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Application of COLTRIMS to Study
Collision Induced Dissociation of
Multiply Charged Benzene
Giorgi Veshapidze, Haruo Shiromaru
Tokyo Metropolitan University
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Outline
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Motivation
Experimental method and apparatus
Data handling methodology
Doubly charged benzene
Multiply charged benzene
Multiply charged difluorobenzene
Summary and conclusions
Motivation
• Benzene  Building block of many
organic molecules
• Low charge fragmentation  Access to
intermediate states
• High charge fragmentation  Access to
initial Geometry
• By varying the charge state and excitation,
useful information might be gained.
TMU-ECRIS facility
Analyzing Magnet
Electron - Ion
Scattering
Switching
Magnet
ECR Ion Source
TMU-ECRIS
Secondary Ions
from Surfaces
ZOO-RISE setup
Coulomb-Explosion-Imaging
0
1(m)
Polarization Spectroscopy
CEI setup
• CEI = Coulomb Explosion Imaging
• ZOO-RISE = ZOOmable Recoil Imaging with
Secondary Electrons
• Both are based on the application of Recoil Ion
Momentum Spectroscopy (RIMS) method to the
molecular fragmentation.
• RIMS = Position-sensitive + Time Of Flight
(TOF) measurement.
• x, y, t  px, py, pz or vx, vy, vz
RIMS principle
PSD for
recoil ions
x, y and TOF

E
x  Vx  TOF
Ions, with kinetic energy more
than εmax can not be detected
with 4π solid angle.
 max
qER 2

L
•q = Ion charge
•E = Extraction field strength
•L = Flight length
•R = PSD radius
y  V y  TOF
a  TOF 2
L  Vz  TOF 
2
Initial velocity vectors are calculated by
simple, classical-mechanical equations.
Increasing E or decreasing L
shortens TOF, thus reducing
time resolution.
Differences between RIMS
and CEI
RIMS
• Atomic target
• Recoil ion energy <
1 eV
• Single ion has to be
detected.
CEI
• Molecular target
• Fragment ion energy >1
eV or >>1 eV (depends
on charge state).
• Several fragment ions
are to be detected in
coincidence.
4π solid angle detection of fragment ions with energies
> 1 eV is required for Coulomb Explosion Imaging.
ZOO-RISE Technique
•PSD
•Ring electrodes
•Electro-magnetic coils
•Aluminum plate
•Magnetic field lines

E
•Fragment ions
•Secondary electrons
Trigger
BMCP
K
B plate
Schematic diagram of electric potential
inside drift tube
+2200 V
0V
-300 V
-2200 V
Aluminum
plate
Collision
region
Mesh
electrode
PSD
Electrons, produced at
aluminum plate, can reach
PSD,
while
those,
produced
at
collision
region, are retarded.
Triggering method
T
A
A = Projectile ion signal
B = Fragment ion signal
C = Stretched A
T = Trigger position
B
C
Trigger if (C while B)
This method ensures that
measurement is trigged only if
projectile and fragment(s) are
detected in coincidence
Comparison
BEFORE
a)
AFTER
b)
TOF Coincidence map for Ar8+ + N2 products. a) – conventional mode (fragment ions are detected on
PSD), b) – ZOO-RISE mode (secondary electrons are detected on PSD).
Points to consider
• Magnetic field at Aluminum plate and MCP
surfaces should be as uniform as possible.
Otherwise mapping might not be linear.
• Due to non-vanishing ExB at non-uniform
magnetic field region, secondary electrons
may acquire considerable transverse
velocity component. This will lower
positional resolution.
d [mm]
d [mm]
Positional linearity
TOF2 – TOF1 [ns]
Calculated
TOF2 – TOF1 [ns]
Measured
Summary of benefits
• Larger “detection area” for the same price.
• Improved detection efficiency.
• Photon imaging can be done in the same
way.
• Ion-Ion, Electron-Ion, Photon-Ion, PhotonElectron or Photon-Electron-Ion
coincidence measurement can be done
with single PSD.
PSD
Conventional
PSD
MCP
MBWC
anode
mesh
Electron
avalanche
Front view
Rear view
Electron avalanche should overlap several wedges, to
obtain positional information.
In magnetic field
MCP
MBWC
anode

B
mesh
Ceramic Plate
with resistive
layer
New PSD

B
Can be used in magnetic
field.
MBWC and Resistive plate
y
x
MBWC anode
Resistive plate
The Mask and the Image
a)
b)
Data Analysis
 I1  I 4 1 
x  Sx  
 
 I
2 
n

 I1  I 2 1 
y  Sy 
 
 I
2 
n

I
Pre-onset level
TOFex
p
Doubly charged C6D6
•Only two charged fragments are produced
•Only double coincidence study is necessary
(and possible) to analyze fragmentation.
•Branching ratios and KERs for various
channels can be readily deduced.
•Dissociation scheme can be studied.
[C3Dx -- C3Dy]2+
[C2Dx -- C4Dy]2+
2+
[CD3 -- C5D3]2+
CD3+ + C5D3+
C2Dx+ + C4Dy+
C3Dx+ + C3Dy+
Specifics
• Slow dissociation.
• “Plenty” of time for rearrangement.
• KER values are sensitive to the
intermediate states.
• Intermediate states can be studied.
Experimental conditions
• Projectile pulse duration < 50 ns.
• Maximal energy with 4π collection angle ~
6 eV.
• Projectile = H+ (15 keV) and Ar8+ (120 keV).
H + + C 6 D6
C3D3+ or
(C6D6)2+
D+
CDx+
C2Dx+
C3Dx+
C4Dx+
C5Dx+
H+ + C 6 D6
C2Dx+
D+
CDx+
C3Dx+
C4Dx+
C5Dx+
Molecular Fragments
In each group, each parallel line
corresponds to the different
number of lost D atoms.
H
H
H
H
H
H
CD3+ + C5D3+
H
H
H
H
H
Number of parallel lines
H
H
C2Dx+ + C4Dy+
H
H
H
H
H
C3Dx+ + C3Dy+
Excitation of the parent
ion
• To test our assumption that number of
parallel lines corresponds to the vibrational
excitation of target molecule, Ar8+
projectile was used.
• Charge capture occurs at a larger distance
and direct vibrational excitation would be
smaller.
• Decrease in the number of parallel lines is
expected.
Ar8+ + C6D6
Only three lines
H
H
H
H
H
H
CD3+ + C5D3+
H
Less excited than in
H
H
H+ + C6D6 case
H
H
H
H
C2Dx+ + C4Dy+
H
H
H
H
H
C3Dx+ + C3Dy+
The trend
Why different number of
parallel lines?!
H
H
H
H
H
H
CD3+ + C5D3+
As an excitation increases,
H
H
H
H
H
fragmentation becomes
more and more symmetric.
H
H
C2Dx+ + C4Dy+
H
H
H
H
H
C3Dx+ + C3Dy+
Some similarity with
nuclear fission.
Fragmentation Mechanism
1.
(C6D6)2+
(C6D4)2+ + 2D
If the structure of parent ion
changes
(C2D2)+ + (C4D2)+
(C2D3)+ + (C4D3)+
E1
2.
(C6D6)2+
KER depends on the
number of lost D atoms
E2
(C2D3)+ + (C4D3)+
E2’
ΔE
E1’ + E2’ < E1 + E2
(C4D2)+ + D
E1’ ΔE
(C2D2)+ + D
KER depends on the
number of lost D atoms
KERs
Expected difference of KER should
have been ~ 10% but no difference
is found
H
H
H
H
H
C3D3+ +
C3D3+
H
CD3+ + C5D3+
H
H
H
C3D+ + C3D+
H
H
H
H
C2Dx+ + C4Dy+
H
H
H
H
H
C3Dx+ + C3Dy+
Alternatives
• If D-loss occurs just before dissociation 
No time for rearrangement  KER does
not depend on the number of lost D-s.
• If D-loss occurs just after dissociation 
Fragments have not acquired significant
kinetic energy yet  No kinematic effect of
D-loss  KER does not depend on the
number of lost D-s.
• D-loss occurs ~ during dissociation.
Comparison of KERs
C3Dx+ + C3Dy+
C2Dx+ + C4Dy+
CD3+ + C5D3+
Vibrational excitation leads to the
increased bond lengths in the
molecule  Decreased KERs
a) P.J. Richardson, J.H.D. Eland and P. Lablanquie, Organic Mass Spectrometry 21 (1986) 289-294.
Conclusions
• Increased vibrational excitation leads to
more symmetric fragmentation of (C6D6)2+.
• D-loss occurs during fragmentation
process.
• KER trend for different ionization
mechanism is consistent with the nature of
excitation.
Multiply charged C6H6
•
•
•
•
Faster dissociation
More Coulombic behavior
More charged fragments are available
Triple coincidence study is possible
CEI Setup
PSD for
fragment ions
Triggering probability

E3  0

E2

E1
Trigger
Number of ejected
Auger electrons
Number of captured
electrons
Charge state of the
target molecule
High charge states of
target are preferentially
detected
Experimental conditions
• Continuous projectile beam.
• Maximal energy with 4π collection angle >
20 eV.
• Projectile = Ar8+ (120 keV).
Planarity test

n12
123

v1

v2
  
 
v1  v2   v3  n12  v3 cos123

v3
n12 is perpendicular to v1 and v2

when v3 is coplanar to v1 and v2,
it will be perpendicular to n12
cos θ123 = 0
v1, v2 and v3 are velocity
vectors of first, second and
third fragment ion, detected
in coincidence
Results
C2H4
-1
-0.5
0
Cos H C 2 H 
Planar
C2H6
0.5
1
-1
-0.5
0
Cos H C 2 H 
Non-planar
C6H6
0.5
1
Cos H  C 2C 
Planar
For planar molecules, velocity vectors of fragments are
also co-planar.
Charge state estimation
Measured
Simulation
Charge states higher than 8+ are
mainly populated in collisions.
Conclusions
• Coulomb explosion Imaging of highly
charged benzene was successfully done
for the first time.
• Quite sensitive tool to explore molecular
geometry.
• Might find application in isomer
identification.
Angle between velocity vectors
F
F
F
F
F
M. Nomura et. al. Int. J. Mass Spectrom. 235 (2004) 43-48
F
General conclusions
• Compact type of PSD, usable in magnetic field,
was developed.
• New type of position-sensitive TOF analyser,
nick-named ZOO-RISE, was developed and
constructed.
• Fragmentation of doubly charged benzene was
studied and fragmentation-excitation trend was
identified.
• Coulomb Explosion Imaging was applied to the
highly charged benzene and planar-nonplanar
molecule distinction was made on the basis of
coincident velocity vector correlation.
おわり
Positional resolution
y
x
-1.0
-0.6
-0.2
0.2
0.6
1.0
-1.0
I ( x)  a  b 0.52  xi2 e ( x xi )
xi
-0.6
-0.2
0.2
0.6
1.0
FWHM:
2
/d
2
Δx = 250μm
Δy = 140μm
Position Calibration
Edge of the
aluminum plate
Rexp
Rreal
K
Rexp
Center of
symmetry
In our experiments
K = 2.66
Typical image on PSD, when Helmholtz coils
are switched off.
TOF and extraction
voltage adjustments
TOFreal
2L
2 L2 m


a
qU
TOF1exp  t
TOF2 exp
m1 q2


 t
m2 q1
2mL2
U
q  TOF 2
TOF1real
m1 q2


TOF2 real
m2 q1
TOF1exp 
t 
m1 q2
  TOF2 exp
m2 q1
m1 q2
 1
m2 q1
Branching ratios and
KERs
H+
Ar8+
hνa
Ratio
(%)
KER
(eV)
Ratio
(%)
KER
(eV)
C+ + C5+
21.6
2.4
21.4
2.8
27.4
3.0
C2+ + C4+
40.8
2.8
40.6
3.3
37.5
3.8
C3+ + C3+
37.4
2.9
36.8
3.5
35.0
4.2
Ratio (%) KER (eV)
a) P.J. Richardson, J.H.D. Eland and P. Lablanquie, Organic Mass Spectrometry 21 (1986) 289-294.
θ and cos θ
The probability that the angle
between two vectors is
between θ and θ+dθ in three
dimensional space is
dP  2  p  sin   d
Histogram of θ will be
dP / d  2  p( )  sin 
d cos 
d  
sin 
dPcos   2  p   d cos
Histogram of cosθ will be
dPcos / d cos   2  p( )
Multiply charged C6H4F2
• Isomers can not be distinguished by massspectrometric methods alone.
• Coulomb Explosion Imaging makes possible to
calculate initial velocity vectors of fragment ions.
• If fragmentation is fast enough (Coulomb
explosion), velocity vectors might reflect initial
geometry of parent molecule.
• Different isomers are expected to have different
velocity vector correlation.
M. Nomura et. al. Int. J. Mass Spectrom. 235 (2004) 43-48
C6H4F2-o
F
F
H+
C2+
Coincidence island is parallel
to bissectrice  two F+ ions
are emitted in almost same
direction.
C+
F+
C2+
C6H4F2-m
F
F
H+
C2+
C+
F+
C2+
Non-linear shape of
coincidence island
Emission directions of two
F+ ions are not correlated.
C6H4F2-p
F
F
H+
C2+
C+
F+
C2+
Coincidence island is
perpendicular to
bissectrice  two F+
ions are emitted in
almost opposite direction.
Acknowledgements
• My supervisors, Prof. N. Kobayashi and Prof. H.
Shiromaru.
• Dr. T. Nishide and Mr. T. Kitamura for the help in
development of new PSD.
• Mr. M. Nomura and Dr. Matsumoto for the help in
construction of ZOO-RISE.
• Dr. F. A. Rajgara, Dr. A. Reinköster, Ms. Y. Takeda, Mr. R.
Hatsuda and Mr. T. Matsuoka for collaboration during
experiments.
• Members of Atomic Physics and Physical Chemistry
groups.
• Monbusho, for initial support of my research.