1. Giorgi Veshapidze, Haruo Shiromaru Tokyo Metropolitan University
Application of COLTRIMS to Study Collision Induced Dissociation of Multiply Charged Benzene
Giorgi Veshapidze, Haruo Shiromaru
Tokyo Metropolitan University

2. Outline Motivation Experimental method and apparatus
Data handling methodology
Doubly charged benzene
Multiply charged benzene
Multiply charged difluorobenzene
Summary and conclusions

3. 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.

4. TMU-ECRIS facility
ZOO-RISE setup
CEI setup

5. 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

6. RIMS principle PSD for recoil ions x, y and TOF
Ions, with kinetic energy more than εmax can not be detected with 4π solid angle.
q = Ion charge
E = Extraction field strength
L = Flight length
R = PSD radius
Increasing E or decreasing L shortens TOF, thus reducing time resolution.
Initial velocity vectors are calculated by simple, classical-mechanical equations.

7. Differences between RIMS and CEI
Atomic target
Recoil ion energy < 1 eV
Single ion has to be detected.
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.

8. ZOO-RISE Technique PSD Ring electrodes Electro-magnetic coils
Aluminum plate
Magnetic field lines
Fragment ions
Secondary electrons
Trigger

9. Schematic diagram of electric potential inside drift tube
+2200 V
0 V
-300 V
-2200 V
Electrons, produced at aluminum plate, can reach PSD, while those, produced at collision region, are retarded.
Aluminum plate
Collision region
Mesh electrode
PSD

10. Triggering method Trigger if (C while B) T A = Projectile ion signal
B = Fragment ion signal
C = Stretched A
T = Trigger position A
Trigger if (C while B) B
This method ensures that measurement is trigged only if projectile and fragment(s) are detected in coincidence C

11. Comparison BEFORE AFTER a) 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).

12. 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.

13. Positional linearity Calculated Measured d [mm] d [mm]
TOF2 – TOF1 [ns]
TOF2 – TOF1 [ns]
Calculated
Measured

14. 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, Photon-Electron or Photon-Electron-Ion coincidence measurement can be done with single PSD.

15. PSD Conventional PSD MCP MBWC anode mesh Electron avalanche
Developed new PSD

16. Front view
Rear view
Electron avalanche should overlap several wedges, to obtain positional information.

17. MCP
In magnetic field
MBWC anode
mesh
Ceramic Plate with resistive layer
New PSD
Can be used in magnetic field.

18. MBWC and Resistive platey x
Resistive plate
MBWC anode

19. The Mask and the Image a) b)

20. Data Analysis I
Pre-onset level
TOFexp

21. 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.

22. [C3Dx -- C3Dy]2+
[C2Dx -- C4Dy]2+ 2+
[CD3 -- C5D3]2+
CD3+ + C5D3+
C2Dx+ + C4Dy+
C3Dx+ + C3Dy+

23. Specifics Slow dissociation. “Plenty” of time for rearrangement.
KER values are sensitive to the intermediate states.
Intermediate states can be studied.

24. Experimental conditions
Projectile pulse duration < 50 ns.
Maximal energy with 4π collection angle ~ 6 eV.
Projectile = H+ (15 keV) and Ar8+ (120 keV).

25. H+ + C6D6
C3D3+ or
(C6D6)2+

26. H+ + C6D6 C5Dx+ C4Dx+ C3Dx+ C2Dx+ CDx+ D+ C3Dx+ C4Dx+ C2Dx+ D+ C5Dx+

27. Molecular Fragments
In each group, each parallel line corresponds to the different number of lost D atoms.
Number of parallel lines
CD3+ + C5D3+
Molecular fragments.
Meaning of parallel lines.
Deprotonation level=Excitation level.
Excited fragments=Excited parent.
Parent excitation=Dissociation Channel.
C2Dx+ + C4Dy+
Excitation of the parent ion
C3Dx+ + C3Dy+

28. 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.

29. Ar8+ + C6D6 Only three lines Less excited than in H+ + C6D6 case
CD3+ + C5D3+
C2Dx+ + C4Dy+
C3Dx+ + C3Dy+
Only three lines
Less excited than in
H+ + C6D6 case

30. The trend Why different number of parallel lines?!
CD3+ + C5D3+
C2Dx+ + C4Dy+
C3Dx+ + C3Dy+
Why different number of parallel lines?!
As an excitation increases, fragmentation becomes more and more symmetric.
Nuclear fission
Some similarity with nuclear fission.

31. Fragmentation Mechanism
1. (C6D6)2+ (C6D4)2+ + 2D
(C2D2)+ + (C4D2)+
(C2D3)+ + (C4D3)+
2. (C6D6)2+ (C2D3)+ + (C4D3)+
(C4D2)+ + D
(C2D2)+ + D
If the structure of parent ion changes
KER depends on the number of lost D atoms E1 E2
Expected behavior of KERs for these two cases.
E1’ + E2’ < E1 + E2
E2’ ΔE
E1’ ΔE
KER depends on the number of lost D atoms

32. KERs
CD3+ + C5D3+
C2Dx+ + C4Dy+
C3Dx+ + C3Dy+
Expected difference of KER should have been ~ 10% but no difference is found
C3D3+ + C3D3+
C3D+ + C3D+

33. 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.

34. 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)

35. 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.

36. Multiply charged C6H6 Faster dissociation More Coulombic behavior
More charged fragments are available
Triple coincidence study is possible

37. CEI Setup PSD for fragment ions Triggering probability
Number of ejected Auger electrons
Number of captured electrons
Charge state of the target molecule
High charge states of target are preferentially detected
Trigger

38. Experimental conditions
Continuous projectile beam.
Maximal energy with 4π collection angle > 20 eV.
Projectile = Ar8+ (120 keV).

39. Planarity test 123 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

40. Results
C2H4
C2H6
C6H6
Planar
Non-planar
Planar
For planar molecules, velocity vectors of fragments are also co-planar.

41. Charge state estimation
Simulation
Measured
Charge states higher than 8+ are mainly populated in collisions.

42. 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.

43. Angle between velocity vectorsF F F
M. Nomura et. al. Int. J. Mass Spectrom. 235 (2004) 43-48

44. 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.

45. おわり

46. Positional resolution x y
FWHM:
Δx = 250μm
Δy = 140μm

47. Position Calibration Rexp In our experiments K = 2.66
Edge of the aluminum plate
Rexp
Center of symmetry
In our experiments K = 2.66
Typical image on PSD, when Helmholtz coils are switched off.

48. TOF and extraction voltage adjustments

49. Branching ratios and KERs
Ar8+
hνa
Ratio (%)
KER (eV)
C+ + C5+
21.6
2.4
21.4
2.8
27.4
3.0
C2+ + C4+
40.8
40.6
3.3
37.5
3.8
C3+ + C3+
37.4
2.9
36.8
3.5
35.0
4.2
a) P.J. Richardson, J.H.D. Eland and P. Lablanquie, Organic Mass Spectrometry 21 (1986)

50. θ and cos θ
The probability that the angle between two vectors is between θ and θ+dθ in three dimensional space is
Histogram of θ will be
Histogram of cosθ will be

51. Multiply charged C6H4F2
Isomers can not be distinguished by mass-spectrometric 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

52. C6H4F2-o F
Coincidence island is parallel to bissectrice  two F+ ions are emitted in almost same direction. H+ C+
C2+ F+
C2+

53. C6H4F2-m F
Non-linear shape of coincidence island Emission directions of two F+ ions are not correlated. H+ C+
C2+ F+
C2+

54. C6H4F2-p F
Coincidence island is perpendicular to bissectrice  two F+ ions are emitted in almost opposite direction. H+ C+
C2+ F+
C2+

55. 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.