Partial
cross
sections
for
dissociative
electron
attachment
to
tetrahydrofuran
reveal
a
dynamics-driven
rich
fragmentation
pattern
R.
Janeˇcková
a,
O.
May
a,
A.R.
Milosavljevi ´c
b,
J.
Fedor
a,∗aDepartmentofChemistry,UniversityofFribourg,CheminduMusée9,CH-1700Fribourg,Switzerland bInstituteofPhysicsBelgrade,UniversityofBelgrade,Pregrevica118,11080Zemun,Belgrade,Serbia
Wereportpartialabsolutecrosssectionsfordissociativeelectronattachment(DEA)totetrahydrofuran C4H8O.Thehighsensitivityofthepresentsetup,quantitativeDEAspectrometerwithtime-of-flight
ana-lyzer,ledtoidentificationofanumberofpreviouslyunreportedfragments(CH2−,OH−,CHO−,C4H5O−,
C3H3−)thusrevealingcomplexdissociationdynamicsoftheinvolvedresonantstates.Thechemical
com-positionoffragmentanionswasassignedbyexperimentswithcompletelydeuteratedtetrahydrofuran. Wealsoshowthatthedominantheavyanionicfragmentwithm/z=41isC3H5−anionratherthanC2HO−
asreportedinthepreviousstudies,inspiteofthefactthatformationofthelatteroneisenergetically morefavorable.Thedynamicsontheresonantstateisthusmoredecisiveforthereactionoutcomethan theasymptoticenergetics.Theassignmentofresonancesandtheirdynamicpathwaysisdiscussedby usingcomparisonwithrecentcomplementaryexperimentsthatcharacterizeelectronicstatesofneutral andionictetrahydrofuran.
1. Introduction
Electroncollisionswithtetrahydrofuran(THF)attracteda sur-prisingamountofattentionduringthelast15years.Thisinterest– whichmayseemunusualforasmallorganicmolecule–hasbeen partofthelineofresearchinelectroncollisionswithmoleculesof biologicalrelevance.Thislineofresearchaimsatelucidatingthe role ofelectron-triggeredprocesses inradiationdamage.Inthis respectTHF,sinceitisthesimplestsaturatedfuranosering,canbe consideredasaprototypefortheDNAsugarbackbone. Addition-ally,incontrarytosugars,THFisaliquidatstandardlaboratory conditionswhichgreatlyfacilitatestheexperiments.
Theabove-mentionedlineofresearchhasbeentriggeredbya groundbreakingdiscoveryofSancheandco-workers[1]that low-energyelectronscausesingleanddoublestrandDNAbreaksevenat sub-ionizationenergy.Inspiredbythisdiscovery,variousresearch groupsstartedtolookattheelectron-inducedreactionsin molec-ularconstituentsofDNA.Oneofthefirstgroupsthatadoptedthis direction–and quicklybecame abeaconinthefield–wasthe groupofT.D.MärkinInnsbruck.Märkrecognizedthehighscientific potentialinthisfieldandtheexperimentsinhisgrouphavegreatly contributed to a renewal of the interest in electron-triggered
∗ Correspondingauthor.Tel.:+41300268697. E-mailaddress:juraj.fedor@unifr.ch(J.Fedor).
physicsandchemistry,especiallythedissociativeelectron attach-ment (DEA). This process, which proceeds via formation of a transientresonantstateisaneffectivebond-breakingreactionand theexperimentsinInnsbruckhaveelucidateditsroleinawhole rangeofmoleculeswithbiologicalrelevance:DNAbases[2,3], sug-ars[4],oraminoacids[5].Theexperimentshaverevealedaplethora ofphenomenaoccurringindissociativeelectronattachment: site-andbond-selectivity[6]orDEAfootprintsofvibrationalFeshbach resonancesinbiomolecules[7].ThegroupofT.D.Märkhasbeen alsothefirstonethatlookedintoDEAprocessinthemoleculeof presentinterest:tetrahydrofuran[8].
Basicallyallexperimentallyaccessibleelectron-triggered pro-cessesinTHFhavebeencharacterizedintherecentyears.Thetotal electronscatteringcrosssectionswerereportedbyZeccaetal.[9], Mozejkoetal.[10]andFussetal.[11].Theelasticscatteringin var-iousenergyrangeswasprobedbyMilosavljevi ´cetal.[12],Dampc etal.[13],Allan[14],Colyeretal.[15],Baeketal.[16],Chiarietal. [17]andHomemetal.[18].Thevibrationalexcitationcrosssections werereportedbyDampcetal.[19]andAllan[14]andelectronic excitationwasprobedbyDoetal.[20]andGiulianietal.[21].The electroncollisionswithTHFattractedalsoconsiderableamount ofattentionfromatheoreticalpointofview.Thescattering pro-cesseswerecharacterizedbyaSchwingermultichannelmethod [22],complexKohnvariationalmethod[23],orR-matrixmethod withBorncorrection[24].Thelaststudyisofspecialinteresthere sinceitidentifiedanumberofcore-excitedresonances. Comple-mentarytothegasphase,seriesofexperimentswithTHFdeposited
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which should be cited to refer to this work.
onasurfacewasperformedbyLepageetal.[25],Anticetal.[26,27], Bretonetal.[28],Jäggleetal.[29],andParketal.[30].
ThedissociativeelectronattachmentchannelsinTHFhavealso beenthesubjectofseveralstudies,Sulzeretal.[8]andIbanescu etal.[31]reportedyieldsofindividualDEAfragmentsasafunction oftheelectronenergyandAflattonietal.[32]measuredtotalDEA crosssectionusingatotalioncollectioninstrument.Surprisingly, thefirsttwostudiesonlypartiallyagreewitheachother:theyagree ontheoccurrenceofthe(strongest)fragmentwithatomicmass 41,whichwasassignedbybothgroupstobetheC2HO− anion.
However,Sulzeretal.alsoreported theoccurrenceof aparent anionTHF−andadoublydehydrogenatedparentanion(THF-2H)− around2eV,whichwerenotdetectedbyIbanescuetal.who,on theotherhand,reportedhydrideanionH−andanionwithmass43 (C2H3O−).TheoccurrenceofstableparentanionsinDEAatenergies
considerablyhigherthan0eVisveryrare,sincetheexcessenergy (electronaffinityandincidentelectronenergy)hastobestored withininternaldegreesoffreedom.Suchsituationoftenleadsto metastabilityoftheparentanioninthemicrosecondtime win-dow[33,34]:theexcitationenergyrandomizesovertheinternal degreesoffreedomandthedecayoftheparentanionresultsfrom theconcentrationofsufficientenergyintherelaxationchannel.
OurprimarymotivationtostudyTHFwastheeventual occur-rence of such metastability, which can be detected with a time-of-flightsetupsuchastheoneavailableinourlaboratory.We reportnegativeevidenceformetastableprocessesinourtime win-dow,however,wehavediscoveredseveralsurprisingaspectsofthe DEAtoTHF.Thehighsensitivityofourinstrumentenabled iden-tificationofanumberofpreviouslyunreportedanionicfragments, thusrevealingmuchmorecomplexfragmentationdynamics.We alsoprovidenewassignmentofthestrongestheavyanionic frag-ments with the mass 41, which turned out to be C3H5−. We
provideabsolutepartialDEAcrosssectionsforallreported frag-mentsanddiscussthepossibleoriginsofresonantstatesleadingto thereporteddissociationpattern.
2. Experiment
ThequantitativeDEAspectrometerhasbeendescribedindetail previously[35–37].Theelectronbeamisproducedbyatrochoidal electronmonochromator,passesthroughacollisioncellfilledwith thesamplegasandiscollectedbyaFaradaycup.Anionscreated byDEAareextractedthroughanarrowslitinthecell’swalltoa time-of-flightmass(TOF)spectrometer.Theexperimentworksin apulsedmode.Theelectronbeampassesthroughthecollisioncell for200nswhileitisfield-free.Afteranother200ns(sothatthe electronsareallowedtoleavethecell)avoltagepulseof−300V isappliedacrossthecelltoexpeltheanions.Theyaredetectedby amicrochannelplatecoupledwithphosphorusscreenand pho-tomultiplier,countedandtheirarrivaltimesareanalyzedusinga delayedcoincidencescheme.Thespectraarerecordedasa two-dimensionalmapoftheionsignalasafunctionofelectronenergy andtheflighttime(whichreflectstheanionmass).Therepetition rateis50kHz.Thepressureinthecollisioncelliscontrolledbya needlevalveandmonitoredbyacapacitancemanometer.Itwas keptintherangeof3×10−4to6×10−4Torr.Sincetheprevious studiesdidnotrevealanyfinestructuresinDEAionyields,the elec-tronmonochromatorwasoperatedtoyieldhighelectroncurrent (around100nAintheCWmode)andthusatotalhighsensitivity. Theelectronenergyresolutionduringthepresentmeasurements was250meV.
TheTOFsetuphastwoaccelerationregionsandadriftregion. Thefirst accelerationregionis betweenthe pulsedpusher and thewallofthecollisionchamber,thesecondaccelerationregion is betweenthewallof the chamber and thefront plateof the
time-of-flighttube.TheTOFtube(driftregion)isnotentirely field-free,itisconstructedasafocusingionlensandtheentiresetupwas designedtokeeptheextractionanddetectionefficiency indepen-dentofthemassandinitialkineticenergyofthedetectedanions [37].Additionally,itsfirstparthasadeflectingcapacityinorderto compensatefortheeffectofthemagneticfield.Ithasbeenshown bysimulationsandextensiveteststhatonlytwotypesofvoltage settingsarenecessaryforthepropercollection:‘lightionsettings’ forH− andD−ionsand‘heavyionsettings’forionswithlarger masses.Themassscaleiscalibratedwithionsofknownmasses fromothercompounds(O−fromO2,H−,C2−andC2H−from
acety-lene)andthecalibrationisbasedonthefactthatthetime-of-flight isdirectlyproportionaltothesquarerootofmass.Themass assign-mentisverifiedbythefactthatallassignedmassesfitperfectlyon thecalibrationcurve.
Theelectronenergyscalewascalibratedusingthe4.4eVO−/CO2
peak.Theshapeofthisband(withathresholdat3.99eV)hasbeen measuredwithhighelectron-energyresolution[38].Wedetermine boththeenergyscalecalibrationandtheelectronenergyresolution byfittingtheconvolutedhigh-resolutiondatatoourmeasurement. Absolutemagnitudesofthecrosssectionswerecalibratedonthe sameO−/CO2bandwithapeakcross-sectionvalueof14pm2[35].
Theerroroftheabsolutemeasurements(twostandarddeviations) istakentobe±25%.Thisisthecombinederrorof±20%ofthe rel-ativemeasurementswiththe±15%erroroftheO−/CO2 absolute
crosssection.We haveshownrecently[39]thatthis calibration approachisnotappropriateiftherearenarrowbandsintheDEA spectraduetoeffectofelectronbeamresolutiononthepeakwidth, however,itisperfectlysuitableforthepresentmoleculesinceonly broadbandsappearinthespectra(seebelow).
3. Resultsanddiscussion
3.1. Anionicfragmentsandpartialcrosssections
Fig.1(top)showsthenegativeionmassspectrumof tetrahy-drofuranoriginatingfromtheimpactof8.5eVelectrons.Onecan observearichfragmentationpatternwithanumberofpreviously unreportedanionic fragments. The chemical composition of all
Fig.1.NegativeionTOFmassspectrumoftetrahydrofuran(top)andfully deuter-atedtetrahydrofuranTHF-d8(bottom).
Fig.2.PartialcrosssectionsforproductionofanionicfragmentsinTHF.
fragmentswasdetermineduponcomparisonwiththemass spec-trumoffullydeuteratedTHF(Fig.1bottom).Fig.2showsthepartial absolutecrosssectionsforallfragments.Inspiteofacareful acqui-sitionwedidnotdetectanysignalbelow5eVelectronenergy.Note thatthemassspectrainFig.1weretakenatheavyionsettings,thus theintensityofH−peakisnotdirectlycomparablewithFig.2.This isalsothereasonforashorterflight-timeshoulderoftheH−signal. The previouslyunreported fragmentsare CH2−,OH−,COH−,
C3H3−andC4H5O−[triplydehydrogenatedtetrahydrofurananion
(THF-3H)−].Thestrongestheavyfragmentwiththemass41has beenreportedbothbySulzeretal.[8]andbyIbanescuetal.[31]. Theshapeandpositionofthebandagreesverywellwithpresent results.Additionally,Ibanescuetal.reportedC2H3O−andH−
frag-ments.Thepositionandshapeofthebandsagainagreewellwith presentresultsforC2H3O−andreasonablywellforH−.Inthelatter
case therelative intensitiesofthetwobands showcertain dis-crepancies.Itshouldbenotedthatthetwoexperimentsarenot directlycomparable–Ibanescuetal.usedaquadrupolemass fil-terwherethetransmissionefficiencyisknowntodependstrongly ontheionmassandontheinitialkineticenergy.Allthenewly reportedfragmentsexceptCH2−have verylowcrosssections–
below0.01pm2whichisprobablythereasonfortheirabsencein
earlierstudies.Comparisonofthetwomassspectraalsoshowsthat thepeakwiththemass41shiftstomass46upondeuterationof theprecursormolecule.ThusitschemicalcompositionisC3H5−,
theallylanion,insteadofC2OH−,deprotonatedketene,asitwas
assignedinbothpreviousmass-resolvedstudies[8,31].Thereason fortheirassignmentwasprobablythefactthattheformationof C2OH−isenergeticallymorefavorable.
Table1summarizesthresholdenergiesforthereactionchannels leadingtoproductionofanionswithmolecularmass41amu.The calculationswereperformedattheB3LYP/6-311++G(2df,2pd)level oftheory.Thismethodhasbeenshowntoyieldreliableasymptotic energiesinarangeofmolecules[40,41].Ithasbeenpointedout innumberoftheoreticalinvestigations(seeRef.[21]forreview), thatneutralTHFhasseveraldifferentconformationscloselyspaced
Table1
DFT[B3LYP/6-311++G(2df,2pd)]thresholdenergies(eV)forvariouschannels lead-ingtoproductionofanionswithm/z=41.
Process Products Eth(eV)
e+THF→ C2HO−+C2H5+H2 1.84
C3H5−+CH3O 2.59
C3H5−+CH2O+H 3.63
inenergy,allofthembeingconnectedviaout-of-planering defor-mation(pseudorotation).Thehigh-levelab-initiocalculations[42] revealthatthetwolowestconformers,withC2andCssymmetry,
haveonly6meVdifferenceinenergy,whichresultsinalmostequal Boltzmannpopulationofthesetwostructuresinthegasphase.We haveoptimizedtheneutralTHFwithintheC2geometry.The
calcu-latedstructureofC3H5−correspondstoallylanion,thecalculated
structuralformulaoftheC2OH−isHCCO−.AscanbeseeninTable1,
thechannelleadingtoC2HO−+C2H5+H2isindeedthe
energeti-callymostfavorable.BothchannelsleadingtoproductionofC3H5−
arehigherinenergy:theoneleadingtoproductionofthemethoxy radicalCH3Oby0.75eV,theoneleadingtoproductionof
formalde-hydeandhydrogenatomby1.79eV.However,asonecanseein Fig.2,theC3H5−bandstartsatapproximately6.7eV,withall
chan-nelseasilyaccessibleanditisnottheasymptoticenergeticsbut ratherthedissociativedirectionoftheinvolvedresonantpotential energysurfacethatdeterminesthefragmentationpathway.This showsdominantroleofthefastnucleardynamics–theinternal energydoesnotredistributeandseekthelowestenergypathway (asonecouldintuitivelyexpectforsuchrelativelylargemolecule) butratherfollowsthedirectpathonthepotentialenergysurface.
ThemassspectrainFig.1alsocontainsomespuriouspeaks.The smallpeakwithmass35thatcanbediscernibleinbothspectrais atomicchlorineanionCl−whichcomesfrominteractionof elec-tronswithtraceamountsofchlorinatedspeciesthatarepresentin theexperimentalsetupfrompreviousexperiments.Wealsobelieve thatalargepartoftheO−signalisnotcausedbytheDEAtoTHFbut rathertospuriouswatermolecules:theenergydependence spec-trumoftheO−anionhasbasicallythesameshapeastheionyield ofO−comingfromDEAtowater[44].Itishighlyimprobablethata signalwiththesameshapewouldoriginatefromtheelectron col-lisionswithTHF,sincethe11.5eVresonanceisnotvisibleinany otherTHFfragment.Waterhasseveralordersofmagnitudelarger DEAcrosssection[45],thepresentsignalcorrespondsto approx-imately0.5%impurityofwaterinoursample.Thewaterimpurity isalsoareasonfortheH−signalvisibleinthespectrumof deuter-atedtetrahydrofuraninFig.1.SimilarlyasintheTHFspectrum,this peakhasashoulderatshortflighttimescausedbytheheavy-ion settingsoftheionlens.
3.2. TotalDEAcrosssection
Fig.3showsthesumofourpartialcrosssectionscomparedthe totalnegativeioncrosssectionofAflatoonietal.[32].Theirdata wereobtainedusingatotal ioncollectiontube,thusatelectron energieslargerthantheionizationenergy,aformationofpositive ionssetsinandinterfereswiththenegativeionsignal,causingthe abruptcutoffofthenegativecrosssection.Forthesakeofthe com-parison,thespectrumofAflatoonihasbeenshiftedby360meV towardshigherelectronenergy.Eventhoughtheinaccuraciesof theenergyscalecalibrationofthetrochoidalmonochromatoron theorderoftensofmeVareexpectable[46],thepresent discrep-ancyisrelativelylargeandwedonotknowthereasonforit.It shouldbenoted,thatourpresentenergypositionofthestrongest mass41 fragmentagreesexcellentlybothwithSulzeretal.and Ibanescuetal.Apartfromtheissueintheenergy-positionofthe resonances,theagreementofthepresenttotalcrosssectionswith
Fig.3.Comparisonofthesumofpresentpartialcrosssectionstototalcrosssection fromAflatoonietal.[32].TheverticalarrowshowstheionizationpotentialofTHF [43].
thetotalcrosssectionofAflatoonietal.isexcellentuptothe elec-tronenergyofapprox.8.5eV.Itshouldbenotedthatsuchhighlevel ofagreementisuptosomelevelfortuitousintheviewoftheerror barsofthebothexperiments(±25%ofthepresentvalues).Thetotal ioncollectiondataarelowerthanthepresentvaluesabove8.5eV. TheionizationpotentialofTHFis9.42eV[43].Wecanonly specu-late,whatcausesthedecreaseofthenegativeionsignalbelowthis value.Thecrosssectionsforpositiveionizationareusuallyorders ormagnitudelargerthanpresentDEAcrosssections.Thepositive signalcouldbecaused,forexample,byevenaveryweak shoul-derofhigh-energyelectronsintheelectron-energydistribution. Anotheroptionwouldbepresenceofanimpuritywithlower ion-izationenergyinthesample–duetolargeionizationcrosssections alreadyatraceamountofsuchimpuritywoulddecreasethetotal negativesignalconsiderably.
3.3. AssignmentofDEAbands
SpectraofalltheanionicfragmentsinFig.2showthreemain bandscenteredaroundtheelectronenergiesof6.6eV,7.7eVand 8.7eV.Thedominantfragmentoriginatingfromthefirstandthe thirdbandisH−,thedominantfragmentappearingatthe
posi-tionofthesecondbandisC3H5−.Intheenergyrangeofinterest
(above6eV)twoclassesofresonanceshavebeensuggestedtooccur inTHF:one-particleshaperesonancesorcore-excited(Feshbach) resonances.
Theshaperesonancesinthisenergyrangewereidentified theo-reticallybyTrevisanetal.[23]at8.6eVandbyWinsteadandMcKoy [22]at8.3and13eV.Also,experimentsonvibrationalexcitation [14]showedshaperesonancesat6.2and10.8eV.However,these resonancesareverybroad(feweV),whichisacommonfeaturefor shaperesonancesinorganicmoleculesinthisenergyrange.They usuallyarisefromtheelectronoccupationofthe*orbitals
associ-atedwithC HandC Cbondsandduetotheirlargeenergywidth theyarenotexpectedtoleadtodetectableDEAsignal.Theirshort autodetachmentlifetimeusuallycausesthattheelectrondetaches beforeatomicnucleimovetosuchconfigurationinwhichthe elec-tronicstatebecomesaboundstate,i.e.,whereoneofthefragments becomesastableanion.
Themoresuitablecandidatesforprecursorresonantstatesin thisenergy range arecore-excited Feshbach resonances.A Fes-hbachresonancehasacationiccoreand adoubleoccupationof Rydberg-likeorbitals.TheyarethuscloselyrelatedtoRydbergand cationicstates, thisrelationis demonstratedinthetoppanelof Fig.4.TheRydberg(parent)stateswhicharevisibleinVUVand
Fig.4.Upperpanel:schematicrepresentationoftherelationbetweenVUV,EELS, PESandDEAspectra(seetextfordetails).Graphs:(a)VUVspectrumofTHFtaken fromGiulianietal.[21],(b)EELSspectrumtakenfromDoetal.[20],(c)PES spec-trumfromGiulianietal.[21],and(d)presentDEAcrosssectionsforH−andC3H5− fragments.ThescaleofthePESspectrum(c)isshiftedby3.1eVasexplainedinthe text.
EELS(electron-energylossspectroscopy)spectraareshiftedwith respecttothecation(grandparent) states,visible inthe photo-electronspectra(PES),byatermenergy.Theresonanceisfurther stabilizedbytheelectronaffinityoftheparentRydbergstatewhich istypically0.3–0.5eVforthes2configuration[47].Recentworkon
excitedelectronicstatesofneutralandionicstatesofTHFby Guil-ianietal.[21]greatlyfacilitatestheassignmentofresonances.Fig.4 showstheVUV,EELSandPESspectratogetherwithpresentcross sectionsforthetwostrongestDEAfragments.Thehigh-level ab-initiocalculationsfromRef.[21]showedthatalltheelectronically excitedstatesofTHFhaveRydbergcharacter.ThefirstVUVand EELSbandbetween6and6.9eVisduetotransitionto3sRydberg state,thebandisbroadenedbyrichvibrationalprogressionsandby thefactthatthetwoTHFconformers(withC2andCsgeometries)
haveverticalexcitationenergiesseparatedby0.25eV.Thesecond bandcontainsamixtureof3pand3dstates,thethirdbandconsists ofprogressionof3dstates.AllthreebandsarevisiblealsoinEELS spectra.InthePESspectrumthefirstbandcorrespondstothehole inthe9bmolecularorbital,comparisonwiththeVUVspectrum showsthatthetermenergyofthecorrespondingRydbergstate
(9b)−13sis3.1eV.ThescaleofthePESspectruminFig.4startsat 9.1eV,by3.1eVhigherthanthescaleofVUVandEELSspectra.If onethenassumes0.2–0.3eVasstabilizationenergyofthe Fesh-bachelectronpair,thenthefirstbandofthePESspectrumoverlaps nicelywiththefirstH−DEAband,andtheelectronicconfiguration oftheresonancewouldbe(9b)−13s2.The8.7eVH−DEApeak
over-lapswiththesecondbroadbandinthePESspectrum.Thisband correspondstoholesin11a,10a,8b,and9aorbitals.TheFeshbach resonancesresponsiblefortheDEAH−thuscorrespondtoa3s2
electronpairaroundioniccorewiththeholeinoneorseveralof theseorbitals.
ItisworthnoticingthatthePESspectrumhastobeshiftedonly byapprox.3.3eVinordertooverlapwiththeDEAbands.Often,this differenceisassumedtobe4.5eV[40].Thisassumptionisbasedon anusualtermenergyof4eVandstabilizationenergyofthes2pair
of0.5eV[47].Thesevalueswhichareoftenconsideredtobethe sameforallmoleculesbecauseoftheweakinteractionofthe dif-fuseRydbergorbitalwiththecationiccore.However,formolecules withlargeralkylchains,thetermenergyisdecreasedduetothe hin-deredapproachoftheRydbergelectrontothecationiccore,thus decreasingtheattractionenergyanddecreasingthetermenergy [48,49].Anextremeexamplewasshownintherecent studyof theTEMPO(2,2,6,6-Tetramethylpiperidine-N-oxyl)[50]wherethe shiftbetweenDEAandPESbandswasonly2.1eV.Inthepresent casewehavetheadvantageofknowingtheshiftbetweenPESand VUVspectratobe3eVduetothehigh-resolutiondataofGiuliani etal.[21].
Theassignmentofthe7.6eVbandleadingtotheproductionof theC3H5−fragmentisapuzzle.Itdoesnotoverlapwithanyband
intheshiftedPESspectrum,soitisnotpossibletodoan assign-mentasaFeshbachresonancewithanelectronpairina3sorbital. Inprinciple,onecouldspeculatethatitcouldbeaFeshbach res-onancewithanelectronpairina3porbital,orwitha3s3ppair. Such resonancesareseeninthetransmissionspectra,however, thestabilizationenergyofsuchelectronpairismuchlower,inthe orderof0.01–0.1eV[47].Thustheirlifetimeisexpectedtobetoo shorttoleadtoobservableDEAsignals.Thisisasimilarsituation, aswiththehigh-energyshaperesonancesmentionedabove:their energiesoverlapwiththeDEAbands,however,suchassignment isimprobableduetoashortlifetimetowardsautodetachment.For completeness,itshouldbementionedthatinthisenergyrangeDEA bandshavealsobeenfoundwhichoriginatefromresonanceswhere ans-likeelectronisboundtothesingletvalenceexcitedstate.Some examplesarenorbornadiene,cyclopentadieneorSO2 [51].
How-ever,noviableprecursorexcitedstateshavebeenidentifiedinTHF [21].
3.4. Noteonthemetastability
Ibanescuetal.[31]studiedDEAbond-breakinginarangeof lin-earethersandfoundthatFeshbachresonancesleadtoveryefficient cleavageoftheC Obond.SinceTHFisacyclicether,theysuggested thatinthefirststeptheC Obondisbreakingaswell,leadingto anopen-chainTHF−parentanion.Sincetheenergyreleasedupon thering-openingisstoredintheinternaldegreesoffreedom,this anionsubsequentlyundergoesdelayeddissociation.
Inatime-of-flightsetup,ifsuchadelayeddissociationhappens duringtheaccelerationstage,beforeTHF−reachesthedriftregion, theanionsaredetectedbetweentheflight-timeoftheparentanion andtheanionintowhichitfragments.Thiseffecthasbeenusedto estimatethelifetimeofvibrationallyhotSF6−[33]or
dinitroben-zeneanions[34].Inoursetupthecorrespondingmetastabilitytime windowis 0.2s<t<1s. Thelowestvaluecorrespondstothe delaybetweentheelectronpulseandtheion-extractionpulse,the largestdelaytothetimepointwhenionsenterthedrifttube.We
havedetectedneitheranyremainsoftheTHF−signalnorthe
sig-nalcorrespondingtoametastabledecay.Thisdoesnotmeanthat theprocesssuggestedbyIbanescuetal.isnotcorrect:duetohigh excessenergyitprobablyhappensonafastertime-scale.Theonly peakwithslightlynon-standardshapeisthatofCH2−whichhasa
tailtowardslongerflighttimes.Thisindicatesthatitisinfactpartly formedduringtheearlystagesoftheacceleration.
Sulzeretal.[8]reportoccurrenceofTHF−anion,however,at lowerelectronenergies,around2eV.Ithasbeenpointedoutby Neumarkandco-workers[52]thattheTHF−signalmighthavebeen causedbyasmallpresenceofclustersinthesample:theparention isstabilizedintheDEAtoacluster,sincetheneighboringmolecules actasaheatbath.Theclusteringmighthavebeenenhancedinthe experimentsofSulzeretal.sincetheyhaveusedamolecularbeam target,whileinotherstudies,includingthepresentone,stagnant gastargetwasused.
4. Conclusions
Inconclusion, wehaveinvestigated thedissociativeelectron attachmentintetrahydrofuran.Thepresentexperimentsusinga time-of-flightsetuprevealedmuchricherfragmentationpatterns thanpreviouslyreported.ThepartialabsoluteDEAcrosssections areverylow,thedominantfragmentH−hasapeakcrosssectionof 2.3pm2,theweakestdetectedfragmentC
3H3−hasthepeakcross
section0.002pm2.Thefragmentationtowardsthemostabundant
heavyfragmentisnotdrivenbyasymptoticenergeticsbutrather byfastdissociativedynamicsoftheresonantanion.Thespectra revealthreemainDEAbandscenteredaround6.6eV,7.7eVand 8.7eV.Basedonthecomparisonwiththeexcitedstatesofthe neu-tralandionicTHF,thefirstandthelastDEAbandcanbeassigned totheFeshbachresonancescorrespondingtotheholesinspecific molecularorbitalsanda(3s)2electronpair.Thesumofpresent
par-tialcrosssectionagreeswithpreviousdataofthetotalDEAcross sections[32]verywellintheenergyrangeupto8.5eV. Addition-ally,wehaveruledoutmetastabledecayprocesseshappeningin the0.2s<t<1stimewindow.
Acknowledgements
ThisworkispartoftheprojectNo.PZ00P2132357oftheSwiss NationalScienceFoundation.A.R.M.acknowledgessupportfrom theSNSFshortinternationalvisitgrantNo.IZK0Z2143984andby theMinistryofEducation,ScienceandTechnologicalDevelopment ofRepublicofSerbiaunderprojectNo.17020.
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