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ANewElectron‐IonCoincidence3DMomentum‐ImagingMethodandItsApplicationinProbingStrongFieldDynamicsof2‐Phenylethyl‐N,N‐Dimethylamine
LinFan1,SukKyoungLee1,Yi‐JungTu1,BenoîtMignolet2,DavidCouch3,KevinDorney3,QuynhNguyen3,LauraWooldridge3,
MargretMurnane3, Françoise
Remacle2,H.BernhardSchlegel1andWenLi1*
1DepartmentofChemistry,WayneStateUniversity,Detroit,48202
2DepartmentofChemistry,B6c,UniversityofLiege,B4000Liege,Belgium
3JILAandUniversityofColoradoatBoulder,Boulder,CO,[email protected]
We report thedevelopmentof a new three‐dimensional
(3D)momentum‐imagingsetup based on conventional velocity map
imaging (VMI) to achieve
coincidencemeasurementofphotoelectronsandphoto‐ions.This
setupusesonlyone
imagingdetector(microchannelplates/phosphorscreen)butthevoltagesonelectrodesarepulsedtopushbothelectronsandionstowardthesamedetector.Theion‐electroncoincidenceisachievedusingtwocamerastocaptureimagesofionsandelectronsseparately.
The time‐of‐flight (TOF) of ions and electrons are read out
fromMCPusing a digitizer. We demonstrate this new system by
studying the dissociativesingle and double ionization of PENNA
(2-phenylethyl-N,N-dimethylamine).
Wefurthershowthecamera‐based3Dimagingsystemcanoperateat10kHzrepetitionrate.
Introduction
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The coincidence detection of photoelectrons and photoions
arising from a singleatom/molecule in gas phase is a powerful tool
for untangling multi‐channelionization/dissociation dynamics. The
early implementation of coincidencetechnique only provided energy
information of each particle using time‐of‐flightmethods.1‐4
Various position‐ and time‐sensitive detectors were introduced
tomeasureboththepositionandtheTOFandthusenabled3Dmomentumdetectionof
all particles.5‐12 Successful coincidence detection requires the
count rate of theexperiment to be less than one event per driving
pulse (electron/ion/photon) tosuppress false coincidence events.
Velocity mapped imaging13‐15 was not
initiallydevelopedtoachievecoincidencedetectionbecauseitdidn’tprovidehighresolutionTOF
informationof individualparticles. Instead, itusedan
imagingdetectorandacamera to measure the positions of particles
with a relative large TOF range(nanosecond to one microsecond),
which identifies the mass of the
particles.Nonetheless,VMIhasbecomeaverypopularmethodinreactiondynamics,ultrafastspectroscopy
and other fields, because of its relatively easy implementation
andsimultaneousmeasurementofenergyandangulardistributionofchargeparticles.Recently,
a new type of VMI imaging system was developed to achieve
3Dmomentum detectionwith a conventional imaging detector.16,17 In
this system, astandard video camera was replaced with a fast frame
CMOS camera
(>1kFrames/s)andthecameraexposureissynchronizedwiththelaserpulses(>1kHz)togetherwithawaveformdigitizer,whichreadsouttheMCPpulsesignal.Inthesecoincidenceexperiments,
because the count rate is lowso that a true
coincidencebetweenhitpositionsfromthecameraandTOFsextractedfromthedigitizercanbe
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establishedtoyieldthreecoordinates(X,Y,
t)requiredfor3Dmomentum(Px,Py,Pz)imaging.Thedemonstratedtimeresolutionforthissystemisexcellent(~30ps).With
this system, sliced velocitymapping of photoelectronswas achieved
for thefirst timewith an imagingdetector. Itwas also demonstrated
that photoelectron‐photoion coincidence could be achieved by
accelerating electrons and ionssimultaneously
inadouble‐sidedVMIsetup towards two imagingdetectorsat theopposite
ends of the spectrometer and applying the same 3D
measurementscheme.18 However, a conventional VMI apparatus features
only one
imagingdetectorandauni‐directionalspectrometer.Isitpossibletoconvertsuchapparatustoaphotoelectron‐photoioncoincidence
imagingapparatus? In
thiswork,wewillshowthatthisispossibleandthesolutionisquitesimplewiththecamera‐based3Dimagingsystem.
PENNAisabifunctionalmoleculewitha‐CH2‐CH2‐bridgelinkingthechromophoregroup(phenyl)andaminegroup.Inthepastdecade,therehavebeengreatinterestsinstudyingthephotoionizationdynamicsofPENNA.ThisismainlybecausePENNAis
oneof the firstmolecules thatwere identified to support anew typeof
chargemigrationprocessthattakesplaceatanextremelyshorttimescale(afewhundredsof
attoseconds to a few femtoseconds) 19‐24. This charge transfer
arises from
acoherentsuperpositionofafewelectronicstatesofthecationanditoccursbeforeany
significant nuclear motion. Resonant two-photon ionization (R2PI)
and
RydbergFingerprintSpectroscopyhavebeenusedtostudytheslowerintramolecularchargetransfer
(~80 fs) at different laser wavelengths. 19,22,25,26 However, such
studieswere limited to tens of femtosecond time resolution. A
strong field IR‐pump‐IR‐
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probemethodhasbeenproposedtoobservetheultrafastchargemigrationwitharesolution
of few femtoseconds.24 So far experimental investigations on
thephotoionization dynamics of PENNA under intense laser field have
been
scarce.Furthermore,thedynamicsofdissociationfollowingionization(singleanddouble)are
crucial for measuring molecular/recoil frame ionization rates
because theyshowwhether the axial recoil approximation is valid or
not.With the axial
recoilapproximation,therecoil‐frameionizationratemeasurementismuchsimplerthanthosemethodsthatrequirepre‐alignmentofmolecules.Here,weappliedournew3D
electron‐ion coincidence technique to study the strong
fieldionization/dissociationofPENNAwithonelaser.Wehaveidentifiedthedissociationpathways
following the strong field double ionization of PENNA. This result
willprovidebackgroundinformationforfuturetime‐resolvedstudies.
Experimentalsetupandcomputationalmethods
Experimentalsetup
AnamplifiedTi:Sapphire femtosecond lasersystemwasusedto
ionizethePENNAmoleculesinmid‐intensitystrongfield(~8×1013W/cm2).Thewavelengthwas800nm
and the repetition ratewas1500Hz. The ultrashort laser pulse (~30
fs)waslinearly polarized along the time‐of‐flight axis. For the
one‐camera setting, theexperiment was carried out with circularly
polarized laser. PENNA (purity
98%)waspurchasedfromSigma‐Aldrichanditwasseededinheliumtobesentintothesourcechamberbyagas
jetwith20μmorifice.ThePENNAsamplewasheated to~30°Candthegas
jetwasheatedto~70°Cfromoutsideofthesourcechamber.A
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skimmer with 0.5 mm orifice was used to skim the center part of
the
expandedmolecularbeambeforeitenteredthemainchamber.Thelaserbeamwassentintothemainchamberperpendicularlywith
themolecularbeamandthe time‐of‐flightaxis, and itwas focusedonto
themolecular beamby a concavemirror (f =5
cm)insidethemainchamber.Afour‐plateelectron‐ionopticswasbuilttovelocitymaptheelectronsandionsarisingfromstrong‐fieldionization.AshortTOFlength(~10cm)
was adopted to allow detection of high‐energy electrons. The
timings
ofelectronsandionshittingthedetectorwerepickedofffromthefrontMCPplatebyahigh‐speeddigitizer(NationalInstruments,PXIe5162)throughasignal‐decouplingcircuit.
The maximum sampling rate of the digitizer is 5 GHz. The positions
ofelectrons hitting the detector were recorded from the phosphor by
acomplementary metal‐oxide semiconductors (CMOS) camera (Basler,
acA640‐750μm).Thiscamera(e‐)wassetat~65cmawayfromthedetector,pointingatthecenter
of phosphor screen with a normal angle. Another CMOS camera
(XIMEA,MQ013MG)was employed to record the ionpositions. Itwas
located at~120
cmawayfromthedetectorandslightlyoff‐centerofthescreen.Thiscamera’spointingdirectionhada~6°angledeviation
to thescreennormal.The justification for
theuseoftwocamerasisdiscussedintheResultsandDiscussionsection.AschematicoftheexperimentalsetupisshowninFigure1(A).
Computationalmethods
ElectronicstructurecalculationswerecarriedoutwiththedevelopmentversionofGaussian27
using the wB97XD functional.28 Relaxed potential energy scans
were
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executed with the 6‐31G(d) basis set.29, 30 The geometries of
neutral PENNA,PENNA+, and PENNA2+, transition states, products were
further optimized by
6‐311++G(d,p)basissetandtheirSCFenergieswerealsocalculatedwiththisbasisset.All
optimized structures were checked by normal mode vibrational
analysis,
andwavefunctionsweretestedforSCFstability.ForPENNAdications,theidentitiesoftheopen‐shellsingletandtripletelectronicconfigurationswereconfirmedbyspin‐squaredexpectationvalues()andspindensitypopulations.GaussView31wasusedtovisualize
isodensityplotsof
thespinpopulations(isovalue=0.004au).ToexplorethepotentialenergysurfacesfordissociationofPENNA2+,relaxedpotentialenergysurfacescanswereperformedbystretchingtheC–Cbondandoptimizingtheremainingcoordinates.Thetransitionstatestructuresontheopen‐shellsingletandtriplet
surfaces of the dication were confirmed to have only one
imaginaryfrequencybyvibrationalmodeanalysis.Toexploretheclosed‐shellsingletpotentialenergy
surface of the dication,we startedwith ground state geometry of
PENNA,vertically ionized to the closed‐shell singlet state of
PENNA2+ and tracked
thedissociationusingDampedVelocityVerlet(DVV)reactionpathfollowing.Uptothetransition
state, the closed shell singlet dication calculations had a
closed‐shell toopen‐shell instability and the wavefunction
optimized to the lower energy open‐shellsingletdication.
The photoelectron spectrumhas been computed bymodeling the
photoexcitationand photoionization dynamics induced by a 22 fs
800nm IR pulse with a fieldstrength of 1012W/cm2 in the PENNA
molecule at a frozen geometry. The
time‐dependentSchrödingerequationwasnumericallyintegratedusingabasisofneutral
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andionizedelectronicstates.24,32Theelectronicstructuresofthelowest30neutralstates
were computed at the TDDFT wB97xD/6–311++G(d,p) level. A
densemanifold of excited states is required to accurately describe
the multiphotonexcitation and ionization of the PENNA ground state.
The ionized states
aredescribedastheanti‐symmetrizedproductofthefield‐freecationicelectronicstatesand
the wavefunction of the ionized electron described by a plane wave.
Thisapproximation can affect the photoelectron spectrum, especially
at low
kineticenergybecausetheinteractionbetweentheionizedelectronandthecationiccoreisneglectedandoverthebarrierandtunnelionizationarenotaccountedfor.ThetenlowestcationicstatescomputedatthewB97xD/6–311++G(d,p)level.Theionizationcontinuaarediscretizedbothinenergy(from0to25eV)andangulardistribution.Intotal,
thereare18000ionizedstatesand30neutralstatesinthebasis.Toaccountfortherandomorientationofthemolecules,wecomputedthedynamicsforasetof50
randomly oriented molecules. From the amplitudes of the ionized
states,
wecomputedthephotoelectronspectrumaveragedovertheorientation.
ResultsandDiscussion
Achieving3Dcoincidencemeasurementwithasingleimagingdetector
ToupgradeaconventionalVMItoacoincidence3Dmomentumimagingapparatus,we
need to address two issues associated with it. The first issue is
the
uni‐directionalspectrometerofaconventionalVMIapparatus.Itcannormallyberunineither
photoelectron or photoion mode by applying different voltages on
theelectrodes.To imageboth ionsandelectronsat thesamedetector,
thevoltagesof
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electrodesshouldbeswitchedveryquicklytoacceleratebothparticlestowardthesamedirection.Thishasbeendemonstratedby
Janssenandcoworkersbypulsingthe electrodes and employing a delay
line detector.33 Owing to the large
massdifferencebetweenelectronsandions,theelectroncanmaintainthesameimagingcondition
as with non‐pulsing electrodes while ions suffer minimum
momentumblurring (SIMIONsimulation~1%).Once ions andelectronsarrive
at the imagingdetector, they both produce flashes on the phosphor
screen, which are
thencapturedbythecamerainthesameframe.However,howtoassociatethepositionsandmeasured
TOFs is not trivial. Previously, in either ion or electronmode,
thebrightnessofthecameraflashshowsastrongcorrelationwiththeintensityoftheTOFpeakinthedigitizedwaveform.16Thiswasexploitedtoassociatethepositionsand
TOFs in a multi‐hit event. In Figure 1(B), it is shown that because
thecorrelationslopesaredifferentforelectronsandions,thisschemecanleadtosomemis‐assignment
of ions in the electron image (or vice versa) when
processingframescontainingbothelectronsandions.
Figure 1. (A) The schematic of the two‐camera VMI setup that is
capable ofcoincidence 3D momentum‐imaging. The electrodes are
pulsed to push both
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electronsand ions toward the imagingdetector.
(B)Withaone‐camerasetup, theseparation between electrons and ions
are not complete. The main features arephotoelectrons from strong
field ionization of PENNA using circularly polarizedlight.
The solution to this is to addone additional camera. Because the
largedifferencebetweentheTOFsoftheelectrons(1us),onecameracanbetriggeredtoonlyexposeforthefirst200nsafterthelaserpulsetocaptureelectronswhilethesecondcamerastartstoexposeafter500nstoonlyimageions.Withthisconfiguration(Figure1(A)),theassociationbetweenthetimeandpositionof
the charged particles becomes quite easy and self‐evident: the
positions ofparticles
intheelectroncameraareassociatedwiththeelectronTOFswhilethosepositions
on the ion camera with the ion TOFs. Both TOFs are measured
bydigitizing theMCPoutputwith a single high‐speeddigitizer. If
there
aremulti‐hiteventssuchasindissociativedoubleionization,thebrightness‐intensitycorrelationcannowbeappliedtoeachcameraframeseparately,asshownpreviouslyineitherelectron
or ion detectionmode.With this new scheme, we can achieve
completeseparation of electrons and ions and this enables
ion‐electron coincidence 3Dimaging measurement for the first time
using a single imaging detector. Weestimated thespatialand temporal
resolutions tobe6%and1% for
ionsand3%and3%forelectrons,respectively.InFigures2and4weshowedtheresultsofion‐electrondoublecoincidencemeasurementofPENNAsingleionizationandelectron‐ion‐iontriplecoincidenceofPENNAdissociativedoubleionization.
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SingleionizationofPENNA
Previous ion mass spectrum of PENNA in an intense laser field
showed
thatdimethylaminemonocation(N(CH3)2CH2+,mass58)isthemajorproductwhiletheyieldofparentionismuchlowerthanN(CH3)2CH2+(~0.05),whichsuggestedmostparent
cations are unstable.34 Figure 2(A) shows the momentum distribution
ofN(CH3)2CH2+intheplaneperpendiculartothelaserpolarizationwhileFigure2(B)and
(C) show the momentum distributions of electron in coincidence
withN(CH3)2CH2+ in the plane of perpendicular and parallel to the
laser polarization,respectively.Theelectronenergydistribution
isshowninFigure3(A). Ithasbeenshowed previously that the single
ionization of PENNA could populate threedifferentelectronic
statesD0,D1andD2,with thedifference in
ionizationenergiesbeinglessthanonephotonenergy(1.6eV).34ThedissociationsoftheexcitedstatesD1andD2arethroughaseriesofconicalintersectionstotheD0stateandthemainproductisN(CH3)2CH2+.Thekineticenergyrelease(KER)ofionmatchedwellwiththatofcalculations.34InFigure3(A),weshowtheelectronenergydistributionandits
comparison with the photoelectron spectrum resulting from frozen
geometrysimulationsoftheelectrondynamicsofrandomlyorientedmolecules,whichtakesintoaccountthemultiphotonexcitationandionizationbythelaserpulse,formoremethoddetails
see ref. 24Theagreement isgood forhighkineticenergyelectrons(above
4 eV). The discrepancy at low kinetic photoelectron energy can
beunderstood from the limitations of themodel used in the
simulations. Themodelonly includes themultiphoton photoionization
process and neglects the
Coulombinteractionbetweenionizedelectronandioniccore,whichisknowntoproducelow
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energy electrons by over‐the‐barrier or tunneling ionization.
This result furthersuggests that in strong field ionization, the
photoelectron spectrum alone
isinsufficienttoidentifytheproducedcationelectronicstates.Acompletetheoreticalmodeling
of the strong field photoionization of big molecules such as PENNA
iscurrentlyoutofreach.
Figure 2. (A) XY momentum distribution of dimethyl amine
monocation(N(CH3)2CH2+). (B) XY momentum distribution of
photoelectrons in
coincidencewithdimethylaminemonocation(N(CH3)2CH2+).(C)Ytmomentumdistribution(tisthe
TOF axis) of coincidence photoelectron in coincidence with dimethyl
aminemonocation(N(CH3)2CH2+).
With the electron‐ion coincidence measurement capability, we can
produce thecorrelation map between the KER of the dissociated
fragments and the electronkinetic energy (eKE) and this is shown in
Figure 3(B). Interestingly, an
apparentcorrelationcanbeseen(thediagonalstructure).However,becausetheenergyscaleis
different between the KER and eKE, it is unlikely that an energy
conservationmechanism (as in single photon ionization) is at play.
Upon further investigation,such a structure persists even for
non‐dissociative channels such aswater single
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ionization,whichsuggeststheobservedfeatureisnottruecorrelation.However,wedid
observe significant difference in momentum distributions of
electrons incoincidence with monocations and dissociative dications
(see Figure 2(C)
andFigure4(C)).Thisvalidatesourmethodfordetectingcoincidenceevents.
Figure 3. (A) The photoelectron spectroscopy of single
ionization (black) andcomparison with theoretical simulations
(red). (B) The energy correlation
mapbetweenionKERandelectroneKE.
DoubleionizationanddissociationdynamicsofPENNA
PENNAdicationswerenotobservedinthemassspectrumandthissuggeststhatalldications
dissociate after production by the strong laser field. Triple
coincidence(ion‐ion‐electron)wasusedtoidentifythedissociationproducts.Wecouldemployquadrupole
coincidence (ion‐ion‐electron‐electron)because the currentmethod
iscapable of highly efficient detection of two electrons35.
However, two‐electronmeasurements do not provide further insight to
the scope of this study while
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require much longer acquisition time. Therefore, we will focus
on the triplecoincidencedata(Figure4).
Figure 4. XY momentum distributions of coincidence ion pairs of
(A).
dimethylaminemonocation(N(CH3)2CH2+,mass58)and(B).benzylmonocation(C6H5‐CH2+,mass91).(C)Ytmomentumdistributionofphotoelectronsincoincidencewiththeionpairs.
It canbe readily identified from thephotoion‐photoion
coincidence (PIPICO)map(Figure 5(A)) that the major dissociation
channel leads to dimethyl
aminemonocation(N(CH3)2CH2+,mass58)andbenzylmonocation(C6H5‐CH2+,mass91).Afterapplyingmomentumconservationcriteriatoremovefalsecoincidence,wecancleanly
select the ion pairs that arise from the dissociation of PENNA
dication.Figure 4(A) and (B) showmomentumdistributions of ionsmass
58 andmass 91after applyingmomentum conservation criteria in the
plane perpendicular to thelaser polarizationwhile Figure 4(C) is
themomentumdistribution of
coincidenceelectronsintheplaneparalleltothelaserpolarization.Theangulardistributionsofboth
ions are isotropic (spherical), which suggests that the
dissociation time is
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longerthanrotationperiod.ThetotalKERoftheionpairsispeakedaround2.9eVandhasacutoffextendingbeyond4eV(Figure5(B)).
Figure 5. (A) Photoion‐photoion coincidence (PIPICO)map of
dissociative
doubleionizationofPENNA.(B)Thekineticenergyreleasedistributionofmass58and91ionpairsfromdoubleionization.
Nowweturntotheorytohelp identifythedicationstates thatare
involvedinthedissociation processes. PENNAdication has three
possible electronic
structures:closed‐shellsinglet,open‐shellsinglet,andopen‐shelltriplet.Inordertofigureoutthe
electronic structure of PENNA2+, transition states, and reaction
pathways ofdouble ionization, density functional theory
calculations were carried out. Westarted with the ground state of
neutral PENNA that was confirmed in previousstudies. 19, 24 From
the neutral PENNA molecule, we calculated the
adiabaticallyoptimizedgeometriesofthePENNAmonocationanddications.Then,relaxedscansby
stretching the C‐C bond of PENNA dications were employed to explore
thedissociationpotentialenergysurfaces.
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Closed‐shell singlet PENNA dications are produced with two
electrons
removedfromthesameorbital,leadingtoanexcitedstatewhileopen‐shellsingletdicationshave
two electrons removed from different orbitals, resulting in
diradicals.
TherelaxedscansbystretchingtheC‐CbondofPENNA2+showthatclosed‐shellsingletPENNAdicationsoverlapwiththesingletopen‐shellafteritgoesthroughtheenergybarrier,
leading to the same products (Figure 6 inset). The energy of the
tripletpotentialsurfaceishigherthanthesingletpotentialsurface(Figure6).Thesinglettransition
state has a shorter C‐C bond (1.92Å) than the triplet transition
state(2.17Å),andtheenergyis0.65eVlower.
Fromtheenergydiagram,wecanseethereversebarrierfortheopen‐shellsingletstate
is 3.85 eV,which is close to themeasuredmaximum kinetic energy
release(KER). The open‐shell triplet state has a reverse barrier of
only 2.67 eV,which
isevensmallerthanthepeakvalueofthemeasuredKER.Theopen‐shellsingletstatealsohas
loweractivationbarrier(0.34eV) thanthatof theopen‐shell
tripletstate(0.96 eV). Both facts prompt us to conclude that
dissociation through
open‐shellsingletisthedominantdissociationchannelofmetastablePENNAdications.Thisinturnalsosuggests
thatstrong fielddouble
ionizationproduceddominantlysingletopen‐shell
dications.Closed‐shell dicationsareunlikely toplaya roledue to
theirhigh excitation energies and barrierless reaction pathways,
which should lead
toanisotropicangulardistributionofthefragments.
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Figure 6. Energy levels of PENNA dication open‐shell states,
transition states
andfinalproducts.TheinsetshowstherelaxedpotentialenergysurfacesalongtheC‐Cbondfordifferentdicationsstates.
Achieving3Dmomentum‐imagingofelectronsat10kHzandbeyond
Finally, we demonstrate another major improvement of the
camera‐basedmomentum‐imagingsystem.Foratypicalcoincidencemeasurement,becauseofthelowcount
rate required forachieving true coincidence, it ispreferred tohave
thewholesystemrunningatarepetitionrateashighaspossible.Thelimitingfactorisusually
the laser system.WhileonekHz laser is verypopularandsuitable for
thecamera‐based3Dmomentum‐imagingsetup,higherrepetitionlasersrunningat10kHzor100kHzdoexistandarebeingused
inmanystrong fieldexperiments.Byexploiting a simple fact of all
coincidence measurement, we can improve the
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repetitionrateofthecurrentimagingsystembyfivetotenfoldswithoutupgradingthecamera.The
lowcountrateofacoincidenceexperimentmeans
thatnoteverycameraframehasevent‐hits.Forexample,roughly80%of
thecamera
frameswillbewithoutevent‐hitifthecameraframerateisthesameasthelaserrepetitionrateandthecountrateiskeptbelow0.2events/lasershot.Thissuggeststhatthecameraframerateisnotfullyutilizedinthisway.Adifferentwayofrunningexperimentsistoexposeacameraframeformorethanonelasershots.Aslongastheaveragehitinthecameraisclosetoonepercameraframe,thecameraeventanddigitizereventcanbecorrelatedtoprovidethethreecoordinatesfor3Dmomentumimaging.Evenif
therearea fewevents inonecamera frame,
thebrightness‐intensitycorrelationwillbeabletocorrelatethetimeswiththepositionsoftheevents.Withthismethod,thecameraframeratecannowbefullyutilizedwhilethesystemrepetitionrateisincreasedfivetotentimesbeyondthehighestcameraframerate.Wedemonstratedthis
using a 10 kHz laser located in the Kapteyn/Murnane group at
University
ofColoradoofBoulder.Thecamerawasrunningat2kFrames/secondandthelaserat10kHz.AstandardVMIsystemwithathree‐lensspectrometerwasusedtodetectthe
photoelectrons arising from strong field ionization of krypton.
Because onlyelectronswereof
interest,noelectrode‐pulsingwasemployed.Figure7showsthe3DmomentumdistributionsoftheelectronNewtoncloud.Theachievedspatialandtemporal
resolutionwasgoodwhile it tookonly fiveminutes toaccumulate
theseevents.
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Figure 7.3Dmomentum distributions of photoelectrons arising from
strong fieldionization of krypton by linearly polarized laser beams
running at a 10
kHzrepetitionrate.Thecamera‐basedimagingsystemrunsat2kFrames/second.
Conclusion
We demonstrate a new method to convert a standard VMI apparatus
to acoincidence 3D momentum‐imaging setup without modifying parts
inside thevacuum chamber or the imaging detector. It should be
noted this setup isautomatically capable of slicing the electron
Newton sphere due to its excellenttemporal resolution.The
additional cost for adding a second camera isminimum.The current
setup requires a high repetition laser in order to expedite the
dataacquisition.Furtherimprovementofthemulti‐hitcapabilitymightenablethistobeusedwithlowerrepetitionlasers.Furthermore,wehaveshownthatahighsystemrepetitionratebeyondthecameraframeratecanbeachieved.
With this new imaging setup, by measuring the KER of
dissociative
doubleionizationandcomparingitwithdensityfunctionalcalculations,weshowthemainproducts
of strong field double ionization of PENNA are singlet diradicals,
whichdissociateintoN(CH3)2CH2+andC6H5‐CH2+.Theisotropicdistributionsof
fragment
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ions suggest a long lifetime of the parent dications, which
poses a challenge
forfutureexperimentsthataimformolecular/recoilframeionizationrate.
Acknowledgement
ResearchsupportedbytheChemicalSciences,Geosciences,andBiosciencesDivision,OfficeofBasicEnergySciences,OfficeofScience,U.S.DepartmentofEnergy,undergrant
numberDE-SC0012628. Computational resources were provided
byWayneState University Grid and the Consortium des Équipements de
Calcul Intensif (CÉCI), fundedby theFondsde
laRechercheScientifiquedeBelgique(F.R.S.‐FNRS)undergrantnumber2.5020.11.W.L.ispartiallysupportedasaSloanResearchFellow.F.R.andB.M.gratefullyacknowledgesupportoftheFondsdeNationaldelaRechercheScientifique(FNRS,Belgium).
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CMOS Camera (e-)
Continuous beam
SkimmerPhosphor/MCP
Computer
High-speedDigitizer
Concave mirror
Image
TOF
Signal decoupler
Linearly polarized light
CMOS Camera (ion)
BA
Ion mis-assignment
YX
t
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0
20
40
60
-60
-40
-20
P y (a
.u.)
0 20 40 60-60 -40 -20
Px (a.u.)0.0 0.2 0.4 0.6 0.8-0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
-0.8
-0.6
-0.4
-0.2
Px (a.u.)
P y (a
.u.)
A B CIon Electron Electron
0.0 0.2 0.4 0.6 0.8-0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
-0.8
-0.6
-0.4
-0.2
Pt (a.u.)
P y (a
.u.)
0510
25
45
80
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-
400
250
130
130
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0
eKE (eV)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
KER
(eV
)
BAExp.Theory
Coun
ts
1600
0.00
200
400
600
800
1000
1200
1400
eKE (eV)15.00 2.5 5.0 7.5 10.0 12.5
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0 50 100 150-150 -100 -50
0
50
100
150
-150
-100
-50
Px (a.u.)
P y (a
.u.)
0.0 0.2 0.4 0.6 0.8-0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
-0.8
-0.6
-0.4
-0.2
Pt (a.u.)
P y (a
.u.)
B C
0 50 100 150-150 -100 -50
0
50
100
150
-150
-100
-50
Px (a.u.)
P y (a
.u.)
×1.5
A Mass=58 Mass=91 Electron
01.53
6
16
24
×1.5
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600 800 1000 1200 1400 1600 1800 2000600
800
1000
1200
1400
1600
BA
Ion TOF (bin/2ns)
Ion
TOF
(bin
/2ns
)
0.0 1.0 2.0 3.0 4.0 5.0
KER (eV)
0
20
40
60
80
100
Coun
ts
01.7
17
34
53
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2.67
3.85
0.34 eV
0.96 eV
-3.51 eV
-1.71 eVProducts
TSPENNA2+
19.33 eV, triplet19.30 eV, singlet
PENNA+
7.43 eV
Neutral GS
20
18
16
14
12
10
8
6
4
2
0
Ener
gy (e
V) 21.0
17.5
18.0
18.5
19.0
19.5
20.0
20.5
C-C Bond Length (Å)1.5 2.0 2.5 3.0 3.5 4.0
Ener
gy (e
V)
PENNA2+
Singlet surface
Triplet surface
Open-shellsinglet
Open-shell triplet
Closed-shell singlet
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0.0 0.2 0.4 0.6-0.6 -0.4 -0.2
Px (a.u.)
A
0.0
0.2
0.4
-0.4
-0.2
P y (a
.u.)
0.0 0.2 0.4 0.6-0.6 -0.4 -0.2
Px (a.u.)
B
0.0
0.2
0.4
-0.4
-0.2
P z (a
.u.)
0.0 0.2 0.4-0.4 -0.2
Pz (a.u.)
C
0.0
0.2
0.4
-0.4
-0.2
P y (a
.u.)
051025
45
80
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