Mössbauer spectroscopy yog cov txheej txheem raws li qhov tshwm sim los ntawm Rudolf Ludwig Mössbauer xyoo 1958. Qhov peculiarity yog tias txoj kev muaj nyob rau hauv rov qab ntawm resonant absorption thiab emission ntawm gamma rays nyob rau hauv solids.
Zoo li sib nqus resonance, Mössbauer spectroscopy tshuaj xyuas cov kev hloov pauv me me ntawm lub zog ntawm cov atomic nucleus teb rau nws ib puag ncig. Feem ntau, peb hom kev sib cuam tshuam tuaj yeem pom:
- isomer hloov, yav tas los kuj hu ua tshuaj hloov pauv;
- quadrupole splitting;
- ultrafine splitting
Vim lub zog siab thiab nqaim heev ntawm gamma rays, Mössbauer spectroscopy yog cov txheej txheem rhiab heev ntawm lub zog (thiab yog li zaus) daws teeb meem.
Basic Principle
Zoo li rab phom thaws thaum raug rho tawm haujlwm, tswj lub zog yuav tsum muaj cov tub ntxhais (piv txwv li hauv cov pa roj) kom rov ua dua thaum nws tawm lossis nqus gammahluav taws xob. Yog hais tias lub atom ntawm so emits ib tug nqaj, nws lub zog yog tsawg tshaj li lub ntuj hloov zog. Tab sis txhawm rau kom cov tub ntxhais nqus cov gamma rays thaum so, lub zog yuav tsum yog me ntsis ntau dua li lub zog ntuj, vim tias nyob rau hauv ob qho tib si lub thrust poob thaum lub recoil. Qhov no txhais tau hais tias nuclear resonance (lub emission thiab nqus ntawm tib lub gamma hluav taws xob los ntawm zoo tib yam nuclei) yog tsis pom nrog dawb atoms, vim hais tias lub zog hloov yog loj heev thiab emissions thiab absorption spectra tsis muaj qhov tseem ceeb sib tshooj.
Nucles hauv cov khoom siv lead ua tsis tuaj yeem thaws vim tias lawv raug khi los ntawm cov iav siv lead ua. Thaum ib qho atom hauv cov khoom emits los yog absorbs gamma hluav taws xob, qee lub zog tseem yuav ploj mus raws li qhov tsim nyog recoil, tab sis qhov no nws ib txwm tshwm sim hauv cov pob ntawv sib cais hu ua phonons (quantized vibrations of the crystal lattice). Txhua tus lej ntawm phonons tuaj yeem raug tso tawm, suav nrog xoom, uas yog lub npe hu ua "tsis muaj rov qab". Nyob rau hauv rooj plaub no, kev txuag ntawm lub zog yog ua los ntawm cov siv lead ua tag nrho, yog li tsis muaj zog poob qis.
Ntxhais nrhiav pom
Moessbauer pom tias ib feem tseem ceeb ntawm emission thiab nqus cov xwm txheej yuav tsis rov qab los. Qhov tseeb no ua rau Mössbauer spectroscopy ua tau, vim nws txhais tau hais tias gamma rays emissions los ntawm ib tug nucleus yuav resonantly absorbed los ntawm ib tug qauv uas muaj nuclei nrog tib isotope - thiab qhov nqus no tuaj yeem ntsuas.
Cov feem seem ntawm qhov nqus tau raug tshuaj xyuas siv nuclearResonant oscillatory method.
Qhov twg yuav coj Mössbauer spectroscopy
Nyob rau hauv nws daim ntawv uas muaj ntau tshaj plaws, cov qauv khoom yog raug rau gamma hluav taws xob thiab lub detector ntsuas qhov hnyav ntawm tag nrho cov nqaj uas tau dhau los ntawm tus qauv. Cov atoms nyob rau hauv qhov chaw emitting gamma rays yuav tsum muaj tib lub isotope raws li nyob rau hauv cov qauv uas absorbs lawv.
Yog hais tias lub radiating thiab absorbing nuclei nyob rau hauv tib lub chemical ib puag ncig, lub zog hloov nuclear yuav sib npaug, thiab resonant absorption yuav raug soj ntsuam nrog ob qho tib si cov ntaub ntawv thaum so. Qhov txawv ntawm qhov chaw tshuaj lom neeg, txawm li cas los xij, ua rau lub zog nuclear hloov mus rau ntau txoj hauv kev.
Thaum lub sijhawm Mössbauer spectroscopy txoj kev, qhov chaw tau nrawm dua li ntau qhov kev tshaj tawm uas siv lub cev muaj zog kom tau txais cov nyhuv Doppler thiab luam theej duab gamma ray zog hauv ib lub sijhawm. Piv txwv li, ib tug raug ntau yam rau 57Fe yuav yog ± 11 mm/s (1 mm/s=48.075 neV).
Nws yog ib qho yooj yim los nqa tawm Mössbauer spectroscopy nyob rau ntawd, qhov twg hauv qhov tau txais spectra qhov kev siv ntawm gamma rays tau nthuav tawm raws li kev ua haujlwm ntawm qhov chaw tus nqi. Ntawm qhov nrawm sib xws rau cov qib resonant zog ntawm cov qauv, qee qhov ntawm gamma rays yog absorbed, uas ua rau poob rau hauv qhov ntsuas siv thiab ib qho kev poob rau hauv spectrum. Tus naj npawb thiab txoj hauj lwm ntawm lub ncov muab cov ntaub ntawv hais txog cov tshuaj ib puag ncig ntawm absorbing nuclei thiab tuaj yeem siv los ua tus qauv. Li noKev siv Mössbauer spectroscopy ua rau nws muaj peev xwm daws tau ntau yam teeb meem ntawm cov qauv ntawm cov tshuaj lom neeg; nws kuj yog siv hauv kinetics.
Xaiv qhov tsim nyog
Qhov xav tau gamma ray puag muaj cov niam txiv xov tooj cua uas lwj mus rau qhov xav tau isotope. Piv txwv li, qhov chaw 57 Fe muaj 57 Co, uas yog tawg los ntawm kev ntes cov hluav taws xob los ntawm lub xeev zoo siab los ntawm 57 Fe. Nws, nyob rau hauv lem, decays mus rau hauv lub ntsiab txoj hauj lwm ntawm lub emitting gamma rays ntawm coj lub zog. Radioactive cob alt yog npaj rau ntawm ntawv ci, feem ntau rhodium. Qhov zoo tshaj plaws, lub isotope yuav tsum muaj ib nrab-lub neej yooj yim. Tsis tas li ntawd, lub zog ntawm cov hluav taws xob gamma yuav tsum muaj tsawg, txwv tsis pub lub kaw lus yuav muaj qhov tsis tshua muaj qhov tsis zoo, ua rau muaj qhov tsis zoo thiab lub sijhawm sau ntev. Daim ntawv teev sijhawm hauv qab no qhia cov ntsiab lus uas muaj cov isotope haum rau MS. Ntawm cov no, 57 Fe yog hnub no cov ntsiab lus feem ntau kawm siv cov txheej txheem no, txawm hais tias SnO₂ (Mössbauer spectroscopy, cassiterite) kuj tau siv ntau zaus.
Kev tsom xam ntawm Mössbauer spectra
Raws li tau piav qhia saum toj no, nws muaj lub zog daws teeb meem zoo heev thiab tuaj yeem kuaj pom txawm tias muaj kev hloov pauv me ntsis hauv ib puag ncig nuclear ntawm cov atoms sib xws. Raws li tau hais los saum no, muaj peb hom kev sib cuam tshuam nuclear:
- isomer hloov;
- quadrupole splitting;
- ultrafine splitting.
Isomeric shift
Isomer hloov pauv (δ) (tseem qee zaum hu ua tshuaj) yog ib qho kev ntsuas txheeb ze piav qhia txog kev hloov pauv hauv lub zog resonant ntawm lub nucleus vim kev hloov ntawm cov hluav taws xob hauv nws cov s-orbitals. Tag nrho cov spectrum yog hloov mus rau qhov zoo lossis qhov tsis zoo, nyob ntawm tus nqi ceev ntawm s-electron. Qhov kev hloov pauv no yog vim muaj kev hloov pauv hauv cov lus teb electrostatic ntawm orbiting electrons uas tsis yog xoom qhov tshwm sim thiab cov nucleus nrog lub ntim tsis yog xoom uas lawv tig.
Piv txwv: thaum tin-119 siv hauv Mössbauer spectroscopy, tom qab ntawd qhov kev tshem tawm ntawm cov hlau divalent uas lub atom pub rau ob lub electrons (lub ion yog xaiv Sn 2+), thiab kev sib txuas ntawm plaub-valent (ion Sn4+), qhov twg lub atom poob txog plaub electrons, muaj qhov sib txawv isomeric hloov.
Tsuas yog s-orbitals qhia qhov ua tau tsis yog xoom, vim tias lawv cov duab peb sab kheej kheej suav nrog lub ntim ntim los ntawm cov nucleus. Txawm li cas los xij, p, d thiab lwm yam khoom siv hluav taws xob tuaj yeem cuam tshuam qhov ntom ntom s los ntawm kev tshuaj ntsuam xyuas.
Isomer hloov pauv tuaj yeem qhia tau siv cov qauv hauv qab no, qhov twg K yog qhov tsis tu ncua nuclear, qhov txawv ntawm Re2 thiab R g2 - zoo nuclear nqi hluav taws xob sib txawv ntawm lub xeev zoo siab thiab lub xeev hauv av, nrog rau qhov sib txawv ntawm [Ψs 2(0)], thiab [Ψs2(0)] b qhov sib txawv ntawm electron ceev ntawm lub nucleus (a=qhov chaw, b=qauv). Tshuaj hloovLub isomer tau piav qhia ntawm no tsis hloov nrog qhov kub thiab txias, tab sis Mössbauer spectra yog qhov tshwj xeeb tshaj yog vim muaj kev sib raug zoo uas hu ua Doppler nyhuv thib ob. Raws li txoj cai, kev cuam tshuam ntawm cov nyhuv no yog me me, thiab tus qauv IUPAC tso cai rau isomer hloov pauv yam tsis tau kho nws txhua.
Kev piav qhia nrog piv txwv
Lub ntsiab lus ntawm lub cev ntawm qhov sib npaug uas pom hauv daim duab saum toj no tuaj yeem piav qhia nrog piv txwv.
Thaum qhov nce ntawm qhov ceev ntawm s-electrons nyob rau hauv spectrum ntawm 57Fe muab qhov kev hloov tsis zoo, txij li qhov kev hloov pauv ntawm cov nqi hluav taws xob muaj txiaj ntsig tsis zoo (vim R e <Rg), nce qhov ceev ntawm s-electrons hauv 119 Sn muab kev hloov pauv zoo vim rau qhov kev hloov pauv zoo hauv tag nrho cov nqi hluav taws xob nuclear (vim R e > Rg).
Oxidized ferric ions (Fe3+) muaj cov isomer hloov me dua li ferrous ions (Fe2+) vim qhov ceev ntawm s -electrons nyob rau hauv lub hauv paus ntawm ferric ions yog siab dua vim qhov tsis muaj zog shielding nyhuv ntawm d-electrons.
Isomer hloov pauv yog qhov muaj txiaj ntsig rau kev txiav txim siab oxidation xeev, valence xeev, electron shielding, thiab muaj peev xwm tshem tawm electrons los ntawm cov pab pawg electronegative.
Quadrupole splitting
Quadrupole splitting qhia txog kev sib cuam tshuam ntawm qib nuclear zog thiab qhov chaw hluav taws xob ambient gradient. Nuclei nyob rau hauv cov xeev nrog ib tug non-spherical nqi tis, piv txwv li, tag nrho cov nyob rau hauv uas lub angular quantum tooj yog ntau tshaj 1/2, muaj ib tug nuclear quadrupole lub sij hawm. Hauv qhov no, qhov chaw hluav taws xob asymmetric (tsim los ntawm asymmetric hluav taws xob xaim hluav taws xob lossis kev teeb tsa ligand) faib cov qib hluav taws xob nuclear.
Nyob hauv cov isotope nrog lub xeev siab ntawm kuv=3/2, xws li 57 Fe lossis 119 Sn, lub xeev zoo siab tau muab faib ua ob lub xeev: mI=± 1/2 thiab mI=± 3/2. Kev hloov pauv ntawm ib lub xeev mus rau lub xeev zoo siab tshwm sim raws li ob lub siab tshaj plaws hauv spectrum, qee zaum hu ua "ob npaug". Quadrupole splitting yog ntsuas raws li qhov kev ncua deb ntawm ob lub ncov no thiab qhia txog qhov xwm ntawm hluav taws xob hauv lub nucleus.
Quadrupole splitting tuaj yeem siv los txiav txim siab oxidation xeev, xeev, sib luag thiab kev npaj ntawm ligands.
Magnetic ultrafine splitting
Nws yog qhov tshwm sim ntawm kev sib cuam tshuam ntawm cov nucleus thiab txhua qhov chaw sib nqus ib puag ncig. Lub nucleus nrog spin kuv splits rau hauv 2 I + 1 subenergy theem nyob rau hauv lub xub ntiag ntawm magnetic teb. Piv txwv li, ib tug nucleus nrog spin state I=3/2 yuav muab faib ua 4 non-degenerate substates nrog cov nqi mI +3/2, +1/2, - 1/ 2 thiab 3/2. Txhua qhov muab faib yog hyperfine, ntawm qhov kev txiav txim ntawm 10-7 eV. Txoj cai xaiv rau magnetic dipoles txhais tau hais tias kev hloov pauv ntawm lub xeev zoo siab thiab lub xeev hauv av tsuas tuaj yeem tshwm sim qhov twg m hloov mus rau 0 lossis 1. Qhov no muab 6 qhov kev hloov mus los ntawm3/2 rau 1/2. Feem ntau, tsuas yog 6 lub ncov tuaj yeem pom hauv cov spectrum uas tsim los ntawm kev sib cais hyperfine.
Qib ntawm kev sib cais yog proportional rau qhov kev siv ntawm txhua qhov sib nqus ntawm lub nucleus. Yog li ntawd, lub magnetic teb tau yooj yim txiav txim siab los ntawm qhov kev ncua deb ntawm lub peaks sab nrauv. Nyob rau hauv cov ntaub ntawv ferromagnetic, nrog rau ntau lub tebchaw hlau, ntuj sab hauv magnetic teb muaj zog heev thiab lawv cov teebmeem dominate lub spectra.
Peb lub ntsiab Mössbauer tsis:
- isomer hloov;
- quadrupole splitting;
- ultrafine splitting.
Txhua peb yam khoom feem ntau tuaj yeem siv los txheeb xyuas qhov sib xyaw tshwj xeeb los ntawm kev sib piv cov qauv. Nws yog txoj haujlwm no uas tau ua tiav hauv txhua lub chaw soj nstuam ntawm Mössbauer spectroscopy. Cov ntaub ntawv loj, suav nrog qee qhov kev tshaj tawm tsis pub dhau, yog tswj hwm los ntawm cov ntaub ntawv chaw. Qee zaum, ib qho kev sib txuas yuav muaj ntau dua ib txoj haujlwm rau Mössbauer active atom. Piv txwv li, cov qauv siv lead ua ntawm magnetite (Fe 3 O 4) tuav ob qhov chaw sib txawv rau cov hlau atoms. Nws cov spectrum muaj 12 lub ncov, ib tug sextet rau txhua qhov chaw muaj peev xwm atomic sib raug rau ob pawg ntawm qhov ntsuas.
Isomeric shift
Mössbauer spectroscopy txoj kev tuaj yeem siv tau txawm tias peb qhov teebmeem raug pom ntau zaus. Hauv cov xwm txheej zoo li no, kev hloov pauv isomeric yog muab los ntawm qhov nruab nrab ntawm txhua kab. quadrupole splitting thaum tag nrho plaubzoo siab substates yog sib npaug biased (ob lub substates yog nce thiab ob tug down) yog txiav txim los ntawm offset ntawm ob sab sab kab txheeb ze rau lub puab plaub. Feem ntau, rau qhov tseeb, piv txwv li, hauv chav kuaj ntawm Mössbauer spectroscopy hauv Voronezh, siv software tsim nyog.
Ntxiv rau, qhov sib txheeb ntawm qhov sib txawv ntawm ntau lub ncov qhia txog qhov sib xyaw ua ke hauv cov qauv thiab tuaj yeem siv rau kev tsom xam ib nrab. Vim hais tias ferromagnetic phenomena yog magnitude nyob ntawm seb, nyob rau hauv tej rooj plaub spectra yuav muab kev pom rau qhov luaj li cas ntawm crystallites thiab grain qauv ntawm cov khoom.
Mossbauer spectroscopy settings
Txoj kev no yog ib qho txawv txav tshwj xeeb, qhov twg cov khoom emitting yog nyob rau hauv cov qauv kuaj, thiab cov khoom absorbing yog nyob rau hauv tus qauv. Feem ntau, txoj kev no yog siv rau khub 57 Co / 57 Fe. Ib daim ntawv thov raug yog tus cwj pwm ntawm cov chaw cob alt hauv amorphous Co-Mo catalysts siv hauv hydrodesulfurization. Hauv qhov no, tus qauv yog doped nrog 57Ko.
