Interaction between guest AgI and host zeolite FAU studied by optical spectra and EXAFS

AgI molecules were dilutely adsorbed into nano-cages of Na⁺, K⁺ and Cs⁺ type FAU zeolites in order to understand the interaction between host zeolite and guest AgI. This interaction was investigated using optical absorption spectroscopy and extended X-ray absorption fine structure (EXAFS). The optical spectra strongly depend on the type of the alkali cations. Compared with the lowest absorption band of AgI molecules in gas phase, the spectra of AgI molecules adsorbed in the zeolite cages shifts to higher energy in the order of Cs⁺, K⁺, and Na⁺. On the contrary, Ag-I bond lengths of adsorbed AgI molecules obtained from EXAFS were independent of the type of the alkali-cations. The bond length was very close to gas phase AgI molecules. Therefore, the interaction between AgI molecules and the zeolite, whose magnitude is Na⁺ > K⁺ > Cs⁺, is important in the photo-excited electronic state.     

Distortions in IR spectra related to the registration conditions: I. The influence of the reflection from interfaces

We show that the contour of an absorption band of a thin layer of a liquid or a film of a solid compound deposited onto a substrate can be strongly distorted as a result of the reflection from the specimen surface or the interface with the substrate if the refractive index of the compound under study changes sharply in the range of the absorption band. We consider the theory of this phenomenon and ways of taking it into account in studies of the absorption spectra of films, liquids, and adsorbed molecules.     

Structure determination of ethylene adsorbed on Ag(100) surface by multiple-scattering cluster method

The carbon K-shell near edge X-ray absorption fine structure (NEXAFS) spectra of gas-phase ethylene molecules and ethylene adsorbed on Ag(100) have been calculated by the multiple scattering cluster method (MSC) for the first time. The calculated results show that carbon-hydrogen scattering resonances exist in the carbon K-shell NEXAFS spectra for both gas-phase ethylene molecules and ethylene adsorbed on the metal surface. By a comparison between the theoretical results and experimental spectra, we have found that ethylene molecules are adsorbed on the Ag(100) surface taking a four-fold hollow-site with the direction of the CC bond parallel to the [001] or [010] direction, the bond lengths of CC and AgC are equal to 1.40 ± 0.02 and 1.96 ± 0.03 Å, respectively. These results can be compared with that of the C₂H₄-Cu(100) system.     

Isotope shifts in spectra of molecular liquids

In the IR absorption spectra of low-temperature molecular liquids, we have observed anomalously large isotope shifts of frequencies of vibrational bands that are strong in the dipole absorption. The same effect has also been observed in their Raman spectra. At the same time, in the spectra of cryosolutions, the isotope shifts of the same bands coincide with a high accuracy (±(0.1–0.5) cm^–1) with the shifts that are observed in the spectra of the gas phase. The difference between the spectra of examined low-temperature systems is caused by the occurrence of resonant dipole–dipole interactions between spectrally active identical molecules. The calculation of the band contour in the spectrum of liquid freon that we have performed in this work taking into account the resonant interaction between states of simultaneous transitions in isotopically substituted molecules can explain this effect.     

The metal-surface selection rule for infrared spectra of molecules adsorbed on small metal particles

The metal-surface selection rule predicts that some infrared absorption bands should be absent from the spectra of molecules adsorbed on metal particles. It predicts that those bands corresponding to molecular vibrations with an oscillating dipole moment parallel to the surface should be suppressed. We calculate that this selection rule should apply to adsorption on metal particles larger than about 20 Å in diameter; for smaller particles the rule is weakened.     

Rotational dynamics of propylene inside Na-Y zeolite cages

We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.      

Low temperature coloration of WO3 and MoO3 thin films

The nature of the optical absorption band arising in amorphous MoO₃ and WO₃ films on insertion of hydrogen atoms was investigated carrying out this process over a wide temperature range. Hydrogen atoms were inserted in oxide films detached under the action of light from organic molecules adsorbed on the oxide surface. The absorption spectra were successfully fitted with three Gaussian functions each being assigned to the definite type of color centers. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Optical spectroscopy on Se clusters and chains confined in zeolites

The incorporation of selenium into the supercages of zeolite Y leads to the formation of Se ₈ rings and distorted Se chains in a ratio which is influenced by the nature of the zeolitic cations. Here we review Raman (including resonance Raman) and UV/vis absorption spectroscopy results on Se encapsulated into a number of cation-exchanged faujasite zeolites. Both rings and chains give rise to characteristic Raman bands. In particular, low-frequency Raman bands are attributed to localized vibrations in ordered segments of distorted chains. The UV/vis absorption spectra indicate an opening of the band gap of selenium upon confinement in these zeolites. This can be reversed through electronic interaction with zeolite cations.Received: 1 January 2003, Published online: 14 October 2003PACS: 78.30.-j Infrared and Raman spectra - 82.75.Mj Measurements and simulation of properties (optical, structural) of molecules in zeolites     

Strong quadrupole interaction of light with molecules and its manifestation in surface-enhanced hyper-Raman scattering spectra

The possibility of the giant enhancement of hyper-Raman scattering by molecules adsorbed on rough metal surfaces is demonstrated. The theory is based on the qualitative consideration of electromagnetic field enhancement near some model rough surfaces and individual irregularities, as well as on the quantum-mechanical features of dipole and quadrupole interactions of light with molecules (as in the theory of surface-enhanced Raman scattering), proposed by the author. A consideration of symmetric molecules makes it possible to obtain selection rules for surface-enhanced hyper-Raman scattering (SEHRS) spectra and establish such a regularity as the occurrence of strong forbidden lines (which are due to totally symmetric vibrations); these lines are transformed according to unitary irreducible representation in molecules with the symmetry groups C _nh , D, and higher. An analysis of the data in the literature for trans-1,2-bis (4-pyridyl)ethylene and pyridine molecules shows that their spectra can be explained in terms of the dipole-quadrupole theory of SEHRS. At the same time, the analysis of the SEHRS spectra of pyrazine revealed the presence of strong forbidden bands due to totally symmetric vibrations. This finding substantiated the proposed theory, which makes it possible to interpret the entire spectrum in detail. These results are in good agreement with the general mechanism of the optical effects enhanced by molecules adsorbed on metal surfaces, which was developed by the author.     

Study of adsorption at the electrode/solution interphase by in-situ infrared reflectance spectroscopy — adsorption of methanol on a platinum electrode —

A detailed study has been carried out on the electrode potential dependence of spectra of adsorbed CO molecules produced by chemisorption of methanol molecules on a platinum electrode between 0.05 and 0.8 V (NHE). It has been shown that, in addition to the effect of surface charge on the band position, there is a considerable effect of co-adsorbed hydrogen species on the spectra which gives rise to the broadening of the band and to the lowering of the wave number for the maximum absorption.     

Very Low Frequency Far-Infrared Spectra of Mixtures of Dioxan with the Halogens and Related Molecules

Abstract

There has been considerable interest recently in the study of the submillimetre (or very far-infrared) absorption shown by both polar^1–4 and non-polar molecules.^2,5–8 Such absorption, in the ～5–100 cm^?1 region, for non-polar molecules is thought^2,6 to be caused by fluctuating dipoles associated with classical quadrupole-induced dipole interactions (in general multipole-induced dipole interactions). These effects are said to be “collision-induced” and can be thought of as being due to “collision” complexes in the liquid phase. For polar molecules the absorption is now thought^1–4 to be due to libration of the dipole in a “cage” of surrounding molecules (either molecules of the same species or of solvent). This so-called Poley-Hill model^9,10 is not inconsistent with the idea^1,2 of a residual rotation of the molecular dipole in the liquid phase. This similarity is underlined by the fact that some simple polar molecules^11,12 have a liquid phase Poley-Hill absorption the general shape and position of which follow quite closely that of the pressure broadened, gas phase spectrum. Our interest in the far-infrared region stems from our studies on halogen complexes with various n and bπ donors.^13,14 The pyridine-I₂ system (in cyclohexane) for example shows two bands in the far in frared not present in either component. The band at -183 cm is thought to be the stretching mode of the iodine, perturbed in the complex. The band at ～96 cm to the v(1-I) band, has so far been interpreted molecular″ stretching modebetween donor and acceptor, v(D-A).     

Spectroscopic studies of folic acid adsorbed on various metal substrates: does the type of substrate play an essential role in temperature dependence of spectral features?

Folic acid is an essential nutrient involved in biosynthetic processes and cell growth. From an analytical point of view, folic acid can be used as an active part of sensors for substances with affinity to folic acid molecules. In biological environment, sensors can be exposed to temperatures that differ from room temperature. Thus, it is important to describe the influence of temperature on adsorbed molecules, especially on orientation of molecules towards the metal surface and on stability of adsorbed layer. Surface‐enhanced Raman scattering spectroscopy is a useful tool for investigation of architecture of molecular layers adsorbed on metal surfaces because the spectral features change with varying orientation of molecules towards the surface, as well as with changes in interactions among adsorbed molecules. In this study, folic acid was adsorbed on electrochemically prepared Au and Ag substrates, and both these substrates were exposed to temperature changes according to the temperature program consisting of stabilization, the substrate temperature at 10°C, at 50°C and back at 10°C. Decomposition of adsorbed folic acid at 50°C can be excluded on both metals, though the metal (Ag or Au) influences the arrangement of adsorbed molecules and its temperature‐induced changes. In the case of Ag substrate, significant and irreversible increase of certain Stokes band intensities is observed, whereas in the case of Au substrate, the band intensity increase is negligible. Experimental spectra were analyzed using principal component analysis and supplemented with calculated data for folic acid molecule adsorbed on small metal cluster using density functional theory. Moreover, it was proved that changes of surface plasmon resonance and collision frequency of Ag substrate are negligible in the studied temperature interval. Slight re‐orientation of adsorbed folic acid molecules was identified as the main source of band intensity variation in the spectra of folic acid adsorbed on Ag. Copyright © 2014 John Wiley & Sons, Ltd.     

Diffusion Collisionnelle Induite Pour Les Bandes Rayleigh Et ν1 Raman Du Liquide Isotopique CCl4

We have studied the depolarized component of the ν₁ Raman band of the isotopic liquid C³⁵Cl₄ at 296 K. We compare the experimental Rayleigh and Raman depolarized scattering with theoretical spectra calculated using dipole induced dipole approximation.     

Absorption and Raman spectra of Se8-ring clusters in zeolite 5A

Samples with different weight ratio of Se to zeolite 5A (Se composition) have been prepared by loading Se into the cages of zeolite 5A and the measurements of the absorption and Raman spectra have been carried out for the prepared samples. The measured absorption edges of the samples are close and blue shifted to the value for monoclinic Se containing Se₈-ring, suggesting the formation of Se₈-ring clusters in the cages. The continuous and broadening features of the absorption spectra are interpreted by the strong electron-phonon coupling in Se₈-ring clusters. The sample with high Se composition has a red shift of the absorption band edge relative to the samples with less Se composition. It is tentatively attributed to the reason that with different Se composition, single Se₈-ring clusters and double Se₈-ring clusters are formed in the cages of zeolite 5A. A single broad band at about 262 cm⁻¹ is observed in the Raman spectra, that gives the further support of the formation of Se₈-ring clusters.     

Dynamics of water molecules adsorbed by silica and resin SGK-7

This paper has presented neutron spectroscopy data on the dynamics of light water molecules adsorbed in the cation exchanger (ion-exchange resin) SGK-7 and on the surface of aerosils (highly dispersed pyrogenic silica) with different levels of hydration. The measurements have been performed on a DIN-2PI spectrometer (Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research, Dubna, Russia). The characteristics of the diffusive and vibrational motions of adsorbed water molecules have been determined from the experimental neutron scattering spectra. The data obtained in the quasi-elastic neutron scattering region have been analyzed using a model accounting for the effects of restricted translational and rotational diffusion. The results have demonstrated a significant decrease in the diffusion mobility of adsorbed water molecules as compared to conventional (bulk) water. In particular, the self-diffusion coefficient decreases several times, and the diffusion rate is the lower, the smaller is the thickness of the hydration layer. The dependences of the intensity and half-width of the quasi-elastic scattering peak on the magnitude of the neutron momentum transfer q in the scattering process exhibit a nonmonotonic character. This indicates manifestation of the effects of restricted translational diffusion, rotational diffusion, and jump diffusion. The partial distributions of vibrational frequencies of hydrogen atoms of water molecules adsorbed by the cation exchanger and aerosils have been obtained from the inelastic neutron scattering data.     

Absorption,fluorescence, and SERS spectra of sanguinarine at different pH values

We have studied the absorption, fluorescence, and surface-enhanced Raman scattering (SERS) spectra of sanguinarine using a silver hydrosol and an electrochemical cell with a silver working electrode for different pH values in the medium. We carried out quantum chemical calculations in order to interpret the electronic and vibrational spectra and to establish their correlations with the structure of the molecules. We optimized the structure and determined the spectral characteristics of the cationic and neutral forms of the sanguinarine molecules in solution and adsorbed on the surface of an anodized Ag electrode for different potentials. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 604–609, September–October, 2007.     

表面分子功能化银纳米粒子的光谱特征

通过玻片表面构筑银纳米粒子二维组装结构研究吸附分子对银粒子表面等离子体共振的影响.吸收光谱结果表明银纳米粒子表面吸附二硫代乙二酰胺分子可导致金属粒子的表面等离子体共振吸收红移,主要与金属粒子的微环境改变以及吸附分子与金属间电荷转移而导致的金属粒子内部电子密度改变有关.二硫代乙二酰胺分子通过其氨基和巯基共同吸附于金属表面.     

Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigating the fine structure of near-edge X-ray absorption in the molecular spectra of C60F18 adsorbed on a single nickel crystal

Angular dependences of the fine structure of near-edge X-ray absorption (NEXAFS) of carbon C1s spectra are obtained for a monolayer film of C₆₀F₁₈ polar molecules on a Ni (100) substrate. The fine structure and angular dependences of these spectra are interpreted using calculation data obtained by the density functional method upon fitting NEXAFS spectra by the set of trial functions. It is shown that during deposition, the dipole moment of molecules is oriented perpendicular to the surface and fluorine atoms are the ones closest to the surface.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.