Alain Monnier

Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures

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.

Molecular and Atomic Arrays in Nano- and Mesoporous Materials Synthesis

Abstract A model is presented to explain the formation and morphologies of 3-d periodic surfactant-silicate mesostructures. The structures of lamellar, hexagonal tubular and a minimal surface cubic liquid crystal/silicate phase are described.

High-resolution transmission electron microscopy of mesoporous MCM-41 type materials

High-resolution transmission electron microscopy of MCM-41 shows for the first time clearly the hexagonal shape of the pores, and allows direct observation of the transition from a lamellar to the hexagonal phase.

High-temperature spectral hole burning on samarium(II) in single crystals of the lead fluorohalide structure family and in thin films of calcium fluoride

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 PbFCI 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 hole burning.

A mass spectrometry and optical spectroscopy investigation of gas-phase ion formation in electrospray

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.

The properties of electrons in sodalite saturated with alkali atoms

We study the properties of the electrons produced by absorbing sodium atoms into a zeolite which we call dry sodalite. Upon absorption, the sodium atom is ionized and the electron is shared with the other ions present in the sodalite cage. When the dry sodalite is saturated with sodium, each cage has one such electron and the system is periodic. We develop a simple model for the electron interaction with the zeolite framework and calculate the energy bands of the electrons in this periodic system. We find that in the one‐electron approximation, the system is a narrow band metal. It is very likely that the electron–electron interactions will transform it into a Mott insulator which is antiferromagnetic at low temperature. Particular attention is paid to understanding how the interactions between the electrons and the zeolite frame affect the electronic properties. Since the properties of the framework can be modified chemically, this understanding can help us determine how to use such modifications to our ...

The Jahn-Teller system Ag2+: NaF, an electron paramagnetic resonance and optical absorption study

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⊗e Jahn–Teller state. The well‐resolved superhyperfine structure due to the F− neighbors is analyzed with a linear combination of atomic orbitals picture. Optical absorption of as‐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.

Luminescence mechanisms of Ag+ cubic centres in strontium fluoride crystals

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 A2g and T2g respectively, are involved in the luminescence processes. The pure spin triplet A2g only emits at low temperatures (T<15 K), whereas the T2g 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.

Silver-doped alkaline earth fluorides: Crystal growth and physico-chemical processes studied by optical spectroscopy and EPR

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.

Electronic structure of Cr3+ in Cs2NaYCl6 and Cs2NaYBr6 lattices. Electron‐paramagnetic resonance and electron‐nuclear double resonance measurements and multiple scattering Xα calculations

The electronic ground state of isolated Cr3+ introduced into the title compounds has been investigated with electron spin resonance and electron‐nuclear double resonance spectroscopy. Simultaneously a multiple scattering (MS) Xα study of the (CrCl6)3− cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupole interaction 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 Cr‐hyperfine structure constant. It is suspected that the second neighboring Cs play a non‐negligible role in the electronic structure of the cluster.