early history of xray and diffraction.docx
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earlyhistoryofxrayanddiffraction
AppliedModernPhysicsI
X-raydiffraction/topographyandspectroscopy,electronmicroscopy,materialsscience
X-rayradiationisveryhighinenergyE=hf(≈210-15J=1,25104eVsoit’sthesameenergyanelectronwouldhaveifitwereacceleratedbyanelectricalforcegoingthroughapotentialof12,500V)that’swhyitpenetratedskinandflesheasily,bonesnotquitesoeasilyandhaveusageinmedicine–isthatthemainusage?
?
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WhowasConradWilhelmRöntgen,discovererofX-rays?
Amedicaldoctor?
Aphysicist,theveryfirstNobelprizewinnerinPhysics?
howdidhediscoverX-rays?
1895,bychance,experimentingwithcathoderays(doingsimilarthingstoJ.JThompson)ononeendofthelaboratory,therewasasheetofpaperthatwascoveredwithaphosphorsittingaroundattheotherendofthelaboratory,experimentinginthedark,henoticedthatphosphorlightsupwhenheswitchesonhiscathoderaytube,draggingoutelectronsandacceleratingthembyapotentialdifference,thecathoderaytubeisexpectedtobeundervacuum,buttherewasjustenoughrestgas(air)thatelectronsgotsloweddownbybeingscatteredbythemolecules,
todayweknow:
whenelectronsaresloweddowntheyradiateofftheirlostinkineticenergy–andthatisX-raysanelectromagneticwave+astreamofhighenergyphotonstravelingatthespeedoflight
atthetimenobodyknowshowtheradiationoriginatesandofwhatkinditwas:
waveorparticles?
Röntgencouldinalongseriesofexperimentsshowed:
-X-rayspenetrateallsubstancestosomeextend,elementsoflowatomicweightareespeciallytransparent,elementsofhighatomicnumbercanbyeusedasshieldsforprotectionagainstX-rays
-X-rayscausemanysubstancestofluoresce,i.e.absorbingthemandradiatingoffelectron-magneticwavesofadifferentwavelength
-X-raysarenotdeflectedinamagneticfield,sotheydonotconsistofchargedparticles
-X-rayscaneffectivelybegeneratedby“cathoderays=acceleratedelectrons”fallonametalanode(andanyothermaterialplacedintheirpath
-X-raysionizethegassesthroughwhichtheypass,causinganincreaseofelectricalconductivity
butnotdiffractionwhichwouldproveX-raysaretobedescribedbywaves!
reason:
then-ofcourse-unknown,thewavelengthofX-raysissosmallthattherewerenomanmadediffractiongratingsavailable
howdoesadiffractionatagratingdifferfromthatofadoubleslitarrangement?
adiffractiongratinghasamuchmuchgreaternumberofslits
forthepositionsofthemaxima
dsinθ=mλ
wherem=0,1,2,…arecalledtheorderofthelineanddiscalledthegratingperiod,i.e.quotientofsomelengthWdividedbythenumberoflines
asthemaximaofthelinesisverywelldefinedandnarrow,onecanmeasurethewavelengthoftheincident(mono-orpolychromaticifitisnottoomany)radiationthatproducedtheinterference/diffractionpattern(foradoubleslitexperimentthebrightfringesarisingfromdifferentwavelengthlightwouldoverlaptoomuchtobedistinguished
by1912,SommerfeldatthePhysicsDepartmentofUniversityofMunich,nominated80timesforNobelprizewithouteverwinningit,diedfromacaraccidenthadcalculatedanorderofmagnitudeforthewavelengthofX-rays:
0.1nm
ifthatweretobetrue,wavelengthwouldbesoshortthatdiffractionof0.1nmX-raysfromastandardopticalgratingofperiod3μmweretogivediffractionangleof0.002°
sopessimism,howissomebodygoingtoproveX-raysarewavesornot
aPhDstudentofSommerfeld,Ewaldworkingonsolidstatetheoryimplicationswhatifcrystalswere,assomeoftheleadingmineralogist(e.g.VictorM.GoldsmithattheMineralogyDepartmentofUofMunich)expectedathree-dimensionalorderedarrangementofatomswithinter-atomicdistancesoftheorderofmagnitude0.1nm–youhavetopausehere,theveryexistenceofatomswasnotwellestablishedatthattimeletalonetheirpossiblearrangementincrystallinesolids,
approachedMaxLaue(anassistantprofessoratSommerfeld’schair)atCafé“Lutz”,wherestudentsandfacultyusedtohangout,onanquiteunrelatedmatter
Lauewasatthattimeworkingonatreatiseonclassicalopticsandheardforthefirsttimebothhypotheses,thatthewavelengthofX-raysmaybeasshortat0.1nmandthatcrystalsmaypossessapossibleperiodicarrangementoftheatomsin3dimensionswithaspacingoftheorderofmagnitude0.1nm
ontheirwaybacktothePhysicsdepartment,theideaoccurredtoLaue,well,thereshouldbeinterferencesifacrystalisexposedbyabeamof“sufficientlymonochromatic”X-rays
hehimselfbeingatheorist,hetalkedapostdoc,Friedrich,whohadpreviouslyworkedwithRöntgen)andapostgraduate,Knipping,intotryingtheexperiment,behindthebackofthebigboss,Sommerfeld,whenSommerfeldheardaboutithewasfuriousespeciallyastheydidn’tgettheresultsLaueexpectedastheydidn’tknowwheretolookfor-sotheprojectcametoanhalt
Lauefinallygotpermissiontogoaheadbutthebigbosswasstillconvinceditwasastupidexperimentashecalculated,eveniftherewereX-rayinterferences,thethermalmotionoftheatomsattheirrespectiveplacesinacrystalwouldsmearthemout.
littledidheknow…
andMaxvonLaue,i.e.hegotknighted,gotforthecorrectinterpretationoftheintensitymaximaandthedevelopmentofthesocalledkinematictheoryofX-raydiffractionhis1914Nobelprize.
whatwasfoundinoneexperimentnotonlythatX-raysdoindeedpossesswavespropertiesandthatcrystalsindeedpossessathreedimensionalarrangementofatoms,butalsonothinglessthanthediscoveryofamethodtoprobetheinnerstructureofcrystallinematter,whichweknowtodayisalmostallsolids,betheymetals,ceramics,semiconductors,….(notglass,rubber,andcertainplastics)
adiffractiongratingisreallyaonedimensionalthing,thenaturallyoccurringcrystalsare,however,3dimensional,thereforewegetaslightlydifferentrelationforthedistributiononintensityonascreenofphotograph
Bragg’sequation:
nλ=2dsinθwherenisorderofreflection
allowingcrystalstobeusedaskindofagratingformeasuringwavelengthofcharacteristicX-rayswhicharisefromelectronictransitionsoftheinnermostelectrons
andanalysisofcrystalsstructures,solidsingeneral
asamatteroffactanewsciencewasenabledwhichgoesbythenamematerialsscience/crystallography
newsciencesalwaysstartoutasaparepistemes(R.W.Cahn)
fromtheancientGreek:
episteme–meaningadomainofknowledge
par-meansamongstotherthingssubsidiary
i.e.asubsidiarysubjectwithinascienceissomethingrightatthefringesofattentionandfundingthatdoesnotprimarilyaimforsolvinganypracticalproblem,peopleonlydoitforthefunofit,becausetheyfinditinteresting.Chandescribeditas:
“lookingattheforest,notjustselectingatreeforlogging…”
lateron,electron,andneutrondiffraction,whicharecomplementarytoX-raydiffraction–thesinglemostimportanttoolsofmaterialsscience
ascienceinitsownright?
It’sasciencewhenithasitsownjournals,conferences,societies,institutionalizeddepartments,degrees,graduatestudents,…whenithassomethinguniquethatdistinguishesitfromthesciencethatgavebirthtoit(whichisprobablyalwaysPhysics)
whatisuniqueaboutmaterialsscience/crystallography?
R.WChan:
“…thematerialsscientisthastoworkatseverallevelsoforganization,eachofwhichisunder-pinnedbythenextlevel.Here,again,heisbrotherundertheskinofthebiologist,whodoesjustthesame:
staringwiththecellwall,say,thegoesontostudythemorphologyandeconomyofthecellasawhole,thentheisolatedorgan(madeofcells),thentheorganismasawhole.…theconceptofmicrostructureisthemostimportantsingledefinitionthemeofMaterialsScienceandEngineering.”
perhapssomethingisasciencewhenithasspunoffanengineeringdiscipline,whatdoyouthink?
perhapssomethingisasciencewhenconsistentlyNobelprizeshavebeenawardedinthatfield,whatdoyouthink?
NobelprizesPhysics:
1914MaxvonLaue,X-raysareanelectromagneticwaveofabout0.1nmwavelength,atomsincrystalsarearrangedinaregular3Dlatticewithspacinginthe0.1nmrange,thereisX-raydiffractionfromcrystals,thepossibilityofanalyzingthearrangementsofmatterincrystals,(Laueequations),1912
1915SirWilliamHenryBraggandWilliamLawrenceBragg(sonoftheformer),X-raycrystallography,W.L.Braggequation,X-raycrystalspectrometer,1913
1937ClintonDavidsonandSirGeorgeP.Thompson(J.J.Thompson’ssonwhofirstshowedelectronsareparticles),electrondiffractionbycrystals,inotherwordselectronsarewaves,1927
1986ErnstRuska,inventingtheelectronmicroscopewhereoneoperationmodeiselectrondiffraction(1931)togetherwithGerdBinningandHeinrichRohrer,scanningtunnelingmicroscope,(completelyunrelated)
1994BertramN.BrockhouseandCliffordG.Schull,pioneeringneutrondiffractionandspectroscopy
NobelPrizesChemistry
1936PeterJ.Debye
X-rayandelectrondiffractionanalysesofmolecules
1962MaxF.Perutz,JohnC.Kendrew
X-raydiffractionanalysisofhaemoglobinundmyoglobin
1964DameDorothyCrowfootHodgkin
X-raydiffractionanalysisofbiochemicalmacromolecules
1976WilliamN.Lipscomb
X-raydiffractionanalysisofborhydrids
1982AaronKlug
X-raydiffractionanalysisofnucleicacidsandproteincomplexes
1985HerbertA.Hauptmann,JeromeKarle
directmethodsofX-raycrystallography
1988JohannDeisenhofer,RobertHuber,HartmutMichel
structureandfunctionofprotein-chromoph