A Physical Chemistry Experiment in Mossbauer Spectroscopy William H. Armstrong. -. Ernest E. Dorlllnger. - . Owen T. Anderson, and Bennett R. Wllleford, Jr. Bucknell University. Lewisburg. PA 17837 The recoilless emission of r-radiation followed by its resonant absorption by a sample was discowred h i Rudolf Mc~shauerin 1958 and is now known as the Mosshawr Kffect. Because of the very narrow line widths which result, extremely small energy changes can he observed now. Significant applications have been found, many of which are of particular interest to the chemist. For this reason, it is desirable that the undergraduate physical chemistry laboratory include an introduction to this spectroscopic technique. Research Mossbauer enuioment is not usuallv available for use in undergraduate teaching laboratories. However, simplified instrumentation adeuuate for significant introductorv exneriments can he assemhlkd a t reasonable cost. An ~nstructiokalScientific Equipment program grant by the National Science Foundation enabled Bucknell University to acquire the necessarv basic e a u i ~ m e nin t January 1976. Suvvlementarv avparatus was consiructed, and thehew expe&ent was introduced during the spring semester, 1977. We make available our experience with this experiment to encourage others to explon. the i~ossihilityoC:lls~,idding M6ssbaut:r exi~eriments totheir undergraduate programs. A
.
Theory' The natural width of a spectral line from a source can be determined from the Heisenberg Uncertainty Principle and the mean lifetime of the excited state. For lifetimes of 10-6-10-10 s, y-rays with natural line widths of 10-9-10-5 ev result. For a solid source, the emitting atom is rigidly bound in the crystal lattice, and a significant fraction of the low energy radiation will he recoil free; the line is not broadened in these cases, and a gamma source of very narrow width results. This precision enables one to observe the very small energy chanees involved in hvnerfine interactions not nreviouslv -. detertahle. Furthermore, the line width issuchastnall fraction of thr r~hutun'stotal enerm -.t h a ~siznificant shiftsot the enerey -. can be made by moving the source relative to the absorber (DoDoler effect) with velocities of the order of mm s-' which are attained easily in the laboratory. When the emitter or absorber is moved toward the other durine a measurement. the frequency of the photon passing he&een them is increased: thus, absorber transitions of energy hieher than that of the emitter can be measured. ~ o n v e r s e l ~ m o away ~ o n from each other leads to measurement of lower energy transitions. A Mossbauer spectrum consists of a plot of y-ray counts versus relative velocity. Resonant absorption a t some particular relative velocitv is indicated bv a dron in the conntine" rate ui 7 rays transm~tted111ruu.h the ahsorhrr. The hli~sshaueroararneters 01 hterest is this e ~ u t r i m e n t are the isomer shifi (IS), the quadrupole splitting (QS), and maenetic hvnerfine structure. The isomer shift (sometimes cafed the chkmical shift) arises from the fact that there is a small difference in the radii of the nucleus in excited and ground states, 6r = r , - rgr. The isomer shift is given by the eqn. (5) e 2 26r [l+(0)ls2-lil.(O)lall = (const) (61+(0)12)
-
.~
):(
where the final factor is the difference in s-electron density at the nucleus between source and absorber. The nuclear factor 6rlr can be either positive or negative; thus, a positive
spherically symmetric field Figure 1. Energy fields.
level
asymetric field
diagram for $'Fe nucleus in presence of
electric
IS for 57Fefor which 6rlr is negative indicates a decrease in s-electron density while, for llgmSnfor which 6rIr is positive, a ~ositiveIS is associated with an increase in s-electron density a t the nucleus. Isomer shift data correlations with s electron densities have man"- ao~lications in chemistrv. in.. cluding studies of electronegativities of ligands, oxidstion states, covalency and ionic bonding, shielding of nuclei from electrons, polarities and polarizati& effects,and the extent of d , back-bonding in metal complexes. Isomer shift data are usually reported relative to some appropriate reference absorber. Quadmpole splitting (QS) arises when a non-homogeneous electric field exists a t a non-spherical nucleus. Any nucleus with $pin I > I/' is non-spherical and possesses a quadrupole I I I ~ , I ~Q. I ~An I electrir field gradient q can br pnrluced by an asymmetric distrihution of electrons around~thenucleus or by the effects of neighboring atoms in the lattice. The quadruvole moment can alinn itself at various mace auantized orientations relative to the gradient. For the case o i 5 7 ~ ethe , excited state has I = 312 while for the ground state I = 112. The resulting energy level diagram is-shown in Figure 1. (Notice that the energies of the *3/2 states are equal and difhwnl from the equal energies of the rllL'states).- or this c a w (IS = tconst)(Q)ty,and the quadrupdr splirting isdirectlv proporrional t o the magnitude of the clectrw field gra(limr. 'l'hus, the appcnr~nwo r quadrupole splitting indicates nn nsvmmetric distrihution ol'electrons around the hlbsshauer nucleus, and the magnitude is an indication of its extent. However. often it is difficult toattribute themaenitude of the electric held gradient to specific factors. ~ ~ p l i c a t i o nofs T h e following brief introduction to the theory of the Mbssbauer Effect is intended to give the student the background necessary to understand and verform the ex~eriment.More informationis readilv n\.ndnhl~.Some rourcrr uh~chnr hare found p?rtic?lllrly useful and to which students are ret'erred nrr given in retrnwrcs (l)-t91.
Volume 58 Number 6 June 1981
515
Figure 3. Block Diagram of t&%sbauw System
Figwe 2. Energy level diagram fa 5'Fe splining of nuclear states.
nucleus shaking
Nuclear
Zeeman
Mossbauer QS measurements have included the establishment of site symmetries in lattices, the nature of bridging atoms and cis-trans isomerism in complexes, distortion from tetrahedral or octahedral geometries in complexes, coordination numbers. and hieh-soin. low-soin eouilihria. Magnetic hyderfine &u&re can be in;estigated by the ao~lication of a maenetic field to the emittine nucleus. The . (21- 1 ) fckl degeneracy 01nuclear spin states I+ not remo\,ed by- the orrscnce of an elrrtr~cfitld nradient hut is removed bs . a magnetic field and is referred
