1C12Photofragment Imaging Apparatus for Measuring Momentum Distributions
in Dissociative Photoionization of Fullerenes*

Department of Structural Molecular Science, Graduate University for Advance Studies, Myodaiji, Okazaki, 4448585, Japan¶
Institute for Molecular Science, Myodaiji, Okazaki, 444-8585, Japan{ Bhim P. Kafle*, Md. S. I. Prodhan*, Hideki Katayanagi*¶, and Koichiro Mitsuke*¶
The yield curves of the fragments C60-2nz+ and C70-2nz+ (n ≥ 1, z ≥ 1) produced by photoionization of solitary
fullerenes of C60 and C70, respectively, have been reported in the range of 45-150 eV [1,2].Comparison
between the experimental results and observation from theoretical studies revealed that the excess energy isstatistically distributed among the internal degrees of freedom of the parent ions and C2 units are ejectedsequentially:(1)
C60+ → C58+ + C2, C58+ → C56+ + C2, … C60-2n+2z+→ C60-2n+ + C2.
Moreover, it is assumed that there exists no potential barrier along the reaction coordinate and that no resonant
state participates during dissociation. To clarify the above argument, we have developed a photofragmentimaging apparatus based on time-of-flight (TOF) mass spectrometry to measure the kinetic energy and angular
distributions of the fragments.
There are only a few experimental studies of product analysis of the fragments. Translational energy
distribution of C60-2n+ have been measured by several groups to gain insight into the energetics andfragmentation mechanism. Hertel and coworkers [3] evaluated the kinetic energies of C60-2n+ (1≤ n ≤ 14)
produced by photoionization of C60+. Later Märk and co-workers [4] fulfilled electron impact ionization of C60
and reported a value of ca. 0.45 eV as the total average kinetic energy release in the decomposition of C60+
into C60-2n+ (1≤ n ≤ 8).These authors suggested that not only sequential C2 ejection of process (1) but also
single-step two-fragment fission of the parent C60+ ions(2)
C60+→ C60-2n+ + C2n.
are possible mechanisms for the formation of C60-2n+.
In the present study, we will develop a new version of momentum imaging spectrometer to obtain reliablevelocity distributions of the fullerene fragments. From photofragment images, we will be able to decide on
which mechanism dominates fragmentation of fullerene ions, since three-dimensional velocity distributions areexpected to considerably differ for different mechanisms. Moreover, closer inspection of the images may allow
us to directly probe the properties of transition states correlated to the dissociation channels. If dissociation
channels involve passage over an exit barrier, the translational energy distribution makes a peak at somefraction of the barrier height. In contrast, if little or no exit barrier exists on dissociation channels, thedistribution shows a peak at close or equal to zero because energy partitioning occurs statistically among all theinternal degrees of freedom of the parent species.
We adopted the Eppink–Parker type three-element velocity focusing lens system (electrodes R,E, and T) [5]to achieve the kinetic energy resolution of ca. 0.01 eV on the photofragment images. Furthermore, to select abunch of fragments having the same mass-to-charge ratio m/z from neighboring bunches (m±24)/z, for example,
to separate C58+ from C60+ and C56+, we utilized a potential switcheable mass gate M and an ion reflector G,
which are kept very close to the imaging detector (PSD) inside the TOF tube, as demonstrated in Fig. 1.
The operating principle of these two components of the spectrometer is as follows: As long as the tube of themass gate is kept grounded, all fragments are reflected back by the ion reflector and do not impinge against
the PSD. In contrast, when an entire bunch of the fragments having an expected m/z arrives inside the massgate, a pulsed voltage is applied there. The potential energies of the ions in this bunch are suddenly elevated, sothat only these ions can pass through the ion reflector and reach the PSD.
For optimizing the dimensions of the setup, we performed ion trajectory simulations utilizing the SIMION
software [6]. We considered that the dissociative ionization of C60 takes place within a region of rectangularparallelepiped ∆x∆y∆z = 1×3×1 mm3 in the ionization region of the spectrometer. The y-coordinate of thisregion ranges from -1.5 to +1.5 mm because the y-direction is assigned to the passage of synchrotron radiation
as shown in the Fig. 1 (the x-direction represents the TOF axis). In ion trajectory simulations the eight corners
and the center of the ionization region were chosen for the starting points of the trajectories. From each point

171 trajectories were generated in the elevation angle range of – 90o to + 90o at intervals of 22.5 o and in theazimuth angle range of 0o to + 180o at intervals of 10o. The amplitude and duration of the pulsed voltage
applied to repeller were 300 V and 7 µs, respectively, and the rising edge of this pulse preceded that of thepulsed voltage applied to the mass gate by 44.5 µs. The amplitude and duration of the pulsed voltage applied tothe mass gate were set to 120 V and 1 µs, respectively. The ratio of voltages between that applied to extractor
and that to repeller was set to be constant at 0.714, while a continuous voltage of 320 V was applied to thecentral electrode of the ion reflector.
With the above parameters, the simulated trajectories of C60+, C58+ and C56+ at initial kinetic energy of 0.1
eV show that the trajectories of both unwanted ions C60+ and C56+ are reflected completely. On the other hand,most of the trajectories of C58+, the ion whose momentum image we wish to measure, are found to go beyond
the ion reflector and reach the PSD. This observation provides direct evidence for exclusive imaging detectionof C58+ after excluding C60+ and C56+ with the same kinetic energies.
SRPSD

yPyx
M
RE

GTFigure 1. Schematic view of the momentum imaging spectrometer in combination
with the mass gate M and ion reflector G. The dimensions of all the electrodes areoptimized by using the SIMION 3D software. R, Repeller; E, extractor; T,Entrance electrode of a drift tube; SR, synchrotron radiation.

At Ejection angle every 15o from angle 0 - 90o
141210864200510Δy15

Figure 2. Quasi-linear relationship between
the y-component of the momentum and thatof the displacement at PSD.

y/mm

Moreover, a quasi-linearship between the y10
component of the momentum (Py) and that of thedisplacement at PSD (Δy) shows linear trend (Fig.
82), which allows us to translate the displacement ofionic fragment on PSD to velocity and angular
6distribution of a desired fragment.
Figure 3 shows the image of C58+ ions on the4PSD at the kinetic energies of 0.1eV (triangles) and0.11eV (circles). We took into account the ion
trajectories generated in the elevation and azimuth
2angle ranges of 0o to + 90o and 0o to + 180o ,respectively, which cover only one quarter of the0full three-dimensional trajectories over the 4π solid
0-2-4-6-8-10
angle. The trajectories with a given elevation angle
z/mmform a horizontal stripe, and the envelope of all theFigure 3. Projection of the three-dimensional
stripes makes an arc, which clearly demonstrates
scattering distribution of C58+ ions on the PSD at thethat scattering distribution in spherical symmetry can
initial kinetic energies of 0.1 (▲) and 0.11 eV (Ο ).
be successfully projected on an image plane. It is+
likely that C58 fragment ions with kinetic energy difference of 0.01 eV are almost separable. Comparison
between the simulations with and without the ion reflector confirmed that the images are not distorted in thepresence of the ion reflector. The present momentum imaging spectrometer will be constructed and installed in
the end station of beam line 2B in the UVSOR facility.

REFERENCES
1. J. Kou, T. Mori, Y. Kubozono, and K. Mitsuke, Phys. Chem. Chem. Phys. 7, 119-123 (2005).
2. K. Mitsuke, H. Katayanagi, J. Kou, T. Mori and Y. Kubozono, Am. Inst. Phys. CP 811, 161-166 (2006).
3. H. Gaber, R. Hiss, H. G. Busmann, and I. V. Hertel, Z. Phys. D 24, 307-309 (1992).
4. D. Muigg, G. Denifl, P. Scheier, K. Becker, and T. D. Mark, J. Chem. Phys. 108, 963-970 (1998).
5. A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477-3484 (1997).
6. D. A. Dahl, SIMION 3D Version 7.0 User’s Manual, Idaho falls: 2000.