Multiple photoionization and fragmentation of C
60
in the 18–280-eV range
P. N. Juranić,
1,
*
D. Lukić,
2,†
K. Barger,
3
and R. Wehlitz
1,‡
1
Synchrotron Radiation Center, University of Wisconsin–Madison, Stoughton, Wisconsin 53589, USA
2
Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA
3
Western Washington University, Bellingham, Washington 98225, USA
Received 4 January 2006; revised manuscript received 7 February 2006; published 3 April 2006
We examined the relative ionization and fragmentation probabilities of C
60
after excitation by monochro-
matized synchrotron radiation in the energy range of 18– 280 eV. The energy dependence of the relative cross
sections of C
60
q+
q =1–4 and the associated fragments C
60–2n
q+
n =1–6 for q =2,3 and n =1,2 for q =4 are
analyzed. We present the ionization and appearance energies for those ions and fragments as well as the partial
cross sections for the C
60
ions.
DOI: 10.1103/PhysRevA.73.042701 PACS numbers: 33.80.Eh, 36.40.Qv, 61.48.c
I. INTRODUCTION
Buckminsterfullerene, C
60
, is of scientific interest because
it is an exceptionally stable and symmetric cluster with a low
sublimation temperature that allows us to study large clusters
in gas phase. Observing the behavior of C
60
after ejecting
one or more electrons by photon impact could give us great
insight into the multiple-photoionization process of other
clusters, and will increase our knowledge about C
60
clusters.
Earlier studies have extensively explored the C
60
2+
ioniza-
tion threshold region 1–3 between 18 and 45 eV. Aksela et
al. 4 and Karvonen et al. 5 investigated C
60
in the higher
photon energy range of 280–340 eV near the K shell, while
Reinköster has explored the intermediate energy region be-
tween 26 and 130 eV 6. Recently, we have investigated the
double-photoionization process of C
60
from threshold to
280 eV, which is published elsewhere 7. However, multiple
photoionization and fragmentation over the full photon en-
ergy range from the double-ionization threshold to the K
shell has not been covered experimentally, with a notable gap
in the 130– 280-eV region. The present study improves on
previous measurements of the double-, triple-, and
quadruple-ionization threshold regions, while also bridging
the gap between the low and high photon energies. More-
over, we present new data on the C
60
q+
q =1–4 ionization
energies and cross sections.
Finally, although the amount of data for ionization and
fragmentation of C
60
by electron impact is large 8–14,
there are less fragmentation data available for photoioniza-
tion. The existence of C
58,56
2+
has been observed in the
60–120-eV range 1 and of C
52
+
at 41 eV 3. Further stud-
ies have observed C
58,56,54
+
ions and double-ionized C
60–2n
n =1–5 fragments in the 26–130-eV range 6. None of
these papers show higher charge states of these fragments,
nor do they present the corresponding appearance energies.
Our measurements in the 18– 280-eV region presented here
show the presence of doubly, triply, and quadruply charged
C
60–2n
n =1–6 fragments along with their appearance ener-
gies obtained through single-photon ionization and ion time-
of-flight TOF spectrometry.
II. EXPERIMENT
A. Beamlines
The data were obtained through a series of experiments
performed at the Aladdin Storage Ring at the Synchrotron
Radiation Center SRC in Stoughton, WI U.S.A.. The first
experiment was performed on the Undulator 4 m Normal
Incidence Monochromator U1 NIM15 using an energy
range of 18–40 eV with an energy resolution of 15 meV at
25 eV. We had found that stray light affected the data at
higher energies 35 eV, and thus restricted our data to the
region between just below the double-ionization threshold
19 eV and 35 eV. The data were acquired in 0.1 eV steps
in the 19–22 eV region, in 0.2 and 0.25 eV steps in the
22–26-eV region, and in 0.5 eV steps beyond that. The ac-
quisition times depended on the photon energy, and they
ranged from up to an hour near threshold to five min in the
midrange energies, and up to half an hour near 35 eV. Most
data points were measured at least twice and the measured
values were averaged.
The second experiment was performed on the 6 meter To-
roidal Grating Monochromator 6 m-TGM16 which has
an energy range of 8–200 eV with an energy resolution be-
tween 54 meV at lower energies and 0.3 eV at higher ener-
gies. Second order light was present at the lower energies
unless suppressed by an appropriate filter. In the case of the
6 m-TGM we had used Al and Si
3
N
4
filters for the
36–70 eV and 70–100-eV range, respectively. Thus, data
below 36 eV were not taken. Similarly, data above 180 eV
were affected by stray light and useful data were limited to
the 36– 180-eV range. The acquisition times were, on aver-
age, 15 minutes to half an hour and were taken in 2–5-eV
steps. The measurements were done typically twice at each
photon energy.
The third experiment was performed on the Wadsworth
Normal Incidence Monochromator beamline 17 that has an
energy range of 7.8– 45 eV with an energy resolution of
*Electronic address: pnjuranic@wisc.edu
†
On leave from Institute of Physics, P.O. Box 57, 11001 Belgrade,
Serbia and Montenegro.
‡
Electronic address: wehlitz@src.wisc.edu
PHYSICAL REVIEW A 73, 042701 2006
1050-2947/2006/734/0427018/$23.00 ©2006 The American Physical Society 042701-1