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C 70 and C 60 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.

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Henrik Cederquist, Physics Department, Stockholm University


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slide1

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

slide2

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

slide3

Projectiles:

Targets:

Xe8+ , Xe16+ , Xe23+ (v= 0.07, 0.10, 0.12 v0)

C60, C70 (T=600 °C)

Important collision features:

A(q-s)+

Hollow ions and atoms

E0+E

Aq+

E0

C70r+

slide4

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

slide5

Fragmentation modes of excited C60 and C70:

Neutral evaporation of small fragments:

C60

C60r+C60-2mr+ + C2m (m=1,2,3,4…)

C70r+ C70-2mr+ + C2m (m=1,2,3,4…)

Asymmetric fission:

C70

C60r+C60-2m(r-1)+ + C2m +(m=1,2,3,4…)

C70r+ C70-2m(r-1)+ + C2m +(m=1,2,3,4…)

Multifragmentation:

C60r+many small fragments in low charge states

C70r+ many small fragments in low charge states

slide6

The experimental technique:

Collimated C70 Jet

Cylindrical analyzer

Xe23+

Vex

0 V

-100 V

TRIG

V1

time-of-flight

V2

V3

PSD1

START

STOP

PSD2

Multi-hit TDC

22+

21+

C705+

C685+

time-of-flight

slide7

(a)

Intensity (arb. units)

(b)

m/q (units of C)

Xe8+ - C70 (24 keV)

Xe8+ - C60 (24 keV)

The C70 powder has high purity (99.4 %)

slide9

Xe23+ + C70 Xe (23-s)+ + C70r+ +...

Xe23+ + C60 Xe (23-s)+ + C60r+ +...

Intensity (arb. units)

m/q (units of C)

slide10

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

C60r+

C70r+

slide11

Maximum fullerene charges

and

Similarities in the fragmentation of C60 and C70 ions

slide12

Xe23+ + C70 Xe 20+ + C70r+ +...

Light fragments:

GATED!

Xe23+ + C60 Xe 20+ + C60r+ +...

Intensity (arb. units)

C709+

GATED!

+

2+

C

C

15

8

+

7+

C

+

C

6+

C

9+

C

C

+

3

C

70

5

70

70

6

8+

C

+

C

Intensity (arb. units)

70

+

C

11

2+

C

4

11

4

6

8

10

12

m/q (units of C)

slide13

Heavy fragments:

4+

3+

5+

C

C

C

70

70

70

5+

C

Xe23+ + C70 Xe 21+ + C70r+ +...

68

4+

66

C

5+

C

70

68

Xe8+ + C70 Xe 6+ + C70r+ +...

70

3+

C

60

5+

6+

C

C

68

70

10

20

4+

3+

5+

C

6+

C

C

C

60

60

7+

60

60

C

60

Xe23+ + C60 Xe 21+ + C60r+ +...

56

58

4+

5+

3+

C

C

C

60

60

60

Xe8+ + C60 Xe 6+ + C60r+ +...

6+

C

60

10

20

slide14

C68(r-1)+

C58(r-1)+

C2+

C2+

Similarities in kinetic energy releases:

slide15

Kinetik energy releases – calibration and resolution

Collimator

Ion beam

Ion beam

Warm Xe target

(high v(Xe))

’Cold’ C60-jet

(low v(C60))

Xe+

Detector

Detector

C60+

~ 40 meV

~ 3 meV

Energy resolution

Energy calibration

slide16

Kinetic Energy Releases (KER’s) for C60r+ asymmetric fission – comparison with other measurements

16

C60r+  C58(r-1)+ + C2+

17+

Present expt. data for Xe

12

KER’s are insensitive to production method!

MIKE

Scheier et al.,

8

Kinetic Energy Release (eV)

MIKE

Senn et al.,

4

Chen et al.,

TOF

TOF

Tomita et al.,

0

1

2

3

4

5

6

7

8

9

Charge of final fragment C

58

slide17

Kinetic Energy Releases for C70r+ asymmetric fission – comparison with other measurements (Xe16+ and Xe23+ projectiles)

C70 KER’s are similar for Xe23+ and Xe16+!

slide18

C68(r-1)+

C58(r-1)+

C2+

C2+

Model:

Comparisons Kinetic Energy Releases C60/C70

C70 data:

25

23+

Present data Xe

16+

Present data Xe

20

15

Kinetic Energy Release (eV)

U

C60 data:

10

23+

Xe

, C

60

5

et al.

Cederquist

, C

60

R

0

2

3

4

5

6

7

8

9

10

11

12

Initial charge of C

/C

, r

70

60

slide20

Xe23+ + C70 Xe 21+ + C70r+ +...

C684+

C685+

C704+

C705+

Xe8+ + C70 Xe 6+ + C70r+ +...

C685+

C705+

C704+

C684+

Surprising no such difference for the C60 target!

slide21

Xe23+ + C70 Xe 21+ +...

Xe8+ + C70 Xe 6+ +...

5+

5+

C

C

68

68

5+

5+

C

C

70

70

200

250

300

200

250

300

Same comparison - projected distributions:

slide23

4th Annual LEIF Meeting Belfast, June 29, 2003

Conclusions:

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!

slide24

Look at the raw data again C60 and C70 really behaves differently with Xe8+ projectiles:

Xe8+ + C70 / C60 Xe6+ + C70r+ / C60r+ +...

C685+

C705+

C704+

C684+

C604+

C605+

C585+

C584+

Note!

slide25

There are some differences even with Xe23+ projectiles:

Xe23+ + C70 / C60 Xe21+ + C70r+ / C60r+ +...

C684+

C685+

C704+

C705+

C584+

C585+

C604+

C605+

slide26

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

C70

C60