Excitation spectrum of neutral molecules adsorbed on dielectric surfaces

It is shown that the excitation spectrum of neutral molecules physisorbed on a dielectric surface consists of two symmetric and two antisymmetric energy modes. The spectral functions of these modes are represented respectively by two lorentzian lines whose spectral widths are described by the radiative decay of the energy modes in question. Numerical results are derived for the energies of excitation and spectral widths for the rare-gas atoms adsorbed on graphite.     

Mechanism of lateral interactions between molecules adsorbed on oxide surfaces

Comparison of the observed and calculated values for static and dynamic frequency shifts due to lateral interactions between CO molecules adsorbed on oxides indicates that these interactions are indirect and performed through a solid. Mechanism of static interaction includes relaxation, i.e. the displacement of surface atoms due to their adsorption.

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Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces

The energy within a vibrationally excited physisorbed molecule often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond to the surface bond causes the surface bond to rupture and the vibrationally relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond, the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbate-substrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guide-lines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters.Let us summarize this guide to spontaneous desorption of physically adsorbed vibrationally excited molecules. The most efficient desorption processes will occur for adsorbates with a small number of bound states (d₀ small) and when released the adsorbate has small translational momentum (small q_m). This momentum gap correlation is most succinctly revealed by fig. 3. Smaller translational momentum will be achieved if the adsorbate can take up energy into its internal motions. Absorption of energy into lattice modes of the substrate will also serve to reduce the translational momentum and provide for more efficient desorption. However, if the vibrational frequency of the adsorbate is in near resonance with surface polarons or plasmons of the substrate, energy transfer to the solid will be so efficient that desorption will be quenched.A test of these possible relaxation channels awaits the first experimental measurements of desorption of vibrationally excited molecules.     

UV-induced protonation of molecules adsorbed on ice surfaces at low temperature

UV irradiation of ice films adsorbed with methylamine molecules induces protonation of the adsorbate molecules at low temperature (50-130 K). The observation indicates that long-lived protonic defects are created in the ice film by UV light, and they transfer protons to the adsorbate molecules via tunneling mechanism at low temperature. The methylammonium ion formed by proton transfer remains to be stable at the ice surface. It is suggested that this solid-phase protonation might play a significant role in the production of molecular ions in interstellar clouds.     

Enhanced raman spectra of molecules adsorbed on small metal particles

The Raman spectra of benzene adsorbed on small nickel and platinum particles have been measured. The spectra are strongly enhanced, compared with those wavelengths, where no enhancement has been observed. The spectra of benzene adsorbed on platinum and nickel particles are rather similar. The most intensive vibrational band is the totally symmetric breathing mode, which dominates the spectrum. Besides the Raman active vibrations, some other weak vibrational modes are observed, which may be assigned to normally infrared and inactive vibrational transitions. The enhanced Raman effect seems to be dependent on the particle size and shape. Therefore it is important to investigate the behaviour of small metallic particles. First results of pyridine bonded to Ag₂ and Ag₃ are reported.     

Displacement of adsorbed functionalized polystyrene-block-butadiene copolymers by small molecules

This work deals with the displacement of end-anchored copolymers by the addition of solvent displacer. The adsorption behavior of functionalized polystyrene-block-polybutadiene diblock copolymers from dilute solution in toluene using silicon wafers as solid substrates is investigated by means of null-ellipsometry. The desorption phenomena are observed by adding displacers of low molecular weight to the mixture. The displacers used are tetrahydrofuran (THF) and acetone. The critical composition of the binary solvent mixture at which the desorption is complete, is determined experimentally.     

Existence of hydration forces in the interaction between apoferritin molecules adsorbed on silica surfaces

The atomic force microscope, together with the colloid probe technique, has become a very useful instrument to measure interaction forces between two surfaces. Its potential has been exploited in this work to study the interaction between protein (apoferritin) layers adsorbed on silica surfaces and to analyze the effect of the medium conditions (pH, salt concentration, salt type) on such interactions. It has been observed that the interaction at low salt concentrations is dominated by electrical double layer (at large distances) and steric forces (at short distances), the latter being due to compression of the protein layers. The DLVO theory fits these experimental data quite well. However, a non-DLVO repulsive interaction, prior to contact of the protein layers, is observed at high salt concentration above the isoelectric point of the protein. This behavior could be explained if the presence of hydration forces in the system is assumed. The inclusion of a hydration term in the DLVO theory (extended DLVO theory) gives rise to a better agreement between the theoretical fits and the experimental results. These results seem to suggest that the hydration forces play a very important role in the stability of the proteins in the physiological media.     

Dissociative chemisorption dynamics of small molecules on metal surfaces

Much progress has been achieved for both experimental and theoretical studies on the dissociative chemisorption of molecules on surfaces.Quantum state-resolved experimental data has provided unprecedented details for these fundamental steps in heterogeneous catalysis,while the quantitative dynamics is still not fully understood in theory.An in-depth understanding of experimental observations relies on accurate dynamical calculations,in which the potential energy surface and adequate quantum mechanical implementation are desired.This article summarizes the current methodologies on the construction of potential energy surfaces and the quantum mechanical treatments,some of which are promising for future applications.The challenges in this field are also addressed.     

SERS from molecules adsorbed on small Ag nanoparticles: A microscopic model

A microscopic approach to surface-enhanced Raman scattering (SERS) from molecules adsorbed on noble-metal nanoparticles is developed. For nanoparticle sizes smaller than 10 nm, the classical electromagnetic enhancement mechanism is modified by quantum-size effects. Using time-dependent local field approximation, we perform systematic analysis of SERS in nanometer-sized Ag nanoparticles. We find that, in small nanoparticles, Raman cross-section enhancement is governed by the interplay between Landau damping of the surface plasmon and interband screening in the nanoparticle surface layer.     

On the interplay between chemical reactions and phase transitions for molecules adsorbed on solid surfaces

We use Monte Carlo simulations to examine the manner in which reagent aggregation affects the reaction rate between molecules adsorbed on a solid surface. We discuss the temperature and concentration dependence of the rate of product formation.     

The influence of hydroxyl groups on the ultraviolet spectra of substituted aromatic molecules adsorbed on silica surfaces

Infrared and uv spectroscopy have been used to study the interactions of a series of mono and di-substituted benzene molecules on both porous and nonporous high surface area silicas. It is confirmed that the strength of adsorption depends upon the presence and type of surface hydroxyl group but shown that the uv spectral shifts are not necessarily related to bond strength. Thus, when the surface OH groups reduce the effect of the electron donating side groups, stronger hydrogen bonds produce larger blue shifts in π-π* transitions. When n-π* transitions are involved, however, it is a dipole-dipole interaction which determines the magnitude of the red shift and not the strength of the hydrogen bond.     

Phase transition of short linear molecules adsorbed on solid surfaces from a density functional approach

A microscopic density functional theory is used to investigate the adsorption of short chains on strongly attractive solid surfaces. We analyze the structure of the adsorbed fluid and investigate how the layering transitions change with the change of the chain length and with relative strength of the fluid-solid interaction. The critical temperature of the first layering transition, rescaled by the bulk critical temperature, increases slightly with an increase of the chain length. We have found that for longer chains the layering transitions within consecutive layers are shifted toward very low temperatures and that their sequence is finally replaced by a single transition.     

Anomalous chemical shifts of water and organic molecules adsorbed on graphitized surfaces of various types

¹H NMR spectroscopy methods have been used to study adsorption of water and organic molecules on graphitized carbon materials. As follows from the results obtained, the adsorbed molecules have anomalously low chemical shifts.     

吸附分子结构单元与溶液吸附常数的关系

本工作利用硅胶自环己烷中附醇.酮.酯. 活性炭自环己烷中吸附芳香化合物的实验结果计算证明,这些化合物吸附的Henry常数K(K=n^s~m.b)~i,式中n^s~m. b 为Langmuir等温式中极限吸附量,b为与吸附热有关的常数,n~i为构成分子的i 结构单元的Henry常数,这一结果给出了在一定条件下预示吸附等温线的可能性.     

Pore-size effects on the fractal distribution of adsorbed acceptor molecules as revealed by electronic energy transfer on silica surfaces

The fractal dimension, D, of the distribution of adsorbed molecules (malachite green, MG, the acceptor) is affected by morphological changes in the support, i.e. by the average pore size of silica. This was found by analysing the kinetics of one-step electronic energy transfer between adsorbed rhodamine B and MG according to the Klafter-Blumen equation.