GENERAL DISCUSSION Mr. P. A. Cox (Oxford University) (communicated) Schatz and his coworkers have attempted to interpret the MCD of hexachloro-osmate on the basis of the ligand-field assignment due to D0rain.l Now the optical spectrum of hexachloro-osmate is very similar to that of hexachloro-iridate and if the spectra are due to ligand-field transitions this is difficult to understand since the osmium compound is d4 and the iridium compound d5. Jarrgensen has shown however that the similarity can be explained if it is assumed (i) that both spectra arise from charge-transfer transitions, and (ii) that because of the high spin-orbit coupling constant of osmium we may apply pure j-j coupling to the osmate.2 In this interpretation the intense band at about 26 kK in the osmate spectrum like the first intense band in the iridate spectrum is due to transitions from the U’ (r,) spin-orbit component of a t l orbital of pre-dominantly ligand pi character into the E” (I?,) component of the f 2 g orbital.The intense band starting at 29 kK contains two overlapping transitions from the E” and U’ components of a t2u ligand pi orbital into the E component of t2g. The charge transfer interpretation of the hexachloro-iridate spectrum has been confirmed by MCD meas~rernents.~ We therefore calculated the MCD expected for hexachloro-osmate on the basis of this assignment. For t l to t2s (U’ to E ) we predict AID = -0.25 BM and for t2 to t2g we find for E” to E” AID = - 1.7 BM, and for U’ to E” AID = + 1.0 BM all values being approximate. In addition, large B terms are expected from mixing the two states arising from the t2 to tZg transition in the magnetic field.If the E” to E” transition is at lower energy than the U’ to E” transition we expect for the former BID = -0.8/AE and for the latter, BID = + 1.7/AE where AE is the energy separation. If the energies are the other way round the signs will be reversed. No experimental parameters have been reported by Schatz and the complexity of the spectrum especially in the band starting at 29 kK must make the extraction of these a difficult task. However we believe that our calculated parameters are in qualitative agreement with the MCD observed for the intense bands of hexachloro-osmate thus supporting the charge-transfer assignment. We are doubtful however, that it is a good approximation to apply pure j-j coupling to this compound and are at present engaged in more elaborate calculations in intermediate coupling which we hope will throw more light on this spectrum.Prof. P. N . Schatz (University of Virginia) (commzmicated) In reply to the com-ment of Cox we would emphasize that we do not believe that our OsC1;- spectrum can be interpreted on the basis of predominantly ligand-field assignments and the preliminary analysis in our paper shows that Dorain’s detailed attempt to do this is contradicted in many particulars by our MCD data. As indicated in our oral presenta-tion we believe that a fruitful starting point for the interpretation is a j-j coupling formulation in terms of charge-transfer transitions as has also just been suggested by Cox.We outline here briefly some of the general features of such an interpretation. Jsrgensen 4 first noted the similarity of Ir4+ and Os4+ hexahalide solution spectra P. B. Dorain et al. J. Chem. Phys. 1968,49 3845. C. K. Jsrgensen Mol. Phys. 1959 2 309. (a)G. N. Henning et aZ. J. Chern. Phys. 1968,48 5656. (6) A. J. McCaffery et a!. J. Chem. Phys. 1969 50 379. C . K. Jsrgensen 2. Naturforsch. 1967 22 945. 9 GENERAL DISCUSSION 93 and attributed this to the predicted similarity of the d4 and d5 systems in the j-j coupling limit. For example both the t&tz configuration of OsC1,2- and the r,5,t,6, configuration of IrCl2- in the j-j limit give two allowed states separated by a[,-,-for the tZu-+tzg transition. (When electrostatic repulsion is included the d4 system gives many more states than d5.) As discussed in our paper Dorain et a1.l concluded from a study of the extensive fine structure in their low temperature Cs2ZrC16 Os4+ crystal spectrum that all transitions below 35,000 cm-1 were d-d.We follow Jarrgensen's general j-j charge-transfer interpretation but do not rule out the presence of d-d transitions in addition. The helium temperature Cs2ZrC16 Ir4+ crystal spectrum has such striking simi-larities with that of CsZZrCl6 Os4+ that it seems clear that the main absorption bands in the spectra arise from analogous transitions. Our MCD of IrC1;- has shown unequivocally that the first and second strong bands in the IrCl2- spectrum arise respectively from tlu+t2g and t2,-+t2 ligand-to-metal charge-transfer transitions.By analogy then we assign the 26,000 cm-l band in Cs2ZrC16 Os4+ to and the 29,000 cm-l band to and A lg-+Tl ,3[(~3~(e3~(24)~+ (~:)~(e':)~(u;)"(e;)]. We assume that these allowed transitions account for the high extinction coefficients observed. Electrostatic interactions will separate the states of the j-j configurations in an unknown manner but we assume these interactions are small since the Ir4+ spectrum is so similar to that of Os4+. The forbidden states undoubtedly account in part for the temperature dependence of the bands while the T, states are responsible for the gross features of the absorption and MCD. with eel- = 590 cm-l 5oS4+ = 2400 cm-l and assuming that all two-centre integrals are zero we find AID = - 1.67 for A1,+T1,2 and A / D = 0.92 for A1,-+T1,3.The value of A / D for Alg-+Tlul varies between -0.25 and - -0.50 assuming a realistic range of 0-IT mixing for the tl orbital. These values are in good qualitative agreement with experiment. Thus the broad-line progression in the totally symmetric vibration through the 26 000 cm-l band and the corresponding progression of negative A terms in the MCD fits the assignment to the allowed Alg+Tlul transition. Likewise the lower energy end of the MCD for the 29 000 cm-l band is reasonably assigned to a totally symmetric progression accompanying the Alg+TlU2 transition since the MCD shows a pro-gression of large negative A terms again in agreement with our calculation. The higher energy end of the 29 000 cm-I band is dominated by a progression of positive A terms in agreement with the assignment to the A1,+TlU3 transition.The fine structure in the MCD of the 29 000 cm-l band corresponds to the fine structure in the absorption spectrum but is not obviously related to the Alg+2&2 and TlU3 transitions. The spacing between the various sharp progressions beginning at - 28 500 cm-1 suggests assignment to parity-forbidden transitions interacting with odd vibrational modes.4 These parity-forbidden transitions together with the A 1 g -+ Tl u 1 E(u:)4(e:)2(~~)4-+(uI) (e32(44(~31, A 1,-+~lu2C(u:)4(ec)2(~~)4-)(U:)4(e~)(~~)4(e~)l Using Griffith's irreducible tensor methods P. B. Dorain H. H. Patterson and P. C . Jordan J. Chenz. Phys. 1968 49 3845. G. N. Henning A. J. McCaffery P. N. Schatz and P.J. Stephens J. Chem. Phys. 1968 48, 5656; A. J. McCaffery P. N. Schatz and T. E. Lester J. Chem. Phys. 1969,50,379. J. S. Grfith The Irreducible Tensor Method for Molecular Syinrnerry Groups (Prentice-Hall Inc. Englewood Cliffs New Jersey 1962). * I. N. Douglas J. Chem. Phys. 1969 51 3066 94 GENERAL DISCUSSION orbitally-forbidden charge-transfer transitions arising from the t2u-) t2g excitation probably account for the temperature-dependent behaviour of the 29 000 cm-I band. Both our temperature dependent data on OsBri- doped into crystals and that of Day supports the assignment of the strong bands in the Os4+ spectra as charge-transfer transitions. In the bromide the strong bands either gain in intensity or stay about the same as the temperature is lowered.Since the solution MCD of 0sBr;-is very similar to that of OsCl;- we are confident that we are observing analogous transitions. Finally assignment of all bands below 35 000 cm-I in Cs,ZrCl Os4+ as d-d transitions is unreasonable on intensity grounds. d-d transitions cannot gain intensity by mixing with states from $tzg (7 = tlu t2J configurations but only by mixing with the states from the higher energy yzttgeg config~rations.~ If one assumes with Dorain et al. that all charge-transfer transitions are above 35 000 cm-l the $t&eg states will be extremely high in energy ; thus the high oscillator strengths of the strong OsC12- bands would seem to be incompatible with their assignment as d-d transitions. However d-d bands almost certainly are present below 35 000 cm-l in the Cs,ZrCl Os4+ spectrum and may account for many of the sharp lines observed.Dr. W. A. Runciman (A.E.R.E. Harwell) said It is known from tables of atomic energy levels that the 6s6d levels of T1+ are at considerably higher energy and will not greatly affect the 6s6p levels. Also the multiplet splitting rule can be compared with the Russell-Saunders value. It is not greatly different being 3.17 compared with the expected value of 2. The Land6 g-factor can be found from the Zeeman spectra. There are many examples which show that charge compensation sometimes does and sometimes does not affect the spectrum of an impurity replacing an ion of different charge. Dr. E. J. Bowen (Oxford University) said Changes will depend on whether the compensating ion is adjacent to or somewhat removed from the excited ion.Dr. P. Day (Oxford University) said In view of Schatz’s discovery that the angular momenta of the 3P1 excited states in a number of B-subgroups ions are substantially quenched it may be worth pointing out that in the lattices of their pure salts the coordination geometries of such ions are frequently distorted. Thus e.g. TI1 is 5-coordinate and TlF has a tetragonally distorted NaCl ~tructure,~ whilst the unusual structures of PbO and Bi203 were commented on in Dunitz and Orgel’s review.6 Under a distortion the s2 ground state might interact with the totally symmetric component of slpl mixing p into former and some more s into the latter. Perhaps it is this type of mixing which Schatz has detected.Prof. P. J. Stephens (Uniu. of S. Calfornia LA.) said The ions Sez+ and Te:+ discussed in my paper had already been identified prior to the MCD work. More recently I have studied the MCD of solutions of S in oleum and obtained evidence for the new species S:+ therein.’ I believe this to be the first instance in which a new P. Day and E. A. Grant Chem. Comm. 123; B. D. Bird P. Day and E. A. Grant J. Chem. SOC. A 1970 100. G. N. Henning Diss. (University of Virginia June 1968). R. F. Fenske J. Amer. Chem. SOC. 1967 89,252. C. E. Moore Atomic Energy Levels NBS circ. no. 467 1958 vol 3 p. 204. A. F. Wells Structural Inorganic Chemistry 3rd ed. (Oxford University Press 1962) p. 900. J. D. Dunitz and L. E. Orgel Adv. Inorg. Chem. Radiochem. 1960 2 1 . Cheni. Comnt.1969 1496 GENERAL DISCUSSION 95 molecular species has been initially identified by MCD and to substantiate further the potential value of MCD in synthetic inorganic chemistry. Dr. W. A. Runciman (A.E.R.E. Harwell) said For anisotropic centres in cubic crystals consisting of trivalent rare-earth ions the orientation can be best determined by the Zeeman effect. Earlier theoretical calculations contain errors and recent articles l* based on a lecture to the Zeeman Centennial Conference described the corrected results. The Zeeman effect is also useful for isotropic centres as illustrated by results for divalent europium in calcium fluoride. Dr. B. Briat (L’l?cole Supkrieure de Physique et Chirnie Paris) said We agree with Dr. Runciman that Zeeman effect experiments do provide the answer to the problem of the orientation (and thus the local symmtery) of anisotropic centres.Such experiments however are restricted to spectral lines having half-widths in the range 1-10 cm-l. This is not so when one considers the Faraday or Voigt-Cotton-Mouton effects. Both allow the study of broad bands. It has been established in particular (see our ref. (la)) that half-widths of a few hundred cm-l would result in magnetic linear dichroism signals which could be easily detected with our apparatus (using a 50 kG field). As far as MCD (or MORD) is concerned bands as broad as a few thousand wavenumbers can be investigated (e.g. in cobaltous salts). Dr. P. Day (Oxford University) said In addition to the bands whose MOR parameters are listed in table 2 of Shashoua’s paper high-spin ferric haemoproteins, though not low-spin ones also exhibit a further band in the near infra-red at about 10 000 cm-l.The latter with Shashoua’s band I have been assigned as transitions from the highest occupied n-orbitals (aln and a,,) of the porphyrin to the d-shell of the iron(III) and they have identical polarizations to the visible hands in a series of metmyoglobin derivatives.5 As the charge transfer states thus have a symmetry E, common to the locally excited n-n* states configuration interaction will be possible between them. Such an effect has been used to explain the change in the visible region of the absorption spectrum on going from low-spin to high-spin in the met-myoglobins and may also account for the large decrease in MOR intensity.Prof. M. Sharnoff (University o j Delaware) said I would remark that the theory developed in the paper by Bird Briat Day and Rivoal should find fruitful application in the study of d-d spectra of transition metal ions. The magnetic susceptibility tensors of the states excited by d-d absorption are often predictable with considerable precision from non-magneto-optical measurements which are inherently free of the complications which the combination of A B and C terms impose upon the analysis of MCD spectra. The comparison of such predictions with the results of MCD studies would not only provide a severe test of the analysis which you have developed in this paper but would also deepen our insights into the bonding of transition metal ions.As an example I would cite the tetrachlorocuprate ion where the results of an e.s.r. study of the ground state taken in conjunction with the energies and polariza-tions of the d-d transitions enable one to predict (ref. (9) of their paper) the magnetic W. A. Runciman Proc. Iizt. Adu. Summer Physics Institute,-1969 Crete (New York Plenum Press) p. 344. D. F. Johnston S. Marlow and W. A. Runciman J. Phys. C. (Proc. Phys. SOC.) 1968,1,1455. W. A. Runciman and C . V. Stager J. Chem. Phys. 1963 38,279. P. Day D. W. Smith and R. J. P. Williams Bioclzenz. 1967 6 1963. ‘ P. Day G. Scregg and R. J. P. Williams Biopolymers Symp. 1963 1 271 96 GENERAL DISCUSSION properties of all the excited d-levels. The principal values of the g-tensor of the upper-most level (the 2A1 level at 9050 cm-l above the ground level) are predicted to be 911 = 2.002 g1 = 1.591.While MCD measurements on d-d bands of tetra-halide complexes necessarily entail the awkwardness of working in the near infra-red, it seems that current technology makes MCD experiments in this region quite feasible. Dr. A. J. McCaffery (University of Sussex) J. A. Spencer and P. N. Schatz (University of Virginia) said The purpose of this contribution is to draw attention to the use of magneto-optical measurements in investigating a wider variety of physical problems than is suggested by the more conventional spectroscopic applications described in the MCD papers presented so far at this Symposium. In this we utilize the unique property of magnetic optical activity viz. that it measures the ground and excited state magnetic moments for each individual electronic transition.Thus if we have a large number of thermally populated ground states we can measure the magnetic moments for each of these provided transitions from them are resolved. This situation occurs when there is exchange coupling between pairs of paramagnetic ions and MCD may be used as a very sensitive probe into the nature of the inter-actions between unpaired spins in solids. The technique is illustrated here to determine the signs of the magnetic interactions between Cr3+ ions in ruby. Oxygen o ~ r 3 ' o r ~ P FIG. 1 .-Spatial ariangement of nearest neighbours in the M203 lattice. Fig. 1 shows the spatial relationships of metal ions in the Al,03 lattice. In a concentrated ruby crystal there will be an appreciable number of chromium ion pairs and they may be classified as first second third etc.nearest neighbours as shown in fig. 1. Each Cr3+ site has one nearest neighbour three equivalent second neighbours, three equivalent third neighbours and six equivalent fourth neighb0urs.l At distances greater than that of the fourth neighbours Cr3+ sites are linked by at least two oxygen atoms and the coupling between these is much weaker. The A1203 Cr3+ system N. Laul-ance E. C. McIrvine and J. Lambe J. Phys. Chem. Solids 1962,23,5 15 GENERAL DISCUSSION 97 has been investigated by a variety of techniques and thus provides a good test of the method. The exchange interaction between unpaired spins on coupled Cr3+ ions may be approximately accounted for by a term in the Hamiltonian of the form H = J,S s2.This may be ferro- or antiferromagnetic in nature depending on whether the exchange constant for the nth nearest neighbour J, is negative or positive. The two possible couplings are shown in fig. 2 with the energies in terms of the exchange constant J. In fig. 3 we show the states which arise from the various couplings of the S = 3 FIG. 2.Ctates produced by ferro-and antiferro-magnetic coupling of the S = 4 spins of two Cr3+ ions. anti-fe r romag n et i c f er rorn ag net i c s=3 s=o *; 3 .... -ground state of Cr3+. The energies of these levels have been determined by stress experiment^,^'^ Stark splitting~,~ temperature dependence of absorption and emis-sion,6s 7 s 2 * * and other techniques.l Here we show only the ground states to illustrate the method.Inclusion of the excited states which are also coupled leads to a much 1st 2nd 3 rd 4t h 2 3 Nearest neighbour 400 . nteradions in A'zo3' Cr3' FIG. 3 . 4 t a m produced by coup-2 ling of first four nearest neighbour pairs in A1201 Cr3+. I - 'i 0- 0 1 0 - 1 0 0 1 2 3 P. Kisluik N. C. Chang P. L. Scott and M. H. L. Pryce Phys. Rev. to be published. L. F. Mollenauer and A. L. Schawlow Phys. Rev. 1968,168 309. A. A. Kaplyanskii and A. K. Przevuskii Soviet Physics Doklady 1962 7 37. A. A. Kaplyanskii and A. K. Przevuskii Soviet Physics-Solid State 1967 9 190. A. A. Kaplyanskii V. N. Medvedev and A. K. Przevuskii JETP Letters 1967 5 347. P. Kisliuk A. L. Schawlow and M.D. Sturge Advances In Quantum Electronics (Columbia Press N.Y. 1964) p. 725. ' P. Kisliuk and W. F. Krupke Appl. Phys. Letters 1963,3,215. * R. C. Powell B. DiBartole B. Birang and C. S. Naiman Phys. Reu. 1967,155,296. s3-98 GENERAL DISCUSSION more complicated picture. The absorption spectrum of a very concentrated ruby crystal shows 110 resolved lines in the region 6850-7065 A and we shall focus on the various groups of lines originating from the different exchange coupled ground states. Fig. 4 shows the MCD of a relatively concentrated ruby sample at low temperature. The signals are very large for very low fields-a characteristic of sharp intra-con-figurational transitions 2-though the lines were too weak to be seen in absorption in our sample. The magnitude of the MCD signal for each transition is sensitive to the magnetic moment of the ground state of that transition.If the transition is 40 kg 0 It II FIG. 4.-MCD of moderately concentrated ruby at approximately 6 K. Only the R1 and R2 lines show up in absorption in this sample. from a ferromagnetically coupled ground state the MCD will increase rapidly with decrease in temperature-more rapidly that is than the 1/T dependence of a para-magnetic state. If from an antiferromagnetic ground state the MCD will conversely decrease with reducing temperature. Thus from the temperature dependence of the MCD we may determine the sign and the magnitude of the exchange constants J, for the various interactions in the solid. Our preliminary temperature- and field-dependence experiments on ruby in the temperature range of around 6-15 K indicate that the groups of bands at 7041 and 6995 A are from antiferromagnetically coupled ground states in agreement with increasing assignments form other experiments as from the third and second nearest neighbours respectively.The group of lines around 6980 A however decreases rapidly with temperature and can be assigned as from the fourth neighbour interaction, in agreement with Stark experiment^.^ The first neighbour interaction is so strongly antiferromagnetic that we could detect no MCD in the known region of these lines in our sample. Ruby is clearly a very complex system and it may be impossible to analyze the complete spectrum due to the large number of overlapping lines. However from S. F. Jacobs Doctoral Thesis (The John Hopkins University Baltimore Maryland 1956). A. J. McCaffery P. J. Stephens and P. N. Schatz Inorg. Chem. 1967 6 1614. P. Kisliuk N. C. Chang P. L. Scott and M. H. L. Pryce Phys. Rev. to be published GENERAL DISCUSSION 99 these and continuing experiments we have been able to confirm the nature of the ground-state couplings in ruby. Since the excited state moment also contributes to the MCD of each transition we can hope to determine the signs and magnitudes of the excited states interactions. One particular region which appears to be tractabla is 6959-6970& where the MCD is relatively simple. Here we have been able to evaluate theg-value for one of the third nearest neighbour excited states and an analysis of other regions may be possible. Through this and continuing work we hope to comment in more detail on the excited state coupling mechanism