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

Outline 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 ZOO-RISE setup 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 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.

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.

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

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

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

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).

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.

Positional linearity Calculated Measured d [mm] d [mm] TOF2 – TOF1 [ns] TOF2 – TOF1 [ns] Calculated 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, Photon-Electron or Photon-Electron-Ion coincidence measurement can be done with single PSD.

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

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

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

MBWC and Resistive plate y x Resistive plate MBWC anode

The Mask and the Image a) b)

Data Analysis I Pre-onset level TOFexp

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+ + C6D6 C3D3+ or (C6D6)2+

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

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+

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

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.

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

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

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 Number of ejected Auger electrons Number of captured electrons Charge state of the target molecule High charge states of target are preferentially detected Trigger

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

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

Results C2H4 C2H6 C6H6 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 Planar Non-planar Planar For planar molecules, velocity vectors of fragments are also co-planar.

Charge state estimation Simulation Measured 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 M. Nomura et. al. Int. J. Mass Spectrom. 235 (2004) 43-48

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 -1.0 -0.6 -0.2 0.2 0.6 1.0 x y -1.0 -0.6 -0.2 0.2 0.6 1.0 FWHM: Δx = 250μm Δy = 140μm

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.

TOF and extraction voltage adjustments

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) 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 Histogram of θ will be Histogram of cosθ will be

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

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

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

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

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.