Capture dynamics in collisions between fullerene ions and rare gas atoms

Chemical Physics 239 Ž1998. 299–308

Capture dynamics in collisions between fullerene ions and rare gas atoms E.E.B. Campbell a

a,)

, R. Ehlich b, G. Heusler b, O. Knospe c , H. Sprang

b,1

School of Physics and Engineering Physics, Gothenburg UniÕersity and Chalmers UniÕersity of Technology, S-41296 Gothenburg, Sweden b Max-Born-Institut fur ¨ Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, D-12489 Berlin, Germany c Institut fur ¨ Theoretische Physik, Technische UniÕersitat ¨ Dresden, D-01062 Dresden, Germany Received 8 June 1998

Abstract Ž . The collision energy dependence of capture in collisions between Cq 60 ions and small rare gas atoms He, Ne is studied in detail and compared with the results of classical molecular dynamics simulations. Additional insight is obtained on the dynamics of the collisions by also studying the kinetic energy loss of the projectile ions. Two capture mechanisms are found for He collisions: penetration of a six-membered ring with no significant cage distortion and scattering from a C 2 unit followed by deflection inside the cage. Good agreement is found with the simulations. Ne capture appears to be mainly the product of collisions with ring-structures on the cage followed by bond-breaking and insertion via a window mechanism. The very low threshold energy for Ne capture by fullerene ions Ž10 eV., reported previously, is attributed to the presence of highly excited, deformed fullerene ions in the beam. A second, higher threshold is found which is in better agreement with other experiments reported in the literature. The simulations of the Ne collisions do not give such good agreement as the He simulations. We attribute this to a too low value of the screening parameter used in the Ne–C potential. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction One of the most fascinating properties of the fullerenes is their hollow, cage-like structure. Already, during the early days of fullerene research in the mid-1980s, the existence of so-called endohedral fullerenes, in which one or more ‘foreign’ atoms are captured inside the cage, was postulated w1x. Early molecular beam experiments in the Smalley group provided convincing evidence for the occurrence of ) Corresponding author. E-mail: [email protected] 1 Present address: Peoplesoft, Munchen, Germany. ¨

such species w2x. In these experiments mass-selected clusters corresponding to the mass of a metal atom plus a fullerene cage were subjected to intense ns laser pulses and photofragmented. In many cases the major fragmentation channel was the loss of C 2 units from the carbon cage leaving the metal atom attached to the carbon cluster. This could be observed down to a minimum carbon cluster size that corresponded to the size of the captured species w2x. Soon after Kratschmer and Huffman discovered a method ¨ for making macroscopic amounts of fullerenes w3x it was shown to be possible to produce endohedral metal–fullerenes in their arc-discharge method w4x or by laser vaporisation under high temperatures w5x.

0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 9 9 - 7

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E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

Although the yield of the endohedral fullerenes obtained in this way is still very low, many experiments on their properties have been carried out. At the same time as the first arc-discharge endohedral fullerenes were being produced, Schwarz and co-workers showed that it was possible to shoot rare gas atoms inside fullerenes in high-energy collisions w6x. Convincing evidence that the rare gas atom was caught inside the carbon cage was obtained in later experiments. The He@Cq 60 formed in a first collision was mass selected and then subjected to further collisions with Xe w7x. Product ions observed had retained the He atom but suffered the loss of C 2 n , analogous to dissociation of Cq 60. This is very different from collision induced dissociation of other adducts of Cq 60 formed in the gas phase which generally lose the substituent atom or group on dissociation w8x. In a similar type of experiment the massselected He@Cq 60 was subjected to two energetic charge-transfer collisions to neutralise and then reionise the fullerene. The endohedral He@Cq 60 could still be detected after this treatment w9x. Other convincing evidence for the endohedral nature of the initial collision product was obtained in experiments in our group where the collisional energy dependence of the cross-section for production of the composite species was studied for collisions between fullerene ions and He and Ne atoms. A clear energetic threshold was observed for the formation of q He@Cq 60 and Ne@C 60 at 6 and 9 eV, respectively w10x. These values were in very good agreement with both ab initio molecular orbital calculations for the barrier for penetration of a benzene ring by the rare gas atom w11x and also with molecular dynamics simulations using empirical two- and three-body potentials w12x. We were also able to show that the capture threshold was dependent on the internal energy of the fullerene projectile ion w13x. This was also in agreement with the molecular dynamics simulations. The experimentally determined thresholds underestimate the true energetic barrier by ; 2 eV for the He case due to the high initial internal excitation of the fullerene ion Ž; 20–30 eV. w13x. The situation for Ne is less straightforward. Complementary experiments by Anderson and co-workers in which neon ions were collided with neutral C 60 gave a significantly higher energetic barrier of ; 28 eV w14x.

Anderson and co-workers have also studied the collisional capture of metallic ions, predominantly alkali ions, under single-collision gas-phase conditions w15x. The results are similar to those found for rare gas ions: a clear energetic threshold is observed, the parent endohedral molecule as well as endohedral fullerene fragments can be observed and the cross-section for endohedral production reaches its maximum at a collision energy which depends on the size of the metal ion. Our group has recently exploited this information in order to produce macroscopic amounts of endohedral alkali fullerenes w16x. Collisions take place between alkali ions and thin layers of fullerenes deposited on a conducting substrate. The conducting substrate serves to efficiently remove the vibrational excitation energy of the fullerene produced on capture on a timescale fast enough to avoid fragmentation. The M@C 60 endohedral production yield as a function of ion beam energy follows the Anderson cross-section curves for the sum of all endohedral species Žparent and fragment. observed in the gas-phase experiments w17x. In this way we have produced, isolated Žby HPLC methods. w18x and studied some spectroscopic properties of Li@C 60 w19–21x. In this paper we extend our earlier studies on the collision energy dependence of the capture of small rare gas atoms in gas-phase collisions with fullerene ions. By studying the detailed collision energy dependence of the relative capture cross-sections together with the measured inelasticity as a function of collision energy, accompanied by molecular dynamics simulations we have been able to obtain considerable insight into the capture dynamics. In particular, we find convincing evidence for two distinguishable capture mechanisms for He.

2. Experimental method and data analysis The experimental set-up used for the experiments reported here has been described in detail before w22x and will be only briefly described here. A beam of neutral C 60 from an oven heated to 4258C is ionised by electron impact and accelerated by a pulsed electric field to an energy of 2 keV. The non-fragmented fullerene ions are mass selected by a second pulsed field and enter a collision cell containing the target

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

atoms. The pressure in the target cell is chosen to ensure single-collision conditions for the experiments. Changing the potential on the collision cell varies the collision energy. After the collision the ions exit the scattering cell and can be post-accelerated before travelling through a 1.45 m field-free region followed by a reflectron and detected by channel plates. Alternatively, for the determination of the kinetic energy lost by the fullerene projectile ions during the collision, the ions are detected without the reflectron, i.e. in a simple linear time-of-flight configuration without post-acceleration. The procedure by which the signal due to endohedral products is extracted from the total product signal is described in detail in Ref. w22x. As we have done in previous reports w10,13,22x we define the experimentally determined relative cross-section for capture as the ratio of intensities RG@C 60r ŽRG@C 60 q C 60 ., where RG stands for He or Ne. The experiments using linear time-of-flight detection were carried out using isotopically pure 12 C 60 in order to improve the resolution. The kinetic energy lost by the projectile during the collision can be determined directly from the time-of-flight distribution of the ions. An example is shown in Fig. 1 for collisions between Cq 60 and He at a laboratory collision energy of 3350 eV. Similar distributions have been reported and discussed previously, notably by

301

Mowrey et al. w23x. The fully inelastic He capture process should appear at position A. Under the experimental conditions used in Fig. 1 the probability for endohedral capture and detection is relatively low and is swamped by other elastic and inelastic scattering processes. The maximum seen at position B is typical for fullerene ion collisions and has been recently discussed in detail in the context of molecular dynamics simulations by Ehlich et al. w24x. It is attributed to the majority of central and near-central Ž b - 0.8 R C60 . collisions that contribute to only a small range of final fullerene kinetic energies. The maximum kinetic energy loss suffered in the collisions is marked by the onset of peak B at position C. This is easier to determine from the experimental results than the exact position of the maximum and is the parameter which we use to characterise the collisions at a given collision energy. Considering the conservation of energy and momentum it can easily be shown that the maximum kinetic energy a projectile of mass m p can lose Ž D Emax . during a collision occurs for a head-on, fully elastic collision with the target of mass m t . 2

D Emax s 4 m p m tr Ž m p q m t . Elab ,

Ž 1.

where Elab is the initial kinetic energy of the projectile ion in the laboratory frame of reference. For a

Ž . Fig. 1. Time-of-flight distribution of Cq 60 projectile ions at a laboratory collision energy of 3350 eV 18.5 eV in centre of mass frame . Lower curve: no target gas, upper curve: with He target gas. A: position at which endohedral product lies, B: typical maximum in kinetic energyrtime-of-flight distribution for fullerene ion collisions, C: maximum kinetic energy loss in collision.

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

302

light target such as He the maximum kinetic energy loss is thus nearly 4 Ecm Žwhere Ecm is the collision energy in the centre of mass frame of reference, Ecm s  m trŽ m p q m t .4 Elab ., assuming that C 60 behaves as a rigid particle. This is a much greater value than what is generally found in the experiments. Following Mowrey et al. w23x we assume that the low-energy onset ŽC. arises from an elastic collision in which the projectile mass is effectively less than 720 u followed by a fully inelastic collision between this, slower, part of the fullerene cage and the rest of the cluster. The experimental determination of the maximum energy loss of the scattered ions Ž D Eexp . can thus give a direct measure of the effective mass, m eff , of the projectile as detailed below. The kinetic energies of both parts of the cage Ž m eff and m rest . before the collision are 0 Eeff s

m eff mp

Elab

Ž 2a .

Fig. 2. Effective mass on carbon cage as a function of relative energy loss for collisions between Cq 60 ions and He or Ne targets. Calculated using Eq. Ž6..

The kinetic energy loss of the projectile fullerene ion in the laboratory frame of reference is thus

° ~1 y ¢

II D Elab s

and m rest

m p y m eff

ž

¶• ßE 2

ž(

m eff K Ž m eff . y 1 q m p

/

mp

/

lab

.

Ž 2b .

Ž 6.

respectively. The kinetic energy of m eff after the first, elastic, collision is

II The experimentally determined value of D Elab is used to obtain m eff by solving Eq. Ž6.. The relationship between the relative energy loss II rD Emax and m eff is illustrated in Fig. D Erel s D Elab q 2 for C 60q He and Cq 60q Ne collisions.

0 Erest s

mp

Elab s

mp

Elab ,

ž

I 0 0 0 Eeff s Eeff y D Eeff s Eeff 1y4

m eff m t

Ž m eff q m t .

2

/

3. Molecular dynamics simulations

0 s Eeff K Ž m eff .

Ž 3.

whereas the kinetic energy of the rest of the cage is unchanged I 0 Erest s Erest .

Ž 4.

Considering the conservation of momentum, the kinetic energy of the projectile ion after the second, fully inelastic collision of the effective mass with the rest of the cage is given by

II Elab s

ž

(

m eff K Ž m eff . q m rest mp

2

/

Elab .

Ž 5.

The molecular dynamics simulations have been described in detail elsewhere w12,24x. The C 60 q rare gas atom system was simulated by classical molecular dynamics in which Newton’s equations of motion are solved with empirical inter-atomic forces. The forces between the carbon atoms were derived from a Brenner-type potential using the first parameter set given in Ref. w25x. The interaction between the carbon atoms and the rare gas atom was described pair-wise by a screened Coulomb potential with a Moliere type screening function w12,24x. The screening radii for the He–C Ž0.2317a 0 . and Ne–C Ž0.1772 a 0 . potentials were chosen empirically according to Ref. w12x in order to reproduce the experi-

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

303

mentally determined threshold energies for capture of He and Ne. As will be discussed later, this is reasonable for He but may be questionable for Ne. The differential cross-section for the kinetic energy of C 60 after the collision, d srd E was calculated according to Ref. w24x ds

2p s

dE

bmax

H dE 0

P Ž E, b . b d b ,

Ž 7.

where P Ž E, b . is the probability to find the scattered C 60 with the kinetic energy E and impact parameter b in the simulated events and the energy bin d E. 10 4 trajectories were calculated at each collision energy for both He and Ne collisions in order to determine the relative abundance of events. The calculation was run using laboratory coordinates with C 60 as projectile and the rare gas as target. The initial kinetic energy of the target atom was set to zero. Including 30 eV initial vibrational excitation in the fullerene simulated the initial thermal excitation of the fullerene ions in the experiment. The starting configuration for each trajectory was chosen randomly using random orientations of C 60 from a set of 100 initial model configurations. The model configurations were obtained after trajectory times of at least 30 ps starting with the ground state C 60 and randomly oriented velocity vectors of similar size for each carbon atom. Each collision trajectory was calculated for a total time of 0.8 ps in order to determine the capture and kinetic energy cross-sections. The number of capture events decreased by - 0.1% if the trajectory time was increased from 0.8 to 1.0 ps. Processes such as shooting through the cage or backward scattering from inside the cage, discussed in w24x, which occur on a shorter time-scale are excluded from the crosssection calculations. Fig. 3 shows some calculated kinetic energy loss results for collisions with He at five different collision energies. The maximum in the kinetic energy distributions, discussed above, can be most clearly seen at intermediate energies. The pattern obtained at a laboratory collision energy of 5 keV is in excellent agreement with the experimental results of Mowrey et al. as was discussed in a recent paper w24x. The white bars correspond to the results from trajectories in which the He atom is captured. The maximum kinetic energy loss found from all the trajectories calculated at a given collision energy is

Fig. 3. Kinetic energy distributions after collision calculated with classical molecular dynamics simulations for Cq 60 qHe collisions at 5 different laboratory collision energies. The white bars indicate endohedral products.

used to compare with the experimental values and discussed in the following section.

4. Results 4.1. Capture cross-sections The data for the relative capture cross-sections of He in collisions with Cq 60 have been considerably improved in terms of mass and energy resolution compared to the original measurements w10x. The new data are plotted in Fig. 4 as a function of the

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304

Fig. 4. Capture cross-section for Cq 60 qHe collisions as a function of reciprocal centre of mass collision energy. Squares: experimental data, circles: molecular dynamics simulations. The experimental data have been normalised to the simulations at a collision energy of 10 eV. Full lines: linear fits to the data using Eq. Ž8.. Dashed lines: sum of the two linear fits for each collision energy.

reciprocal collision energy in the centre of mass reference frame. The results of the molecular dynamics simulations are also shown in the figure. The experimental data have been normalised to the molecular dynamics simulations using the values at 10 eV. The observed linear behaviour is consistent with what would be expected from a simple absorbing sphere model. This model assumes that reaction occurs if the collision energy is equal to or exceeds the effective potential energy at the critical distance for the reaction given normally, for simplicity, by the sum of the radii of the two collision partners. The cross-section close to threshold is then given by the expression

ž

2 s s p R 12 1y

VB Ecm

/

,

model in the context of the fusion reaction in fullerene ion–fullerene collisions. There appear to be two separate processes, both showing a linear dependence on the reciprocal collision energy. This is observed for the simulations as well as for the experimental data. The extrapolated threshold for the low-energy process is 6 " 0.5 eV in excellent agreement with the threshold value reported previously w10x. This value is dependent on the initial internal energy of the fullerene ion w13x which was similar in the different experiments. The molecular dynamics simulations are in very good agreement with the experimental data and linear fit in this energy range. There is also, clearly, a second, higher-energy process contributing to the results. The extrapolation of the experimental results gives a threshold value of 17 " 0.5 eV. The molecular dynamics simulations, on the other hand, show a similar gradient but a much lower threshold of 11 eV. A similar plot is shown in Fig. 5 for Cq 60 q Ne collisions. In this case the new improved apparatus actually gave cross-section data with significantly poorer signal to noise ratio than the original measurements due to the much smaller signals Žlower target pressure. and longer flight-times Žincreased metastable fragmentation.. For this reason we show the original data w10x in the figure. The situation is not as clear-cut as that for He but there is still evidence for two processes. The low-energy extrapolation yields the same threshold value, within the

Ž 8.

where VB is the energetic barrier to reaction and Ecm the centre of mass collision energy. A linear extrapolation of the data to the x-axis thus gives a reliable estimation of the threshold energy. For fullerenes, due to the large number of competing channels and complexity of the fullerene projectile, the factor 2 p R 12 is generally considerably less than the geometrical cross-section of the collision partners. See Campbell et al. w26x for a recent discussion of this

Fig. 5. As Fig. 4 for Cq 60 qNe collisions and normalisation at 28 eV.

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

error bars, as reported previously Ž10 " 0.3 eV compared to 9 " 1 eV.. Unfortunately, there are only two simulation values in this energy range: one at the threshold and one at the high-energy limit of the extrapolation. The parameters used in the simulation were chosen to provide agreement with the measured experimental threshold Žsee Section 3. so it is not surprising that good agreement is found here. The high-energy process seen in the experimental data onsets at a collision energy of 18 " 3 eV. There are rather few molecular dynamics simulation points but the data, with the exception of the very first point at the lowest threshold, are consistent with a linear behaviour with the same threshold as the higher experimental threshold but a larger gradient.

305

Fig. 7. As Fig. 6 for Cq 60 qNe collisions.

4.2. Kinetic energy loss The results of the analysis Žsee Section 2. of the maximum kinetic energy loss measurements for Cq 60 q He collisions are shown in Fig. 6. Note that this analysis is concerned with collisions that do not lead to capture. Those that do lead to capture appear with a characteristic energy loss corresponding to a completely inelastic collision which is less than the energy loss values being discussed here Žsee, e.g., Fig. 3.. However, as will be discussed below, the results from this analysis provide important information on the overall collision mechanisms as a func-

Fig. 6. Effective mass as a function of centre of mass collision energy for Cq 60 qHe collisions, determined from kinetic energy loss data using Eq. Ž6.. Squares: experimental data, circles: classical molecular dynamics simulations.

tion of energy and are very relevant to the understanding of the capture dynamics. There are extremely large errors in the determination of m eff ŽFig. 6. when the relative energy loss is 0.95 or larger, due to the rapid change in m eff with D Erel in this range Žsee Fig. 2.. This is the reason for the large scatter in the data points up to a collision energy of ; 15 eV. The effective mass of the fullerene cage is large at low collision energies but is seen to decrease as the collision energy increases until it levels off at a value of ; 24 u for energies larger than ; 17 eV. Thus for collision energies beyond this value the target atom effectively ‘sees’ only a C 2 unit in the cage from which it scatters elastically. The analysis of the molecular dynamics simulations is shown on the same figure for comparison. The simulations also show a large m eff at low energies decreasing to ; 20 u for energies larger than 15 eV. However, the rapid decrease in m eff occurs at significantly lower collision energy Ž- 10 eV. than seen for the experimental data. The situation for collisions with Ne is shown in Fig. 7. Again there is a large error in determining m eff for high D Erel leading to the scatter in the data for effective masses larger than ; 100 u. The effective mass also decreases with increasing collision energy but much slower than the He case. It reaches a minimum of ; 60–100 u for collision energies in the range 40–55 eV and then increases rapidly to reach the total mass of the cage at 70 eV. The molecular dynamics simulations give no agreement

306

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

with the experimental data yielding a value of ; 12–24 u over the entire energy range investigated.

5. Discussion For collisions between Cq 60 and He there are clearly two different mechanisms leading to capture of the He atom ŽFig. 4.. The efficiency of the mechanism contributing at low collision energies increases relatively slowly with increasing collision energy, indicating a low probability for this mechanism in comparison to other competing processes ˚ .. We interpret this proŽeffective R 12 of only 0.5 A cess as being due to the penetration of a He atom through a six-membered ring on the fullerene cage. The majority of collisions in this energy range lead to elastic scattering with practically all of the cage so that the probability of capture is critically dependent on the impact parameter and initial orientation of the collision partners. This scenario is confirmed by the molecular dynamics trajectories. The mechanism with the experimentally determined energetic threshold of 17 eV is much more efficient as witnessed by the significantly larger gradient compared to the low-energy process ŽFig. 4.. The higher efficiency of this process rules out the possibility of it being due to penetration of a fivemembered ring which would be expected to have a similar energetic barrier of ; 17 eV w27x but a lower efficiency than capture through a six-membered ring. This new capture mechanism coincides with the dramatic change in the scattering process seen in Fig. 6. At this energy and beyond, the He atom scatters predominantly with a C 2 unit on the fullerene cage. If the impact conditions are favourable the He can produce a larger opening in the cage by momentarily deflecting the C 2 unit and be deflected inside the cage rather than back-scattered from it. Such trajectories are observed in the molecular dynamics simulations for energies below ; 40 eV and can lead to capture for impact parameters significantly larger than those producing endohedral fullerenes with the first penetration mechanism. At energies beyond 40 eV, not probed by the present experiments, the simulations predict an increasing tendency for the occurrence of direct fragmentation. This produces small ŽC 1 –C 3 . non-thermal fragments w24x.

The molecular dynamics simulations are able to reproduce the main features of the He experimental results and help to visualise the dynamics of the processes involved. However, the details of the collision energy dependence are not reproduced accurately. The simulations predict the onset of the second, more efficient, capture process at a collision energy that is significantly lower than that observed in the experiments Ž11 eV compared to 17 eV.. The situation with Ne is less satisfactory. The simulations ŽFig. 5. reproduce the cross-section data reasonably well but are unable to reproduce the kinetic energy loss measurements ŽFig. 7.. It should be recalled that the screening radius used to describe the Ne–C potential in the simulations was chosen to reproduce the experimentally determined energetic threshold for capture and was ; 20% lower than literature values would indicate w12x. The threshold was determined in the simulations by looking for the lowest collision energy at which a Ne atom would penetrate a six-membered ring on the cage. Later simulations by other groups have indicated that the ring-penetration mechanism in which the cage remains basically undamaged does not occur for an atom as large as Ne w28x. The energetic thresholds calculated in this case are generally much larger than the threshold determined in our experiments. The experimental value which we observe is, however, much lower than the value favoured by Anderson and co-workers w14x. They carried out complementary experiments in which Ne ions were collided with neutral fullerenes. The threshold value which they favour lies at ; 28 eV w14x. Another series of fullerene ion–Ne collision experiments by Caldwell et al. gives a very rough upper limit for endohedral production of 22 eV w29x. The main difference in all these experiments is the initial internal energy of the fullerenes. The Anderson experiments have the coldest fullerenes since a neutral fullerene target at a temperature of 3408C is used. Caldwell et al. use electron impact ionised fullerenes which are certainly much hotter and can probably be regarded as having internal energies in the 20–30 eV range, similar to the fullerene ions we are using in the present experiments. Our Ne cross-section data were obtained using a laser desorption ionisation source w10x. It is difficult to put an accurate figure on the internal excitation but at the conditions used in the

E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

initial experiment it is possible that the internal energy was up to 10 eV higher than in the electron impact ionisation studies. At such high excitation energies we would expect to have a proportion of highly deformed fullerene ions in our beam. It is possible that the low threshold we observe is due to capture of Ne atoms by fullerenes that are already strongly deformed Ži.e., with open hole-structures. on collision, thus making penetration by the Ne atom much easier. A similar interpretation has been used by us previously to explain the desorption laser fluence dependence of He capture by negatively charged fullerenes w22x. Unfortunately, neither the cross-section data obtained for Ne with the electronimpact ionised fullerenes in the new apparatus Žnot shown here. nor the laser desorption studies with He w10x were of sufficient quality to confirm or deny this hypothesis. Assuming that the low-energy process seen in Fig. 5 is due to capture by deformed fullerenes, the ‘true’ capture process would appear to onset at a collision energy of ; 18 eV. This is very close in energy to the C 2 deflection capture of He, discussed above. The m eff data in Fig. 7 indicate, however, that the mechanism may be somewhat different. The effective mass of the fullerene cage seen by the Ne starts to decrease at a collision energy close to the capture threshold but much more slowly than for the He case and stabilises at a value of ; 60–100 u for collision energies beyond 40 eV. The reason for the rapid rise in the apparent effective mass at energies beyond 55 eV is not known. However, the effective mass seen by the Ne atom in the energy region corresponding to the cross-section maximum is certainly larger than in the He case and is on the order of a C 6 unit or even larger. This could be consistent with a window mechanism in which the impact induces C–C bond-breaking around a six-membered ring thus allowing the Ne to enter w28x. This second threshold at 18 eV could be consistent with the Anderson data when the different internal energies of the fullerenes are taken into consideration. Our previous work showed that there is an internal energy dependence on the capture threshold. The early molecular dynamics simulations studied this dependence for different values of the screening radius used in the Ne–C potential w12x. Taking a radius of 0.2222 au which is the value given by the

307

expression of O’Connor and MacDonald w30x the results indicate that the threshold determined by Anderson should lie ; 4–6 eV higher than ours. This certainly brings the two sets of measurements into agreement within the bounds of experimental error. A slight reduction of this radius, on the same order as that found for the He radius Ž10%. would also bring the simulated threshold in line with the second process found in the experiments.

6. Conclusions We have studied in detail the collision energy dependence of the capture of rare gas atoms in collisions with Cq 60 ions and compared this dependence with the maximum kinetic energy loss the projectile ion undergoes. Such a comparison allows us to make some conclusions about the dynamics of the capture process. Two different mechanisms contribute to capture in collisions with He. Penetration through a six-membered ring with no significant cage deformation is seen for low collision energies but has a relatively low probability. A second, much more efficient process has a threshold lying 11 eV higher in energy and is attributed to the scattering of He by a C 2 unit in the carbon cage followed by deflection inside the cage. The experimental results and interpretation are in good qualitative agreement with classical molecular dynamics simulations although the quantitative energy dependence is not perfect. For the Ne case we attribute the low-energy threshold at 10 eV to be due to capture in highly excited fullerenes which were strongly deformed before collision. The ‘true’ capture process has a threshold at ; 18 eV under our experimental conditions Žinitial internal excitation of the fullerene ion in the 20–30 eV range. and is in much better agreement with the results of Anderson and co-workers w14x. The mechanism appears to be different from the He case and is interpreted as being due to collisions with a ring-unit on the cage leading to bond-breaking and insertion of the Ne through the ensuing window. The molecular dynamics simulations are not in good agreement with the experimental results on the dependence of m eff on collision energy. We suggest that the value of the screening radius used in the

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E.E.B. Campbell et al.r Chemical Physics 239 (1998) 299–308

Ne–C potential is too low making the Ne atom ‘behave’ more like a He. A value closer to the expected literature value should give better agreement. As well as providing information on capture dynamics the results presented here help to clear up a controversy in the literature concerning the energetic threshold for capture of a Ne atom by C 60 . We have also shown that results of the type presented here provide a rigorous test of molecular dynamics simulations and can be used to fine-tune the potential parameters used in such models.

Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft ŽCa 127rA and Sfb 337. is gratefully acknowledged.

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