Time-resolved X-ray diffraction is utilized to follow phase transitions in nanostructured silica/surfactant composites in real time under hydrothermal conditions. The data allow us both to obtain kinetic parameters and to observe intermediate phases. In all cases, changes in the packing of the organic component of these composites drives the transformation, indicating that surfactant packing is a dominant factor in determining the overall structure of these materials. For materials heated in pure water, however, high activation energies for transformation were measured, suggesting that large kinetic barriers can stabilize structures against surfactant-driven rearrangements. Matching between the interfacial charge density of the inorganic silica framework and the charge density of the surfactant headgroups is also found to affect the kinetics of transformation. Lamellar-to-hexagonal transitions, which complement condensation-induced changes in charge density, are observed to be continuous, while hexagonal-to-lamellar transitions, which proceed contrary to these charge density changes, are discontinuous. For materials heated in their high-pH synthesis solutions, more complex phase behaviors are observed. Hexagonal (p6mm) structures transform either to a bicontinuous cubic phase (Ia3d) or to a lamellar structure. Lamellar phases are observed at either long or short polymerization times, while cubic phases dominate at intermediate polymerization times. The production of these different phases can be understood by considering the interplay between organic packing, charge density matching, and changing activation energies. At short times, high charge on the inorganic framework favors transformation to the low-curvature lamellar structures. At very long times, silica condensation both reduces this charge density and cross-links the framework. This cross-linking raises kinetic barriers for transformation and again favors the topologically simpler hexagonal-to-lamellar transition. Transformations to the cubic phase are only observed at intermediate times, when these effects are balanced.

In this study, phase transformation of the hexagonal mesostructure MCM-41 to the cubic mesostructure MCM-48 is examined by in situ X-ray diffraction (XRD) of the transforming mesostructure and by XRD of products from bulk transformation experiments in Parr autoclaves. Transformations were studied under conditions of high pH and temperatures between 100 and 190 °C. Heating events took place after the hexagonal mesophase had assembled, but before it had fully polymerized. On the basis of these and previous results on transformations in silica−surfactant−water and surfactant−water systems, a model is proposed to explain the expected hexagonal → cubic transformation as well as the brief existence of a lamellar phase during the transformation. Additional experiments to establish synthetic parameters for the transformation included varying the silicon alkoxide source, replacing the supernatant prior to heating, and adding fluoride or aluminum to the reaction mixture. The results, taken together, illustrate the strong cooperativity between the organic and inorganic regions in controlling the assembly of the mesostructure and provide a better understanding of the effects that control phase transformations in these systems.

The optical and paramagnetic properties of X-irradiated silver doped SrF₂ crystals were investigated. The freshly irradiated crystals show a complex absorption spectrum between approximately 200 and 650 nm. Subsequently, systematic heat treatments were applied and absorption, photoluminescence and its polarization dependence, thermo- and radio-luminescence experiments have been undertaken. The resulting experimental data were mutually correlated with the aid of the factor analysis technique and six different origins of the observed spectra were identified. Models of the underlying silver-defect structures are discussed and crystal preparation is further presented.

The growth of thin films made from Samarium-doped alkaline earth fluoro halides (AEFH) of composition Sr_xCa_1−xFCl:Sm²⁺ (0 ⩽ x ⩽ 1) is presented and the possibilities are studied to increase significantly the inhomogeneous width of the Sm²⁺ optical zero phonon transitions. The best films were obtained when grown with a molecular beam deposition (MBD) method involving two separate molecular beams: one for the alkaline earth fluoride, the other one for the alkaline earth halide (Cl or Br). The results demonstrate that the double beam MBD technique employed is able to produce pure and mixed Matlockite films with targeted composition. The results of mainly optical studies of the samarium f–f transitions and of other complementary techniques are used to assess the composition and homogeneity of the films. With the aid of a model the composition dependence of the positions of specific optical f–f emission lines is established. Their inhomogeneous linewidth is compared with that of corresponding emission lines obtained from bulk samples of the same chemical composition. The linewidths of the films are only slightly larger (∼1.5–2 times). Thus, the film morphology cannot be exploited to increase substantially the inhomogeneous broadening of the luminescence lines. A novel approach to increase this broadening was devised, theoretically modeled and successfully tested by using multilayered sandwich-type thin films in conjunction with interdiffusion. Films with cation disorder of composition Sr_xCa_1−xFCl (x = 0.5 /0/ 0.5/ 0/..) were grown. The ⁵D₁→⁷F₀ Sm²⁺ emission linewidth is thereby increased to 70 cm^−1 full width half maximum. A width of 100cm^−1 may be obtained within the composition range x = 0, x = 1. This represents an enhancement by a factor of 3–5 in comparison with the largest values obtained in appropriate mixed bulk AEFH of constant composition. A factor >50 is gained in comparison with pure bulk AEFH hosts. The room temperature (RT) homogeneous linewidths, on the other hand, are similar to those found in bulk mixed crystals of constant composition. The intrafilm host cation diffusion during film growth of the sandwich structures was further studied. A diffusion constant of 2â‹…10^−19m²s^−1 for the Sr and Ca ions was deduced from this observation. These films are among the most promising materials for optical mass data storage through RT hole-burning.

The results of a detailed optical and paramagnetic-resonance study performed on copper in CaF₂ and silver in BaF₂ are presented. Two different Ag⁺ centers were identified in BaF₂. One is associated with an interstitial F^- ion whereas the other one has a cubic surrounding. The Cu²⁺ ion in CaF₂ was shown to reorient at 4.2 K between 6 equivalent minima of D_2h symetry. This fact is interpreted with the aid of a T_2gx(T_2g+E_g) type Jahn-Teller effect. The nonlinear mixed coupling terms are shown to play an important role. The Cu⁺ impurity in CaF₂ is presumably off-center in the F^- sublattice without associated defect or impurity.

To determine the limitations of electrospray mass spectrometry for the study of condensed-phase chemistry, it is important to understand the origin of cases for which the electrospray mass spectra, which are a measure of the relative abundances of gas-phase ions, do not reflect the equilibrium ion abundances in the solution electrosprayed. One such divergent case is that of free-base octaethylporphyrin. Under conditions for which this porphyrin is present in solution predominantly as the doubly charged, diprotonated molecule, the predominant ionic species observed in the electrospray mass spectrum is the singly charged, monoprotonated molecule. In this paper, direct optical spectroscopic measurements of the ions in solution (absorption spectra) and in the electrospray plume (fluorescence excitation spectra) are correlated with the ion distribution observed in the gas-phase (as reflected in the electrospray mass spectra) to determine at what point in the electrospray process and by what mechanism(s) the transformation from dication to monocation occurs. The data indicate that the major portion of the doubly protonated porphyrin species originally present in solution are converted to singly protonated species relatively late in the electrospray process, during the latter stages of droplet desolvation in the atmospheric/vacuum interface of the mass spectrometer, via the loss of a charged solvent molecule/cluster.

Optical hole burning, a potential technique for spectrally selective recording, was demonstrated in Sm-doped MBE-grown thin films of CaF₂/Si(111). The inhomogeneous broadening of the corresponding Sm²⁺ 5d(T_1u) ↞ 4f(⁷F₀, A_1g) transition (690 nm) was investigated as a function of substrate temperature and film thickness. The MBE apparatus is briefly described as well as the thin film growth procedure.

Oxygen-free strontium fluoride crystals containing single monovalent silver ions in a cubic site were grown. Our experiments showed that the Ag⁺ ion remained chemically stable upon optical irradiation at 222 nm (KrCl excimer). The ion exhibits a strong UV luminescence which presents no thermal quenching up to RT. At this temperature, the emitting level time-constant is 12 μs. An explanation is proposed for the silver photostability by relating it to the large electronic bandgap of the host (11.4 ev). The 222 nm absorbing level lies below the conduction band in a way such that photoionization of Ag⁺ is avoided, as well as other optically-induced opacity phenomena. A minimum source intensity at threshold is estimated at some 276 kW, when using a Fabry-Perot cavity. This power can normally be achieved with the excimer laser.

When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCl family, doped with samarium(II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the samarium(II) and the time evolution of the hole burning.

Single crystals of the alkaline earth fluorides doped with silver were grown successfully. This paper presents details of the methodology. The as-grown crystals consist of colorless transparent and yellowish regions. The former were shown to contain the Ag⁺ ion and the latter also silver pairs, small clusters and probably colloidal aggregates. Complex optical absorption bands were observed in the samples of the former parts after they had been X-irradiated. The samples were subsequently exposed to extended series of physico-chemical treatments with the aim to obtain information regarding the electronic structures involved. The evolution was monitored with the aid of optical absorption experiments. Factor analysis technique is presented and was applied to uncover mutually independent contributions to these absorption spectra. We identified the Jahn-Teller Ag⁺ [Bill H. et al., Solid State Commun.70, 511 (1989)] and several centers which formally involve an Ag^− ion.

The spectroscopic properties of Ag⁺-doped strontium fluoride crystals were investigated at various temperatures, using absorption and fluorescence spectroscopies. The system exhibits a strong ultraviolet emission upon excitation into the two principal absorption bands. The azimuthal dependence of the degree of polarization of this luminescence is analysed, as well as its dynamics. The monovalent silver ions are shown to substitute for a host cation, with cubic symmetry. This is the first reported example of a cubic coordination for the Ag⁺ ions in an insulator. This cubic field, together with the strong ionic character of the framework, confers rather original spectroscopic properties to this system. The luminescence mechanisms are interpreted on the basis of the measured decay times and with the aid of energy diagram calculations. Two closed thermalized spin-orbit levels, with symmetry A_2g and T_2g respectively, are involved in the luminescence processes. The pure spin triplet A_2g only emits at low temperatures (T<15 K), whereas the T_2g level ( approximately 2% spin singlet character) emits in turn upon warming the crystal. One-dimensional configuration coordinate diagrams are proposed to interpret the peculiar temperature dependence of the emission band maximum.

A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q²³⁰ proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.

The ion Ag²⁺ introduced into NaF shows a tetragonal electron paramagnetic resonance spectrum at 4.2 K which dynamically averages above ≂40 K. Uniaxial stress is used to show that the ground state is a strongly coupled E⊗ϵ Jahn–Teller state. The wellâ€�resolved superhyperfine structure due to the F^− neighbors is analyzed with a linear combination of atomic orbitals picture. Optical absorption of asâ€�grown and treated crystals is further presented. The former ones show peaks at 202, 213, 219 nm due to Ag⁺. The latter ones present complex absorption spectra related to silver.

The electronic ground state of isolated Cr³Â ⁺ introduced into the title compounds has been investigated with electron spin resonance and electronâ€�nuclear double resonance spectroscopy. Simultaneously a multiple scattering(MS) Xα study of the (CrCl₆)³Â ^−cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupoleinteraction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xα model and the experimental superhyperfine interaction constants but not with the Crâ€�hyperfine structure constant. It is suspected that the second neighboring Cs play a nonâ€�negligible role in the electronic structure of the cluster.