Spindynamicsofwatericeand
theOPRofgaseouswaterdesorbedfromice
T.Hama,N.Watanabe,A.Kouchi,
InstituteofLowTemperatureScience,HokkaidoUniversity
NuclearSpinEffectsinAstrochemistry,2-4May2017,
Grenoble,France
Orthoandpara water
H
(1)Ortho-H2Oandpara-H2Omustexistin
differentrotationalstates.(Protonisafermion)
O
H
H
O
H
Spintemperature(Tspin):Ortho-to-para ratio(OPR)
assumingthermodynamicequilibrium 3.0
O
H
Para-H2O (I = 0, JKaKc = 000)
Ortho-to-para ratio
2.5
2.0
1.5
1.0
0.5
0.0
0
H
Rotationalenergy
difference
34.2K
Miani andTennyson,JCP(2004).,Caccianietal.,Phys.Rev.A(2012).
Rotational
energy
O
Ortho-H2O (I = 1, JKaKc = 101)
(2)Nuclearspinconversionisveryslowinthegasphase
byradiationornonreactivecollisions.
Spinand
rotational
degeneracy
H
H
5
10
15
20
25
30
35
40
45
50
Temperature (K)
BecauseofΔErot (23.8cm−1,34.2K),para-H2O(JKa,Kc =000)enrichmentbelow50K.
TheanomalousOPRsofgaseousH2Ohavebeenobservedinspace.
3.0
Ortho-to-para ratio
2.5
2.0
Comets
OPR=2.0~3.0
1.5
1.0
Interstellarclouds
Proto-planetarydisks
OPR =0.1~3.0
0.5
0.0
0
5
10
15
20
25
30
35
40
45
50
Temperature (K)
ThemeaningoftheobservedTspin remains
atopicofcontinuingdebate.
Lis etal.,Astron.Astrophys.521,L26 (2010).,Hogerheijde etal.,Science334,338(2011).
Choi etal.,Astron.Astrophys.572,L10(2014).,Willacyetal.,SpaceSci.Rev.197,151(2015).
Hypothesis:Tspin reflectstheformationtemperatureofice?
Interstellarclouds/Proto-planetarydisks
H2Oiceisformedthroughdustsurfacereactions,
e.g.,HatomadditiontoO,O2,andOH.
O
H
H
Dust
Dust
30K
30K
O2
H2Oice
H
H2O
Dust
H2Oice
VUV
Photodesorption
Ice-covereddust(<50K)istoocold
forthethermaldesorption.
Comets
(Thermaldesorption)
H2O
（Tspin＝30K）
Whatistherealmeaningof
theOPR?
↓
MeasurementsoftheOPRof
H2Odesorbedfromice.
TesttherelationbetweenTspin andtheiceformationtemperature:
H2OicewasproducedinsituthroughthehydrogenationofsolidO2 at10K.
InsituproductionofH2Oiceat10K
byco-depositionofHandO2
H
H
O2
VUVphoton
at157nm
O2
H
Resonance-EnhancedMultiPhoton
Ionization(REMPI)
Heating
O2
H2Oice
Substrate
H2Oice
Substrateat10K
O2 +H→HO2,
HO2 +H→H2O2,
H2O2 +H→H2O+OH,
OH+H→H2O,
OH+H2 →H2O+H.
(1)
(2)
(3)
(4)
(5)
H2O（OPR?)
H2Oice
Substrate
Surfacereactions(3)– (5)are
theformationprocessesof
interstellarH2Oice.
VanDishoeck,Herbst,andNeufeld,Chem.Rev.113,9043(2013).
Hama, andWatanabe,Chem.Rev.113,8783(2013).
Experiment with RASCAL
Infraredreflection–absorptionspectraat4000–800cm−1.
(A) Vapor-depositedH2Oiceat10K.
(B) H2Oiceproducedbyco-depositionofO2 withatomicHat10Kfor420min.
Wavelength (µm)
(A) Vapor-depositedH2Oiceat10K.
0.15
A
5
7
9 11
Vapor-deposited H2O ice at 10 K
Absorbance unit
H2Oice
Substrate
3
0.10
0.05
0.00
H
H
O2
B
O2
H
H2Oice
Substrate
O2
0.15
O2 +H→HO2,
HO2 +H→H2O2,
H2O2 +H→H2O+OH,
OH+H→H2O,
OH+H2 →H2O+H.
In situ-produced H2O ice at 10 K
H2O + H2O2
Absorbance unit
H2Oiceproducedby
hydrogenationofO2 at10K
H2O
H2O2
0.10
H2O2
H2O
0.05
0.00
4000 3600 3200 2800 2400 2000 1600 1200 800
-1
Wavenumber (cm )
Detector
(TOFmass+MCP)
“kick-out”photodesorption:
MomentumtransferfromahotHatom
photodissociated fromaneighboringH2O.
H2O(ice)+hν (157nm)→hotH +OH
ExcessEnergy:220kJmol−1
hotH+H2O(ice) →H +H2O(gas)
ΔH =45kJmol−1
Hamaetal.,JCP.132,164508(2010).
REMPIlaser(248-249nm)
Focused(1mJ pulse-1)
↓
Ortho/Parastateselective
detection
H2O+ ionization
157nm
Photodesorption laser
Unfocused(0.1mJ pulse-1)
1013-1014 photonscm-2 pulse-1
~1mm
AmorphousH2Oice
DCO,D2CO,CD
3OH
Alsubstrateat10K
H2O REMPI signal (arb. units)
0.4
0.2
0.0
-0.2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
REMPIrotationalspectrumof H2Ophotodesorbed fromiceat10K
B H2O desorbed from in situ-produced H2O ice at 10 K
REMPI
(Trot=200K,OPR=3)
VUV
157nm
C Simulation with Trot = Tspin = 200 K
321(o)
221(o)
221(o)
423(o)
322(p)
220(p)
OPR=3
312(o),
212(o)
110(o) 212(o)
Assignments: JKaKc (ortho orpara)
211(p)
1.6
1.4 D Simulation with Trot = 200 K, Tspin = 10 K OPR=0.3
1.2
212(o)
220(p)
1.0
1
(o)
10
322(p) 221(o)
0.8
211(p)
3
(o),
0.6
12
423(o)
321(o) 2 (o)
0.4
212(o)
21
0.2
0.0
-0.2
80450
80500
80550
80600
80650
124.3nm
(248.6)
-1
Two-photon wavenumber (cm )
H2Oice
Substrate
Strongerortho-H2Olinesthan
para-H2Olines.
Well-reproducedwithOPR=3.
↕
Para-H2Osignalsaretoostrong
withOPR=0.3(10K).
TheOPRisnotrelatedto
120.0nm theiceformationtemperature.
(248.0)
H2O REMPI signal (arb. units)
0.4
0.2
0.0
-0.2REMPIrotationalspectrumof
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
H2Othermallydesorbedfromiceat150K
B H2O desorbed from in situ-produced H2O ice at 10 K
REMPI
(Trot=150K,OPR=3)
Heat
C Simulation with Trot = Tspin = 150 K
221(o)
321(o) 221(o)
423(o)
322(p)
220(p)
OPR=3
312(o),
212(o)
110(o)
212(o)
211(p)
1.6
D Simulation with Trot = 150 K, Tspin = 10 K OPR=0.3
1.4
1.2
220(p)
110(o) 2 (o)
12
1.0
322(p) 221(o)
211(p)
0.8
312(o),
0.6
4
(o)
23
212(o)
321(o) 221(o)
0.4
0.2
0.0
-0.2
80450
80500
80550
80600
80650
124.3nm
(248.6)
H2Oice
Substrate
-1
Two-photon wavenumber (cm )
Strongerortho-H2Olinesthan
para-H2Olines.
Well-reproducedwithOPR=3.
↕
Para-H2Osignalsaretoostrong
withOPR=0.3(10K).
TheOPRisnotrelatedto
120.0nm theiceformationtemperature.
(248.0)
Largedifferenceof“rotationalstateofH2O”
betweenthegasandsolidphases.
Thedefinitionofspintemperature.
3.0
Spinandrotationaldegeneracy
Ortho-to-para ratio
2.5
2.0
1.5
1.0
0.5
0.0
0
5
10
15
Rotationalenergyis
important!!
DE =34.2K.
Inrotational-hinderedsystem,
the“rotational”energydifference(ΔErot)becomessmall.
EveninasolidAr matrix,theDErot becomessmall~20cm−1.
Turgeonetal.,JPCA,121,1571(2017).
Michaut etal.,Vib.Spec.3483(2004).
20
25
30
Temperature (K)
35
40
45
50
OPR-Tspin curvesinrotationalhinderedsystems
Approximated(crude)OPR-Tspin curve;
Rotationally
hindered
Free
rotation
DE =1,10,20,23.8cm−1
3.0
Ortho-to-para ratio
2.5
Onlyconsiderthelowesttworotationallevels.
(sincewecannotknowallrotationallevels.)
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
25
30
35
Temperature (K)
AstheDE valuedecreased,theOPRreaches3atlowtemperature.
® Theenergiesofortho-,andpara-H2Obecomecomparable.
TheTspin curveonlyappliestoH2Ointhegasphase(freerotors).
40
45
50
Inice,H2Ohasahighbarrier(6700K)torotation,
becauseofhydrogen-bondnetwork.
Wittebort etal.,JACS.110,5668(1988).
Bothortho- andpara-H2Oarenolongerfreerotors.
Thequenched DE value:10−13cm−1 (10−13K)
(Thesameenergylevel!!)
Buntkowskyetal.,Z.Phys.Chem.222,1049(2008).
Ortho- andpara-H2Oareenergeticallycomparableat10K
instronglyrotational-hinderedsystems(ice).
H2O in ice
Gaseous H2O
DErot :
23.8 cm-1
DErot :
H
10-13 cm-1
H
O
H
O
H
Ortho
H
O
Para
H
NuclearspinconversionofH2O
NSC:ortho-para statemixingbymagneticperturbation(e.g.,paramagneticO2).
Protonhasamagneticmoment(10－26 JT－1)
Intermolecularproton–protondipolarinteractioncaninduceNSC.
NSCofwater(■)andmethane(△)inanAr matrix:
Enhancedbyintermolecularproton-protondipolarinteraction.
10−7 to10−6 cm−1 (10−30J)
(IntermsofNMR,10kHz)
Rapid
conversion!!
O
H
H
O
H
H
Increaseofconcentration
at4.2K
Fillionetal.,EASPub.Ser.,(2012).,FukutaniandSugimoto,Prog.Surf.Sci.,(2013).
Asaresultofrotationalquenching,
fastnuclearspinconversionofH2Ocanoccurinice
Inice,eachprotonfeelsmagneticfieldscreatedbyallprotonsinice.
↓
Ortho- andpara-statesarestronglymixed:
Intermolecularprotonmagneticinteraction>> rotationalenergydifference(DErot).
10−7 to10−6 cm−1 >>10−13cm−1
H
O
Conversiontimescale:0.1ms!!
H
Para-H2O
H
O
H
Ortho-H2O
Limbach etal.2006,
Chem.Phys.Chem.,7,551.
Buntkowsky etal.2008,
Z.Phys.Chem.,222,1049.
TheOPRofH2Oiniceisindynamicequilibriumatthestatisticalvalueof3.
(ThespinstateofH2Oissuperpositionofortho- andpara-H2O)
H
O
H
H
O
H
H
O
H
H
Ortho-H2O
23.8 cm-1
DErot :
H
10-13 cm-1
H
H
(2)Fastcontinuous
ortho-para interconversion
Gaseous H2O
DErot :
O
H
Para-H2O
(1) Comparableenergyofortho- andpara-H2O
byrotationalquenching
H2O in ice
O
O
H
Ortho
H
O
Para
O
H
H
H
O
H
H
TheoriginoftheanomalousOPRsofinterstellarH2Oisstillanopenquestion.
Nuclear-spineffectsingas-chemistryarekey.
(1)NSCviaion-moleculereactions,e.g.,H+ orH3O+.
Ortho-H2O(JKa,Kc =101)+H+ → Para-H2O(JKa,Kc =000) +H+
Ortho-H2O (JKa,Kc =101) +H3O+ → H3O++Para-H2O (JKa,Kc =000)
Thesetworeactionscanbeendothermicatlowtemp.because∆Erot =34.2K.
Mightleadtopara-enrichmentofH2O.
(2)Gas-formationprocesses.
OH+ ＋H2 →H2O+ ＋ H
H2O+ ＋H2 →H3O+ ＋ H
H3O+ ＋ e →H2O＋ H
SinceH2 canbepara-enrichedininterstellarclouds,
theH2Oproductsmightbealsopara-enriched.
H2O+,H3O+ havealsonuclear-spinisomers.
Nuclear-spineffectsshouldbeconsidered.
Accuratestate-to-stateratecoefficientsatlowtemperatureareunknown…
Gas-phaseconverion:
Nuclear-spinselectionrules:
Liqueetal.,Int.Rev.Phys.Chem.33,125(2014).
Gerlich etal.,Proc.R.Soc.A.364,3007(2006).
Summary
(1)H2O desorbedfromiceat10K shows astatisticalOPRof3,
evenwhentheiceisproducedinsituat10K.
(2)Reinterpretationofpreviousobservationsisnecessary.
(3)Importanceofthegas-phasechemistry.
(a) NSCviaion-moleculereactions,e.g.,H+ orH3O+.
(b) Gas-formationprocesses.
Hama, and Watanabe,Chem.Rev.113,8783(2013).
Hama,Kouchi,Watanabe,Science 351,65-67 (2016).
Financialsupports:
JSPS(grant24224012), MEXT(grant25108002).
AnotherexplanationfornuclearspinstateofH2Oinice:NMR
Waterice:adiamagneticinsulator(Nomacroscopicnuclearmagnetization).
↓
Thedirectionoftheprotonspinangularmomentumisdistributeduniformlyinice.
(Nonetmagneticmoment).
Eachprotonfeels
magneticfieldscreatedbyallprotonsinice,while
Proton-protondipolarinteractions <<thermalenergy
H
H
10−30
J(10 kHz) <<
10−22
J(10 K)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
WhenanH2Omoleculeisdesorbedfromtheiceintothegasphase,
itisrestoredas“acoupledhomonuclear spin-1/2pair”,ortho (I=1),orPara(I=0).
↓
SincethedesorbedH2Omoleculeshavesufficientenergy,
(>34.2K,ΔErot inthegasphase),
theOPRcanbestatistical.
Abragam:PrinciplesofNuclearMagnetism(1961).
Levitt:SpinDynamics:BasicsofNuclearMagneticResonance(2008).
Rotationalspectrum
Detector
Trot andTtrans aredifferent.
¯
Notinthermodynamicequilibrium.
¯
Nolocalheating(non-equilibriumprocess)
+
H
O
2
¯
“kick-out”Ionization
H2O(ice)+hn à hot H+OH
hot H+H2O(ice)à H+H2O(gas)
REMPIlaser
157nm
2mm
Photodissociation
icefilm
Au
AnderssonandvanDishoeck,A.& A.491,907(2008).
Trot=350K
80500 80600 80700
-1
Wavenumber / cm
Laser-delayspectrum
Ttrans=1800K
Photodesorption mechanism
“kick-out”:AnH2Omoleculeisdesorbedwithoutintramolecular bonddissociation
bythemomentumtransferfromanenergeticHatomphotodissociated fromaneighboringH2O.
H2O(ice)+hν →hotH+OH,
hotH+H2O(ice)→H+H2O
Andersson,andvanDishoeck,Astron.Astrophys.,491,907(2008).
kick-out:D2Odesorptionfrom“D2O+H2Slayeredice”by193nmirradiation.
H2S(ice)+193nm→hotH+SH
hotH+D2O(ice)à H+D2O(gas) !!
NoD2Ofollowing193 nmirradiationof
pureD2Oice(noabsorption).
1.0
m/z =20
¯
D2 O
0.5
0.5
0.0
D2O signal intensity / arb. unit
D2O signal intensity / arb.units
1.0
0
2
4
6
8
10
12
time-of-flight / us
No D2O desorption
193nm
14
0.0
×
0
2
4
6
8 10 12
Time of flight / µs
14
D2O desorption
193nm
D2O
Neat D2O ice
H2S
D2O
Experiment(MyPhDwork):Hamaetal.,JCP.132,164508(2010).
ResonanceEnhancedMulti-PhotonIonization(REMPI)spectroscopy
Ordinaryphotoionization techniquescannot
distinguishortho/para H2O(m/z =18)
Onephoton Multiphoton
ionization 2+1REMPI
ionization
In2+1REMPIofH2O,2-photonabsorptionexcitespopulation
fromthegroundX 1A1 (v=0,JKa,Kc)statetotheintermediateC 1B1(v’=0, J’Ka’,Kc’)state.
Theabsorptionofonefurtherphotontransferspopulationintotheionizationcontinuum.
SincetheREMPItransitionisrotationally(i.e.,OPR)resolved,
wecangettherotationalandspintemperatureofdesorbedH2O.
PhysicalChemistryResearchLabCourseATUBraunschweig,InstituteofPhysicalandTheoreticalChemistrySS2014
G.Meijer,J.J.ter Meulen,P.Andresen,andA.Bath,TheJournalofChemicalPhysics85,6914(1986).
ParamagneticcatalyticeffectsontheNSCofH2Oduringphotodesorption.
REMPIspectraofphotodesorbed H2Ofrom
H2O/O2(1:1)mixediceat10K.
Strongerortho-H2Olines
thanpara-H2Olines.
Reproducedbythesimulationwith
Trot=Tspin =200K.
Althoughtheelectronmagneticmomentis
about−658timeslargerthantheproton
magneticmoment,thetimescalefor
photodesorption (femto-s)ismuchshorter
thanthatforNSCthroughmagnetic
interactions(micro-sec).
+
H2O signal intensities / arb. units
2+1REMPIspectrumofH2OviatheC 1B1(v=0)←X 1A1 (v=0)transition.
(J’Ka’,Kc’←JKa,Kc)arerotationalassignments.
1.50
(a) Gaseous H2 O at room temperature
1.25
1.00
0.75
0.50
0.25
0.00
-0.25
1.50
101 -220
212 -313
1.25 (b) Simulations
202 -321
2
-2
3
-3
02
21
03 22
1.00
000 -221
0.75 212 -211 313 -312
0.50
0.25
0.00
-0.25
248.10
248.20
248.30
248.40
248.50
Wavelength / nm
JKaKc (ortho orpara)
Trot =Tspin =300K
Gray;difference
Trot =Tspin =300K
Trot =300K,Tspin =30K
Trot =300K,Tspin =20K
Trot =300K,Tspin =15K
Trot =300K,Tspin =8K
248.60