1. C70 and C60 colliding with slow highly charged ions – a comparison
Henrik Cederquist, Physics Department, Stockholm University
Atomic Physics SU: H. Zettergren, H. T. Schmidt, J. Jensen
and H. Cederquist
CEA Ganil: B. Huber, and B. Manil
Aarhus University: S. Tomita, S.B. Nielsen, J. Rangama,
and P. Hvelplund
C60
C70
4th Annual LEIF Meeting Belfast, June 29, 2003

2. Part of the team (in Stockholm in December 2002):
J. Jensen
S. Tomita
P. Hvelplund
S.B. Nielsen
J. Rangama
H. Cederquist
Missing in the picture: H. Zettergren, H.T. Schmidt, and B. Huber

3. Important collision features: A(q-s)+ E0+E Aq+  E0
Hollow ions and atoms
E0+E
Aq+  E0
C70r+
Projectiles:
Xe8+ , Xe16+ , Xe (v= 0.07, 0.10, 0.12 v0)
Targets:
C60, C70 (T=600 °C)
The electronic response of C60 is manifested in its
1, Ionization
2, Excitation (electronic and of course also the vibrational excitations through the coupling between electronical and vibrational degrees of freedom).
and 3, Fragmentation
and the time-scales for these processes.
Methods to probe the response involve, e.g., collisions with photons, electrons, positrons, slow and fast atomic ions and, lately, also collisions with other (multiply charged) fullerenes. In some respects, the method of energy transfer plays a minor role as it has been noted that fragmentation spectra of C60 have many common features for widely different excitation methods (involving photons, electrons, fast and slow ions etc.). This in turn shows that inherent properties of the C60 molecule itself may be deduced from such collision experiments.
Slow highly charged ions has the unique feature of being able to remove many electrons gently from the C60.
High q: Efficient multiple electron transfer at large distances and formation of Hollow Atoms and Ions
Low v: Low momentum transfers to target electrons in direct head-on hits of the projectile no direct ionization is expected (but rather multiple capture followed by autoionization of the hollow projectile ion ).
The relevant time scales of todays discussion is of the order of one femtosecond as given by the varairion of the field strength from the projectile at the target. This number seems to be to small even for the fastest (projectile based) autoionization processes to compete but in spite of this I will argue , and this is one of my main points, that ultra-fast electron emission during the collision s a very important process for sufficiently small impact parameters.

4. Charge stability limits
Similarities and differences in ion-induced fragmentation of C60 and C70
C60
C70
Charge stability limits
Kinetic Energy Releases (KER)
Fission barriers
Competition asymmetric fission/evaporation

5. Fragmentation modes of excited C60 and C70:
Neutral evaporation of small fragments:
C60
C60 r+ C60-2mr+ + C2m (m=1,2,3,4…)
C70 r+ C70-2mr+ + C2m (m=1,2,3,4…)
Asymmetric fission:
C70
C60 r+ C60-2m(r-1)+ + C2m + (m=1,2,3,4…)
C70 r+ C70-2m(r-1)+ + C2m + (m=1,2,3,4…)
Multifragmentation:
C60 r+ many small fragments in low charge states
C70 r+ many small fragments in low charge states

6. The experimental technique:
Collimated C70 Jet
Cylindrical analyzer
Xe23+
Vex
0 V
-100 V
TRIG V1
time-of-flight V2 V3
START
PSD1
PSD2
STOP
Multi-hit TDC
22+
21+
C705+
C685+
time-of-flight

7. Xe8+ - C70 (24 keV) Xe8+ - C60 (24 keV) (a) (b)
m/q (units of C)
Intensity (arb. units)
Xe8+ - C70 (24 keV)
Xe8+ - C60 (24 keV)
The C70 powder has high purity (99.4 %)

8. Similarities in the production of intact C60 and C70 ions:

9. Xe23+ + C70  Xe (23-s)+ + C70r+ +...
Xe23+ + C60  Xe (23-s)+ + C60r+ +...
Intensity (arb. units)
m/q (units of C)

10. Relative cross sections for producing intact C70r+ and C60r+ ions in Xe23+-C60 and Xe23+- C70 collisions.
C60r+
C70r+

11. Maximum fullerene charges
and
Similarities in the fragmentation of C60 and C70 ions

12. Light fragments: C C C C C C C C C C C C
Xe23+ + C70  Xe C70r+ +...
Light fragments:
GATED!
Xe23+ + C60  Xe C60r+ +...
Intensity (arb. units)
C709+
GATED! + C 2+ C 15 8 C + C + 9+ C 7+ 6+ 3 C + C C 5 70 70 70 6 C 8+
Intensity (arb. units) C + C + 70 11 2+ 4 C 11 4 6 8 10 12
m/q (units of C)

13. Heavy fragments: Xe23+ + C70  Xe 21+ + C70r+ +...5+ C 4+ C C 3+ C 5+ 70 70 70
Xe23+ + C70  Xe C70r+ +... 68 4+ 5+ C 66 C 70
Xe8+ + C70  Xe 6+ + C70r+ +... 70 68 C 3+ 60 C 6+ C 5+ 70 68 10 20 6+ C 5+ C C 4+ C 3+ 7+ 60 60 60 C 60 60
Xe23+ + C60  Xe C60r+ +... 56 58 C 5+ C 4+ C 3+ 60 60 60
Xe8+ + C60  Xe 6+ + C60r+ +... C 6+ 60 10 20

14. Similarities in kinetic energy releases:
C58(r-1)+
C2+
C68(r-1)+
C2+

15. Kinetik energy releases – calibration and resolution
Detector
Ion beam
C60+
Collimator
’Cold’ C60-jet
(low v(C60))
Xe+
Warm Xe target
(high v(Xe))
~ 3 meV
~ 40 meV
Energy resolution
Energy calibration

16. Kinetic Energy Release (eV)
Kinetic Energy Releases (KER’s) for C60r+ asymmetric fission – comparison with other measurements 16
C60r+  C58(r-1)+ + C2+
Present expt. data for Xe
17+ 12
KER’s are insensitive to production method!
Scheier et al.,
MIKE 8
Kinetic Energy Release (eV)
Senn et al.,
MIKE 4
Chen et al.,
TOF
Tomita et al.,
TOF 1 2 3 4 5 6 7 8 9
Charge of final fragment C 58

17. Kinetic Energy Releases for C70r+ asymmetric fission – comparison with other measurements (Xe16+ and Xe23+ projectiles)
C70 KER’s are similar for Xe23+ and Xe16+!

18. Comparisons Kinetic Energy Releases C60/C70
Model:
Comparisons Kinetic Energy Releases C60/C70
C58(r-1)+
C2+
C70 data:
C68(r-1)+
C2+ 25
Present data Xe
23+
Present data Xe
16+ 20 15
Kinetic Energy Release (eV) U
C60 data: 10 Xe
23+
, C 60 5
Cederquist
et al.
, C 60 R 2 3 4 5 6 7 8 9 10 11 12
Initial charge of C /C
, r 70 60

19. Differences in fragmentation pathways:

20. Surprising no such difference for the C60 target!
Xe23+ + C70  Xe C70r+ +...
C704+
C684+
C705+
C685+
Xe8+ + C70  Xe 6+ + C70r+ +...
C704+
C684+
C705+
C685+
Surprising no such difference for the C60 target!

21. Same comparison - projected distributions:
200
250
300 C 5+ 68 70
Xe23+ + C70  Xe
Xe8+ + C70  Xe

22. Data for Xe8+ impact!
Note!!

23. Conclusions:
4th Annual LEIF Meeting Belfast, June 29, 2003
The relative cross sections for production of intact C60r+ and C70r+ ions are almost identical as functions of r for a given projectile.
The fragmentation spectra are very similar for C70 and C60 indicating similar roles played by asymmetric fission and evaporation – exception: Enhanced production of C60 from C70
Measured Kinetic Energy Releases (KER) for asymmetric fission C70r+  C68(r-1)+ + C2+ are significantly lower than those reported earlier in the literature.
KER’s are close for C60 and C70 ionized by Xe23+ - in agreement with the fission process being controlled by a barrier
Fission barriers and stability limits are similar for C60 and C70
C70 in intermediate charge states fission after collisions with Xe16+ or Xe23+ but evaporate after collisions with Xe8+ - new phenomena not observed with C60!

24. Xe8+ + C70 / C60  Xe6+ + C70r+ / C60r+ +...
Look at the raw data again C60 and C70 really behaves differently with Xe8+ projectiles:
Xe8+ + C70 / C60  Xe6+ + C70r+ / C60r+ +...
C704+
C684+
C705+
C685+
C604+
C584+
C605+
C585+
Note!

25. There are some differences even with Xe23+ projectiles:
Xe23+ + C70 / C60  Xe21+ + C70r+ / C60r+ +...
C704+
C684+
C705+
C685+
C604+
C584+
C605+
C585+

26. Fission barriers are lower for C70r+ than for C60r+
thus
Activation energies for neutral C2-emission must be lower for C70r+ than for C60r+
k=r
Ea (C60r+) = Ea (C60) + (Ik(C58)-Ik(C60) )
k=1
k=r
Ea (C70r+) = Ea (C70) + (Ik(C68)-Ik(C70) )
k=1
C60
C70