Photoreactions of aromatic compounds—XXIX : Pathway and intermediates of the photoreactions of 3,5-dinitroanisole with nucleophiles

The photoreactions of 3,5-dinitroanisole with nucleophiles (mostly hydroxide ion) have been studied by sensitization and quenching as well as by flash photolysis experiments. The photosubstitution starts mainly from the π-π* triplet excited state (λ_max ～ 475 nm; τ 5·10^?8s or shorter depending on the concentration of nucleophile). The formation of substitution product is completed within ～ 10^?6s. The occurrence of the radical anion (λ_max ～ 550 nm; τ ～ 10^?2-10^?4s depending on the nature of the nucleophile) could be established. This cannot be intermediate in the photosubstitution reaction but is so in the formation of reduction products. The radical anion and, probably also, the substitution product seem to originate from a complex (λ_max ～ 410 nm; τ ～ 5·10^?7s) between the triplet excited aromatic compound and its nucleophilic reaction partner. The formation of this complex is a very fast process.     

Green Luminescence of Divalent Europium in the Hydride Chloride EuHCl

Luminescence properties of divalent europium in the mixed‐anion hydride chloride EuHCl were studied for the first time. Olive‐green single crystals of EuHCl (PbFCl‐type structure: tetragonal, P4/nmm, a = 406.58(3) pm, c = 693.12(5) pm, c/a = 1.705, Z = 2) resulted from the reaction of elemental europium (Eu), sodium hydride (NaH) and sodium chloride (NaCl), while powder samples were prepared from the binary components europium dihydride (EuH₂) and dichloride (EuCl₂). Low temperature X‐ray powder diffraction proved the absence of phase transitions for 12(2) K ≤ T ≤ 295(2) K. Bright green emission was observed under UV‐excitation and assigned to the 4f⁶5d¹–4f⁷ transition of divalent europium. Temperature‐dependent luminescence absorption and emission, as well as lifetime measurements were carried out on single crystal and powder samples. Surprisingly, only limited concentration quenching was found. Additionally, two emission bands (485 and 510 nm) are observed, whose intensity ratio depends strongly on temperature. In order to explain this behavior for a single Eu²⁺ site, we suggest either a dynamical Jahn–Teller effect in the excited 5d¹ state or emission from both a 4f⁶5d¹ state and a trapped exciton state.     

Properties of barium chlorophosphate (apatite) luminophors activated by divalent europium

A series of powder materials of barium and calcium chlorophosphate activated by divalent europium have been prepared. These luminophors are isomorphous representatives of the hexagonal chlorapatites (Ba, Ca, Mg)₁₀(PO₄)₆Cl₂. In addition to x-ray diffractograms, excitation and emission spectra of luminescence have been studied at room and low temperatures (T ∽ 4 K). An increasing calcium content affects the luminescence properties of the solid solutions by changing the crystal field acting upon the Eu²⁺ ion. The appearance of a new low-temperature emission band with higher Ca²⁺ content indicates that the Eu²⁺ ions substitute on two different barium sites thus forming two types of luminescent centres. The luminescence arises from transitions between 4f⁶5d and 4f⁷ configurations of Eu²⁺.     

Improvement of the Water Resistance of a Narrow‐Band Red‐Emitting SrLiAl3N4:Eu2+ Phosphor Synthesized under High Isostatic Pressure through Coating with an Organosilica Layer

A SrLiAl₃N₄:Eu²⁺ (SLA) red phosphor prepared through a high‐pressure solid‐state reaction was coated with an organosilica layer with a thickness of 400–600 nm to improve its water resistance. The observed 4f⁶5d→4f⁷ transition bands are thought to result from the existence of Eu²⁺ at two different Sr²⁺ sites. Luminescence spectra at 10 K revealed two zero‐phonon lines at 15377 (for Eu(Sr1)) and 15780 cm^?1 (for Eu(Sr2)). The phosphor exhibited stable red emission under high pressure up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for the Eu^2+/3+ result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70 % of their initial EQE after aging for 5 days under harsh conditions. White‐light‐emitting diodes of the SLA red phosphor and a commercial Y₃Al₅O₁₂:Ce³⁺ yellow phosphor on a blue InGaN chip showed high color rendition (CRI=89, R9=69) and a low correlated color temperature of 2406 K.     

Sensitization of Tb3+ luminescence with Ce3+ in LaOBr: Tb3+, Ce3+

Luminescence emission and uv-excitation properties of LaOBr: Tb³⁺, LaOBr: Ce³⁺, and LaOBr: Tb³⁺, Ce³⁺ phosphors were studied. The visible emission spectra of La_0.995Tb_0.005OBr consists of⁵D_3,4 →⁷F_3–6 transitions in the wavelength range of 410–630 nm. The excitation of the Tb³⁺ ion gives a broad 4f → 5d transition band at 254 nm and weaker4f → 4f transition lines above 300 nm. The uv-excitation and emission of La_0.995Ce_0.005OBr at 290, 315, 355 (excitation), and 440 nm (emission) originate from transitions between the 4f-ground state and the four crystal field components of the5d²D excited state. The sensitization of Tb³⁺ luminescence in LaOBr with Ce³⁺ at varying concentrations is described and discussed. With increasing Ce³⁺ concentration the ⁵D₃ →⁷F transitions of Tb³⁺ quench totally and the⁵D₄ →⁷F transitions begin to quench gradually. The excitation spectrum of the⁵D₄ →⁷F₅ transition of Tb³⁺ consists of four bands due to Tb³⁺ and Ce³⁺, of which the three Ce³⁺ bands increase in intensity and the Tb³⁺ band decreases as the Ce³⁺ concentration is increased.     

Mechanistic studies of the photorearrangement of o-nitrobenzyl esters

α-Methyl-o-nitrobenzyl isobutyrate and copolymers containing α-methyl-o-nitrobenzyl acrylate were irradiated in dilute solution with 20 ns flashes of 347 nm light or with 60 ps flashes of 355 nm light. The formation of the absorption spectrum of a nitronic acid with τ = 5 ns was observed in each case. A transient absorption spectrum similar to that of the nitronic acid was formed with τ ⩽ 0.5 ns. This spectrum is assumed to belong either to the triplet biradical formed upon intramolecular triplet state hydrogen abstraction or to nitronic acid formed very rapidly by intramolecular singlet state hydrogen abstraction, the latter mechanism being operative in parallel with the triplet mechanism.In 60vol.%EtOH-40vol.%H₂O the nitronate anion was formed by dissociation of the nitronic acid with τ = 3 μs as indicated by a build-up of a new absorption band with λ_max = 420 nm and by an increase in electrical conductivity. The conversion into the end products (nitroso compound and carboxylic acid) occurred at a much faster rate from the nitronic acid than from the nitronate anion: τ = 80 μs (CH₂Cl₂), τ = 360 μs (60vol.%EtOH-40.vol.%H₂O, 10⁻⁴ M H₂SO₄) and τ = 10 ms (60vol.%EtOH-40vol.%H₂O, no H₂SO₄). Conversion of the nitronate anion into another transient was inferred from the partial decrease in the electrical conductivity (τ ≈ 15 μs).Irradiation of the copolymers gave the same results as for the low molecular weight model compound, indicating that there is no polymer effect with respect to the kinetics or the mechanism of the photorearrangement. This conclusion was substantiated by the quantum yields for acid formation (0.24 – 0.25) measured with both the model compound and the copolymers.     

Magnetic field-induced ΔF=±2,3,4 transitions in the Eu I-spectrum

The occurrence of ΔF=±2, ±3, and ±4 transitions in the hyperfine structure of the Eu I lineλ 629.1 nm (4f ⁷6s ² a ⁸ S _7/2?4f ⁷6s6p z ⁸ P _5/2) was investigated with the application of high-resolution laser-atomic-beam spectroscopy. It was possible to show that the appearance of such transitions depends on the magnitude of an external magnetic field. Calculations of the hyperfine Zeeman splittings of the excited and the ground state were performed. This allowed the identification of the forbidden transitions.     

Interferences in and lifetime measurement of a resonance electronic Raman effect using tunable pulsed laser techniques

The excitation profile for the intensity of electronic Raman transitions of terbium aluminum garnet (TbAlG) in the spectral range of 483.0–680.0 nm is reported. The electronic Raman transitions take place between the crystal field levels of the split ⁷F₆ ground manifold of TbAlG with shifts of 73 cm^?1 and 83 cm^?1 and the electronic Raman process is induced with tunable pulsed and fixed wavelength cw lasers. The tunability of the former was employed to obtain detailed information of the behaviour of the Raman intensity if the wavelength of the exciting source is tuned throughout the region of 483.0–490.0 nm where ⁵Da₄ ← ⁷F₆ absorptions of TbAlG occur and the data reveal the occurrence of interference effects. We also report measurements of the shape of the pulse — due to resonance enhanced electronic Raman scattered light — in real time. These studies reveal that the lifetime of the resonating state (which is responsible for the enhancement of the Raman intensity) as determined from the tailing end of the said pulse is within experimental error equal to the lifetime τ = 33.5 ± 1 μs of this state measured in a direct way from the intensity decay of an appropriate fluorescence transition of TbAlG.     

Luminescent Nitridophosphates CaP2N4:Eu2+, SrP2N4:Eu2+, BaP2N4:Eu2+, and BaSr2P6N12:Eu2+

Nitridophosphates MP₂N₄:Eu²⁺ (M=Ca, Sr, Ba) and BaSr₂P₆N₁₂:Eu²⁺ have been synthesized at elevated pressures and 1100–1300 °C starting from the corresponding azides and P₃N₅ with EuCl₂ as dopant. Addition of NH₄Cl as mineralizer allowed for the growth of single crystals. This led to the successful structure elucidation of a highly condensed nitridophosphate from single‐crystal X‐ray diffraction data (CaP₂N₄:Eu²⁺ (P6₃, no. 173), a=16.847(2), c=7.8592(16) Å, V=1931.7(6) ų, Z=24, 2033 observed reflections, 176 refined parameters, wR₂=0.096). Upon excitation by UV light, luminescence due to parity‐allowed 4f⁶(⁷F)5d¹→4f⁷(⁸S_7/2) transition was observed in the orange (CaP₂N₄:Eu²⁺, λ_max=575 nm), green (SrP₂N₄:Eu²⁺, λ_max=529 nm), and blue regions of the visible spectrum (BaSr₂P₆N₁₂:Eu²⁺ and BaP₂N₄:Eu²⁺, λ_max=450 and 460 nm, respectively). Thus, the emission wavelength decreases with increasing ionic radius of the alkaline‐earth ions. The corresponding full width at half maximum values (2240–2460 cm^?1) are comparable to those of other known Eu²⁺‐doped (oxo)nitrides emitting in the same region of the visible spectrum. Following recently described quaternary Ba₃P₅N₁₀Br:Eu²⁺, this investigation represents the first report on the luminescence of Eu²⁺‐doped ternary nitridophosphates. Similarly to nitridosilicates and related oxonitrides, Eu²⁺‐doped nitridophosphates may have the potential to be further developed into efficient light‐emitting diode phosphors.     

The 3[ndσ*(n+1)pσ] Emissions of Linear Silver(I) and Gold(I) Chains with Bridging Phosphine Ligands

The complexes [Au₃(dcmp)₂][X]₃ {dcmp=bis(dicyclohexylphosphinomethyl)cyclohexylphosphine; X=Cl^? ( 1 ), ClO₄^? ( 2 ), OTf^? ( 3 ), PF₆^? ( 4 ), SCN^?( 5 )}, [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ), and [Ag₃(dcmp)₂Cl₂][ClO₄] ( 7 ) were prepared and their structures were determined by X‐ray crystallography. Complexes 2 – 4 display a high‐energy emission band with λ_max at 442–452 nm, whereas 1 and 5 display a low‐energy emission with λ_max at 558–634 nm in both solid state and in dichloromethane at 298 K. The former is assigned to the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺, whereas the latter is attributed to an exciplex formed between the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺ and the counterions. In solid state, complex [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ) displays an intense emission band at 375 nm with a Stokes shift of ≈7200 cm^?1 from the ¹[4dσ*→5pσ] absorption band at 295 nm. The 375 nm emission band is assigned to the emission directly from the ³[4dσ^*5pσ] excited state of 6 . Density functional theory (DFT) calculations revealed that the absorption and emission energies are inversely proportional to the number of metal ions (n) in polynuclear Au^I and Ag^I linear chain complexes without close metal???anion contacts. The emission energies are extrapolated to be 715 and 446 nm for the infinite linear Au^I and Ag^I chains, respectively, at metal???metal distances of about 2.93–3.02 Å. A QM/MM calculation on the model [Au₃(dcmp)₂Cl₂]⁺ system, with Au???Cl contacts of 2.90–3.10 Å, gave optimized Au???Au distances of 2.99–3.11 Å in its lowest triplet excited state and the emission energies were calculated to be at approximately 600–690 nm, which are assigned to a three‐coordinate Au^I site with its spectroscopic properties affected by Au^I???Au^I interactions.     

BrNO—thermodynamic properties,the ultraviolet/vis spectrum,and the kinetics of its formation

The equilibrium has been studied between 275°and 363°K. Third-law calculations lead to ΔH°₂₉₈(1) = -11.50 ±0.17 kcal/mol, from which Absorption bands of BrNO in the ultraviolet with e (γ_max = 215 nm) = 1.84±0.17 × 10⁴ 1/mol·cm, and in the red with e (γ_max = 708 nm) = 7.7±1.9 1/mol·cm at 298°K have been investigated. The rate of formation of BrNO has also been measured between 275°and 363°K.     

Hyperfinestructure and isotope shift in the configuration 4f 7 6s8s of Eu I

The investigation of the isotope shift (IS) and hyperfine structure (hfs) is extended from Eu 4f ⁷ 6s7s to the complete configuration 4f ⁷ 6s8s, by means of the transitions 432.3, 456.5, 463.0 and 493.8 nm to 4f ⁷ 6s6p. A thorough experimental and theoretical analysis — including two further lines 492.8 and 498.7 nm from 4f ⁷ 5d ² — is carried out which e.g. confirms former fine structure calculations of one of us (J.F.W.) concerning some reclassifications. The discussion of the IS of the four levels (4f ⁷)6s8s with the sharing rule manifests again the need for inclusion of crossed-second-order effects. For the parameters we evaluated:g ₃(4f, 6s)=?1, 2(2) mK andd=79, 3(1.0) mK. The ratiog ₃/G ₃=?5, 9(1.0)·10^?6 is again in full agreement with those found by us in other Eu configurations. The single electron hfs splitting constantsa _4f ¹⁰ =?2, 3(4) mK,a _6s ¹⁰ =389(4) mK anda _8s ¹⁰ =49 (4) mK were also evaluated and compared to those found in 4f ⁷ 6s7s.     

INO thermodynamic properties and ultraviolet spectrum

The equilibrium I₂(g) + 2NO(g) = 2INO(g) has been studied at room temperature by ultraviolet absorption spectroscopy. The equilibrium constant has been measured as K_p = (2.7 ± 0.3) × 10^?6 atm^?1 at 298 K. Third-law calculations lead to ΔH°_f,298 (INO) = 120.0 ± 0.3 kJ/mol. The relative absorption spectrum of INO has been measured between 225 and 300 nm. Quantitative measurements gave ?(λ_max = 238 nm) = (1.79 ± 0.5) × 10⁴ L/mol·cm and ?(410 nm) = 234.7 ± 21 L/mol·cm.     

VUV spectroscopic properties of rare-earth (RE3+ = Sm3+, Eu3+, Tb3+, Dy3+) -activated layered borate Ba6Gd9B79O138

Vacuum ultraviolet (VUV) spectroscopic properties of rare-earth RE³⁺- activated (RE³⁺ = Sm³⁺, Eu³⁺, Tb³⁺ and Dy³⁺) Ba₆Gd₉B₇₉O₁₃₈ borates (BGBO) are investigated. The strong absorption bands in the VUV range of un-doped and RE³⁺-activated BGBO were observed. The band range from 140 to 200 nm with a peak at about 173 nm results from the host lattice absorption. For Sm³⁺-activated BGBO, the charge transfer transition from O^2- to Sm³⁺ was observed at 202 nm. In addition, it exhibits bright red emission originating from the Sm³⁺ f-f transitions of ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2). The O^2--Eu³⁺ charge transfer (CT) at 249 nm is observed in the excitation spectrum for Eu³⁺-doped BGBO. For Tb³⁺-activated BGBO, the broad bands around 208 and 230 nm are due to the spin-allowed and spin-forbidden f-d transitions of Tb³⁺, respectively. In addition, the absence of the f-d transitions of Sm³⁺ and Dy³⁺ in the excitation spectra probably due to the photo-ionization effect. It is demonstrated that there are energy transfers from the BGBO host lattice to the luminescent activators depending on the activators.     

Phosphorescence at Low Temperature by External Heavy-Atom Effect in Zinc(II) Clusters

Luminescent Zn^II clusters [Zn₄L₄(μ₃-OMe)₂X₂] (X=SCN ( 1 ), Cl ( 2 ), Br ( 3 )) and [Zn₇L₆(μ₃-OMe)₂(μ₃-OH)₄]Y₂ (Y=I⁻ ( 4 ), ClO₄⁻ ( 5 )), HL=methyl-3-methoxysalicylate, exhibiting blue fluorescence at room temperature (λ_max=416≈429 nm, Φ_em=0.09–0.36) have been synthesised and investigated in detail. In one case the external heavy-atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (λ_max=520 nm, τ=95.3 ms) at 77 K. Single-crystal X-ray structural analysis and time-dependent density-functional theory (TD-DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand-centred (¹LC) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH−I interactions.     

Structure and charge-transport properties of LnBaCuCoO<Subscript>5 + δ</Subscript> (Ln = Y,Dy) cuprocobaltites

LnBaCuCoO_{5 + δ} (Ln = Y, Dy) cuprocobaltites were prepared. Their unit cell parameters were determined and their thermal expansion, electrical conductivity (σ), and Seebeck coefficient (S) were studied in air in the range 300–1100 K. The compounds have tetragonal structures (space group P4/mmm). Their unit cell parameters are a = 0.3867(2) nm, c = 0.7550(7) nm, V = 112.9(2) × 10^?3 nm³ for YBaCuCoO_4.98; and a = 0.3872(2) nm, c = 0.7562(7) nm, V = 113.4(2) × 10^?3 nm³ for DyBaCuCoO_5.01. They are p-type semiconductors. The electrical conductivity of DyBaCuCoO_{5 + δ} is slightly lower and its Seebeck coefficient is 1.5–2 times higher than the respective values for YBaCuCoO_{5 + δ} apparently because of different electronic configurations of the rare-earth cations in LnBaCuCoO_{5 + δ} (4d ⁰ for Y³⁺ and 4f ⁹ for Dy³⁺). Dilatometric measurements show that the LnBaCuCoO_{5 + δ} phases in the range 300–1100 K do not experience structural phase transitions, and their linear thermal expansion coefficients (LTEC) are 14.3 × 10^?6 K^?1 for Ln = Y and 14.7 × 10^?6 K^?1 for Ln = Dy.     

Determination of the mechanism of the oxidation of the trichloromethyl radical by the photolysis of chloral in the presence of oxygen

The photooxidation of chloral was studied by infrared spectroscopy under steady-state conditions with irradiation of a blackblue fluorescent lamp (300 nm < λ < 400 nm, λ_max = 360 nm) at 296 ± 2 K. The products were hydrogen chloride, carbon monoxide, carbon dioxide, and phosgen. The kinetic results reveal that the reaction proceeds via chain reaction of the Cl atom: The results lead to the conclusion that mechanism (B) is confirmed to be more likely than mechanism (A), which was favored at one time by Heicklen for the mechanism of the oxidation of trichloromethyl radicals by oxygen molecules: The ratio of the initial rates of CO and CO₂ formation gave k₇/k₆ = 4.23M^?1, and the lower limit of reaction (5) was found to be 3.7 × 10⁸M^?1 sec^?1.     

Simulation of isotopic selectivities and isotope ratio enhancement factors for gadolinium and lanthanum using narrow band laser excitation

Isotopic selectivities and isotope ratio enhancement factors have been calculated for 4f⁵5d6s^{2 9}D ₅ ⁰ -4f⁷5d6s6p ⁹D₆ (585.622 nm) transition of gadolinium and for 5d6s^{2 2}D_3/2-5d6s6p ⁴F _3/2 ⁰ (753.9 nm) transition of lanthanum by simulating the spectra assuming natural isotopic abundance of the samples. The atomic line shapes are assumed to be Lorentzian. Isotopic selectivities and isotope ratio enhancement factors for single step excitations have been computed. The isotope ratio enhancement factors estimated by us are half the value obtained by Young. et al. This is because the isotope ratio enhancement obtained by them is for a two colour RIMS where as our computations for selectivities are for a single step excitation. Considering the statistical error in the experimental values, our estimated isotopic selectivities are in good agreement with the available experimental data.     

Isotope shifts in odd and even energy levels of the neutral and singly ionised gadolinium atom

Isotope shift studies in the gadolinium spectra have been extended in the region 4140–4535 Å. Isotope shift Δσ(156–160) have been measured in 315 lines of the neutral and singly ionised galolinium atom using a recording Fabry—Perot Spectrometer and gadolinium samples enriched in ¹⁵⁶Gd and ¹⁶⁰Gd isotopes. Some of the Gd I lines studied involve transitions from newly identified high odd levels of 4f⁸6s 6p, 4f⁷5d 6s 7s and 4f⁷5d³ configurations to low even levels of 4f⁸6s² and 4f⁷6s²6p configurations. Electronic configurations of the energy levels have been discussed on the basis of observed isotope shifts. In some cases assigned configurations have been revised and probable configurations have been suggested.     

Emission spectra of Cs2NaTbCl6 and Cs2NaYCl6: Tb3+

The emission spectra of microcrystalline Cs₂NaTbCl₆ and Cs₂Na(Y_0.99Tb_0.01)Cl₆ have been measured at room temperature and at 77 K. The crystal structures of these compounds are face-centered cubic and the terbium (III) ions lie at sites of octahedral (O_h) symmetry surrounded by six chloride ions. Emission is observed from both the ⁵D₃ and ⁵D₄ excited states of Tb³⁺. Assignments have been made for nearly all of the magnetic-dipole transitions split out of the Tb³⁺⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃, ⁷F₂, ⁷F₁ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃ transitions. These assignments are based on the calculated transition energies and relative magnetic-dipole strengths and intensities obtained from a weak-field crystal-field analysis of octahedral TbCl₆^3? units. Magnetic-dipole lines dominate the spectra for transitions of ΔJ = ±1 free-ion parentage, whereas both magnetic-dipole lines and vibronically induced electric-dipole lines contribute significantly to the emission intensities of the ΔJ = 0, ±2 transitions. The crystal-field sub-levels of both ⁵D₃ and ⁵D₄ appear to reach a Boltzmann thermal equilibrium prior to emission. Emission from ⁵D₃ is partially quenched in going from low temperature to high temperature and in going from Cs₂NaYCl₆: Tb³⁺ (1%) to Cs₂NaTbCl₆.This study has led to the identification and assignment of nearly all of the pure magnetic-dipole transitions split out of the Tb³⁺⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃, ⁷F₂, ⁷F₁ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃ transitions in crystal-line Cs₂NaTbCl₆. The assignments were based on calculated transition energies and relative magnetic-dipole strengths (and intensities) obtained from a (weak-field) crystal-field analysis of octahedral (O_h) TbCl₆^3? clusters. Excellent agreement between the calculated and observed relative intensities of the magnetic-dipole lines was achieved by assuming a Boltzmann equilibrated set of crystal-field sub-levels for both the ⁵D₄ and ⁵D₃ emitting states. Furthermore, the experimental results suggest that ⁵D₄ ← ⁵D₃ relaxation is temperature-dependent.The energy levels calculated and displayed in table 1 appear to be qualitatively correct and are in semiquantitative agreement with the emission results (as interpreted in section 4). Calculated and observed transition energies for the assigned magnetic-dipole transitions generally agree to within 0.2%.One of the most remarkable features of the emission spectra obtained on Cs₂NaTbCl₆ is the absence of any vibrational structure in the ΔJ = ± 1 transitions (⁷F₆, ⁷F₃ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃), and the presence of extensive vibrational structure in the ΔJ = O, ±2 transitions (⁷F₆, ⁷F₄, ⁷F₂ ← ⁵D₄). If other than OO vibronic transitions do contribute to the ΔJ = ±1 emissions, their intensities must be at least two or three orders-of-magnitude weaker than the OO magnetic-dipole lines. Vibronically induced electric-dipole transitions appear, however, to make substantial contributions to the ⁷F₆, ⁷F₄, ⁷F₂ ← ⁵D₄ emission spectra. A clear-cut theoretical explanation for the absence of vibrational structure in the ΔJ = ±1 transitions is not readily apparent. We are presently examining this problem in greater detail.