B. Briat

Chiral reflection from a naturally optically active medium

The differential reflection of left and right circularly polarized light from a naturally chiral liquid was enhanced and quantitatively measured under conditions of total reflection. In accord with recent predictions, the observed signal at the critical angle was some 3 orders of magnitude larger than the intrinsic chiral parameters (~10−7). This is to our knowledge the first observation of molecular optical activity by coherent light scattering.

Magnetic circular dichroism and absorption spectra of d° tetrahedral oxyanions and thioanions: MoS4 2-, MoO4 2-, WS4 2-, ReS4 -, VS4 3-, VO4 3- and OsO4

Absorption and magnetic circular dichroism spectra have been measured for the tetrahedral d° ions MoS4 2-, MoO4 2-, WS4 2-, ReS4 -, VS4 3-, VO4 3- and OsO4 in the visible and ultra-violet spectral range. Comparison of the experimentally obtained MCD parameters with regard to their relative signs leads to the conclusion that probably the first two allowed longest wavelength electronic transitions of all these species have the same assignment (i.e., t 1 →2e and 3t 2 →2e, which seems likely on energetic grounds). An estimation of the MCD parameters by using eigenvectors from recently published molecular orbital treatments gave contradictory results.

Measurement and interpretation of magnetic circular dichroism and magnetic linear dichroism spectra

This paper deals with three problems of a different nature. We first discuss the conditions required to obtain reliable and usable magneto-optical and absorption data at any temperature down to that of liquid helium; this discussion includes the description of our cryostat and absorption device, as well as a critical study of the optimization of the signal-to-noise ratio of the instruments. Secondly, a new fitting procedure is proposed for the analysis of absorption and magnetic circular dichroism curves and the extraction of useful parameters. Finally, the significance of these parameters is examined for anisotropic centres isolated in an isotropic matrix; the results thus obtained are illustrated by means of magneto-optical experiments on CaF2+Nd3+ crystals.

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

Correlations between microscopic properties and the photorefractive response for vanadium-doped CdTe

We present a summary of the experimental results, the attempts of interpretation at the present stage, and the conclusions of coupled investigations on semi-insulating, n-type and p-type vanadium-doped CdTe crystals. Combining electron paramagnetic resonance and magnetic circular dichroism permits the monitoring and the quantitative assessment of [V2+] and [V3+] defect concentrations. Photoabsorption results are presented and are discussed in terms of photoionization processes at various wavelengths. The temperature and the spectral dependence of the steady-state photoconductivity are carefully studied in the near infrared. The results are discussed in terms of electronic transitions for the two vanadium charge states. Finally, the photorefractive performances of the semi-insulating sample are analyzed in a two-beam-coupling experiment and are correlated to the properties of the V2+/V3+ donor states to derive photoionization cross sections.

Magnetic circular dichroism and the optical detection of magnetic resonance for the Bi antisite defect in Bi12GeO20

We report on the optical detection of electron paramagnetic resonance at 36 GHz, 67 GHz and 70 GHz for all the absorption bands of Bi12GeO20 (BGO) in the visible spectral range. This completes our previous studies and demonstrates clearly that the three major magnetic circular dichroism features do belong to the same isotropic centre, i.e., a Bi3+Ge+h defect with the hole being delocalized onto the four surrounding oxygen atoms. We provide a simple explanation for the unusual sign of ODMR and the transient behaviour of the magnetic circular dichroism at resonance.

The electronic absorption and magnetic circular dichroism spectra of cobalt (II) bromate hexahydrate

The magnetic circular dichroism (M.C.D.) and absorption spectra between 14 000 and 37 000 cm-1 of a single crystal of cubic [Co(H2O)6] (BrO3)2 have been measured at various temperatures between 5 and 300 K. The most important contributions to the M.C.D. b and c parameters of the vibronic d-d transitions of the cobalt (II) hexaquo ion have been calculated and an assessment of the magnetic dipole contributions has been made. The form of the temperature variation of the dipole strength and the signs of the M.C.D. c-terms show that the intensities of the 4 a T 1g → 4 A 2g and 4 b T 1g transitions are provided mainly through the two cobalt-oxygen t 1u vibrations. Semi-quantitative calculations of b and c for the latter transition are in reasonable agreement with the experimental values. A comparison of the spectra with ‘complete’ crystal field/spin-orbit coupling calculations leads to an assignment of many spin-forbidden bands. It is shown in particular, that the two relatively intense bands above the 4 a T 1g...

Magnetic circular and linear dichroism of iron garnets thin films

Abstract Low temperature absorption, MCD and MLD spectra are reported for pure and substituted Y3Fe5O12 thin films in the 13000–33000 cm−1 spectral range. The materials were obtained by liquid phase epitaxial growth on a Gd3Ga5O12 substrate. Above 19000 cm−1 we demonstrate that our transmission experiments on garnet films provide the same information as do reflexion measurements on bulk crystals. Below 19000 cm−1 however, the former are much more reliable and accurate. All our data indicate that the films are of excellent optical quality. MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum and help in sorting out most of its numerous overlapping components. Finally we prove unambiguously that Clogstons theoretical interpretation is qualitatively correct in the 13000–23000 cm−1 region and even probably at higher wavenumbers.

Charge states and optical transitions of vanadium in Bi4Ge3O12 identified by MCD and ODMR

Abstract The spectroscopic properties of V-doped Bi4Ge3O12 have been investigated in detail via several complementary techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly modified under UV-illumination and the initial state can be restored optically with visible light. Optical absorption and magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UV-illumination while a paramagnetic one is created. The latter shows a very characteristic S-shaped MCD pattern in the near-IR, which is readily assigned to the 2 E → 2 T 2 internal transition of tetragonal V4+ centers at the Ge sites. This assignment was further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at 35 GHz. The g tensor of V4+ was found to be anisotropic with principal values g||=1.81±0.03 and g⊥=1.94±0.02. Two additional MCD bands in the visible spectral region could be attributed to V4+ via ODMR measurements. Ionization thresholds for holes (V5+) and electrons (V4+) were determined by optical absorption experiments and the V4+/5+ donor level was positioned 1.9 eV below the conduction band. The presence of the V4+/3+ acceptor state in the forbidden band is also suggested.

Magnetic phase transitions in CsCoBr3

Elastic neutron‐scattering measurements have been performed on CsCoBr3, an Ising‐like antiferromagnet on a stacked triangular lattice. Three magnetic phase transitions are seen in both zero field and a magnetic field of 2.2 T applied along the stacking direction c. The field does not qualitatively change the behavior near the phase transitions and the transition temperatures are affected only slightly. The high‐temperature ordered phase is known to be partially paramagnetic and the transition from the completely paramagnetic phase to this ordered phase was carefully studied. The transition is found to be continuous and characterized by the critical exponent β=0.22±0.01, a result that disagrees with previous measurement. The disagreement can be attributed to a large amount of temperature‐dependent paramagnetic scattering that exists in both the paramagnetic and the partially paramagnetic ordered phase.