IL NUOVO CIMENTO VOL. 110 A, N. 9-10 Settembre-Ottobre 1997
Ionisation and fragmentation of fullerene ions
by electron impact
(
)
V. SCHAFER¨ , D. HATHIRAMANI, K. AICHELE, U. HARTENFELLER F. SCHEUERMANN, M. STEIDL, M. WESTERMANNand E. SALZBORN
Institut f¨ur Kernphysik, Justus-Liebig-Universit¨at Giessen - D-35392 Giessen, Germany (ricevuto il 31 Luglio 1997; approvato il 15 Ottobre 1997)
Summary. — Absolute cross-sections for electron impact induced fragmentation,
fragmentation-single-ionisation and fragmentation-double-ionisation were measured for the fullerene ions C+
60 and C +
58. For the fullerene ion C 2+
60,
fragmentation-single-ionisation was measured with a loss ofnC
2molecules
(n=1;2;3). The cross-sections
were determined from the respective threshold up to an electron energy of 1 keV using the animated crossed-beams technique.
PACS 34.80.Gs – Molecular excitation and ionisation by electron impact. PACS 36.40.Qv – Stability and fragmentation of clusters.
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
Many collision experiments with fullerene ions have been carried out in the past few years due to the possibility of the production of intense mass-to-charge-selected fullerene ion beams. Interesting and unexpected results were discovered in these studies. One of the most remarkable features of a singly ionised C+
60
cluster is its exceptional sta-bility against unimolecular decomposition. Nevertheless, the main fragmentation chan-nel of the Buckminster fullerene ion is the sequential loss of neutral C2 molecules [1].
Collision induced fragmentation of C+
60 by photons [2, 3], atoms [4], fullerenes [5],
sur-faces [6] and electrons [7-10] has been investigated. In this context we have measured ab-solute cross-sections for electron impact induced fragmentation, fragmentation-ionisation and fragmentation-double-ionisation for the fullerene ions C+
60 , C+ 58 and C2+ 60 . The cross-sections were determined from below the respective threshold up to an electron energy of 1 keV.
(
)Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and
Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.
1224 V. SCH ¨AFER, D. HATHIRAMANI ETC.
Fig. 1. – Absolute cross-sections for the electron-impact induced fragmentation of C+ 60
!C +
56[7]
(open circles) and the fragmentation of C+ 58
!C +
54
(full triangles). The error bars show the total experimental error. The arrows indicate the thresholds of the processes given by W¨org¨otter et al. [15].
2. – Experiment
The measurements were performed at the Giessen heavy ion beam facility with the electron-ion crossed-beams set-up which has been described in detail earlier [11]. With an especially developed oven a sample of C60soot (with a purity greater than 96 %) was
heated up to a temperature of about 800 K. The C60 vapour was evaporated into the
plasma of a 10 GHz ECR ion source [12]. The ion source was operated at a low microwave power (less than 5 W). The so produced Cq+
60
fullerene ions were extracted at energies of 6 keV forq=1and 20 keV forq=2. After mass-to-charge analysis and a collimation to
2.52.5 mm 2
, the fullerene ion beam was crossed with an intense electron beam [13] at an angle of 90
. The product ions of the observed reactions were magnetically separated from the primary fullerene ion beam and counted in a single-particle detector 1 m behind the interaction region. The flight time of the carbon cluster ions from the ECR ion source to the interaction region was approximately 122s forq = 1and 67s for q = 2. The
produced fragment ions were detected in a channeltron electron multiplier roughly 24s
(primary charge stateq=1) and 13s (primary charge stateq=2) after interaction with
the electron beam. The cross-sections were measured using the animated crossed-beams technique [14] where the electron beam is moved up and down through the ion beam with simultaneous registration of both actual beam currents and the signal of the observed fragment ion. The total experimental uncertainties are typically10 % at the maximum
of the cross-sections resulting from the quadrature sum of the non-statistical errors of about 8 % and the statistical error at 90 % confidence level.
3. – Results
The comparison of the cross sections C+ 60 !C + 56 [7] and C+ 58 !C + 54
(fig. 1) clearly shows the higher stability of the C+
IONISATION AND FRAGMENTATION OF FULLERENE IONS BY ELECTRON IMPACT 1225
Fig. 2. – Absolute cross-sections for the electron-impact induced fragmentation-single-ionisation of C+
60 !C
2+
56
[7] (open circles) and C+ 58
!C 2+
54
(full triangles) and the fragmentation-double-ionisation of C+
60 !C
3+
56 (full circles) and C +
58 !C
3+
54 (full diamonds). The error bars show the total
experi-mental error. The arrows indicate the thresholds of the processes given by Scheier et al. [16].
Fig. 3. – Absolute cross-sections for the electron-impact induced fragmentation-single-ionisation of C2+ 60 !C 3+ 58 (circles), C2+ 60 !C 3+ 56 (squares) and C2+ 60 !C 3+ 54
(triangles). The error bars show the total experimental error. The arrows indicate the thresholds of the processes given by Scheier et al. [16].
1226 V. SCH ¨AFER, D. HATHIRAMANI ETC.
and the shape of the cross-sections at electron energies above 35 eV are nearly identical for both processes, the cross-section for C+
58 !C
+
54
exceeds the results for the reaction C+
60 !C
+
56
by almost a factor of 2 in the maximum.
Figure 2 shows cross-sections for processes with a loss of two C2 molecules from
C+ 60
and C+ 58
ions as in fig. 1. But here the reactions additionally include single- (up-per cross-sections) and double-ionisation (lower cross-sections). Whereas the cross sec-tions for the pure fragmentation (fig. 1) show a significant difference in their maximum value, the cross-sections for fragmentation-single-ionisation and fragmentation-double-ionisation for C+
60
and C+ 58
ions are nearly identical (fig. 2).
Different fragmentation degrees for the fragmentation-single-ionisation of primary C2+
60
ions are shown in fig. 3. The observed cross-sections decrease by about 15 % over the whole energy range with each C2 molecule evaporated. Note that the cross-sections do
not converge at higher electron energies within the range observed. Only cross-sections for the parent fullerene ions C+
60 , C+ 58 and C2+ 60 could be measured, but it can be concluded that the influence of the mass of the fullerene ions seems to be less important than expected. Only the cross-sections for pure fragmentation processes show a significant dependence on the ion masses. Furthermore, the reaction channel de-termines the shape of the relevant cross-section. The measurement of cross-sections for electron impact induced pure evaporation of only one C2molecule was not possible with
the present experimental set-up because of the unfavourably similar m/q ratios of the parent and product ions.
REFERENCES
[1] SCHEIER P., D ¨UNSER B., W¨ORGOTTER¨ R., MUIGGD., MATTS., ECHTO., FOLTINM. and M¨ARKT. D., Phys. Rev. Lett., 77 (1996) 2654.
[2] WURZP., and LYKKEK. R., J. Phys. Chem., 96 (1992) 10129.
[3] HOHMANNH., EHLICHR., FURRERS., KITTELMANNO., RINGLINGJ. and CAMPBELL E. E. B., Z. Phys. D, 33 (1995) 143.
[4] EHLICHR., WESTERBURGM. and CAMPBELLE. E. B., J. Chem. Phys., 104 (1996) 1900. [5] ROHMUNDF., GLOTOVA. V., HANSENK. and CAMPBELLE. E. B., J. Phys. B, 29 (1996)
5143.
[6] BECKR. D., ROCKENBERGERJ., WEISP. and KAPPESM. M., J. Chem. Phys., 104 (1996) 3638.
[7] V¨OLPEL R., HOFMANN G., STEIDL M., STENKE M., SCHLAPP M.,TRASSL R. and SALZBORNE., Phys. Rev. Lett., 71 (1993) 3439.
[8] MCELVANYS. W., ROSSM. M. and CALLAHANJ. H., Acc. Chem. Res., 25 (1992) 182. [9] SCHEIERP., D ¨UNSERB. and M¨ARKT. D., J. Phys. Chem., 99 (1995) 15428.
[10] D ¨UNSERB., LEZIUSM., SCHEIERP., DEUTSCHH. and M¨ARKT. D., Phys. Rev. Lett., 74 (1995) 3364.
[11] TINSCHERTK., M ¨ULLER A., HOFMANNG., HUBERK., BECKERR., GREGORYD. C. and SALZBORNE., J. Phys. B, 22 (1989) 531.
[12] LIEHR M., SCHLAPP M. ,TRASSL R., HOFMANN G., STENKE M., V¨OLPEL R. and SALZBORNE., Nucl. Instrum. Methods B, 79 (1993) 697.
[13] BECKERR., M ¨ULLER A., ACHENBACH C., TINSCHERT K. and SALZBORN E., Nucl. Instrum. Methods B, 9 (1985) 385.
[14] M ¨ULLERA., TINSCHERTK., ACHENBACHC. and SALZBORNE., Nucl. Instrum. Methods B, 10/11 (1985) 204.
[15] W¨ORGOTTER¨ R., D ¨UNSER B., SCHEIER P., M¨ARK T. D., FOLTIN M., KLOTS C. E., LASKINJ. and LIFSHITZC., J. Chem. Phys., 104 (1995) 1225.
[16] SCHEIERP., D ¨UNSERB., W¨ORG¨OTTERR., LEZIUSM., ROBLR. and M¨ARKT. D., Int. J. Mass Spec. Ion. Phys., 138 (1994) 77.
