Hyperthermal chemistry and cluster collisions

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Nuclear Instruments

and Methods in Physics Research B 112 (1996) 48-54

NOMB

Beam Interactions with Materials 8 Atoms

ELSEVIER

Hyperthermal chemistry and cluster collisions E.E.B. Campbell

* , I.V. Hertel ’

Max Born Institute fir Nichtlineure Optik und Kurzzeitspektroskopie, Posrfach 1107, D-12474 Be&n. Germany Abstract In collision experiments with cluster ions it is possible to reach conditions of extreme temperature and pressure which may lead to the occurrence of novel chemical reactions which cannot take place under normal conditions. In this paper we review the reactions which can occur in hyperthermal collisions of fullerene ions with gas phase or solid targets.

1. Introduction

2. Experimental

Collisions of energetic cluster ions with gaseous or solid targets can lead to conditions of extreme temperature and pressure. From the point of view of basic research this can provide a fascinating tool to study materials under such extreme conditions. One hopes eventually e.g. to be able to carry out exotic chemical reactions which are not possible using standard chemical techniques. In addition, one would like to gain more insight into the dynamical behaviour of systems with a very large but finite number of degrees of freedom, which at the present time are not well understood. Collision experiments should be able to provide some of the answers. From a more applied point of view, energetic cluster collisions on surfaces are proving to be an interesting method for producing good quality thin films [I]. Due to the high temperature induced locally at the impact zone, the method can be used to form compact, smooth and strongly adhering thin films on room temperature substrates [2]. At even higher impact energies surface erosion can occur via shock wave induced crater formation [3]. The main advantage of surface modification by cluster impact is the restriction of the phenomena to the surface region, avoiding damage or defects in the bulk of the target. Many collision experiments with C& have been carried out in the past few years due to the ease of production of intense, mass selected fullerene ion beams. In this paper we will review some of the work that has been carried out in our lab. A number of completely unexpected phenomena has turned up which helped to further stimulate progress in cluster collisions, both from an experimental and a theoretical point of view.

A schematic diagram of a typical cluster collision experiment is shown in Fig. 1. The clusters are normally produced either in a supersonic jet or via gas aggregation techniques and can either form around ions or be ionised after their formation. The interested reader is referred to Ref. [4] for a review of cluster production methods. For fullerene collisions, which are the topic of this paper, two simple methods are generally used to produce beams of ions. The most straightforward method is to use a small oven to heat the fullerene powder to a temperature of around 400°C and ionise the gas phase fullerenes to produce either positively or negatively charged ions via electron bombardment. Alternatively, a pulsed laser (pulse width on the order of 15-20 ns) can be used to rapidly heat a fullerene film. The ions produced directly by this process have high internal energies (ca. 30 eV corresponding to a temperature of approximately 3000 K) and leave the surface with a velocity on the order of 1000 ms-‘, strongly directed along the surface normal. The cluster ions are then accelerated, mass selected and collide with the target which may be either in the gas phase or a solid surface. The reaction products are, typically, mass analysed by time-of-flight or quadropole mass spectrometry. In many situations it is also desirable to analyse the kinetic energy of the products and/or the angular distributions.

Corresponding author. Tel. + 49 30 6392 1210, fax + 49 30 6392 1229, e-mail: [email protected]. I Also at Fachbereich Physik, Freie Universitit Berlin, Germany. l

0168-583X/95/$09.50 SD1 0168-583X(95)01

3. Gas phase collisions 3. I. Production

of endohedral jidlerenes

Saunders et al. recently found that if C,, is heated to a temperature of about 6OO”C, under a pressure of 2500 atm of He, for a few hours one can detect the formation of so-called endohedral He@& compounds where the rare gas atom is trapped inside the fullerene cage [5]. The yield of these endohedral compounds is approximately 0. I % for

0 1995 Elsevier Science B.V. All rights reserved 133-l

E.E.B. Cumpbell. I.V. Hertel/Nucl.

Fig. I. Schematic ment.

diagram

of a typical

cluster collision

Instr. and Meth. in Phys. Rex B 112 11996) 48-54

experi-

He [6]. A few years earlier endohedral rare-gas fullerene complexes had been discovered in high energy (8 keV) collisions between C& and small rare gas atoms [7]. Due to the large mass of the Cl, projectile compared to the light target atom this laboratory collision energy corresponds to a fairly low energy in the centre of mass frame of reference (44 eV>, which is the amount of translational energy transferred to internal degrees of freedom in the collision. In our laboratory, detailed measurements of the collision energy dependence of the formation of these compounds have been carried out [8]. Fig. 2 shows the relative intensity of the endohedral complex to the projectile beam intensity for collisions between C& and He as a function of centre of mass collision energy. The energetic threshold for capture lies between 3 and 8 eV depending on the internal energy of the projectile ion (Sprang et al. [8]). The lower limit is observed in collisions with very hot ions produced in a laser desorption source and may be due to the existence of a .‘window mechanism” proposed by Murry and Scuseria [9] in which a large hole (C, or C,s ring) can form in an electronically excited fullerene, making it easier for the atom to penetrate the cage. Interesting structure can be seen in the capture cross sections as a function of collision energy. The second maximum that can be clearly seen at about 18 eV in Fig. 2 is due to passage of the He atom through a C, ring which has a larger energy barrier than passage through a C, ring, responsible for the maximum at 10 eV. There is also evidence for additional, reproducible structure superimposed on the two maxima in Fig. 2 which has, however, not yet been satisfactorily interpreted. The drop in signal intensity beyond about 10 eV is due to the thermal emission of the “extra” electron, which occurs sometime

5

10

15

20

Collision Energy

25

30

/ eV

Fig. 2. Centre of mass collision energy dependence by C, (from Sprang et al. [8]).

of He capture

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during the passage to the ion detector, producing a neutral product which cannot be detected in the mass spectrometer. Endohedral products of collisions with positively charged C,, can be detected at higher collision energies since the energy required to fragment the cage (the main loss mechanism in this case) is much larger than the electron detachment energy. The negative ion capture cross sections thus provide a sensitive probe for the efficiency of energy transfer to vibrational excitation of the fullerene with the minima in the cross section corresponding to maximum vibrational excitation. Considerable insight into the collision dynamics can be obtained by carrying out molecular dynamics simulations [IO]. Fig. 3 shows a trajectory calculated for collisions between C,, and Ne at a centre of mass energy of 80 eV. After penetrating the carbon cage the Ne atom collides many times with atoms in the cage and the energy in the system oscillates between the kinetic energy of the atom and deformation and vibrational energy of the fullerene. One fascinating result of these simulations is the evidence for the extreme stability of the fullerene structure. This can be seen in the two lower pictures of Fig. 3. After about a picosecond a ring of carbon atoms starts to leave the cage and one would expect a fragmentation to occur. Instead, however, despite the very large excitation energy of 80 eV. the carbon atoms rejoin their cage partners and close inspection of the situation after 2 ps reveals that the Ne is still surrounded by 60 C atoms. When one considers the relative intensities plotted in Fig. 2, along with the fact that only ca. 30% of the fullerenes in the ion beam collide with atoms, one can see that the yield at the collision energy corresponding to the cross section maximum is much larger than that produced in the high pressure experiments. If one could find a means of removing the vibrational energy in the system before fragmentation occurs it may be possible to produce macroscopic amounts of endohedral fullerenes in this way. 3.2. Reactions

offullerenes

with SF,

The fragmentation mass spectra of positively charged fullerenes for excitation energies beyond about 100 eV invariably show a bimodal structure related to the stabilities of the fragment molecules, regardless of whether the fullerenes are excited by collisions with atoms [I 11, molecules [12], electrons [13] or photons [14]. For C,, j,, only molecules with even numbers of carbon atoms are observed, indicating that the fragments have a fullerenetype structure (only even numbers of carbon atoms can form closed cages [ 151).There is a minimum around CjO and then the fragment intensities increase again towards lower masses with typical “magic numbers” at n = 7, I I, 15, 19, 23. All the fragment masses below n = 30 have very similar appearance energies [ 14,161. Fig. 4 shows the transition region from fullerene to non-fullerene structures for collisions with Ar atoms. The mechanism for the

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Insrr. and Merh. in Phys. Res. B 11.2 (1996) 48-54

t = 267 fs

t =

1030 fs

t = 2000 fs

Fig. 3. Calculated trajectory for capture of a Ne atom in a collision with C, at a centre of mass collision followed by Ne is shown by the full line. For description of the calculation see Ref. [IO].

5 Time of Flight / l~s Fig. 4. Section of a fragmentation obtained in collisions between C& at n= 32 from fullerene-type to The numbers refer to the number of

time-of-flight mass spectrum and Ar showing the crossover

non-fullerene-type fragments. carbon atoms in the fragment.

energy of 80 eV. The path

production of these fragments is not yet fully understood. Recent experiments in which neutral fragments produced in collision experiments were post-ionised to cations or anions in a second, charge transfer collision provided evidence for a cleavage of complete CT,,, moieties from the fullerene ions with the maximum observable neutral fragment from C& being C,, [17]. This fragment is the complement of the lowest observable fullerene fragment ion CT,. We also observe evidence for a cleavage mechanism in collisions with SF, molecules [18]. Fig. 5 shows an extract of a fragment mass spectrum obtained for a centre of mass collision energy of 233 eV. Reaction products consisting of a carbon chain attached to a fluorine atom can be clearly seen. Since the experiments are carried out under single collision conditions and we see no evidence for attachment of a fluorine atom to the large fragments with a fullerene-type structure, carbon chains must leave the fullerene cage and react with SF, on the timescale of the collision (sub-ps). Evidence from moIecu-

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b)

Mass 1 u

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8

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Fig. 5. Section of a mass spectrum showing reaction products obtained in collisions between C& and SF, at a centre of mass collision energy of 233 eV. 0

lar dynamics simulations of fullerene collisions suggests that the excitation energy is very rapidly converted to thermal, vibrational excitation (a timescale on the order of lo- I3 s) so that during the SF, collisions, leading to the fragment spectrum shown in Fig. 5, the temperature of the system could be as high as 15000 K. 3.3. C&-C,,

collisions

Interesting effects are to be found in collisions between C& and C, at energies of a few hundred eV. The reaction channels observed in the collision experiments show some astounding similarities with those observed in nuclear collisions [ 19-211. For example, Fig. 6 shows a time-of-flight mass spectrum obtained for a centre of mass collision

C prc$ctile

50

40i

I. 160

Ii

I.

160

I.

200

1.

220

I.

246

I

266

.,

286

Time of Flight I ps Fig. 6. Reflectron time-of-flight mass spectrum of the reaction products from C& +C6c collisions at a centre of mass collision energy of 153 eV. (See text for details).

Fig. 7. Snapshots of a quantum C& +C,, at a centre of mass the very dense CT,, cluster in (c) and cd). Adapted from Ref.

cd+

molecular dynamics calculation of collision energy of 500 eV. Note (b) and the multifragmentation in [21].

energy of 153 eV. Masses larger than C& can be clearly seen and are due to “fusion” of the two fullerene molecules to produce a CT, which can survive long enough (> 50 ps) to be detected in the mass spectrometer. Some fragmentation is also observed to occur as indicated by the shoulder on the fusion peak which corresponds to a mass of around Czo which has fragmented from a CT,,. A reflectron mass spectrometer was used in these experiments to allow the fusion products (including their fragments) to be separated from the projectile ions [19]. A fusion signal CCTZoand fragments) has been observed in the collision energy range between 80 and 200 eV with the cross section maximum lying between 100 and 150 eV. Beyond 200 eV the cross section drops rapidly to zero. The results are in very good qualitative agreement with quantum molecular dynamics simulations which predict a rapid fall off in intensity of the fusion signal for collision energies beyond approximately 2.50 eV due to the presence of an “L-window”. This is a feature predicted theoretically for heavy ion collisions [22] but not yet experimentally observed, and is a consequence of a non-zero lower limit to the angular momentum in collisions leading to fusion reactions. As the collision energy is increased further the temperatures and pressures prevailing inside the collision complex can reach very extreme values. In Fig. 7 snapshots of the early stage of a typical collision event with an impact parameter of 1 a.u. and a centre of mass collision energy of 500 eV, as calculated with quantum molecular dynamics [21], are illustrated. A super-dense state is formed during the approach phase (Fig. 7(b)) which leads to a pressure inside the cluster which is at least an order of magnitude

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larger than values that can be reached macroscopically [2 11.This state explodes on a timescale 5 100 fs, undergoing a spontaneous multifragmentation. Experiments are planned for the near future which should be able to look for evidence of such processes.

4. C&surface

iq. ? ‘;

l Graphite * Diamond

collisions

A summary of the variety of processes observed in C& with clean crystal surfaces [23] is given in Fig. 8. For impact energies between 100 and 450 eV on graphite it was possible to detect intact, backscattered C& ions. These clusters lose almost all their kinetic energy on collision with the surface and are reflected with only lo-20 eV kinetic energy, almost independently of impact energy in this range. The process can be described in terms of a simple rolling ball model in which the tangential component of the initial velocity is partially transformed into rotational energy of the C& during the interaction with the surface [24]. The normal component of the initial velocity vector determines the amount of energy transferred to vibrational and deformational energy of projectile and target. At low collision energies neutralisation of the cluster on the surface is an important process and the rise in ion scattering yield as a function of energy seen in the upper part of Fig. 9 is attributable to two effects: an increasing contribution from thermionic emission as the vibrational energy of the neutralised cluster increases with ion collisions

0

CSOfIon

400

sions of C& with Ni (100). See text for details.

increasing impact energy [25] and a decreasing probability for adsorption on the surface. The fall in scattering yield as the energy increases beyond 300 eV is due to deposition

%J

Thermionic Electron Emission

Backscattering a

of the

Intact Cluster -h-_

*

Emission P, Secondary

of Ions

Deformation and Heating of C,, and Target Fig. 8. Summary

000

Fig. 9. Upper: Scattering yield of all ions for C& collisions on graphite and diamond (111). Lower: Sticking coefficient for colli-

Vln> “out

k

600

Primary Energy I eV

Rotation ,

200

of the processes observed in collisions of C& ions with surfaces.

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Nucl. Instr. and Meth. in Phys. Res. B 112 (1996) 48-54

processes leading to the production of amorphous carbon structures. A rough estimate of the pressure reached in C& surface collisions gives a value of ca. 40 GPa for an impact energy of I keV. Under these conditions amorphous, diamond-like, hydrogen-free carbon films can be produced at room temperature which are very smooth and strongly adhesive 1261. The films show the presence of graphitic nanocrystallites embedded in amorphous carbon and preferentially oriented with the c-axis parallel to the substrate surface. The percentage of sp3-bonding in the amorphous material can be as high as 50%. Very different results were obtained for scattering of C& on a Ni (100) surface [27]. In contrast to collisions with graphite and diamond, no backscattered ions could be detected for collision energies in the range 90-700 eV. A combination of Auger spectroscopy and LEED measurements showed that the C& ions disintegrated completely on the Ni surface forming a ~(2 X 2)C-Ni(lO0) superstructure with p4g-symmetry [27]. The decreasing sticking coefficient with increasing impact energy in the range 90-300 eV for these collisions, plotted in Fig. 9b, indicates that the adsorption reaction is exothetmic. Simulations [28] and experiment have shown that for impact energies beyond 130 eV the C,, is squashed flat on impact. The potential energy that can be stored in the deformation of the cluster has thus reached its maximum at this energy and a further collision energy increase simply leads to heating of the target surface which supresses the reaction. The increase in the sticking coefficient for energies above 300 eV has the same origin as the decrease in the scattering yield for collisions with graphite and diamond.

5. Conclusion By taking as an example some of the collision experiments carried out with fullerene ions in our group in recent years we have tried to show the potential of cluster collisions for accessing conditions of extreme temperature and pressure. The field of energetic cluster ion impact is still in its infancy, however, the new insights being obtained both in basic research as typified by the kinds of experiments described here and also in more applications oriented work [l-3] indicate that this field has a very interesting future ahead of it.

Acknowledgements We would like to thank all present and past members of our group who have contributed to the work presented here, in particular Dr. H.-G. Busmann for his contribution to the surface experiments. We would also like to thank Prof. R. Schmidt and his group for a fruitful ongoing

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collaboration concerning the molecular dynamics simulations of gas phase collisions. The work has been financially supported by the DFG through projects SIB 276 (TP C7), SFI3 337 (TP Al2), Ca 127-I ,2 and the BMFf under contract number 13N6073.

References [l] H. Haberland, M. Kanais, M. Mall and Y. Thumer, J. Vat. Sci. Technol. A 10 (1992) 3266. [2] H. Haberland. Z. lnsepov and M. Moseler, Z. Phys. D 26 ( 1993) 229. [3] J. Gspann, Z. Phys. D 26 (1993) S174. [4] H. Haberland. in Clusters of Atoms and Molecules, ed. H. Haberland, Springer Series in Chem. Phys. 52 (1994) 205. [5] M. Saunders, H.A. Jimenez-Vazquez and R.J. Cross, J. Am. Chem. Sot. 116 (1994) 2193 H.A. Jimenez-Vazquez, R.J. Cross, S. [6] M. Saunders, Mroczkowski, D.I. Freedberg and F.A.L. Anet, Nature 367 (1994) 256. t71 See e.g.: T. Weiske, J. HNS& D.K. Bohme and H. Schwarz, Helv. Chim. Acta 75 (1992) 79. 181 E.E.B. Campbell, R. Ehlich, A. Hielscher, J.M.A. Frazao and I.V. Hertel, Z. Phys. D 23 (1992) I; R. Kleiser, H. Sprang, S. Furrer and E.E.B. Campbell, Z. Phys. D 28 (1993) 89; H. Sprang, A. Mahlkow and E.E.B. Campbell, Chem. Phys. Lett. 227 (1994) 91. 191 R.L. Murry and G. Scuseria, Science 263 (1994) 791. [lOI R. Ehhch, E.E.B. Campbell, 0. Knospe and R. Schmidt, Z. Phys. D 28 (1993) 153. 1111 See e.g.: K.A. Caldwell, D.E. Gibhn and M.L. Gross, J. Am. Chem. Sot. 114 (1992) 3743. 1121 See e.g.: R.J. Doyle and M.M. Ross, 1. Phys. Chem. 95 (1991) 4954. 1131 See e.g.: R. Mlpel, G. Hofmann, M. Steidl, M. Stenke, M. Schlapp, R. Trassl and E. Salzbom, Phys. Rev. Lett. 71 (1993) 3439. 1141 H. Hohmann, R. Ehlich, S. Furrer, 0. Kittelmann. J. Ringling and E.E.B. Campbell, Z. Phys. D 33 (1995) 143. 1151 E.E.B. Campbell, in Clusters of Atoms and Molecules, ed. H. Haberland, Springer Series in Chem. Phys. 52 (1994) 331. [161E.E.B. Campbell, R. Ehlich, M. Westerburg and I.V. Hertel, in The Physics of Electronic and Atomic Collisions, eds. T. Andersen, B. Fastnip, F. Folkmann, H. Knudsen and N. Andersen, AIP Conf. Proc. 295 (1993) 697; R. Ehlich, M. Westerburg and E.E.B. Campbell, J. Chem. Phys., in press. [17] K.J. McHale, M.J. Polce and C. Wesdemiotis, J. Mass Spectrum 30 (1995) 33. [IS] H. Sprang. R. Ehlich, M. Westerburg, M. Henyk and E.E.B. Campbell, to be submitted to Chem. Phys. Lett. [19] E.E.B. Campbell, V. Schyja, R. Ehlich and IV. Hertel, Phys. Rev. Lett. 70 (1993) 263. [20] G. Seifert and R. Schmidt, J. Mod. Phys. B 96 (1992) 3845. 1211 R. Schmidt, J. Schulte, 0. Knospc and G. Seifert. Phys. Lett. A 194(1994) 101. [22] R.V. Jolos, R. Schmidt and J. Teichert, Nucl. Phys. A 429 ( 1984) 139.

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[23] T. Lill, H.-G.

E.E.B. Campbell, I.V. Hertel/Nucl.

Instr. and Meth. in Phys. Res. B 112 (1996) 48-54

Busmann, F. Lather and I.V. Hertel, Chem. Phys. Lett. 193 (19951 199. 1241 T. Lill, H.-G. Busmann, B. Reif and I.V. Hertel, Appl. Phys. A 5.5 (19921 461. [2.5] C. Yeretzian, K. Hansen, F. Diederich and R.L. Whetten, Nature 44 (1992) 359. [26] H. Gaber, H.-G. Busmann, R. Hiss, I.V. Hertel, H. Romberg,

J. Fink, F. Bruder and R. Brenn, 1. Phys. Chem. 97 (1993) 8244. [27] T. Lill, H.-G. Busmann and I.V. Hertel, Z. Phys. B 91 (1993) 267. [28] R.C. Mowrey, D.W: Brenner, B.I. Dtmlap, J.W. Mintmire and C.T. White, J. Phys. Chem. 95 (1991) 7138.