Two-dimensional Raman spectra of atomic solids and liquids

We calculate third- and fifth-order Raman spectra of simple atoms interacting through a soft-core potential by means of molecular-dynamics (MD) simulations. The total polarizability of molecules is treated by the dipole-induced dipole model. Two- and three-body correlation functions of the polarizability at various temperatures are evaluated from equilibrium MD simulations based on a stability matrix formulation. To analyze the processes involved in the spectroscopic measurements, we divide the fifth-order response functions into symmetric and antisymmetric integrated response functions; the symmetric one is written as a simple three-body correlation function, while the antisymmetric one depends on a stability matrix. This analysis leads to a better understanding of the time scales and molecular motions that govern the two-dimensional (2D) signal. The 2D Raman spectra show novel differences between the solid and liquid phases, which are associated with the decay rates of coherent motions. On the other hand, these differences are not observed in the linear Raman spectra.     

Containerless reaction monitoring in ionic liquids by means of Raman microspectroscopy

Reaction monitoring by Raman microspectroscopy in levitated room temperature ionic liquid (RTIL) droplets is reported. Due to their non-volatility, RTIL droplets are well-suited to act as wall-less microreactors. The droplets were produced by a piezoelectric flow-through microdispenser connected to an automated flow injection system and were levitated by an acoustic trap. Taking advantage of the flow system versatility, the sequence of reagents was easily changed to study a model organic reaction: the Knoevenagel condensation. The reaction was followed by Raman microspectrometry and the obtained spectra were analysed using multivariate curve resolution to retrieve the concentration profiles and pure spectra of reactants, intermediates and products involved in the reaction. In addition, information about solvation interactions was obtained by monitoring the desolvation process taking place when a volatile co-solvent evaporated from the droplet.     

Qualitative features of the two-dimensional Raman spectrum in liquids

The theory presented earlier [J. Kim and T. Keyes, Phys. Rev. E 66, 051110 (2002)] is analyzed to determine the information available from the two-dimensional Raman spectrum R((5))(t(2),t(1)) in liquids. The known spectra are well represented by the sum of two products of ordinary time correlations predicted by the theory. The shape of R((5)) is related in general to the values of simple same-time averages and concepts amenable to physical intuition. Using standard models for the time correlations entering the theory, specific analytic expressions for the spectrum are obtained depending on two parameters and a time scale, and the behavior of the spectrum is mapped out in the parameter space.     

Polarization selectivity of third-order and fifth-order Raman spectroscopies in liquids and solids

Polarization selectivity of third-order and fifth-order Raman spectroscopies is examined for both isotropic liquids and periodic lattices. Our approach directly applies the symmetry property of the probed system to decompose the polarization tensor elements into independent components. The polarization selectivity predicted by symmetry analysis is rigorous and applicable to higher-order Raman spectroscopy. The different polarization selectivities of isotropic systems and periodic lattices can be used as a signature of the liquid-solid phase transition.     

Raman spectra of ionic liquids: interpretation via computer simulation

Theoretical Raman spectra of the complex-forming ionic liquids LaCl3 and ScCl3, derived from molecular dynamics computer simulations, are presented. These simulations, which use polarizable ion interaction models, have previously been shown to predict structural properties in excellent agreement with diffraction experiments. The dependence of the polarizability of the melt on the ionic positions, which determines the Raman spectrum through the time dependence of the polarizability correlation function, is modeled on the basis of ab initio electronic structure calculations carried out on alkali chlorides. New simulation techniques are introduced in order to allow the spectrum to be calculated with acceptable statistics. The calculated spectra are in semiquantitative agreement with experimental data. The distinctive bands which appear in the spectra of such complex melts are linked to the vibrations of the transient coordination complexes which form in these melts and new interpretations for the origin of several well-known features are proposed. The simulations thus enable a link between the structure of a melt as perceived through Raman spectroscopy and through diffraction experiments to be made.     

A model for low frequency Raman scattering in hydrogen- bonded solids

We briefly review the properties of the disorder-induced light scattering spectrum observed in Raman scattering experiments from disordered solids; in particular, we concentrate on the region ω→0. From this analysis it appears that ice I_h shows an anomalous behaviour which may be ascribed to the contribution of multiphonon terms; in this paper these are introduced via an explicit non-linear coupling between radiation and matter. The results of this model are reported, and the underlying physical mechanism examined in some detail. Even if the results are somewhat qualitative in character, they give indirect evidence for the presence of activated hexagonal hydrogen loops in ice I_h.     

Analyzing the adsorption of blood plasma components by means of fullerene-containing silica gels and NMR spectroscopy in solids

The results from studying the adsorption of blood plasma components (e.g., protein, triglycerides, cholesterol, and lipoproteins of low and high density) using silica gels modified with fullerene molecules (in the form of C₆₀ or the hydroxylated form of C₆₀(OH) _x ) and subjected to hydration (or, alternatively, dehydration) are presented. The conditions for preparing adsorbents that allow us to control the adsorption capacity of silica gel and the selectivity of adsorption toward the components of blood plasma, are revealed. The nature and strength of the interactions of the introduced components (fullerene molecules and water) with functional groups on the silica surface are studied by means of solid state NMR spectroscopy (NMR-SS). Conclusions regarding the nature of the centers that control adsorption are drawn on the basis of NMR-SS spectra in combination with direct measurements of adsorption. The interaction of the oxygen of the hydroxyl group of silica gel with fullerene, leading to the formation of electron-donor complexes of C₆₀-H, C₆₀-OH, or C₆₀-OSi type, is demonstrated by the observed changes in the NMR-SS spectra of silica gels in the presence of fullerene.     

A standardised intensity scale for Fourier transform Raman spectra of liquids

A method is proposed for producing a standardised intensity scale for liquid spectra measured on Fourier transform Raman spectrometers. The method uses hexachlorobutadiene as an external standard, and relies upon the high degree of reproducibility available between spectra on a Fourier transform Raman instrument. This procedure is designed primarily to be simple to use and easy to automate in order that it may be applied in routine analytical laboratories with a minimum of recording and processing time. Limitations to the technique and alternative means of standardisation are discussed.     

Orientational environments in liquids and solids

Local orientational order in liquids and solids is examined using the average “distribution of cosines” of angles subtended at particles in the liquid by pairs of nearest-neighbour particles, a techninue first introduced by Scott and Mader. This average information is compared with recent, more complete theories and simulations of orientational order. The lowest-order approximation to the “distribution of cosines” is shown to fail for dense systems, but the true distribution may measured very efficiently in computer simulators.     

Characterization of human cells exposed to deltamethrin by means of Raman microspectroscopy and atomic force microscopy

Raman spectroscopy is a powerful technique for studying cellular biochemistry. In fact, each toxic chemical induces biochemical changes related to the own action mechanism. In this investigation Raman microspectroscopy has been used, in correlation with atomic force microscopy images, to detect biochemical and structural damages occurring in cultured human cells as a consequence of deltamethrin exposure. Cultured human keratinocyte cells have been exposed at increasing concentrations of deltamethrin from 10⁻³ M to 10⁻⁶ M for 24 h. A viability test indicated that the cytotoxic dose corresponds to exposure at deltamethrin solution for 24 h with the chemical concentration between 10⁻⁴ M and 2.5 10⁻⁴ M. The compared analysis of Raman spectra and AFM images allows to state that an evident damage occurs in the plasmatic membrane and it is already detectable after exposure of keratinocytes at the lowest investigated deltamethrin concentration (10⁻⁶ M). The most important modifications are related to the breakdown of CH₂ bonds of lipidic chains, whereas proteineous bonds are less involved in the deltamethrin action. On the whole, cellular damage starts after exposure to deltamethrin doses well lower than that established as cytotoxic.     

Low frequency Raman spectra and librational lattice mode of triclinic n-paraffins

Polarized Raman spectra of a single-crystal specimen of triclinic n-C₂₄H₅₀ were measured. A weak band at 63 cm⁻¹ (at room temperature) was found to exhibit (ab′) polarization and to become very sharp and distinguishable at liquid helium temperature. A similar weak band was found in other members of triclinic n-paraffins at almost the same frequency independently of chain length. This characteristic band series was thought to be due to the librational lattice mode about the chain axis. The observed polarizations of the Raman bands corresponding to the transverse acoustic modes of the triclinic (or monoclinic) polyethylene lattice suggested an appreciable mixing between the in-plane and out-of-plane vibrations. The observed frequencies of n-C₂₄H₅₀ were compared with those obtained from the calculated dispersion curves of the monoclinic polyethylene lattice.     

Raman spectra of ammonia borane: low frequency lattice modes

We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes. The analysis of the spectroscopic measurements taken across the phase transition seems to be consistent with the increasing orientational disorder of the molecular components and seems to be compatible with the equalization of the a and b lattice constants characteristic of the tetragonal phase.     

Stimulated echoes and two-dimensional nuclear magnetic resonance spectra for solids with simple line shapes

Nuclear magnetic resonance (NMR) experiments on ion conductors often yield rather unstructured spectra, which are hard to interpret if the relation between the actual translational motion of the mobile species and the changes of the NMR frequencies is not known. In order to facilitate a general analysis of experiments on solids with such spectra, different models for the stochastic evolution of the NMR frequencies are considered. The treated models involve random frequency jumps, diffusive evolutions, or approximately fixed frequency jumps. Two-dimensional nuclear magnetic resonance spectra as well as stimulated-echo functions for the study of slow and ultraslow translational dynamics are calculated for Gaussian equilibrium line shapes. The results are compared with corresponding ones from rotational models and with experimental data.     

A frequency domain existence proof of single-molecule surface-enhanced Raman spectroscopy

The existence of single-molecule surface-enhanced Raman spectroscopy (SMSERS) is proven by employing a frequency-domain approach. This is demonstrated using two isotopologues of Rhodamine 6G that offer unique vibrational signatures. When an average of one molecule was adsorbed per silver nanoparticle, only one isotopologue was typically observed under dry N2 environment. Additionally, the distribution of vibrational frequencies hidden under the ensemble average is revealed by examining the single-molecule spectra. Correlation with transmission electron microscopy reveals that SMSERS active aggregates are composed of multiple randomly sized and shaped nanoparticles. At higher coverage and in a humid environment, adsorbate interchange was detected. Using 2D cross correlation, vibrational modes from different isotopologues were anti-correlated, indicating that the dynamic behavior was from multiple molecules competing for a single hot spot. This allows hot-spot diffusion to be directly observed without analyzing the peak intensity fluctuations.     

Preconcentration of trace elements in potable liquids by means of a liquid membrane emulsion for flame atomic absorption determination

Summary A liquid membrane emulsion was developed for the simultaneous extraction and preconcentration of traces of Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn in potable liquids. After preconcentration, the eight elements were determined by flame atomic absorption spectrometry (FAAS). The results of analyses of potable water, beer and soft drinks, each from five or six different sources are listed. Data from the preconcentration method were compared with corresponding data obtained from the direct determination of the elements by graphite furnace atomic absorption spectrometry (GFAAS). Differences in results for trace elements between the liquid membrane emulsion-FAAS method and the GFAAS method were in the ranges of ±10% (water), ±9% (beer) and ±14% (soft drinks) for most of the trace elements. The satisfactory agreement meant that analyses of such liquids for trace elements can be carried out accurately with less expensive and widely available FAAS equipment.     

Interface-specific chi(4) coherent Raman spectroscopy in the frequency domain

We demonstrate interface-specific fourth-order (chi(4)) coherent Raman spectroscopy in the frequency domain for the first time. Because the chi(4) Raman spectroscopy uses only visible (vis) or near-IR light, it is expected to be a potential alternative to the widely utilized IR-vis sum frequency generation spectroscopy that cannot be applied to interfaces buried in thick IR absorbers such as water. We present the vibrational absolute value(chi(4))2 spectrum of rhodamine 800 at the air/water interface in a wide spectral range 100-3600 cm(-1). Comparison of the absolute value(chi(4))2 spectrum with the absolute value(chi(3))2 spectrum leads us to conclude that the present chi(4) spectroscopy successfully probes the interface distinguished clearly from the bulk.     

Raman spectra of ionic liquids: a simulation study of AlF3 and its mixtures with NaF

Theoretical Raman spectra of the melts of NaF/AlF3 mixtures have been obtained from computer simulations in order to examine how the Raman spectra reflect the coordination structure around the Al3+ ions. The Raman spectra, both polarized and depolarized, are calculated from a model for the dependence of the polarizability of the system on the ionic coordinates which was inspired by electronic structure calculations of the polarizabilities of ions in a condensed-phase environment. The shapes of the spectra and their evolution with composition in the mixtures conform remarkably well to those seen experimentally, and we discuss the relationship between the bands seen in the spectra and the vibrational modes of the AlFn(3-n) coordination complexes which are found in the melts. Finally, we calculate quantities which relate to the degree of cross-linking between these coordination complexes and their lifetimes.