Alkali Halide Matrices Research Articles

Study of alkali halide/FHF − systems at 10 – 290 K, 0 – 8 kBAR

The bifluoride ion FHF −, (and FDF −), has been substitutionally isolated within single crystal samples of several alkali halides. Infrared and Raman spectra of these crystals have been studied at variable temperature and pressure. The infrared absorptions are strong, whereas the Raman is weak. At low temperatures the bands are very sharp with halfwidths less than 1 cm −1. On applying pressure, ν 3 increases in frequency whereas ν 2 decreases. On reducing temperature, ν 3 decreases in frequency whereas ν 2 increases. Hence the effect of volume contraction is overridden in the temperature dependent case. The deuterated spectra confirm that the bifluoride ion is well isolated within the alkali halide matrix.

Photochemical reactions in alkali halide matrix, decarboxylation of maleic acid into ethylene by uranyl nitrate in a pressed potassium bromide disk

The feasibility of quantitative photochemical reactions of organic compounds in the solid alkali halide disk, usually used for the study of infrared spectroscopy has been explored. The disks were prepared by mixing maleic acid, uranyl nitrate and potassium bromide. Ultraviolet light was used for the irradiation of disk and the products were identified by infrared spectroscopy. It has been observed that the rate of reaction increases with the time of irradiation. The results indicate that the ethylene and uranyl (VI) complex of carboxylate are formed with the decarboxylation of maleic acid, sensitized by uranyl nitrate.

M-center production in europium-doped NaCl and KCl

Abstract The first stage of the M-center coloring has been investigated in europium-doped NaCl and KCl at room temperature, as a function of impurity concentration in the range 30–800 ppm. The results indicate that the M-center production can be described by a statistical process in which the relationship M=K1F2 is obeyed. It was also ascertained that the value for the constant K1 is dependent on the impurity concentration and not very sensitive to the type of the alkali halide matrix.

About the critical transition from dispersed to continuous state of the metallic layer induced in LiF by ion implantation: A percolation or a “Mott” transition?

We give a description of the system formed by Na + and K + ions implanted in LiF crystals at low temperature by a simple model of a lattice of impurities embedded in a host matrix. The transition from a random distribution of isolated implanted impurities to a continuous phase, which is due to the coalescence of the impurities for a critical composition of the composite material, is discussed in terms of a non-metal to metal (NMM) transition. We have to take into account the cluster structure of each interstitially implanted alkali ion, with the next neighbouring alkali ions of the host lattice. Because of this cluster structure the classical model of a “Mott” transition needs to be modified by the use of an appropriate cluster wavefunction. This wavefunction must be very compact to take into account the strong localization of the electron on the alkali complex embedded in the alkali halide matrix. A percolation model is used to give a description of the NMM transition.

CO_2 laser saturation of internal-vibrational modes of molecular impurities in alkali halides

CO2 laser saturation of the ν3 internal-vibrational mode of ReO4− molecules embedded in alkali-halide matrices has been observed near 10.8 μm. Excited-state lifetimes (T1) of the ReO4− internal-vibrational mode as a function of temperature between 1.3 and 300 K were extracted from the saturation data.

Infrared absorption and reflection studies of organic radical salt: K+TCNQ−

The bulk reflectivity and absorption (using alkali halide matrices and thin film dispersions) of polycrystalline K+TCNQ− in the wavelength range between 30 and 4000 cm−1 have been examined in detail. The sharp reflectivity peaks have been analyzed using a classical oscillator fitting procedure to give the peak and oscillator parameters. Most of the observed infrared peaks have been assigned to the fundamental modes of TCNQ− and lattice vibrations. The temperature dependence of the CN stretching peaks in particular have been studied in the 300–433 °K temperature range. The reversible and gradual disappearance of one set of CN stretching peaks in K+TCNQ− has been tentatively linked to a possible cell halving phase transition at Tc∼379 °K. The results are compared with the spectrum of Rb+TCNQ− which is isomorphous with the postulated high temperature phase of K+TCNQ−.

Kinetics studies of the reactions of KMnO 4 and of KIO 4 in alkali halide disks

The kinetics of the solid state reactions of KMnO 4 in alkali halide matrices and KIO 4 in alkali halide matrices were studied. All reactions were found to be first order with respect to the species being reduced. Rate constants were determined at 25, 50 and 100°C, and activation energies were determined for the reactions of KMnO 4 in KI and KIO 4 in KI. At 25°C, the rate constants for the reaction of KMnO 4 in KI and KMnO 4 in RbI were approximately the same. But the rate constant at 25°C for the reaction of KIO 4 with I − was about 2 1 2 times larger in a RbI disk than in a KI disk. Possible mechanisms for the solid state processes are suggested.

A Study of the intensities of the impurity vibrational spectra at different concentrations of NO 2 ? and NO 3 ? ions in alkali halide matrices

A Study of the intensities of the impurity vibrational spectra at different concentrations of NO 2 ? and NO 3 ? ions in alkali halide matrices

Model for OH − in alkali halide matrices—II cases of b.c.c. and silver halide lattices

This paper presents the calculations for the potential barrier hindering angular motion and possible off center displacement of OH − impurity in LiF, NaF, CsBr, AgCl and NaI matrices. The have been used to explain the librational and tunneling levels of the impurity in these matrices. The unexplained result for the Nal-OH − system has been discussed in detail in terms of the recent trend of work in the field.

Infrared Spectrum of Matrix-Isolated Nitrous Oxide

Nitrous oxide, generated by the in situ thermal decomposition of hydroxylamine hydrochloride or ammonium nitrate, was isolated in some pressed alkali halide matrices. Its three infrared active fundamentals appeared at room temperature as single peaks and without the gas phase rotational structures. Cooling of the host matrix caused these bands to sharpen and to shift to higher frequencies. At any given temperature, however, the peak frequencies were essentially independent of the host matrix. The temperature dependence of the most intense ν3 fundamental was examined in detail, and the results indicated that there were two dominant trapping sites for N2O in pressed KCl, KBr, and KI matrices. Furthermore, the relative concentrations of N2O occupying these sites changed reversibly as the temperature was varied from about 90°K, our lower limit, to over 200°K. From these concentration changes, it was deduced that the transfer of a N2O molecule from the low temperature trapping site to the high temperature site was an endothermic process by 1.2 ± 0.2 kcal/mole and was accompanied by a large increase in entropy of 9.0 ± 1.0 eu in all three potassium halide matrices.

Off-Center Direction of Certain Atomic Impurities in Alkali Halide Matrices

The direction of the off-center displacement of certain atomic impurities in alkali halide matrices has been determined through calculations of certain barrier parameters. The controversial results for the RbCl: ${\mathrm{Ag}}^{+}$ system and other systems like KCl: ${\mathrm{Li}}^{+}$ and NaCl: ${\mathrm{Li}}^{+}$ have been suitably discussed in the light of present calculations.

Theoretical formulation of the effects of charge transfer states on optical transitions in s2ions

In an attempt to include the effects of charge transfer states into the existing theory for optical transitions in ions embedded in an alkali halide matrix and possessing a closed s2 outer electron shell, group theoretical arguments are employed in order to provide the simplest possible generalization of the existing theory whilst introducing the minimum number of new parameters necessary for its description. Expressions are developed which permit not only an experimental estimate of these new parameters, but also show their utility in accounting for known experimental results which appear anomalous in terms of the existing theoretical structure. Finally further experiments are suggested on specified impurity ion systems in which the advantages of the present formulation over that previously adopted can be subjected to experimental investigation.

Reactions of mercury-like ions in potassium halide lattices

Cadmium and zinc enter the alkali halide matrices as M 2+ ions and exhibit characteristic absorption bands in the uv. Often these divalent species aggregate to form dimers (and polymers). After additive coloration of the doped single crystals, reduction occurs and monovalent monomers or dimers, and neutral atomary centers are formed. The monovalent dimers exhibit a broad ESR signal with a g value of ∼2.2.

Theory for the Non-Devonshire Lines Observed in the Hydroxyl-Ion-Doped Alkali Halide Matrices

The isotope effect of the non-Devonshire lines can be well understood if one takes the origin of these as transitions corresponding to the quantized translational motion of the impurity in the matrix cage and considers the various perturbing mechanisms. The present model in which rotation-translation coupling and coupling through the anisotropic part of crystalline field are considered as the chief perturbing mechanisms proves to be good in explaining the isotope effect of the near-infrared lines in the KCl and KBr matrices. It provides a good explanation of the multiplet line structure observed in the KCl-O${\mathrm{H}}^{\ensuremath{-}}$ system and absence of such a structure in the KCl-O${\mathrm{D}}^{\ensuremath{-}}$ system. The more complex case of the NaCl matrix is also explained in a satisfactory way. There has not been an attempt to explain the RbCl-O${\mathrm{H}}^{\ensuremath{-}}$ system because of the lack of sufficient experimental data.

Model for OH−, OD−, and CN−Impurities in Alkali Halide Matrices

A model has been developed for calculating the potential barrier-hindering angular motion and possible off-center displacement of the polar impurities in the alkali halide matrices. The general method of the multipole expansion of the intermolecular interaction has been employed. The calculations have been done for the O${\mathrm{H}}^{\ensuremath{-}}$, O${\mathrm{D}}^{\ensuremath{-}}$, and C${\mathrm{N}}^{\ensuremath{-}}$ impurities in the KCl, KBr, NaCl, RbCl, and KI matrices. It has been found that the O${\mathrm{H}}^{\ensuremath{-}}$ and O${\mathrm{D}}^{\ensuremath{-}}$ impurities are displaced bv about 0.2-0.3 \AA{} from the normal-lattice site in the different alkali halide lattices. For the C${\mathrm{N}}^{\ensuremath{-}}$ case, no appreciable displacement of the impurity c.m. is evidenced. In this case, the effect of the relaxation of the lattice near the impurity is also considered. This has been found to add a small tetragonal term of the type ${(4\ensuremath{\pi})}^{\frac{1}{2}}{K}^{\ensuremath{'}\ensuremath{'}}{Y}_{0}^{2}$ to the octahedral potential. The addition of such a small tetragonal term has recently been demonstrated by Pompi and Narayanamurti to explain successfully the rather unexpected infrared results and also the anomalous specific-heat results of Harrison et al. The calculated values of the barrier and the off-center displacement have been used successfully to explain the librational frequency and the tunneling splitting of these impurities in the matrices. The general validity of different approaches has been discussed in the light of the librational frequencies and their isotope effect.

Metallisotopen-effekt bei metalloporphyrin-spektren im fernen infrarot

Metal derivatives of octaethylporphyrine (OÄP) with 58Ni, 64Ni, 64Zn, and 68Za isotopes have been prepared and investigated IN alkali halide matrices at 300 and 80‡ K by far IR spectroscopy. In NiOÄP, the vibration at 356 cm −1 which exhibits the largest isotopic shift has more ring bending than NiN stretching character whereas in the 202 cm −1 vibration of ZnOÄP metal-ligand stretching character predominates.

Effect of Localized Lattice Vibrations on the Motional States of Polar Impurities in Alkali Halide Matrices

The effect of localized lattice vibrations on the librational and tunneling frequencies of diatomic polar impurities trapped in alkali halide matrices has been worked out. It is shown that Devonshire energy-level scheme corrected for the lattice-vibrational effects gives a good account of the librational and tunneling levels of the impurity.

Possibility of Rare-Gas Color Centers in Ionic Crystals

It is argued that the defect consisting of a rare-gas atom at a negative-ion vacancy in an ionic crystal should trap an electron to form an electron-excess color center. The basis for this speculation is that the potential for such defects, as for the $F$ center, must approach a Coulomb form at large distances. The principal optical absorptions for these hypothetical rare-gas centers have been calculated in the point-ion approximation for alkali-halide matrices. The transitions are predicted to lie in a range from 0.9 to 1.8 eV, depending on the rare-gas and the host crystal. In all cases, the absorption lies to the low-energy side of the corresponding $F$ band and has an oscillator strength of the order of unity. Although these centers possess bound electronic states, it is not clear that they are chemically stable against decay into an $F$ center and an interstitial rare-gas atom. However, from studies of the conversion of ${U}_{3}$ centers to ${U}_{2}$ centers, it is argued that the hypothetical rare-gas centers may be stable in the iodides or bromides of intermediate-weight alkalis. The possiblity of observing both substitutional rare-gas atoms and ions by means of $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-band-like transitions it also discussed.

THE TRANSFER AND CONVERSION OF ELECTRONIC ENERGY IN SOME ‘MODEL’ PHOTOCHEMICAL SYSTEMS*

Abstract— Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively ‘simple’ organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of u.v. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substrates ortho‐nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented.A series of ‘model’ ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor‐phase photodissociation into radicals is shown to be strongly dependent on molecular structure.Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type II split) as a “probe” to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. Φc2H4 is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in the p‐CH3, p‐OCH3 and p‐NH2 derivatives respectively, and to 0.00 in both ortho and para hydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being 3(π, π*) rather than 3(π, π*) in character.While several ‘classic’ photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the ‘model’ cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecular environment effect are pointed out.

Photochemical Studies in an Alkali Halide Matrix. I. An o-Nitrobenzaldehyde Actinometer and Its Application to a Kinetic Study of the Photoreduction of Benzophenone by Benzhydrol in a Pressed Potassium Bromide Disk

Photochemical Studies in an Alkali Halide Matrix. I. An <i>o</i>-Nitrobenzaldehyde Actinometer and Its Application to a Kinetic Study of the Photoreduction of Benzophenone by Benzhydrol in a Pressed Potassium Bromide Disk