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Abstract:: The inelastic neutron scattering technique was employed to study the magnetic excitation spectra in the diluted one-dimensional Heisenberg antiferromagnet CsMn1-xMgxBr3 (x =0, 0.05, 0.10, 0.25, 0.50). The spectral response is interpreted in terms of spin-wave excitations in finite chain segments of Mn2+ ions, which are found to exist as long as the chain length exceeds twice the wavelength of the spin excitation. This limit determines the crossover into the mesoscopic regime.

The purpose of this paper is to show that the Mn4+ 2E ? 4A2spin forbidden emission transition (R-line) is intensified in Gd3+based compounds with the perovskite ABO3and double perovskite A2BB'O6structures. The intensity of the R-line is compared in compounds with non-magnetic ions such as Y, La and Eu. and those with Gd ion. The significant increase in the intensity of the spin forbidden transition is explained by a simple model of exchange coupling in the dimeric unit, Mn4+?O2??Gd3+. It is also pointed out that exchange interaction between the Mn4+and Gd3+ions cause significant broadening of the R-line in compound having the pyrochlore structure.

The influence of Excited-State Absorption (ESA) on the green laser transition and the overlap of Ground-State Absorption (GSA) and ESA for 970 nm upconversion pumping in erbium is investigated in Er3+ : BaY2F8 and Cs3Er2Br9. Results are compared to Er3+ : LiYF4. In Er3+: BaY2F8, a good overlap between GSA and ESA is found at 969 nm in one polarization direction. The emission cross section at 550 nm is a factor of two smaller than in LiYF4. In Cs3Er2Br9, the smaller Stark splitting of the levels shifts the wavelengths of the green emission and ESA from4I1 3/2 off resonance. It enhances, however, ground-state reabsorption. The emission cross section at 550 nm is comparable to LiYF4. Upconversion leads to significant green fluorescence from2H9/2. A significant population of the4I11/2 level and ESA at 970 nm are not present under 800 nm pumping.

Neutron inelastic scattering is introduced as a new experimental technique for the study of exchange interactions in polynuclear transition metal complexes. Measurements were performed on deuterated μ-hydroxo-bis[pentaamminechromium(III)]chloride monohydrate at temperatures ranging from 5 K to 290 K. Transitions between split levels of the electronic ground state were observed in both energy-gain and energy-loss configurations. The values for the splittings and the resulting exchange parameters are found to be temperature dependent. Comparisons are made with values obtained from optical spectroscopic studies and magnetic susceptibility measurements. The observed intensity ratios of the neutron inelastic scattering transitions are in good agreement with values calculated on the basis of a simple spin-spin coupling model for the chromium pair.

Crystals of KZnF3 and KMgF3 doped with Mn2+ and Ni2+ were used to study the spectroscopic properties of Mn2+-F--Ni2+ pairs. Pair transitions to the doubly excited states 4Ega, 4A1g(Mn)3T1ga(Ni) and 4Egb(Mn)3T1ga(Ni) were observed. The participation of the spin-allowed 3A2g →3T1ga excitation on Ni2+ in the pair transition is explained by spin-orbit mixing between 3T1ga and 1Eg. The prominent electronic origins are assigned to the double spin-flip transitions 6A1g(Mn)3A2g(Ni) →4Egu(Mn)3T1ga(Γ3)v(Ni) and 4A1g(Mn)3T1ga(Γ3)v(Ni). The former lie at lower energy and are more intense than the corresponding 6A1g(Mn)3A2g(Ni) →4Egv(Mn)3T1ga(Γ3)u(Ni) transitions involving two orbital jumps. The well-resolved vibronic structure is composed of three basic vibrations of ∼ 150 cm-1, ∼ 294 cm-1 and ∼ 508 cm-1 in the KZnF3 host.

A powder neutron diffraction study of the crystal structure of K₂[RuCl₅NO] in the electronic groundstate is presented in the temperature range from 25 to 295 K, confirming the orthorhombic space group Pnma. Neither a structural nor a magnetic phase transition has been observed. A shortening of the axial Ru-Cl₂ bond occurs with increasing temperature, while the Ru-N bond gets longer. The metal to chloride bonds do not change uniformly. Copyright 1993, 1999 Academic Press

Groundstate and electronic excited state (MSI) of deuterated sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) have been investigated by neutron diffraction as well as by optical and Mössbauer techniques. Significant structural changes occur predominantly in the O-N-Fe-C-bond. It has been shown, that the N-O bond-length is not the order parameter, as expected from other studies. We found an increase in the bond lengths Fe-N4 of 0.019(2) Å and N-O of 0.004(4) Å respectively, which is in qualitative agreement with changes determined by Raman spectroscopy and predictions based on diatomic correlations (Badger/Herschbach/Laurie). Additionally we observed a change in the Fe-C1 bond length of 0.012(3) Å in agreement with Raman meaurements.

A study of the absorption of light by pairs of exchange coupled manganese ions in the perovskite fluorides KMgF3 and KZnF3 has been made using the technique of high resolution phosphorescence excitation spectroscopy. Transitions to the following pair states have been observed: 6A1g4T1ga; 6A1g 4Ega; 6A1g 4A1g; 6A1g4Egb; 4T1ga 4T1ga; 4Ega 4T1ga and 4A1g 4T1ga. Quantitative estimates of intensities have been made for some of these transitions as well as estimates of the effective exchange parameters. A detailed analysis of the transition 6A1g 6A1g → 6A1g 4A1g provides confirmation of the Tanabe mechanism of exchange induced absorption of light by pairs of transition metal ions. It is shown that absence of MCD in this pair absorption is predicted by the Tanabe mechanism.

A combination of high resolution MCD and phosphorescence excitation techniques has been used to determine, for the first time, the optical spectra of single Mn2+ ions in the cubic perovskite hosts KMgF3 and KZnF3. Particular attention has been paid to transition mechanisms, spin-orbit effects and vibronic structure, and the experimental results have been compared with theoretical crystal field calculations based on the complete d5 configuration. All of the states arising from the 4G, 4P and 4D terms have been observed and a dynamic Jahn-Teller effect has been observed in the 4T1ga state. Both magnetic dipole and electric dipole (vibrationally induced) absorption processes occur and the former is most important for transitions to the 4T1ga, 4A1g, 4T1gb states. Calculated and observed magnetic dipole intensities are in good agreement. A complete analysis of the zero phonon (magnetic dipole) lines and vibronic (magnetic and electric dipole) side-bands associated with transitions to 4A1g and 4Ega states has been made.

The low temperature spectra of TcO4-and ReO4-both show two band systems with pronounced vibrational structures. The bands are identified as1A1?1T2 transitions. No other bands are observed with certainty. It seems likely that the KClO4 crystals contain KReO4 crystallites. They are therefore not pure mixed crystals. It is concluded that the virtual orbital (2e) used in the construction of the low lying states resembles an atomic nd orbital more and more when going from n=3, Mn to n=5, Re.