Study of the precipitation behavior of NaCl:Pb and KCl:Pb by optical spectroscopy2

The aggregation and precipitation behavior of lead in NaCl and KCl crystals has been systematically investigated by absorption and luminescence spectroscopy. For NaCl, absorption bands at 265 and 285 nm have been identified which correspond to intermediate stages in the formation of a double band (262–266 nm) associated with PbCl₂ precipitates. The kinetics of growth during ageing as well as the thermal annihilation behavior have been obtained for all bands. The emission spectra for excitation at those absorption bands have also been determined. For KCl, absorption bands at 265 and 280 nm develop during ageing at 160 and 200°C and their growth-rate is markedly enhanced by plastic straining. Their emission spectra involve a main band at $\text{\$}\text{?}$550 nm. In contrast to NaCl, no stage has been found where those absorption bands feed into a final band, which could be associated with PbCl₂ precipitates.     

Band structure and optical properties of alkali metal halides

Optical spectra, photoemission spectra, and photoconductivity spectra of alkali metal halides are analyzed using energy band structure calculations and selection rules for direct and indirect transitions. The main feature of the band structure of MeN₃ (Me: Na, K, Rb, Cs) as proposed by the authors is the presence of two conductivity bands, one anionic and the other cationic in nature. A weak dispersion of the valence subband indicates that phonons may yield a significant contribution to the observed spectra. All of the optical and photoemission spectra so far reported for the metal azides may be explained on the basis of the proposed band model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1995.     

Influence of precipitation processes on the optical spectra of Pb2+ in alkali halides

The influence of the aggregation and precipitation processes of lead ions on optical (absorption and luminescence) spectra have been studied for KBr, KI and RbCl. Bands presumably associated with aggregates and precipitates have been identified and compared with those observed for NaCl and KCl. The thermal annihilation behavior of precipitate bands as well as the effect of precipitation on yield-stress have also been determined. As a consequence, a more complete scheme of the influence of aging processes on optical spectra of lead-doped alkali halides has now become available.     

Photoelectron spectroscopy studies of the band structure of silver halides

X-ray photoelectron spectroscopy has been used to probe the valence bands of the silver halides. Previous ultraviolet photoemission and optical absorption experiments together with the theoretical band structure calculations form the basis for interpretation of the spectra. Density of states maxima from the halogen p levels are clearly resolved from those of the silver 4d states. Additional splittings due to k space symmetry are observable and in the case of AgCl and AgBr, give excellent conformation of existing band structure calculations. The role of spin-orbit coupling is shown to be unimportant in determining the primary shape of the photoelectron spectrum. The spectrum of AgF shows the inversion of silver 4d and halogen 2p levels suggested by the optical absorption spectra and predicted by several recent band calculations. A new interpretation is proposed for the AgF optical spectrum in which the observed excitons are due to the forbidden Γ₁₂→ Γ₁ and Γ_25′ → Γ₁ tr the high temperature body centered cubic structure. These spectra clearly show the broadening of the I 5p levels which causes the band gap to decrease above the transition temperature.     

Evaluation of the Mn Lα and Lβ Spectra of Mn Metal and of Various Mn-Oxides

The Mn Lα and Lβ spectra of Mn, MnO, Mn₃O₄, Mn₂O₃, and MnO₂ have been measured. Each Mn La spectrum has been deconvoluted into two bands, Lα_A and Lα_B, and the integrated Lα_A/Lα_B intensity ratio was found to be inversely proportional to the Mn-0 inter-atomic distance, but proportional to the oxidation number. This finding tends to indicate the likelihood of crossover transition from oxygen to manganese. The intensity ratio Lβ/Lα_B of the pure metal was found to coincide closely with the statistically predicted value of 0.5. Furthermore, for Mn-metal, as well as for the oxides, the shift of the Lα_A band from the Lα_B band was found to agree with the change in the Lβ/Lα peak intensity ratio as a function of oxidation number.     

The photoelectron and absorption spectra of allyl halides

The ultraviolet photoelectron spectra (5–20 eV) absorption spectra (1400–2500 Å) of allyl C₃H₅X (X = F, Cl, Br, I) have been measured. The ionisation potentials between 5–16 eV are determined and the type of orbital is interpreted with the aid of the vibration pattern and the band form.From these spectra several conclusions can be drawn concerning the iodide: (1) the ionisation potential of the π_C_C system of allyl iodide is lower than expected when compared to the ionisation potential of the corresponding band of the other halides. (2) The splitting of its np lone pair photoelectron bands is not in accordance with the expected splitting associated with spin-orbit coupling. (3) One of the two components of the lone pair bands has been broadened. These observations can be explained by a through-space interaction of one of the iodine “lone pairs” with the π-system.     

Optical transitions of Ag− centers in alkali halides

Ag^? centers in alkali halides give rise to a strong absorption band in the 300 nm region (formerly called “B band”). Its resolved triplet structure in CsCl suggests that it corresponds to the C band of the isoelectronic In⁺ center. Two very weak bands are found in several alkali halides in the 400 nm region. These new bands are assigned to the A and B transitions of the In⁺-type centers. This is supported by the doublet structure in the A band, and by the temperature dependence of the oscillator strength of the B band. In KCl∶Ag^? the ratios of the oscillator strengths are found to bef _c /f _A =610 andf _c /f _B =3,400 at low temperatures. The energy parameters of Ag^? centers are computed and compared with those of others ²-type centers. The electron-lattice coupling parameters are estimated from the Jahn-Teller splitting of the C band in CsCl and of the A band in KC1. The temperature dependence of the lifetime of the visible fluorescence suggests that a metastable state is involved in the emission process after a C band excitation.     

X-ray photoemission of valence electrons in cuprous halides,and lead and cadmium iodides.

The XPS spectra of valence bands in CuCl, CuBr, and CuI show the existence of two separate bands after appropriate deconvolution. The evaluation of the average p-d mixing rates on the basis of the tight-binding approximation with a few simplifying assumptions indicates, in accordance with optical data, that the upper valence band arises mainly from the 3d state of copper. The XPS spectra coincide qualitatively but not quantitatively with the spectra obtained by excitation with monochromatized synchrotron radiation at 40, 61, and 76 eV. A major difference is that the ratio of integrated intensity of the upper band to that of the lower band is larger in the case of excitation at 40, 61, or 76 eV. The XPS spectra of PbI₂ and CdI₂ have also been measured. In both materials, the valence band spectra have composite structures and give two well-defined peaks after deconvolution. The profile of the spectrum of PbI₂ appears to have some deviation from the reported energy band structure.     

Absorption spectra of NaCl: Pb2+ at the A-band region

The origin of the absorption spectrum of NaCl:Pb²⁺ at the A-band region has been investigated. Main peaks are observed at 265, 273 and 290mμ in an untreated sample. In addition to the effect of lead concentration and thermal treatments, the luminescence and excitation spectra associated to the various bands have been determined. It is concluded that the 290 mμ band should be a composition of a number of bands corresponding to different types of simple dipole aggregates. The 265 mμ band, as well as the structure appearing in the region 220–260 mμ, is consistent with the existence of PbCl₂ precipitates.     

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals

Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship T_M1nτ^?1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu²⁺ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.     

Transverse optic mode Gruneisen parameter and dielectric properties of alkali halides

The well known Szigeti relations for the dielectric theory of ionic crystals have been critically examined by investigating the volume dependence of dielectric behaviour of alkali halides. It has been predicted that the strain derivative of effective ionic charge parameter should be nearly equal to zero. Values of transverse optic mode Gruneisen parameter for NaCl structure alkali halides have been estimated.     

Photoluminescence of O2 and S2 ions in synthetic sodalities

Photoluminescence spectra of oxygen-doped chloro- and bromosodalites and sulfur-doped chloro-, bromo- and iodosodalites were measured at temperatures between 4.2 and 300 °K. At 4.2 and 77 °K, the emission spectra of oxygen-doped sodalites consisted of a series of peaks in the wavelength range 400–700 nm, with an average energy separation of ∼ 1000 cm^-1. In addition, fine structure, attributed to lattice modes, was observed in each vibrational band. At 4.2 and 77 °K, the sulfur-doped samples showed a multiband spectrum in the 500–750 nm range, with an average separation of ∼ 570 cm^-1 between bands. The spectrum at 4.2 °K exhibited some asymmetry not observed at 77 °K, but no fine structure was resolved. At 300 °K weak, broad-band luminescence was observed from both oxygen- and sulfur-doped samples, with no vibrational structure evident. The results compared very favorably with those reported for oxygen- and sulfur-doped alkali halides, and by analogy the spectra were attributed to luminescence from O^-₂ and S^-₂ molecular ions.     

The reflection spectra of cuprous halides in the 4.5–7.5 eV range

The reflection spectra of cuprous halides have been measured on cleaved samples at various temperatures from 300 K up to 6 K. The experimental device is improved by a minicomputer based data acquisition system. The appearance of structures at low temperature, the evolution of the spectra versus temperatures permit us to interprete some experimental features in terms of a band structure scheme where the valence bands are calculated in the tight binding approximation, the spin orbit splitting being introduced as a perturbation.     

Luminescent properties of LiCl-Cu single crystals

The absorption, photoluminescence, x-ray luminescence, and thermoluminescence spectra, and the photoluminescence excitation spectra of LiCl-Cu single crystals with different activator concentration were investigated at temperatures of 79–450°K. The absorption spectrum at room temperature has two bands with maxima at 237 and 259 nm. The absorption coefficient of the 237 nm band is proportional to the copper concentration in the crystal (C_Cu ≤ 7·10^?4 mole %). The photoluminescence and x-ray luminescence spectrum at room temperature consists of one emission band at 324 nm, which conforms with the Mollwo-Ivey rule in the homologous series RbCl → KCl → NaCl → LiCl. The copper ions create trapping levels for electrons and holes at different depths in the forbidden band of the LiCl crystal. The correlation between the thermoluminescence peaks and the recombination-luminescence excitation bands (infrared stimulation) is investigated.     

The effect of vacancies on the electronic structure of Nbo

The crystal structure of NbO is considered as a NaCl structure with 25% ordered vacancies in each sublattice. Self-consistent energy band structure calculations using the augmented plane wave (APW) method have been carried out for a hypothetical NbO with the full NaCl structure and for the structure with the vacancies. We find that the introduction of the vacancies leads to (1) a lowering of the Nb-d like bands with respect to the O-p bands, (2) an almost completely filled “vacancy band”, and (3) a splitting and broadening of the O-p like band. These effects are shown to have a significant influence on both the theoretically calculated X-ray photoemission spectra (XPS) and the X-ray emission spectra (XES). All theoretical spectra obtained with the vacancy-structure are found to be in good agreement with experiments in contrast to those spectra calculated for hypothetical NbO in the NaCl structure.     

Analysis of the repulsive softness parameter in alkali halides

An analysis of the repulsive softness parameter for alkali halides has been presented on the basis of the additivity rule. The softness parameters for individual ions have been obtained using data from three different sources viz. overlap data, molecular data and crystal data. The softness parameters so derived are used to evaluate the bulk modulus and its pressure dependence for 20 alkali halides with NaCl and CsCl structures adopting Smith's distortion model for repulsive interactions. Some attractive features of the present approach have been discussed in order to demonstrate its superiority over the previous work.     

Luminescence induced by x-ray in lithium hydride

Luminescence spectra and decay times of a single crystal LiH were measured with X-ray and pulsed electron excitation at low temperature. The emission band at 4.4 eV with a decay time of 220 nsec and the one at 3.3 eV with decay times of 17 and 150 μsec are tentatively ascribed to correspond to the σ- and π-type transitions, respectively, of the (X^?₂ + e) in alkali halides.     

Two- and three-photon spectroscopy of solids under high pressure

Abstract

The present article reviews measurements under hydrostatic pressure on the tetrahedrally-bonded semiconductors CuCI, CuBr, ZnO, ZnS, ZnSe, ZnTe, Cds, and AgGaS₂, by two-photon spectroscopy and on the alkali halides NaC1, KBr, KI, and RbI by two- and three-photon spectroscopy. It is shown that these nonlinear techniques yield a much higher precision than linear spectroscopy in the determination of the pressure dependence of the electronic band gap. Additionally, it is often possible to determine the pressure dependence of other parameters of the electronic band structure like exciton binding energy, spin-orbit coupling, crystal-field interaction, and Luttinger parameters. In the case of the alkali halides with their rather large band gaps there is a further advantage of three-photon spectroscopy, namely that pressure-cell windows need only be transparent for photons of one third of the band gap energy.     

New infrared integrated band intensities for HC3N and extensive line list for the ν5 and ν6 bending modes

The infrared spectrum of cyanoacetylene (also called propynenitrile) has been investigated from 400 to 4000 cm⁻¹ at a resolution of 0.5 cm⁻¹. Integrated intensities of the main bands and a number of weaker bands have been obtained with an uncertainty better than 5%. Inaccurate values in previous studies have been identified in particular concerning the intensity of the strong ν₅ stretching band at 663.2 cm⁻¹. Former results on the temperature dependence of integrated intensities have also been revisited.Synthetic spectra calculation has been performed for the ν₅ and ν₆ bands on the basis of the best available high resolution data. It has been shown that the GEISA line parameters for HC₃N are not sufficient to reproduce the band intensities and some hot band features observed in our experimental spectra at room temperature. As a first step, the model spectra has been improved by including a number of missing hot subbands and by calculating accurately the hot band relative intensities. Finally, a perfect agreement between calculated and observed spectra was achieved on the basis of a global analysis of HC₃N levels up to 2000 cm⁻¹ combined with the new integrated intensity measurements. A new extensive line list for the ν₅ and ν₆ bending modes of HC₃N has been compiled.