Magnetic circular dichroism and absorption spectra of Cs2ZrCl6:Mo4+

Absorption and magnetic circular dichroism (M.C.D.) spectra of Mo⁴⁺ in Cs₂ZrCl₆ have been recorded under high resolution at liquid helium temperature over the range 20 000–38 000 cm^-1. All of the bands can be assigned to parity-allowed, ligand-to-metal charge-transfer transitions in both MoCl₆ ^2-(4d²) and an impurity ion which has been identified as MoOCl₅ ^2-(4d¹). The d¹ and d² systems are easily distinguished by the temperature dependence of their M.C.D. spectra, and detailed assignments are proposed for the latter. The absorption spectrum in the region 23 000–26 000 cm^-1 has recently been attributed to a d→d transition in the ion MoCl₆ ^-(4d¹) [1], but the M.C.D. spectrum proves this assignment to be incorrect, and we find no evidence for the presence of this singly-charged species. From the temperature dependence of the M.C.D. spectrum of MoCl₆ ^2-, we estimate the second-order spin-orbit splitting of the E and T ₂ components of the ³ T _1g ground state to be 27 ± 5 cm^-1 with E lowest. The electrostatic splitting of the ³ T _2u and ⁵ T _2u charge-transfer states arising from the t _1u (π + σ) →t _2g excitation is estimated to be close to 515 cm^-1.     

Magnetic circularly polarized emission and magnetic circular dichroism of resolved vibronic lines in Cs2GeF6: Mn4+

Magnetic circularly polarized emission (M.C.P.E.) and magnetic circular dichroism (M.C.D.) techniques have been used to study at low temperatures resolved vibronic lines of the ⁴ A _2g ?² E_g transition in octahedral Mn⁴⁺(3d³) in the cubic host Cs₂GeF₆. Measurements have been made with applied magnetic field (and light propagation) along the [001] (F-Mn-F bond) and [111] directions. Though the Zeeman energy patterns are isotropic, the intensity patterns are not, and U′(Γ₈) eigenvectors for arbitrary field orientation have been derived. These have been used to calculate Zeeman intensity patterns for the [001] and [111] crystallographic directions, and the observed intensity variations with orientation are found to provide useful information about intensity mechanisms. In the case of the magnetic dipole origin, the intensity patterns as a function of orientation can be well accounted for by a first-order mechanism which, however, does not predict the small positive M.C.D. observed at the zero-field energy in the [111] orientation. A detailed analysis of this feature and previously measured energy spacings suggest that ζ_3d(Mn⁴⁺) should have a value of ～360 cm^-1 rather than the value 240 cm^-1 usually assumed. Electric dipole vibronic sidebands have been observed corresponding to v ₆(t _2u ), v ₄(t _1u ), v ₃(t _1u ), v ₄(t _1u ) + v ₅(t _2g ) and v ₂(e_g ) + v ₆(t _2u ). Using a U′→U′ vibronic intensity mechanism with spin-orbit mixing (Appendix A), the M.C.P.E. and total emission patterns for the first two of these regions can be quite well accounted for quantitatively. The corresponding M.C.D. in both cases, while in qualitative agreement with the M.C.P.E., shows much more complicated splitting patterns which are not explicable in terms of a simple k = 0 model. The other three vibronic regions can be accounted for qualitatively. In Appendix B a formula is derived which explicitly relates the M.C.P.E. of a vibronic emission line to its M.C.D. absorption counterpart.     

A ligand field theory analysis of the spectra of the t 2g 3 levels of IrF6

Calculations and experimental data are presented for the t _2g ³ molecular levels of IrF₆ which imply that a substantial interaction takes place between these 5d³-configuration states and charge transfer states at ca. 20 000 cm^-1. This t _2g ³-charge transfer state configuration interaction is so extensive that the five t _2g ³ levels can only be fit with physically unreasonable values of the Racah (electrostatic) parameters and a spin orbit coupling parameter that is not consistent with expectations for 5d-series transition metal hexafluorides. Parameter values presented for IrF₆ are thus determined from the lowest three levels only, as these should experience the smallest relative shift due to configuration interaction: B = 297 cm^-1, C = 1167 cm^-1, ζ_5d = 4182 cm^-1, and 10Dq = 35 000 cm^-1. In order to corroborate the assertion that t _2g ³-charge transfer configuration interaction is an important factor for the determination of IrF₆ crystal and molecular properties, a number of t _2g ³ spectroscopic properties, which turn out to be quite sensitive to charge transfer state admixture, were studied theoretically and experimentally. These include gas-to-crystal shifts, site splittings, and Jahn-Teller interactions; comparison of calculated and experimental values clearly substantiate the conclusion that there is a strong influence of charge transfer states on the nature of the 5d³-t _2g ³ manifold. Finally, new data for the Γ_8g (² E_g ) and Γ_6g (² T _1g ) states at 1·2 μm are given, completing the data set for the t _2g ³ transitions of IrF₆.     

Electronic pair excitations of Mn2+ and Ni2+ in perovskite fluorides: involvement of the 3 T 1g a state of Ni2+

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 ⁴ E_g ^a , ⁴ A _1g (Mn)³ T _1g ^a (Ni) and ⁴ E_g ^b (Mn)³ T _1g ^a (Ni) were observed. The participation of the spin-allowed ³ A _2g →³ T _1g ^a excitation on Ni²⁺ in the pair transition is explained by spin-orbit mixing between ³ T _1g ^a and ¹ E_g . The prominent electronic origins are assigned to the double spin-flip transitions ⁶ A _1g (Mn)³ A _2g (Ni) →⁴ E_gu(Mn)³ T _1g ^a (Γ₃)v(Ni) and ⁴ A _1g (Mn)³ T _1g ^a (Γ₃)v(Ni). The former lie at lower energy and are more intense than the corresponding ⁶ A _1g (Mn)³ A _2g (Ni) →⁴ E_gv(Mn)³ T _1g ^a (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.     

4 Ag ↔2 Eg Transition of Mn4+ in Cs2TiF6: MnF6 2-

We report and interpret the ⁴ A_2g ?² E_g absorption, emission, linear dichroism, MCD and MCE of the MnF₆ ^2- octahedra at sites of D_3d symmetry in single crystals of Cs₂TiF₆. The spectra show many sharp features but a simplified calculation using cubic basis functions successfully accounts for all of the major features and much of the detailed structure. The electronic levels are not split by the trigonal field in first order but trigonal distortion splits the three-fold degenerate v ₆(t_2u ) vibration of the octahedron into a two-fold degenerate v ₆(e_u ) and a non-degenerate v ₆(a_1u ) vibration and these two vibrational modes give rise to the features 213 cm^-1 and 246 cm^-1 from the zero-phonon line respectively. Similarly the v ₄(t_1u ) vibration is split into a v ₄(a_2u ) and v ₄(e_u ) vibration and give the features at 313 cm^-1 and 340 cm^-1 respectively.

Several transitions involving two or more quanta of vibration are clearly seen in the emission and MCE spectrum. At an energy shift of greater than 700 cm^-1 from the zero-phonon transition these features are found to consist of progressions in the v ₁(a_1g ) breathing mode and the v ₂(e_g ) two-fold degenerate Jahn-Teller active mode. This structure can again be explained, with one exception, in terms of a calculation which assumes cubic basis functions. The exception is the reversal in sign of the MCE at a frequency v ₆(e_u ) + v ₂(e_g ) compared with that at v ₆(e_u ) which is not fully understood.     

Electron-vibration effects in Cs2SiF6 : Mn4+

We report a study of the ⁴ A _2g →² T _1g absorption band of Mn⁴⁺ in Cs₂SiF₆. The band shows several lines or groups of lines associated with transitions from the ⁴ A _2g ground state to the spin-orbit components (² T _1g )Γ₈ and (² T _1g )Γ₆ coupled to the three odd-parity vibrations v ₆(t _2u ), v ₄(t _1u ) and v ₃(t _1u ). The absorptions associated with the (² T _1g )Γ₈ electronic state have structure whereas those associated with the (² T _1g )Γ₆ do not. It is shown that the structure is a consequence of splitting of the Γ₈ × v vibronic multiplets by electron-vibration interaction. The intensity of the ⁴ A _2g →(² T _1g )Γ _i + v_j vibronic transitions are expressed in terms of a small number of parameters; two parameters for v(t _1u ) modes and three for v(t _2u ) modes. Plausible but not good fits to the low temperature Zeeman data and vibronic splitting patterns are obtained. The excitation spectrum of the Cs₂SiF₆ : Mn⁴⁺ in the region of the ⁴ A _2g →² E_g and ⁴ A _2g →² T _1g is recorded using a c.w. dye laser. This reveals numerous weaker lines involving combinational modes and even-parity modes v₅ (t _2g ), v ₂(e_g ) and v ₁(a _1g ). Several interesting electron-vibrational effects are observed. These are illustrated and discussed qualitatively.     

Absorption of Ni-centres in alkali chlorides

Absorption spectra of Ni²⁺ doped NaCl, KCl, and RbCl were measured in the spectral range from 55,000 to 5,000 cm^?1. The bands in the UV region are ascribed to the transition 3t _1u(σ, π)→3e _g(σ) of NiCl₆ complex ion. The connection of the intensity of charge transfer andd-d transitions has been discussed.     

The magnetic circular dichroism spectrum of the hexaquocobalt (II) ion in the infra-red region

The magnetic circular dichroism (MCD) and absorption spectra in the range 5500–10 250 cm^-1 of a single cubic crystal of [Co(H₂O)₆](BrO₃)₂ have been measured at various temperatures between 5 and 300 K. An analysis of the temperature variation of the zeroth absorption and MCD moments shows that the intensity of the E′ (⁴αT _1g ) → ⁴ T _2g transition is provided mainly through the two cobalt-oxygen t _1u vibrations. The signs and magnitudes of the overall b and c term are in reasonable accord with previously published calculations. The low energy side of the MCD band exhibits some zerophonon lines and a complex vibronic structure which appears to involve low energy lattice vibrations as well as the cobalt-oxygen skeletal vibrations.     

Magneto-optical study of ferromagnetically coupled cobalt (II) pairs in cadmium bromide

Low temperature polarized absorption and magnetic circular dichroism spectra are reported for crystals of CdBr₂ doped with varying concentrations of Co²⁺ in the region near 17 700 cm^-1. Sharp zero phonon lines are identified as the trigonal field components of U_g ′(² T _1g , ² H) in dilute crystals, and as states arising from ferromagnetically coupled in-plane pairs in the concentrated ones. Measurements of the temperature dependence of the lines, and Zeeman spectra recorded with applied fields up to 50 kG parallel and perpendicular to the crystal c-axis, lead to estimates of the anisotropy of the exchange interaction in the ground state, and to both isotropic and anisotropic exchange in the excited state. The preferred direction of spin alignment in both states is parallel to the c-axis.     

Probable location of a 1E2g state of the benzene molecule

Photoexcitation spectra of benzene in rare gas matrices show a previously unreported transition near 46000 cm^?1. The observed bands are not explicable in terms of site splittings, impurity states, aggregation effects, intermediate radius states of the matrix, triplet states, excimer states, exciplex states or $\text{σ-π}{}^1$ transitions. The vibronic spacings in these spectra could be those expected for a ¹E_2g ← ¹A_1g transition and on this and other evidence we argue that the ordering of origins of the first four spin allowed intravalence states of benzene is ¹B_2u (38086 cm^?1), ¹E_2g (near 46400 cm^?1), ¹B_1u (48450 cm^?1) and ¹E_1u (55430 cm^?1). Our data also show that the transition ¹B_1u ← ¹A_1g accounts for most of the intensity of the 210 nm absorption band system. Our ordering of the spin allowed states permits interpretation of experimental data of others, confirms certain semi-empirical and ab initio SCF MO CI calculations in which account is taken of higher excitations and illustrates the necessity of including such higher excitations. The intensity of the ¹E_2g ← ¹A_1g transition is at least an order of magnitude less than previously calculated indicative of the difficulty of choosing suitable wavefunctions for the ¹E_2g state and of calculating “forbidden” transition probabilities.     

Absorption and fluorescence spectra of C<Subscript>60</Subscript> fullerene concentrated solutions in hexane and polystyrene at 77–300 K

The locations of the 0₀⁰ 0_0^0 -bands for S₁← S₀ and S₁ → S₀ transitions have been found for C₆₀ solutions in hexane. It is shown that the profile of the S₁ ← S₀ band is mainly shaped by h_u(4), t_1u(4)- and h_g(1), a_g(2)- modes that are active in absorption. Bands involving the h_u(4)- and t_1u(4)-modes in the emission process have also been identified in the fluorescence spectrum. The appearance of the 0₀⁰ 0_0^0 -band in the forbidden 1¹T_1g ← 1¹A_g transition is explained by symmetry reduction in the C₆₀+environment system due to the interaction of electrons with local phonons. The temperature coefficients of the red shift for the 256.3- and 328.3-nm bands of allowed ¹T_1u ← 1¹A_g transitions for C₆₀ in hexane are equal to –1.45 and –0.46 cm^–1·K^–1, respectively. The peak and half-width values of the 337.2-nm band for C₆₀ in polystyrene remain unchanged on cooling to 77 K. Absorption in the 700–800-nm region for concentrated hexane solutions of fullerene at 292 K results from the production of (C₆₀)_n-clusters.     

High-resolution optogalvanic study of the c4(0)1Πu, c′5(0)1Σu+, and a″(0)1Σg+ Rydberg states of N2

A new optogalvanic technique with an rf discharge was applied to a high-resolution study of the Rydberg states of N₂. The Ledbetter band, c₄(0)¹Π_u ← a″(0)¹Σ_g⁺, and a new visible band, c′₅(0)¹Σ_u⁺ ← a″(0)¹Σ_g⁺, were studied at a Doppler-limited resolution of 0.05 cm^?1. A Doppler-free method was also applied to resolve overlapped lines. Precise wavenumbers were determined for the rotational transitions of the two Rydberg bands. The rotational and the centrifugal constants for the lowest Rydberg state, a″(0)¹Σ_g⁺, were determined to be B₀ = 1.913748(42) cm^?1 and D₀ = 6.088(99) × 10^?6 cm^?1, where the numbers in parentheses are the standard deviation and apply to the last digits.     

Electronic absorption spectrum of Ni2+ in lithium potassium sulphate single crystal

Single crystals of nickel-doped lithium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the bands could be made assumingO _h symmetry for the Ni²⁺ ion in the crystal. The bands have been assigned transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The crystal field parameters derived areDq=910cm^–1,B=890cm^–1 andC=3560cm^–1.The authors wish to express their thanks to Prof. K. Sreerama Murthy for his constant encouragement throughout this investigation. The authors are also thankful to Prof. Mihir Chowdhury, Indian Association for the cultivation of Science, Calcutta for giving permission to take the spectra.     

Exciton-magnon interactions in NicMg1−c O single crystals

The effect of chemical composition and temperature on exciton-magnon interactions in Ni_cMg_1?c O single crystals was studied using optical absorption spectra in the region of the magnetic-dipole ³ A _2g (G)→³ T _2g (F) and electric-dipole ³ A _2g (F)→¹ E _g (D) transitions. The two zero-phonon lines at ～7800 and ～7840 cm^?1 on the low-energy side of the magnetic-dipole transition band were assigned to a pure exciton transition and an exciton-one-magnon transition at the center of the Brillouin zone. The intensity of the exciton-one-magnon peak decreases rapidly with increasing magnesium ion concentration and/or temperature, to vanish altogether at T=6 K for c<0.95 and at T=130 K for c≥0.99. Thus, the contribution of long-wavelength magnons in optical absorption spectra of Ni_cMg_1?c O becomes negligible at temperatures substantially lower than the Néel point T _N (the antiferromagnetic ordering temperature). This observation can be explained as being due to a substantial decrease in the characteristic spin-spin interaction length with increasing concentration of the diamagnetic magnesium impurity ions (static disorder) and/or with increasing amplitude of thermal atomic vibrations (dynamic disorder). At the same time, the peak at ～14500 cm^?1, which lies in the electric-dipole transition region and corresponds to excitation of an exciton and two magnons at the Brillouin zone edge, remains visible up to the Néel temperature. This is accounted for by the short-wavelength magnons being sensitive to short-range magnetic order, which persists up to T _N .     

Optical absorption spectrum of Ni2+ ions doped in sodium potassium sulphate single crystal

Single crystals of nickel-doped sodium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the observed bands could be made assuming the octahedral symmetry for the Ni^{2 +} ion in the crystal. The bands have been ascribed to transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The experimental and calculated energies are in good agreement. The crystal field parameters derived areDq= =880 cm^–1,B= 900cm^–1 andC=3600 cm^–1.One of the authors, Sujatha John expresses her thanks to the Secretary and the Principal, R. B. V. R. Reddy College, Hyderabad for according her permission to pursue the M. Phil. course.     

High resolution MCD spectroscopy of transition metal ions in fluoride crystals

Measurements of the MCD spectra of Ni²⁺ in KMgF₃ and KZnF₃ have confirmed and extended previous assignments of spin-orbit structure based on similarities of vibrational patterns. The measurements also reveal the presence of a ¹ A _1g (¹ G) state which interacts strongly with the ³ T _1g ^b (³ P) state. The MCD spectra of the tetragonal crystals K₃Ni₂F₇ and K₂NiF₄ have been measured for the transitions ³ A _2g →³ T _1g ^a , ¹ E_g . The spectrum of K₃Ni₂F₇ is characterized by the appearance of electric dipole zero phonon lines, consistent with the C _4v site symmetry of the Ni²⁺ in the paramagnetic phase. An analysis of the MCD spectrum allows a positive assignment of six of the seven states (from ³ T _1g ^a and ¹ E_g ) to which transitions are allowed by symmetry in the axial spectrum. A complete tetragonal crystal-field analysis has been made which shows that the distortion is an axial compression of the fluoride octahedron. The absorption spectrum of K₂NiF₄ shows much less vibrational structure, but the MCD spectrum allows a definitive assignment of six states (from ³ T _1g ^a and ¹ E_g ) through a _2u vibrations in the usual vibronic mechanism. False origins based on e_u phonons have not been identified. The tetragonal field is larger than for K₃Ni₂F₇ and corresponds to an axial compression, in agreement with the known crystal structure.     

MCD spectroscopy of transition metal ions in fluoride crystals

Absorption spectra of crystals of KZn _x Fe_1-x F₃ (0·006 < 1 - x < 0·07) have been measured in order to obtain information about Fe²⁺-F^--Fe²⁺ pairs. However, by means of high resolution MCD spectroscopy, it has been established that very small amounts of Fe³⁺ are present in the crystals. The absorption spectra then show features which are due to (a) Fe²⁺, (b) Fe³⁺ and (c) Fe²⁺, Fe³⁺ pairs. In addition to the characteristic Jahn-Teller split ⁵ T _2g → ⁵ E_g band of Fe²⁺, there is a weak, spin-allowed, vibronically assisted transition near 40 000 cm^-1, which is assigned to ⁵ T _2g (d⁶) → ⁵ A _1g (d⁵s). The MCD of this band is consistent with this assignment. The remainder of the visible and near ultra-violet absorption intensity is associated with Fe²⁺/Fe³⁺ pairs. Most of the bands are due to excitations localized on the Fe²⁺ though some weaker ones are assigned as excitations to ⁴ A _1g on the Fe³⁺ of the pair. There is also a strong, broad underlying absorption probably due to t _2g → t _2g charge transfer in the pair.     

Electronic Absorption Spectrum of Fe3+ Doped in Ammonium Perchlorate Single Crystal

Abstract

Single crystals of Iron - doped ammonium perchlorate were grown at room temperature. The electronic absorption bands observed at room and liquid air temperatures have been assigned transitions from the ground ⁶A_1g state to the excited ⁴A_1g, ⁴E_g, ⁴T_1g and ⁴T_2g states. The crystal field parameters Dq = 870 cm^?1, B = 615 cm^?1 and C = 4.2 B are found to give a good fit to the observed band positions.     

The absorption and magnetic circular dichroism of spectra of Cs2NaPrCl6

The room temperature absorption and magnetic circular dichroism spectra and the absorption spectrum at liquid helium (liquid He) temperature have been measured for Cs₂NaPrCl₆. At room temperature the crystal is cubic and the Pr³⁺ sites have O _h symmetry. All terms above 15 000 cm^-1, except ¹S₀, have been assigned and a previous assignment in PrCl₆ ^3- has been shown to be incorrect. The transition at 20 631 cm^-1 is assigned to ³H₄(A _1g ) →³P₁(T _1g ), in contradiction to previous assignments of Pr³⁺ spectra in other systems. A rich vibrational structure was observed in every transition. Vibrations have been assigned using the site group approximation and there is substantial agreement with the vibrational assignments in Cs₂NaEuCl₆. A crystal phase transition takes place between room temperature and liquid nitrogen temperature and the O _h forbidden transitions, A _1g →E_g and T _2g , are observed. At lower temperatures many additional lines are observed but it is unclear presently whether they are due to lower symmetry or a breakdown of the site group approximation.     

Laser photoacoustic spectroscopy of iodine molecule: Single and two-photon absorption

Photoacoustic spectroscopy of iodine molecule has been studied in gas phase using nitrogen laser-pumped tunable dye laser. The experiment yielded the vibrational spectrum corresponding toX ¹Σ⁺(0 _g ⁺ )→B ³Π(0 _g ⁺ ) transition up to the convergence limit. The photo-acoustic spectrum in the region 17580–18850 cm⁻¹ is presented along with the vibrational analysis. Five of the vibrational bands reported earlier by Venkateswarlu, Kumar and McGlynn have been partially resolved and the structure of one of them has been analyzed and shown to be due to an overlap of (14, 2) and (12, 1) bands. The analysis was based on a comparison with the highly resolved spectrum of Gerstenkorn and Luc. The structure observed in the region 20200–20750 cm⁻¹ which is beyond the convergence limit of the transitionX ¹Σ⁺(0 _g ⁺ )→B ³Π(0 _u ⁺ ) has been analyzed as due to two-photon absorption. Most of the bands could be assigned to two transitions both originating in the ground state and terminating in two different electronic states 1 _g andE(0 _g ⁺ ), atT _e=40821 cm⁻¹ (orT ₀=41355 cm⁻¹) andT _e=41411 cm⁻¹ (orT ₀=41355 cm⁻¹) respectively.