Lucjan Dubicki

Exchange interactions in a trigonal chromium(III) pair

The singly excited 2 E pair states in Cs3Cr2Br9 exhibit fine structure which has been analysed by absorption, MCD and transverse Zeeman spectroscopy and the theory given in part I (reference [1]). The corresponding transitions in Cs3Cr2Cl9 overlap with the 4 T 2 band and are weak and broad. Both singleion and exchange-induced transition electric dipoles are important in the pair transitions. There is evidence for the breakdown of the Lande interval rule for the exchange-split excited pair levels. The non-vanishing intensity observed in one ΔS = 2 pair transition requires the inclusion of anisotropic exchange terms in the pair hamiltonian. MCD and Zeeman spectroscopy give, in many cases, a precise identification of the spinor pair states.

Absorption and magnetic circular dichroism spectra of the compressed copper (II) ion in K2ZnF4

The polarized absorption and magnetic circular dichroism spectra of 1–3% copper(II) doped into the tetragonal host K2ZnF4 are measured over the temperature range 1.8-300 K. Sharp origins for the spin-orbit components of the 2A1g(z2→ 2Eg(xz, yz) transition and accompanying vibrational fine structure are observed. The spectra are interpreted on the basis of doped Cu(II) ions occupying a tetragonally compressed octahedron of six-coordinated fluoride ions. This system presents an excellent opportunity for the study of a highly structured spectrum of a Cu(II) complex with an unusual geometry.

Circular dichroism of cobalt(III) complexes: dependence on the outer sphere configuration

Abstract The single crystal axial CD of three Co(III) completes has been measured between 295 and 9 K. For Λ-Co (sepulchrate) (NO3)3 the rotational strength R(1T1(E) ← 1A1) decreases from ≈11 × 10−40 c.g.s. units at 295 K to ≈0 at 80 K. The main features of the single crystal CD spectra can be explained by a static coupling model of optical activity.

Luminescence and excitation line narrowing in tetragonal Cr3+ complexes: Confirmation of a large 2Eg splitting

Abstract A long standing controversy concerning the 2Eg splitting in [Cr(NH3)5H2O](CLO4)3, [Rh(NH3)5Cl]Cl2:Cr3+ 1% and [Cr(NH3)5Cl]Cl2 is resolved by the use of luminescence, excitation line narrowing techniques and site selective spectroscopy. Large 2Eg splittings for these tetragonal Cr3+ complexes are confirmed. A conventional ligand field calculation is clearly inadequate for this class of compounds. Non-spherical contributions to the electronic repulsion probably explain the observed splittings. Large non-spherical electronic repulsions require low symmetry distortion in the first coordination sphere.

The Optical-Spectrum of Ba2zn[cu]f6

Abstract The low temperature polarized absorption and magnetic circular dichroism spectra of ≈3% copper (II) doped into the tetragonally compressed sites of Ba 2 ZnF 6 are reported. The interpretation of the optical spectrum requires that the CuF 6 4− octahedra are tetragonally compressed. This implies that the host site strain dominates with respect to the higher order Jahn-Teller coupling. Considerable vibrational fine structure was observed on sharp electronic origins for the spin-orbit components of the 2 A 1g ( z 2 ) → 2 E g ( xz, yz ) transition. It has been found that cubic anisotropy in the orbital reduction parameters is necessary to interpret the spin-orbit splitting of the 2 E g ( xz, yz ) excited state. The magnitude of the splitting has been used to derive the tetragonal components of the ligand field at the equilibrium geometry of the excited state, and this has been compared with values obtained for other similar systems.

Structured d-d fluorescence from CuF64− doped in cubic and tetragonal perovskites

Abstract Structured fluorescence, with sharp magnetic dipole zero-phonon lines at 6830 and 7498 cm−1 for KCu0.01 Zn0.99F3 and K2Cu0.01Zn0.99F4, respectively, is reported. Resonant features are seen in absorption, clearly identifying the emission process as metal-centered (2T2g → 2Eg) in nature. Lifetimes at 4.2 K were 1.8 and 0.5 ± 0.1 μs for the two systems. This appears to be the first reported incidence of d-d emission from six-coordinated copper.

Electronic-Structure of the Copper(ii) Ion Doped in Cubic Kznf3

The absorption and magnetic circular dichroism spectra of 1%–3% copper(II) doped into the cubic perovskite host KZnF3 are measured over the temperature range 1.8–300 K. Sharp magnetic dipole allowed transitions 2Eg(Γ8)→2T2g(Γ7,Γ8) are observed together with accompanying vibrational fine structure. The spectra are interpreted on the basis of a tetragonally elongated CuF4−6 ground state geometry which arises from strong Jahn–Teller coupling. This results in a statistical distribution of three equivalent elongations which can undergo reorientation on the electron paramagnetic resonance time scale. The Jahn–Teller coupling within the 2T2g multiplet is partially quenched by spin–orbit coupling and the lowest Kramers doublet Γ7 has an octahedral geometry, while the higher lying Γ8 state has a small tetragonal distortion. This system presents an excellent opportunity to study the spectroscopy of the copper(II) ion in a cubic environment.

Spectral hole burning in emerald

Spectral hole burning in the 4A2–2E(2A) zero-phonon line for Cr3+ in Be3Al2(SiO3)6 is reported. Persistent holes with a linewidth of 0.5 GHz were observed when the crystal temperature was below 4 K. The burning is attributed to photoionization of Cr3+ from the 2E state and trapping of the released charge at other sites. The changes in the Coulomb field are the main factors contributing to the hole linewidth. High-resolution axial and transverse Zeeman measurements of the 4A2–2E(2A) transition performed by using the spectral hole are found to be consistent with the known g values.

Paramagnetism of caesium titanium alum

Caesium titanium alum, CsTi(SO4)2⋅12H2O, is a β alum and exhibits a large trigonal field and a dynamic Jahn–Teller effect. Exact calculations of the linear 2T2⊗e Jahn–Teller coupling show that in the strict S6 site symmetry the ground multiplet consists of a Kramers doublet 2Γ6 with magnetic splitting factors g∥=1.1 and g⊥=0, a Γ4Γ5 doublet at ∼60 cm−1 with g∥=2.51 and g⊥=0.06 and another Γ4Γ5 doublet at ∼270 cm−1 with g∥=1.67 and g⊥=1.83. The controversial g values observed below 4.2 K, g∥=1.25 and g⊥=1.14, are shown to arise from low symmetry distortions. These distortions couple the vibronic levels and induce into the ground state the off-diagonal axial Zeeman interaction that exists between the first excited and the ground vibronic levels.

Electron paramagnetic resonance of chromium(III) ion in α, β and γ alums

Abstract The sign of the zero-field splitting (ZFS) of the ground state of Cr 3+ ion in the α alums CsCr(SeO 4 ) 2 .12H 2 O and CsAl(SeO 4 ) 2 .12H 2 O has been determined by axial stress electron paramagnetic resonance. The angular overlap model is used to explain the signs and magnitudes of the ZFS in alums. For β alums the ZFS is mainly determined by the trigonal compression of the Cr(OH 2 ) 6 3+ ion with a minor contribution coming from the outer coordination sphere. The same obtains for α alums but, in addition, there is a larger contribution of opposite sign that arises from the misaligned valence orbitals and dipoles associated with the pyramidal geometry of the OH 2 ligand.