Application of COLTRIMS to Study Collision Induced Dissociation of Multiply Charged Benzene

Giorgi Veshapidze, Haruo Shiromaru Tokyo Metropolitan University

OutlineMotivationExperimental method and apparatusData handling methodologyDoubly charged benzeneMultiply charged benzeneMultiply charged difluorobenzeneSummary and conclusions

MotivationBenzene Building block of many organic moleculesLow charge fragmentation Access to intermediate statesHigh charge fragmentation Access to initial GeometryBy varying the charge state and excitation, useful information might be gained.

TMU-ECRIS facilityZOO-RISE setupCEI setup

CEI = Coulomb Explosion ImagingZOO-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 principlePSD for recoil ionsq = Ion chargeE = Extraction field strengthL = Flight lengthR = PSD radiusIncreasing E or decreasing L shortens TOF, thus reducing time resolution.Ions, with kinetic energy more than max can not be detected with 4 solid angle.Initial velocity vectors are calculated by simple, classical-mechanical equations.x, y and TOF

Differences between RIMS and CEIAtomic targetRecoil ion energy < 1 eV

Single ion has to be detected.Molecular targetFragment ion energy >1 eV or >>1 eV (depends on charge state).Several fragment ions are to be detected in coincidence.4 solid angle detection of fragment ions with energies > 1 eV is required for Coulomb Explosion Imaging.RIMSCEI

ZOO-RISE TechniqueTrigger

Schematic diagram of electric potential inside drift tube-2200 V+2200 V-300 V0 VAluminum plateCollision regionMesh electrodePSDElectrons, produced at aluminum plate, can reach PSD, while those, produced at collision region, are retarded.

Triggering methodABCTrigger if (C while B)A = Projectile ion signal B = Fragment ion signalC = Stretched AT = Trigger positionThis method ensures that measurement is trigged only if projectile and fragment(s) are detected in coincidenceT

ComparisonTOF 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).BEFOREAFTERa)b)

Points to considerMagnetic 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 linearityTOF2 TOF1 [ns]d [mm]d [mm]TOF2 TOF1 [ns]CalculatedMeasured

Summary of benefitsLarger 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.

PSDMBWC anodeMCPElectron avalanche mesh

Front viewRear viewElectron avalanche should overlap several wedges, to obtain positional information.

MCPCeramic Plate with resistive layer meshIn magnetic fieldMBWC anodeCan be used in magnetic field.

MBWC and Resistive platexyResistive plate

MBWC anode

The Mask and the Imageb)a)

Data AnalysisTOFexpIPre-onset level

Doubly charged C6D6 Only two charged fragments are producedOnly 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.

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

SpecificsSlow dissociation.Plenty of time for rearrangement.KER values are sensitive to the intermediate states.Intermediate states can be studied.

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

H+ + C6D6C3D3+ or(C6D6)2+

H+ + C6D6D+CDx+C2Dx+C3Dx+C4Dx+C5Dx+D+CDx+C2Dx+C3Dx+C4Dx+C5Dx+

Molecular FragmentsCD3+ + C5D3+C2Dx+ + C4Dy+C3Dx+ + C3Dy+In each group, each parallel line corresponds to the different number of lost D atoms.Number of parallel lines

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+ + C6D6Only three linesLess excited than inH+ + C6D6 case

The trendAs an excitation increases, fragmentation becomes more and more symmetric.Why different number of parallel lines?!Some similarity with nuclear fission.

Fragmentation Mechanism1.(C6D6)2+(C6D4)2+ + 2D

(C2D2)+ + (C4D2)+

(C2D3)+ + (C4D3)+

2.(C6D6)2+(C2D3)+ + (C4D3)+

(C4D2)+ + D

(C2D2)+ + DE1E2E1E2EEE1 + E2 < E1 + E2KER depends on the number of lost D atomsKER depends on the number of lost D atomsIf the structure of parent ion changes

KERsExpected difference of KER should have been ~ 10% but no difference is foundC3D3+ + C3D3+C3D+ + C3D+

AlternativesIf 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 KERsCD3+ + C5D3+C2Dx+ + C4Dy+C3Dx+ + C3Dy+a) P.J. Richardson, J.H.D. Eland and P. Lablanquie, Organic Mass Spectrometry 21 (1986) 289-294. Vibrational excitation leads to the increased bond lengths in the molecule Decreased KERs

ConclusionsIncreased 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 dissociationMore Coulombic behaviorMore charged fragments are availableTriple coincidence study is possible

CEI SetupPSD for fragment ionsTriggerHigh charge states of target are preferentially detectedTriggering probabilityNumber of ejected Auger electronsNumber of captured electronsCharge state of the target molecule

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

Planarity test123v1, v2 and v3 are velocity vectors of first, second and third fragment ion, detected in coincidencen12 is perpendicular to v1 and v2when v3 is coplanar to v1 and v2, it will be perpendicular to n12cos 123 = 0

ResultsC2H4C2H6C6H6PlanarNon-planarPlanarFor planar molecules, velocity vectors of fragments are also co-planar.

Charge state estimationMeasuredSimulationCharge states higher than 8+ are mainly populated in collisions.

ConclusionsCoulomb 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 vectorsM. Nomura et. al. Int. J. Mass Spectrom. 235 (2004) 43-48

General conclusionsCompact 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.

Positional resolutionFWHM:x = 250m y = 140m

Position CalibrationCenter of symmetryEdge of the aluminum plateTypical image on PSD, when Helmholtz coils are switched off.RexpIn our experiments K = 2.66

TOF and extraction voltage adjustments

Branching ratios and KERsa) P.J. Richardson, J.H.D. Eland and P. Lablanquie, Organic Mass Spectrometry 21 (1986) 289-294.

and cos The probability that the angle between two vectors is between and +d in three dimensional space isHistogram of will beHistogram of cos will be

Multiply charged C6H4F2Isomers 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-oH+C2+C+F+C2+Coincidence island is parallel to bissectrice two F+ ions are emitted in almost same direction.

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

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

AcknowledgementsMy 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. Reinkster, 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.

Developed new PSDMolecular fragments.Meaning of parallel lines.Deprotonation level=Excitation level.Excited fragments=Excited parent.Parent excitation=Dissociation Channel.Nuclear fissionExpected behavior of KERs for these two cases.