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Z. Phys. D 40, 371–374 (1997) ZEITSCHRIFT F ¨ UR PHYSIK D c Springer-Verlag 1997 Electron detachment and fragmentation in collisions between 50 keV C - n (1 n 86) clusters and H 2 H. Shen, C. Brink, P. Hvelplund, M.O. Larsson Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark Received: 5 July 1996 / Final version: 14 September 1996 Abstract. The destruction cross section for 50 keV negative carbon clusters C - n (1 n 88) in collisions with H 2 is reported. The dominant destruction channel is believed to be electron detachment. The measured cross section values are compared with theoretical values based on a simple geomet- rical model of the carbon cluster, and structural information is obtained. Fragment spectra of both positive and negative clusters are also recorded and fragmentation patterns are dis- cussed in relation to fragmentation energies and ionization potentials. PACS: 36.40.+d; 34.90.+q 1 Introduction Photoelectron spectra of negative carbon cluster ions from C - 2 to C - 84 have been reported by Yang et al.. Based on these spectra estimates of vertical electron affinities have been ob- tained [1, 2]. It is normally accepted wisdom [3] that clus- ters C n with n 10 take the form of linear chains, while for 11 n< 32 cyclic structures dominate (see however [4]). For n 32 the cage structure (fullerenes) may domi- nate depending on the production method, especially so for the negative clusters [5, 6]. The interplay between electron affinities and cluster geometry is most likely an important factor for a better understanding of collisional electron de- tachment in energetic collisions between negative clusters and static gas targets. If one goes a step further and asks for explanations of positive and negative fragmentation patterns resulting from such collisions, also ionization energies and fragmentation energies come into play. However, a first hint as to the structure of the cluster ion is obtained from ob- servation of loss of neutral fragments from charged clusters. Loss of C 2 is associated with fullerene structures whereas loss of C 3 is a dominant fragment pathway for chains and rings [3]. The linear chain clusters were predicted [7] to show a strong odd-even dependence of the electron affinity with odd clusters having the lower electron affinity. For clusters with cyclic structure, C n rings with n = 13, 17, 21, 25 and 29 were found to have relative large electron affinities [2]. In a general discussion of the n dependence in processes involving C n clusters, the possible influence of isomers has to be taken into consideration [4, 5, 8]. A characterization of the ions in a negative ion beam has been attempted in a few experiments, but a precise assignment of the extent of isomeric ions is in general not available. In the present communication we present measurements of total destruction cross sections for 50 keV C - n (1 n 86) in collisions with H 2 . We also present a few cases of both positive and negative fragmentation patterns resulting from such collisions. Problems concerning structure isomeric states and stability of C - n clusters will be addressed in con- nection with simple model calculations. 2 Experimental Negative carbon clusters were produced in two different ways. C - n (32 n 86) and n even were produced in a plasma ion source [9, 11]. A fullerene mix was placed di- rectly in the ion source and heated to vaporize the fullerenes, and the negative fullerene ions are then produced as a re- sult of electron attachment or in some cases fragmentation. C - n clusters with 1 n 16 were produced in an ANIS sputter ion source using ordinary graphite as sputter material and cesium for coating and sputtering [11]. The intensity of fullerene ions varied over 8 orders of magnitude in the n in- terval from 32 to 86, with C - 60 as the most intense beam [10]. For n 30 no traces of negative cluster ions were found. With the sputter ion source the small C - n clusters were eas- ily identified but for n 11 great care has to be exercised since clusters of the type C n Cs are quite abundant. Further- more, for n> 16 well mass resolved beams turned out to contain large fractions of unwanted molecules and clusters and we refrained from performing measurements with C - n in the region 17 n 31. It shall be stressed that cluster ions in this interval do exist and can be produced in other types of ion sources [3]. The apparatus (Fig. 1) used in these measurements is the same as that described in previous publications, see e.g. [12]. The negative cluster ions produced in the ion source are electrostatically accelerated to an energy of 50 keV. The

Electron detachment and fragmentation in collisions between 50 keV $C_n^- (1 leq n leq 86)$ clusters and H $_2$

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Z. Phys. D 40, 371–374 (1997) ZEITSCHRIFTFUR PHYSIK Dc© Springer-Verlag 1997

Electron detachment and fragmentation in collisions between 50 keVC−n (1≤ n ≤ 86) clusters and H2

H. Shen, C. Brink, P. Hvelplund, M.O. Larsson

Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark

Received: 5 July 1996 / Final version: 14 September 1996

Abstract. The destruction cross section for 50 keV negativecarbon clustersC−

n (1 ≤ n ≤ 88) in collisions withH2 isreported. The dominant destruction channel is believed to beelectron detachment. The measured cross section values arecompared with theoretical values based on a simple geomet-rical model of the carbon cluster, and structural informationis obtained. Fragment spectra of both positive and negativeclusters are also recorded and fragmentation patterns are dis-cussed in relation to fragmentation energies and ionizationpotentials.

PACS: 36.40.+d; 34.90.+q

1 Introduction

Photoelectron spectra of negative carbon cluster ions fromC−

2 toC−84 have been reported by Yanget al.. Based on these

spectra estimates of vertical electron affinities have been ob-tained [1, 2]. It is normally accepted wisdom [3] that clus-tersCn with n ≤ 10 take the form of linear chains, whilefor 11 ≤ n < 32 cyclic structures dominate (see however[4]). For n ≥ 32 the cage structure (fullerenes) may domi-nate depending on the production method, especially so forthe negative clusters [5, 6]. The interplay between electronaffinities and cluster geometry is most likely an importantfactor for a better understanding of collisional electron de-tachment in energetic collisions between negative clustersand static gas targets. If one goes a step further and asks forexplanations of positive and negative fragmentation patternsresulting from such collisions, also ionization energies andfragmentation energies come into play. However, a first hintas to the structure of the cluster ion is obtained from ob-servation of loss of neutral fragments from charged clusters.Loss ofC2 is associated with fullerene structures whereasloss ofC3 is a dominant fragment pathway for chains andrings [3].

The linear chain clusters were predicted [7] to show astrong odd-even dependence of the electron affinity with oddclusters having the lower electron affinity. For clusters withcyclic structure,Cn rings with n = 13, 17, 21, 25 and 29were found to have relative large electron affinities [2].

In a general discussion of then dependence in processesinvolving Cn clusters, the possible influence of isomers hasto be taken into consideration [4, 5, 8]. A characterizationof the ions in a negative ion beam has been attempted in afew experiments, but a precise assignment of the extent ofisomeric ions is in general not available.

In the present communication we present measurementsof total destruction cross sections for 50 keVC−

n (1 ≤ n ≤86) in collisions withH2. We also present a few cases ofboth positive and negative fragmentation patterns resultingfrom such collisions. Problems concerning structure isomericstates and stability ofC−

n clusters will be addressed in con-nection with simple model calculations.

2 Experimental

Negative carbon clusters were produced in two differentways. C−

n (32 ≤ n ≤ 86) andn even were produced ina plasma ion source [9, 11]. A fullerene mix was placed di-rectly in the ion source and heated to vaporize the fullerenes,and the negative fullerene ions are then produced as a re-sult of electron attachment or in some cases fragmentation.C−n clusters with 1≤ n ≤ 16 were produced in an ANIS

sputter ion source using ordinary graphite as sputter materialand cesium for coating and sputtering [11]. The intensity offullerene ions varied over 8 orders of magnitude in then in-terval from 32 to 86, withC−

60 as the most intense beam [10].For n ≤ 30 no traces of negative cluster ions were found.With the sputter ion source the smallC−

n clusters were eas-ily identified but forn ≥ 11 great care has to be exercisedsince clusters of the typeCnCs are quite abundant. Further-more, forn > 16 well mass resolved beams turned out tocontain large fractions of unwanted molecules and clustersand we refrained from performing measurements withC−

n

in the region 17≤ n ≤ 31. It shall be stressed that clusterions in this interval do exist and can be produced in othertypes of ion sources [3].

The apparatus (Fig. 1) used in these measurements isthe same as that described in previous publications, see e.g.[12]. The negative cluster ions produced in the ion sourceare electrostatically accelerated to an energy of 50 keV. The

372

Fig. 1. Schematic of the experimental setup

Fig. 2. Mass-divided-by-charge spectra (M/q spectra) for 50 keVC−11 col-liding with H2. The target gas pressure is around 1 mTorr, correspondingto a 30% attenuation of the primary beam

energetic cluster ions are magnetically mass selected and di-rected to a gas cell where they collide with theH2 targetmolecules. After exiting the target cell charged primary ionsor fragments (positive or negative) are electrostatically de-flected into a single particle counter (channeltron) by a 180◦hemispherical analyzer. In order to obtain positive or neg-ative fragmentation spectra, the deflection voltage is sweptover the appropriate range to detect ions that extend from theinitial cluster ion to atomic fragments. Due to the high en-ergy of the incident clusters, the fragment energy measuredby its deflection voltage is a direct measure of its mass.Typical fragmentation spectra are shown in Fig. 2 for an in-cidentC−

11 beam. The gas cell pressure was set to attenuatethe incident beam by about 30 %. Note that the intensityaxis for the positive and negative fragments are normalizedto the same beam intensity. The peak corresponding toC−

11represents the part of the primary beam which is not inter-acting destructively when passing the target cell. The totaldestruction cross section for theC−

n clusters is obtained viaattenuation measurements of the primary beam as a functionof target thickness.

3 Results and discussion

The measured total destruction cross sections are shown inFig. 3 as a function of cluster size (n). It is clear from this fig-

Fig. 3. Total cross section for destruction ofC−n clusters as a function ofn. The lines represent the calculated cross sections, (2) and (3), cf. text.The dominating process is believed to be electron detachment

ure that the destruction cross section versusn can be approx-imated by two linear functions, one for smalln and one forlargen (n ≥ 32). It is assumed that electron detachment isthe dominating destruction mechanism, but experimentally,electron detachment accompanied by fragmentation leadingto neutral fragments cannot be ruled out. It should, however,be noted that, based on energetics, electron detachment ismore likely to take place than fragmentation. We have triedto model the two linear parts in Figs. 3 by assuming that thesmall clusters take the form of a linear chain and that thelarge clusters are spherical objects (fullerenes). It is furtherassumed that the detachment cross section is geometrical inthe sense that the cross section for the processes

C− +H2 → C +H2 + e (1)

can be used for estimating the maximum distance of clos-est approach, resulting in electron detachment from theC−-ion. Our measured cross section for this process is∼ 10−15cm2, resulting in a maximum distance of closest ap-proach,r = 1.78A. The geometrical radiusr′ of a fullerenecan be estimated assuming that the surface area of this spher-ical object is proportional to the number of carbon atomsand that the radius ofC60 is 3.5 A. It is thus found thatr′ = 0.452· √nA.

The total detachment cross section for a fullerene is nowgiven as

σ = π(r′ + r)2 = π(0.452· √n + 1.78)2 (A2) (2)

For the small string clusters we approximate the geometricalshape by a cylinder with two hemispherical end caps. If weassume that the distance between carbon atoms is 1.3A[16]then the total cross section for hitting this object within adistance of 1.78A from a carbon atom is

σ =π

4(n− 1) · 1.3 · 2 · 1.78 + 10 (A

2), (3)

the factorπ/4 originates from the assumed random orienta-tion of the “cigar” shaped cluster with respect to the beam

373

Fig. 4. Electron affinities ofC−n clusters as a function ofn. The filledsymbols are based on photodetachment threshold forC−n clusters [1, 2].Open circles refer to measurements of electron affinities based on equilib-rium constants [13], and open triangles to photodetachment of stored ions[14]

direction. The two above formulas can to a good approxi-mation be written as

σ = (0.036· n + 0.07) · 10−14cm2 (4)

for linear clusters with smalln and

σ = (0.01 · n + 0.27) · 10−14cm2 (5)

for spherical fullerenes.The two lines shown in Fig. 3 are based on these esti-

mated cross sections and the good agreement with experi-mental values indicates that the size of the detachment crosssections are determined mainly by the geometry of the neg-ative clusters.

A natural question at this point is to what extent vari-ations in electron affinities, as a function of cluster size,is reflected in then dependence of the detachment crosssection? In Fig. 4 then dependence of electron affinity isdemonstrated. The data are taken from [1, 2, 13, 14]. Themost dramatic variation in the electron affinity is found in theregion fromn = 1 ton = 10, where an odd-even variation ofmore than 1 eV is observed. As can be seen from Fig. 3, thisvariation is reflected in the detachment cross sections in away that the even numbered clusters with large affinity havecross sections smaller than the one for the neighbouring oddnumbered clusters. For clusters withn larger than 10, thisodd-even alternation continues but now the odd numbers cor-respond to the larger electron affinity. In thisn domain, andalso for the fullerenes, no variation in electron detachmentcross sections correlated with variation in electron affinityis observed. For the fullerenes, onlyC−

60 andC−70 seem to

possess electron affinities that deviate from the general trend.As has been argued, we believe that electron detachment

is the dominating process in these attenuation measurements.Such a process can be written as

C−n +H2 → Cn +H2 + e. (6)

Two other processes, which can be recorded with the presentexperimental equipment, are

Fig. 5. M/q spectra for 50 keVC−15 colliding with H2. The positive andnegative fragment patterns are measured with the same incident beam in-tensity and the same target gas pressure. The peak related to the primarybeam is not shown

C−n +H2 → C−

(n−x) +Cx +H2 (7)

and

C−n +H2 → C+

(n−x) +Cx +H2 + 2e. (8)

These are the most likely routes leading to positive or neg-ative fragments but other, more exotic processes can bethought out. In Fig. 5 is shown a positive and a negativefragmentation pattern forC−

15. The cross section for for-mation of these fragment peaks is two to three orders ofmagnitude smaller than the detachment cross sections, andwe believe that also the probability for fragmentation result-ing in only neutral fragments is small compared with thatfor electron detachment alone. The fragmentation spectrashow that for these small clusters loss ofC3 is a dominat-ing process. For the negative pattern almost only loss ofC3is recorded whereas positive fragments can be created alsoin other loss processes, butC3 loss is still the dominatingloss mechanism. Note that unlike in Fig. 2 the intensity axisfor the positive and negative fragments is not normalized tothe same beam intensity. This has the effect that very smalltraces of negative cluster ions withn=10, 9 and 8 can beseen in the spectrum.

The positive fragmentation spectrum forC−60 is shown

in Fig. 6. The dominating peakC+60 is associated with two-

electron loss and the cross section for this process is only 1 %of the total detachment cross section. AlsoC2 loss is foundto contribute significantly to the destruction of fullerenes.In fact, this fragmentation spectrum looks very similar tofragmentation spectra recorded for the collision processC+

60+H2. Such spectra were discussed by us [17] earlier, and itseems that the power law concept can be used also to modelthe fragmentation spectra resulting fromC−

60+H2 collisions.This subject will be discussed in a coming publication. Witha factor of∼20 less in intensity we observe fragments inthe region 1≤ n ≤ 30. These are believed to originate fromlarge energy transfer collisions, resulting in an “explosion”of the fullerene ion.

374

Fig. 6.M/q spectrum of positive fragments for 50 keVC−60 colliding withH2. Note that the base line of the expanded scale has been displaced forclarity

We have earlier reported [15] a negative fragment spec-trum resulting fromC60 collisions. The cross sections areseveral orders of magnitude smaller and only fragments withn ≤ 20 were observed.

4 Conclusion

Carbon cluster anions (C−n , 1≤ n ≤ 86) have been produced

either by sputtering of graphite or by fragmentation/electronattachment of vaporized fullerene mix. Electron detachmentand positive and negative fragmentation spectra were studiedfor 50 keV collisions between size selected clusters andH2.We observe that the electron detachment cross section scaleslinearly with cluster sizen, but that the linear and cage-likeclusters fall on two different curves. The general behaviourof the cross section dependence onn is explained by a ge-ometrical model of the carbon cluster. Positive and nega-tive fragmentation spectra show that loss ofC3 is the dom-inant fragmentation channel for small clusters withn ≤ 16

whereasC2 loss dominates for the fullerenes. Processes lead-ing to charged fragment ions are found only to account for∼ 1% of the total destruction cross section.

This work has been supported by the Danish National Research Foundationthrough the Aarhus Center for Advanced Physics (ACAP).

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