J.C. Rivoal

Magnetic circular dichroism studies of the charge-transfer transitions in tetrachloro and tetrabromocomplexes of transition metal ions

We report on a detailed study of the temperature dependence of the magnetic circular dichroism spectra of the tetrabromo and tetrachloro-complexes of Cu(II), Fe(III) and Co(II) through their charge transfer absorption bands. All M.C.D. and absorption spectra were measured as far as about 44 000 cm-1 on polymer films obtained by evaporating an acetonitrile solution of cellulose diacetate doped with the metal complex. This investigation considerably extends our previous room temperature M.C.D. work and leads to more reliable and detailed spectroscopic assignments than absorption spectra alone. A moment analysis of the FeBr4 - spectra further indicates that non-cubic metal-halogen stretching modes play an important role in the band broadening mechanism. Our data also confirm that the highest occupied orbitals of mainly ligand character occur throughout the series in the order 1e<3t 2

The low temperature magnetic circular dichroism spectra of iron-sulphur proteins II. Two-iron ferredoxins

Variable temperature magnetic circular dichroism (MCD) spectra of a number of two-iron ferredoxins have been measured. The spectra of fully oxidised spinach and Spirulina maxima ferredoxin are independent of temperature between room temperature and 18 K, showing that no contribution to the room temperature MCD spectrum arises from the small population of low-lying excited states originating from the exchange coupling. However, the low temperature MCD spectra of the half-reduced proteins spinach and Spirulina maxima ferredoxin and adrenodoxin are all reasonably intense and temperature dependent. An interpretation of the spectrum of the charge-transfer region is suggested by starting with the assignments previously obtained from rubredoxin.

Exchange interactions in polynuclear complexes

We present detailed optical and magneto-optical data on Cs3Cr2Cl9 as a single crystal containing only pairs of Cr3+ ions. Measurements have been carried out in a wide range of temperatures down to pumped helium and in the whole spectral region covering all singly and several doubly excited states. MCD helped considerably in resolving the absorption spectra into their numerous components, and proved especially useful in demonstrating the occurrence of a double excitation to 4 T 2. It provides also parallel g factors for several excited terms. The main conclusions of our work are: (i) all transitions are electric dipole allowed; (ii) transitions to singly excited doublet states are essentially promoted via a single ion mechanism; (iii) vibronic interaction must be invoked to understand the absorption data on sharp lines; (iv) the trigonal plus spin-orbit interaction competes favourably with exchange interaction to produce excited states splittings. We present also a new non-phenomenological model to describ...

The low temperature magnetic circular dichroism spectra of iron-sulphur proteins I. Oxidised rubredoxin

Variable temperature magnetic circular dichroism spectra have been measured on oxidised Clostridium pasteurianum rubredoxin. Evidence has been obtained for the presence of two one-electron charge-transfer transitions, sulphur to ferric ion, in the region 15 000 to 28 000 cm-1. The first moment of the lower energy band is consistent with it being the orbital transition t1 non-bonding sulphur orbital, to the 2 e ferric d-orbital. The magnitude of the spin-orbit coupling constant in the lower excited state has been determined and shown to be small compared with the axial distortion. The splitting of the low energy band observed in the absorption spectrum can therefore be equated directly with the axial distortion of the lowest excited charge-transfer state. Finally, the potential utility of making saturation experiments at very low temperatures has been examined.

Frequency dependence of Tf and short time remanence for Eu0.4Sr0.6S spin glass

We report on the frequency variation of the freezing temperature and on the time variation of the magnetization of the insulating spin‐glass Eu0.4Sr0.6S. Faraday rotation was used to monitor optically the susceptibility or the magnetization and is well adapted to measurements in a large frequency range (10−2–107 Hz) or at various time scale (down to 25 ns). The temperature Tf(ν) associated with the maximum of the susceptibility peak exhibits a linear dependence with Log(ν) in the range 102–107 Hz. At lower frequencies (down to 10−2 Hz) it approaches a ’’static’’ limit equal to 1.55 K. This result cannot be explained by a model of superparamagnetic clouds. The frequency shift of the freezing temperature is experimentally shown to be related to the occurrence of a remanent magnetization during the time associated with the measuring frequency. Finally the time dependent behavior of the magnetization was investigated down to 25 ns after cutting a weak magnetic field (∼2 Oe). The main part of the magnetization is shown to relax more quickly than 25 ns even at 1.26 K. The only change when lowering the temperature below Tf is the emergence of the remanent magnetization; its time dependence is briefly discussed.

Magnetic circular and linear dichroism of cubic or tetragonally distorted high spin d5 systems

The low temperature, single crystal and polymer matrix absorption, magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) of the d-d electronic transitions in the tetrahalomanganates (II), [(C2H5)4N]2MnX4 (where X = Cl, Br and I), have been investigated. The effect of crystal rotation about the incident radiation propagation direction is theoretically examined and shown to yield anisotropic MLD terms for 4 T 1 and 4 E states and isotropic ones for 4 A 2 and 4 T 2 states. The combination of the sign of the MCD zeroth order moment and the MLD crystal rotational dependence is used to assign the observed bands with a high degree of certainty. It is shown that although conventional crystal field theory correctly predicts the d-d state ordering in the lowest energy range (4 T 1 < 4 T 2 < 4 E) it fails badly in the higher energy region (4 T 1 < 4 T 2 < 4 A 2). An improved model including both charge-transfer and d electron configurations is proposed to explain the discrepancy. Spin-orbit and tetra...

Magneto‐optical determination of magnetic ordering and magnetic structure of CS3MnCl5 below 0.6 K

We report on magneto‐optical determination of the magnetic phase transition as well as the magnetic structure of crystalline CS3MnCl5. A new type of optical cryostat, with a closed cycle of He3 has allowed us to study continuously the variation of the absorption bands down to 0.4 K. The temperature variation of hot and cold bands attributed to the 6A1→4E, 4T2 transitions proves that antiferromagnetic ordering arises at 0.6 K. Studying the MCD spectrum of the 6A1→4T2 zero phonon line in low magnetic field, we have observed that the antiferromagnetic phase reverts to paramagnetic state. From this we deduce a value of the weak exchange field (0.9 T) and the energy of the magnon (ω0 = 0.28 cm−1). Using the latter value we show that the origin lines of the 4E and 4T2 states can be fairly well understood in terms of magnon sidebands. In contrast the σ polarized absorption spectra under an applied magnetic field, perpendicular to the c axis, show no discontinuity. A log‐log plot of the absorbance versus magnetic...

Magnetic circular dichroism and the assignment of charge transfer transitions in tetrahalide complexes

Magnetic circular dichroism (MCD) spectra are reported for the charge transfer transitions in a large number of tetrahalide complexes of first transition series ions. Spectra of all the compounds have been obtained from solutions at room temperature and also, for CoI42– and NiI42–, from evaporated films and doped crystals at various temperatures down to liquid helium temperature. Assignments are discussed in terms of models based on Russell-Saunders, first-order spin-orbit or j,j-coupling, depending on the size of the halogen spin-orbit coupling constant.