Changes In Band Shape Research Articles

Photophysical investigations of Langmuir–Blodgett mono- and multilayer films of pheophorbide-a

Absorption, fluorescence and time-resolved fluorescence spectroscopy has been applied to quasi two-dimensional Langmuir–Blodgett monolayers and three-dimensional arrangements of Langmuir–Blodgett films with different numbers of layers. Pure dye films of pheophorbide-a and films in which pheophorbide-a molecules were embedded in a behenic acid matrix have been investigated. Optical spectroscopy is used in order to obtain evidence for intra- and interlayer interaction in the membrane-like environment and distinguish between these different types of interactions. Depending on how the dye molecule is embedded a broad range of Π-system interactions occur. Optical band shifts as well as band shape changes and the formation of new bands are indicators for these molecular interactions [M.B. Grieve, A.J. Hudson, T. Richardson, Thin Solid Films, 1994, 243, 581.]. Among such interactions, exciton coupling and excimer formation are considered to be possible reasons for the observed major differences between spectra and kinetics of mono- and multilayer Langmuir–Blodgett films. Exciton coupling is indicated by red shifts of the Q-band of about 600 cm −1 in the absorption spectra of mono layer films. The observed broad red shifted fluorescence in the near-infrared region and their short lifetimes (≤ 100 ps) are assigned to excimer formation. Time-resolved fluorescence spectra of bilayer films as well as polarization dependencies of the Langmuir–Blodgett film absorption are presented.

Crystallinity Determination of Polylactide by FT-Raman Spectrometry

The complete Raman spectra of isotropically crystalline and amorphous polylactide (PLA) have been successfully obtained with the use of a Fourier transform instrument equipped with a near-IR laser source. The Raman spectra of PLA were recorded at a resolution of 4 cm−1 from a backscattering sampling geometry. The changes in band intensity and shape of Raman spectra in the crystalline PLA samples facilitated the quantification of the crystallinity, which was primarily determined by the change in enthalpy via differential scanning calorimetry (DSC). A comparison of quantitative analysis of crystallinity by a multivariate technique, partial least-squares (PLS), and a univariate method at the carbonyl stretching band is described. Quantitative analysis of PLA crystallinity was performed after the data preprocessing with the standard normal variate (SNV) method in the combined spectral regions 3100–2800 cm−1 and 2000–200 cm−1 that were determined by the analysis of the correlation spectrum. The use of the calibration model containing mostly 1% d-PLA samples to separately analyze the samples with 1% and 5% d-PLA is discussed. Superior results obtained from the multivariate over the univariate method make PLS the preferred method of choice in quantification of crystallinity. The standard error of prediction (SEP) for the analysis of 1% d-PLA samples with the use of the calibration model described is 0.85 (—J/g), and the SEP for 5% d-PLA samples is 1.35 (—J/g) with the outliers excluded. The discrepancy in analyzing these two types of PLA samples with the use of the same calibration model is also discussed.

Colloidal Silver Nanoparticles: Photochemical Preparation and Interaction with O2, CCl4, and Some Metal Ions

The optical absorption of the colloidal nanoparticles is investigated, formed by the UV illumination of aqueous solutions containing AgClO4 ((1−4) × 10-4 M), acetone (2 × 10-2 M), 2-propanol (1 M), and various polymer stabilizers. The 7 nm particles, which are produced in the presence of polyethyleneimine, possess an unusally narrow plasmon absorption band. The wavelength and shape of this band are affected by various adsorbed solutes; adsorption phenomena can therefore be studied spectrophotometrically. The changes in band shape that occur in the presence of oxygen and of carbon tetrachloride are attributed to a partial oxidation of the silver particles by these solutes. During the oxidation, the Fermi level in the nanoparticles shifts to a more positive potential, until the oxidation comes to a halt. Chemisorbed metal cations (Cd2+, Ni2+, Ag+, Hg2+) affect the plasmon absorption band of the silver nanoparticles more strongly the more electropositive is the metal. The effect is interpreted in terms of the donation of electron density from the silver particles to the adsorbed cations. In the case of Hg2+ ions, the electron donation leads to partial Ag oxidation and amalgam formation. UV illumination of a sol that contains Cd2+ ions produces Cd metal on the surface of the silver particles.

Phonon confinement effects in the Raman scattering by TiO2 nanocrystals

Nanocrystalline TiO2 has been obtained by a sol-gel process by controlling the crystal size through the water/alkoxide ratio. Raman spectra of anatase nanocrystals with average sizes of 9.5–13.4 nm are reported and the correlation between the Raman band shape (peak position and linewidth) of the main feature at 144 cm−1 and the crystals dimension is discussed. While in this system a minor role is played by nonstoichiometry and pressure effects, a model based on the phonon confinement, which takes into account the size distribution as determined by the transmission electron microscopy images, correctly reproduces the Raman band shape change.

Integrated infrared absorption coefficients of CF 3OCF 2H and CH 3OCF 2CF 2H at 297, 253 and 213 K

Infrared absorption coefficients have been measured for gas-phase spectra of CF 3OCF 2H and CH 3OCF 2CF 2H at 297, 253 and 213 K. in the 650–1600 cm −1 region using a Fourier transform spectrometer. The mean values of the integrated coefficients for CF 3OCF 2H over this wavenumber range were found to be 3.34 × 10 −16 cm 2 molecule −1 cm −1 at 297 K and 3.17 × 10 −16 cm −1 molecule −1 cm −1 at 213 K. For CH 3OCF 2CF 2H, the mean values were 1.87 × 10 −16 cm 2 molecule −1 cm −1 at 297 K and 1.83 × 10 −16 cm 2 molecule −1 cm −1 at 213 K. The effects of the decreased overall intensity and changes in band shape on their absorption in the atmosphere have been discussed briefly.

On the number of observable species, observable reactions and observable fluxes in chemometric studies and the role of multichannel integration

It is assumed that preliminary experiments are performed to measure the absorbance of a liquid in a chemically reactive system. Further, given the exploratory nature of the study it is assumed that no further information is available concerning the species present nor the reactions occurring. The principle chemometric goals of such an exploratory experimental study can be stated as follows; 1. (I) determine the number of statistically significant factors associated with the observable species in solution and 2. (II) determine the number of statistically significant factors associated with the observable reactions, consistent with the set of spectroscopic measurements. In the case of infrared (IR) spectroscopy, numerous difficulties are encountered due to the sensitivity of band positions and band shapes to changes in the liquid phase composition during reaction. Although the above mathematical problem and its subsequent goals are typically formulated in terms of the Lambert-Beer-Bouguer Law (LBBL) followed by factor analysis, the merits of utilizing the integral absorption law (IAL) for infrared spectroscopy in order to achieve goals I and II are examined in detail. First, the possibility of transport involving species into and out of the liquid phase is considered, and the associated fluxes are defined. Second, terms representing the model errors for goals I and II arising from the use of both the LBBL and the IAL are specified. Third, well-posed regions R xTP ∗ for each model are defined in which the contribution of model error is less than the experimental error associated with the measurements. Fourth, the existence of well-posed solutions for goals I and II for both the LBBL and the IAL representations are presented in terms of the singular value decomposition (SVD). Based on literature data for the sensitivity of LBBL and IAL absorptivities, it appears that goals I and II are considerably more difficult to achieve in the case of a LBBL formulation compared to a IAL formulation, i.e. R xTP ∗( LBBL) ⊂ R xTP ∗( IAL) . Furthermore, and perhaps most important, it seems that only the IAL representation is suitable for systems where changes in band positions and shapes occur. As a generalization, multichannel integration for goals I and II involving other absorption spectroscopies are also considered. The consequences of both non-destructive and destructive absorption spectroscopies on goals I and II are addressed.

Infrared Spectroscopy: Instrumental Factors Affecting the Long-Term Validity of Chemometric Models

Data from instrumental techniques such as mid-infrared spectroscopy are increasingly being analyzed for sample identification and classification by chemometric methods based on principal component analysis (PCA). However, even modern spectrometers are subject to instability. This may affect PCA, because PCA selects the variables with the largest variance. This paper investigates the relative effects of sources of instrumental instability using a model developed for fruit puree classification. Single-beam spectra, potentially useful for on-line analysis, saw their overall intensity decrease as the infrared source output and/or the detector sensitivity declined. Consequently, single-beam spectra were mainly differentiated by their overall intensity and had to be used with caution in the long term because this strongly affected the analyses. Absorbance spectra were not sensitive to source or detector decay but showed, in the long term, subtle band shape changes and frequency shifts. While these changes were not found to influence analyses involving very different samples, they diminished the success of analyses of data sets with small intrinsic variance. Where there was large spectral differences between sample classes, instrument-related factors were insignificant. However, where spectral differences were more subtle (with a single class), instrumental effects became more important. Suggestions are given to reduce the instrumental and experimental interferences on chemometric analyses, both when recording spectra and for managing spectral databases.

Electrochromism of methylene blue absorbed on the basal plane of graphite

Reflectance measurements were obtained in situ of methylene blue films on a stress-annealed pyrolytic graphite electrode. The optical response in the charging region of the double layer was observed for p polarization in the form of a small bipolar band at 690 nm, red shifted 0.08 eV with respect to the monomer species in solution. The band intensity was found to depend linearly on the modulation amplitude. On the contrary, only tiny changes in band shape and frequency with the bias potential were detected. The average orientation of the adsorbed dye is discussed together with the role of the Stark effect as a likely band source.

High-pressure laser spectroscopy of Cr3+:Gd3Sc2Ga3O12 and Cr3+:Gd3Ga5O12.

We report on the effect of high pressure on the room-temperature emission spectra and lifetimes of ${\mathrm{Cr}}^{3+}$:GSGG (${\mathrm{Gd}}_{3}$${\mathrm{Sc}}_{2}$${\mathrm{Ga}}_{3}$${\mathrm{O}}_{12}$) and ${\mathrm{Cr}}^{3+}$:GGG (${\mathrm{Gd}}_{3}$${\mathrm{Ga}}_{5}$${\mathrm{O}}_{12}$). In both systems we observed a dramatic change of the overall emission band shape upon increasing pressure, from a nearly structureless broadband ${(}^{4}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$) to a highly structured narrow band ${(}^{2}$E${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$). From the peak energy of the broadband emission, we estimated the pressure-induced blueshift of the $^{4}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$ transition to be 10 (\ifmmode\pm\else\textpm\fi{}2) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar. High-resolution measurements in the R-line region (\ensuremath{\sim}700 nm) revealed that the $^{2}$E${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$ transition hardly shifts at low pressures (40 kbar), whereas at higher pressures (>60 kbar) a nearly linear redshift of 0.65 (\ifmmode\pm\else\textpm\fi{}0.05) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar is observed. Besides pressure-induced spectral changes, an enormous increase in the emission lifetime with increasing pressure was found for both systems. In the case of ${\mathrm{Cr}}^{3+}$:GSGG, the lifetime changed from 110 \ensuremath{\mu}s at ambient pressure to 4.4 ms at 125 kbar. For ${\mathrm{Cr}}^{3+}$:GGG, the lifetime increased from 168 \ensuremath{\mu}s to 7.3 ms for the same pressure range. The pressure-induced spectral and lifetime changes are described by a single configurational coordinate model that considers the effect of pressure on the thermal and spin-orbit coupling of the $^{2}$E and $^{4}$${\mathit{T}}_{2}$ states. A previously reported pressure-induced R-line-shift reversal in ${\mathrm{Cr}}^{3+}$:GSGG and the effect of high pressure on the lifetime in ${\mathrm{Cr}}^{3+}$:YAG are also discussed within the same framework.

Calculation of local insulating structure of CaCuO2 with interplanar oxygen defects

The electronic band structures of CaCuO2 with various planar structures have been computed with the spin polarized, full potential pseudofunction method. The perfect tetragonal structure has metallic bands. If the CuO plane is puckered as proposed on the basis of pulsed neutron experiments, a semi-metallic-like behavior is found. When some of the oxygen atoms are moved from the basal plane into interplanar sites that are coplanar with Ca, an insulating ground state is obtained with antiferromagnetic moments of 0.21 μ B . Around this interplanar oxygen defect site, the charge and magnetic moment distributions resemble those of a CuO chain structure with strong coupling to Cu atoms from different basal planes. Small displacements of defect oxygens at interplanar sites along the CaO planar directions produces large changes in band shape near E F , suggesting strong electron-lattice coupling. The coupling is much weaker for displacements of O atoms within the CuO basal plane. Such interplanar defects could occur in superconducting cuprates and thus create small insulating regions for the pinning of magnetic flux lines.

Weathered and unweathered surface spectra of rocks from cold deserts: Identification of weathering processes and remote sensing implications

Abstract Comparisons of the reflectance spectra of weathered and unweathered surfaces of rocks from the Canadian Shield reveal that significant spectral changes can be effected by even the low weathering rates prevalent in cold desert regions. These changes include variations in overall spectral slope, the appearance/disappearance of absorption bands, shifts in absorption band minima wavelength positions, and changes in band shape. The most important alteration processes involve the oxidation of ferrous to ferric iron and the formation of hydrated mineral species. Ferric iron absorption bands are most intense for the most iron-rich lithologies. This may have important implications for the remote sensing detection of iron-rich lithologies such as iron formations and ultramafics. Aluminium- and magnesium-rich lithologies give rise to A1-OH and Mg-OH absorption bands in weathered surface spectra. Dramatic spectral variations betwen interior and exterior spectra occur even when weathered surfaces are thin eno...

Raman spectroscopic studies on the dynamic and equilibrium processes in binary mixtures containing methanol and acetonitrile

Raman isotropic band profiles of the ν 2 mode of acetonitrile in binary mixtures with methanol have been studied over the whole concentration range and between 196 and 330 K. Attempts have been made to understand the spectral behaviour in terms of variations in vibrational dephasing as a function of environment and in terms of rapid chemical exchange between complexed and non-complexed acetonitrile molecules. If exchange dynamics are assumed to be important it is found that the dissociation rate constant ( k 21) for this reaction is of the order of 10 11 s −1. This rate seems unrealistically high although similar rates have been obtained for other hydrogen-bonded systems. Nevertheless, the band shape changes dramatically across the temperature range and this demonstrates clearly that a “merging” band profile does not necessarily prove that exchange dynamic processes are important. Bandwidth and frequency shifts across the concentration range could be attributed to increases in exchange rate as the amount of methanol increases or the temperature increases. However, the most probable explanation is that there is a change in vibrational dephasing rate due to environmental fluctuations. We clearly demonstrate that even at 0.001 molar fraction of CH 3CN in CH 3OH a finite number of CH 3CN molecules are “free” (on the vibrational timescale) from the hydrogen-bonded interaction. An explanation for this rather surprising behaviour has been sought (and found) in terms of multiple hydrogen-bonding equilibria in this system. The effect has been shown to be associated with extensive methanol aggregation. An equilibrium model has been devised which predicts accurately the relative intensities of the two ν(CN) bands and the unusual behaviour in binary mixtures of this type.

Molecular stress distribution and creep of high-modulus polyethylene fibres

Raman spectroscopy has been used to measure the molecular stress distribution in high-modulus, high-strength polyethylene fibres (Allied Spectra 1000 and Spectra 900) at low temperature and as a function of time during creep at room temperature. Increase in band width or change of band shape is interpreted as due to a distribution of stress or strain in molecules in the all- trans configuration. Fibres loaded at 77 K show two peaks in the stress distribution, as was found by van Eijk et al. at 240 K. About one-sixth of the all- trans molecules in a Spectra 1000 fibre as produced carry little load. The rest carry from half to twice the mean load. Spectra 1000 fibres loaded at over 1 GPa for a few minutes at room temperature show two well defined peaks in each of the C-C stretching Raman bands at 1064 and 1130 cm −1 and thus a sharp bimodal distribution of stress. At long creep times, the higher stress peak increases in width while keeping the same area. Eventually, the stress distribution has a single peak with a broad tail on the high-stress side. This result unifies the previous two observations, one of which found only a single asymmetric peak and the other only two peaks in the stress distribution. As the tail broadens, the number of very highly strained molecules increases, and a significant number reach strains of 10%. It is not surprising that this leads to fracture of the fibre. After loading for less than a minute at room temperature or for a few minutes at 273 K, there is only a single peak in the stress distribution. The fibre structure is thus undergoing complex changes during creep, and the stress distribution seen at room temperature is not simply related to the original structure.

Raman spectroscopic studies of vibrational relaxation and chemical exchange broadening in hydrogen bonded systems

The v 1 Raman band of pyridine in binary mixtures with ethanol has been studied across the whole concentration range and over a temperature range between 98 K and 370 K. The band intensities are readily explained in terms of a multiple equilibrium model which includes ethanol trimers as a measure of the alcohol network. It is this aggregation phenomenon which leads to the detection of free (i.e., non-hydrogen bonded) pyridine even in solutions containing a very large proportion of ethanol. The bandwidth data have been interpreted in terms of the molecular dephasing caused by hydrogen bonding interactions. At low temperatures, the Gaussian band shapes are formed in the slow modulation limit but, as the temperature is raised, motional narrowing occurs and the band shape changes from Gaussian to Lorentzian. The dephasing rate therefore slows up due to more rapid thermal fluctuations. At temperatures above room temperature the bandwidths increase again, and this is considered to be an indicator of the importance of chemical exchange dynamics for this system.

Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes

Variable temperature 1H and 13C{ 1H} NMR studies on 1,1′,3,3′-tetra(alkyl)-ferrocenes and -ruthenocenes (alkyl = t-pentyl,t-butyl) have provided accurate barrier energies for restricted rotations of the substituted cyclopentadienyl rings. Energies (Δ G† (298.15 K)) are in the range 40–57 kJ mol −1 and are dependent on the ‘sandwich’ metal (Fe ⋙ Ru) and alkyl substituent (t-pentyl < t-butyl). Ring rotation in 1,1′,3.3′-tetra(phenyl)ferrocene was too rapid for measurement even at 173 K. Spectral changes were analysed primarily on the basis of exchange between a mirror pair of structures having both 5-membered rings eclipsed and the alkyl substituents staggered. In the case of 1,1′,3,3′-tetra(t-butyl)ferrocene a pair of staggered ring rotamers also contributed to the observed NMR bandshape changes.

The effect of apodization and finite resolution on Fourier transform Raman spectra

The effects that finite resolution and choice of apodization function have on Fourier transform (FT) Raman spectra are illustrated by the 839 cm −1 (ν 1) and 914 cm −1 bands of KMnO 4. FT-Raman spectra were recorded at 0.5, 1, 2, 4, 8, 16 and 32 cm −1 resolution using boxcar, Norton—Beer (strong, medium and weak) and triangular apodization functions at each resolution. The results show the dramatic changes in bandshape that occur as the ratio (resolution/true full width at half height of band) increases. The changes were measured in terms of the full width at half height of the band, the height of the band, the area of the band and the bandshape (expressed as a sum of Lorentzian and Gaussian lines). At a given resolution the degree to which each of these characteristics is affected is strongly dependent on the choice of the apodization function.

Spectral Variation of Amorphous As2S3after the Initial Annealing and Photodarkening

The initial annealing at low temperature, e.g. 443 K and 373 K, increased slightly the absorption around the top of the fundamental absorption band at ∼6 eV, while at high temperature, e.g. 463 K, increased largely the absorption near the top. The light illumination in the edge of the band at 90 K increased the absorption around the tail (∼3 eV) of the band, while depressed that around the top. This light illumination brought the identical absorption shape independent of the initial annealing temperature. Once the light illumination was made, spectral variations under repetitions of further thermal annealing at 463 K and light illumination at 90 K were reversibly reproduced in several times. It was made clear that the thermal annealing and the light illumination caused not only the edge shift, but total change of absorption band shape which arose from heavy distortion of the layer structure of films.

A study of a novel degenerate carbocation rearrangement of the 4,9‐dimethyl‐9‐barbaralyl cation by dynamic 13C and 1H NMR spectroscopy in superacid

AbstractThe 4,9‐dimethyltricyclo [3.3.1.02,8]nona‐3,6‐dien‐9‐yl (4,9‐dimethyl‐9‐barbaralyl) cation (3) was generated from 4,9‐dimethyl‐9‐barbaralol (5) at −135°C in two different superacid mixtures [FSO3HSO2CIFSO2F2(1:6:1) and FSO3HSO2CIFSO2F2CHCl2 (2:10:10:1 by volume)]. Its 1H and 13CNMR spectra show a strong temperature dependence in the range −150 to −125°C. The changes in band shapes with temperature show that the following exchanges take place: 4‐methyl with 9‐methyl, C‐4 with C‐9, C‐1 with C‐3 and C‐2 with C‐8. C‐5, C‐6 and C‐7 are found not to exchange rapidly either with each other or with the other carbons in 3. The mechanism of this novel rearrangement is suggested to involve the bicyclic 2,7‐dimethylbicyclo [3.2.2] nona‐3,6,8‐trienyl cation and the secondary barbaralyl cation 4,6‐dimethyltricyclo [3.3.1.02,8] nona‐3,6‐dien‐9‐yl as intermediates rather than 7,8‐dimethyl bicyclo [3.2.2] nona‐3,6,8‐trienyl cation, which does not have a methyl group on the allyl cation moiety. Comparisons with rearrangement mechanisms for other barbaralyl cations were also made. The rate constant for the degenerate rearrangement of 3 is 160 s−1 at −140°C, which corresponds to ΔG≠s = 26 kJ mol−1 (6·3 kcal mol−1). At −125°C ion 3 rearranges non‐degenerately to the 1,4‐bishomotropylium cation 1,8‐dimethylbicyclo [4·3·0] nona, 2,4,7‐trienyl (4) with k = 3 × 10−4 s−1. A mechanism for this rearrangement and the synthesis and purification of the ion precursor 4 are also reported.

Absorption coefficients for the 6190-Å CH 4 band between 290 and 100°K with application to Uranus' atmosphere

Absorption coefficients for the CH 4 6190-Å band as a function of temperature from 290 to 100°K are reported. We have developed and used a laser intracavity photoacoustic spectroscopy method which permits accurate control of the sample temperature due to the small sample volume required. Over the temperature interval from 290 to 100°K, we found the peak absorption coefficient to increase from 0.6 to 1.0 cm −1 (km-am) −1. The peak absorption increase was accompanied by significant band shape changes. The low-temperature data were then used to constrain further the Baines and Bergstralh (1986, Icarus 65, 406–441) standard model atmosphere for Uranus and found to yield an excellent fit to the bandshape near the minimum of the 6190-Å band. The new low-temperature coefficients reduce the pressure level needed to fit the Neff et al. (1984, Icarus 60, 221–235) photometric data, as well as the Voyager Uranus observations, to ∼3 bar, rather than the 7–10 bar required by the room temperature CH 4 absorption coefficients, thus producing agreement between models derived from individual H 2 quadrupole lines and CH 4 lines. The limitations of the present data are discussed in the context of the observations.

Raman spectral studies of ion–molecular interactions of lithium bromide and lithium iodide with dimethyl sulphoxide

AbstractRaman spectra in the CH and SO stretching and SO bending regions are reported for solutions of lithium halides and tetrabutylammonium bromide (TBAB) in dimethyl sulphoxide (DMSO) and DMSO‐d6. The CD stretching band of DMSO‐d6 shows two components in solutions of TBAB with DMSO‐d6, whereas the CH stretching band of DMSO for solutions of LiX (X = Br and I) in DMSO show some broadening and asymmetry in the low‐frequency region. The SO stretching bands show changes in band shapes and band contours in solutions of electrolytes in DMSO. Fourier deconvolution of CH and SO stretching bands were carried out to gain more information regarding number of components present under the broad structured bands. All the solutions show a significant decrease in the intensity of the SO bending bands when compared with the neighbouring CS bending band. The results are discussed in terms of anion‐ and cation–molecular interactions with DMSO.