral acetoneclusters, see above) and reaction (1b) represents a reactionwith surface hydrogen containing adsorbates (dissociativeH-pick-up reaction).

Important insight into the origin of the protonatedacetone formation can be gained by performing an ad-ditional experiment using completely deuterated ace-tone dimer ions (CD3COCD3)+

2 as projectiles. Prelim-inary results indicate in this case, that the monomerion group in the secondary ion mass spectrum consistsof three peaks, corresponding to the deuteronated ace-tone ion (CD3COCD3)D+, the protonated acetone ion(CD3COCD3)H+, and the acetone monomer ’fragment’ion CD3COCD+

3 . It is clear, that in this case the deuteron-ated acetone ion is produced in analogy to reaction (1a)via a surface induced intra-cluster ion molecule reaction,whereas the protonated acetone ion is produced in analogy

C. Mair et al.: Low energy acetone dimer ion/surface collisions studied with high energy resolution 555

to reaction (1b) via a reaction of the impinging dimer ionwith hydrogen containing surface adsorbates.

In order to rationalize the occurrence of these reac-tion products for the interaction of the acetone dimer ionwith the surface, we have to assume that the hydrogen-bonded dimer ion may alternatively exist before the im-pact in two different isomeric configurations separatedby a small barrier (for details see [23]), i.e., one consist-ing of (CD3COCD2) (CD3COCD3)D+ and one consistingof (CD3COCD3) (CD3COCD3)+. Surface collisions con-vert translational energy very efficiently into internal en-ergy [1]. For the present example, the sudden increase ininternal energy results in dissociation with localization ofthe charge on either the deuteronated acetone structure(CD3COCD3)D+ or on the molecular ion (CD3COCD3)+.Similar to the monomer case [19], in the latter case most ofthe molecular acetone cations may then react with surfacehydrocarbons to give by hydrogen abstraction protonatedacetone (CD3COCD3)H+ as its major product. The intactmolecular ion (CD3COCD3)+, is also observed as a lessabundant dissociation product. Some of those ions maybe ions which after dissociation at the surface have notreacted to (CD3COCD3)H+, but some of those ions mayalso have been formed in slow dissociations of the recedingdimer ion to (CD3COCD3)+ + (CD3COCD3), after the in-teraction with the surface.

In conclusion, we have here extended previous stud-ies about the interaction of an acetone monomer ion witha hydrocarbon covered stainless steel surface [19] to stud-ies concerning surface induced reactions of the acetonedimer ion. In particular, we have been able - as predictedby Cleveland and Landman [14] (see also [15, 36–39]) usingmolecular-dynamics simulations - to observe here besidessurface induced dissociation (SID) also surface inducedreactions (SIR) of acetone dimer ions upon impact ona hydrocarbon covered stainless steel surface. Using fullydeuterated acetone dimer ions we obtained evidence for theoccurrence of two competing surface-induced reactions,i.e., on the one hand intra-cluster ion molecule reactionsleading to the production of the deuteronated acetonemonomer ion (CD3COCD3)D+ and on the other hand hy-drogen pick-up reactions leading to the formation of theprotonated acetone monomer ion (CD3COCD3)H+.

Work supported by FWF, OAW/EURATOM, ONB, BMWV,Wien, Austria. It is a pleasure to thank Prof. Zdenek Herman,

Praha, Prof. Helmut Schwarz, Berlin and Prof. Jean Futrell,Delaware.

References

1. R.G. Cooks, T. Ast, M.A. Mabud: Int. J. Mass Spectrom.Ion Processes 100, 209 (1990); A. Amirav: Comments At.Mol. Phys. 24, 187 (1990)

2. L. Hanley (Ed.): Special issue on Polyatomic ion-surfaceinteractions, Int. J. Mass Spectrom. 174 (1998)

3. W.O. Hofer, J. Roth (Eds.): Physical Processes of the Inter-action of Fusion Plasmas with Solids (Academic Press, SanDiego 1996)

4. J.P. Booth: Proc. XIVth European Sectional ConferenceAtomic and Molecular Physics in Ionized Gases (ES-CAMPIG), Malahide, Ireland (1998)

5. D.B. Graves, M.J. Kushner, J.W. Gallagher, A. Garscad-den, G.S. Oehrlein, A.V. Phelps: Database needs for mod-eling and simulation of plasma processing, Report by thepanel on database needs in plasma processing (NationalResearch Council, National Academy Press, Washington,DC 1996)

6. R.K. Janev (Ed.): Atomic and Molecular Processes in Fu-sion Edge Plasmas (Plenum, New York 1995)

7. S. Matt, T.D. Mark, K.H. Meiwes-Broer (Eds.): ClusterSurface Interactions, Book of Abstracts of ESF Confer-ence, Cargese, Corse (1998)

8. T.D. Mark, A.W. Castleman: Adv. Atom. Molec. Phys. 20,66 (1985)

9. H. Haberland (Ed.): Clusters of Atoms and Molecules I andII (Springer, Berlin 1994)

10. W. Christen, U. Even, T. Raz, R.D. Levine: Int. J. MassSpectrom. 174, 35 (1998)

11. V. Grill, R. Worgotter, J.H. Futrell, T.D. Mark: Z. Phys. D40, 111 (1997)

12. R.D. Beck, P.S. John, M.M. Alvarez, F. Diederich, R.L.Whetten: J. Phys. Chem. 95, 8402 (1991); H.G. Busmann,T. Lill, B. Reif, I.V. Hertel: Surf. Sci. 272, 146 (1992);H. Yasumatsu, U. Kalmbach, S. Koizumi, A. Terasaki,T. Kondow: in Structure and Dynamics of Clusters, ed.by T. Kondow, K. Kaya, A. Terasaki (Universal AcademyPress, Tokyo 1996) pp. 485

13. R.D. Beck, J. Rockenberger, P. Weis, M.M. Kappes: J.Chem. Phys. 104, 3638 (1996)

14. C.L. Cleveland, U. Landman: Science 257, 355 (1992)15. T. Raz, R.D. Levine: Chem. Phys. Lett. 246, 405 (1995)16. V. Grill, R.G. Cooks: Contributions to Symposium on

Atomic and Surface Physics SASP 98 , ed. by A. Hansel,W. Lindinger, (Going 1998) p. 129

17. S.B. Wainhaus, E.A. Gislason, L. Hanley: J. Am. Chem.Soc. 119, 4001 (1997)

18. T.M. Bernhardt, B. Kaiser, K. Rademann: Z. Phys. Chem.195, 273 (1996); Z. Phys. D 40, 327 (1997); B. Kaiser,T.M. Bernhardt, K. Rademann: Nucl. Instrum. MethodsPhys. Res. B 125, 223 (1997)

19. R. Worgotter, V. Grill, Z. Herman, H. Schwarz, T.D. Mark:Chem. Phys. Lett. 270, 333 (1997)

20. R. Worgotter, C. Mair, T. Fiegele, V. Grill, T.D. Mark,H. Schwarz: Int. J. Mass Spectrom. Ion Processes 164, L1(1997)

21. R. Worgotter, J. Kubista, J. Zabka, Z. Dolejsek, T.D. Mark,Z. Herman: Int. J. Mass Spectrom. 174, 53 (1998)

22. C. Mair, T. Fiegele, R. Worgotter, J.H. Futrell, T.D. Mark:Int. J. Mass Spectrom. Ion Processes 177, 105 (1998)

23. C. Mair, Z. Herman, T. Fiegele, J.H. Futrell, T.D. Mark:Int. J. Mass Spectrom. 188, 21 (1999)

24. C. Mair, T. Fiegele, F. Biasioli, T.D. Mark, W.R. Hess: In-ternational Congress on Plasma Physics, Book of Ab-stracts, ed. by J. Badalec, J. Stockel, P. Sunka, M. Tendler,Prague (1998) p. 171

25. T. Fiegele, F. Biasioli, C. Mair, T.D. Mark, F. Aumayr,G. Betz, HP. Winter: Book of Abstracts ERC Symposiumon Cluster surface interaction, ed. by S. Matt, T.D. Mark,K.H. Meiwes-Broer, Cargese (1998) p. 63

26. K. Togelhofer, F. Aumayr, H. Kurz, HP. Winter, P. Scheier,T.D. Mark: J. Chem. Phys. 99, 8254 (1993)

556 The European Physical Journal D

27. HP. Winter, M. Vana, G. Betz, F. Aumayr, H. Drexel,P. Scheier, T.D. Mark: Phys. Rev. A 56, 3007 (1997)

28. F. Aumayr, M. Vana, HP. Winter, H. Drexel, V. Grill,G. Senn, S. Matt, P. Scheier, T.D. Mark: Int. J. Mass Spec-trom. Ion Processes 163, L9 (1997)

29. H.R. Siekmann, C. Luder, J. Faehrmann, H.O. Lutz, K.H.Meiwes-Broer: Z. Phys. D 20, 417 (1991)

30. K.A.G. McNeil, J.H. Futrell: J. Phys. Chem. 76, 409 (1972)31. A.S. Blair, A.G. Harrison: Can. J. Chem. 51, 703 (1973)32. M. Kumakura, T. Sufiura: J. Phys. Chem. 82, 639 (1978)33. T.D. Mark: in Linking the Gaseous and Condensed Phases

of Matter , ed. by L.G. Christophorou, E. Illenberger,

W.F. Schmidt (Plenum Press, New York 1994) pp. 15534. C. Mair, T. Fiegele, F. Biasioli, Z. Herman, T.D. Mark: J.

Chem. Phys. 111, 2770 (1999)35. J.H. Futrell: private communication (1998)36. U. Even, I. Schek, J. Jortner: Chem. Phys. Lett. 202, 303

(1993)37. T. Raz, I. Schek, M. Ben-Nun, U. Even, J. Jortner, R.D. Le-

vine: J. Chem. Phys. 101, 8606 (1994)38. I. Schek, T. Raz, R.D. Levine, J. Jortner: J. Chem. Phys.

101, 8596 (1994)39. I. Schek, J. Jortner, T. Raz, R.D. Levine: Chem. Phys.

Lett. 257, 273 (1996)