Local Structure of Amorphous Ice as Revealed by O K‐Edge EXAFS

SummaryandOutlook TheFouriertransformoftheOK-edgeEXAFSspectrumofanamorphousicefilmrevealsseveraldistinctpeakssuggestivelyattributedinthisworkto: * O Hcontactsduetooverlappingcontributionsfromintra-molecularcovalentandintermolecularhydrogenbondssi-mulatedwithasetofthreedistancesat1.01(effectiveco-ordinationnumberN=0.6),1.41(N=1.6),and1.89a(N=1.8); * closestintermolecularO¥¥¥Ocontactsat2.73a(N=4); * longer intermolecular O¥¥¥O contacts at 3.61, 5.36, and6.57a.Thisisthefirsttimethatbothcovalentandhydrogen-bridgeO HbondsinthelocalenvironmentofoxygenatomsinicewererevealedbyOK-edgeEXAFS.Apossibledistortionofthereportedbest-fitparameters,duetoacontributionfrompho-toelectronscatteringbyoxygenlonepairsorotherAXAFS-re-latedeffects,hasyettobeevaluated.TheanalysisofthelongerO¥¥¥Ocontactssuggeststhatthelocalenvironmentaroundtheoxygenatomsinthenear-sur-faceregionoftheamorphousicefilmdifferssubstantiallyfromthebulkstructureoflow-densityamorphousiceandcrystallineiceI,andisclosertothatobservedinhigh-densityamorphousice(HDAandVHDA)orhigh-pressurecrystallinephasesofice,(iceII,III,orIX).TheresultshencedemonstratethatsoftX-rayEXAFSgivesvaluableinformationaboutthelocalatomicstructureofmo-lecularsolidssuchasamorphousice,eventhoughtheyconsistonly of light atoms. Due to its surface sensitivity, O K-edgeEXAFSmakesavaluablecontributiontotheongoingdiscus-sionofthestructuralpropertiesoficeandwater,complemen-tarytotraditionaldiffractiontechniques.Nevertheless,thisstudyleavesseveralimportantquestionsunansweredandweplantocontinueworkinginthisfield.Inparticular,acomparativeanalysisofEXAFSspectraofcrystal-line ice, amorphous ice, and liquid water (measured underidenticalconditionsusingthebulksensitiveX-rayfluorescenceyieldacquisitionmode)wouldbeveryinteresting.