Single-nanocrystal spectroscopy of white-light-emitting CdSe nanocrystals

We report the observation of broad-spectrum fluorescence from single CdSe nanocrystals. Individual semiconductor nanocrystals typically have a narrower emission spectrum than that of an ensemble. However, our experiments show that the ensemble white-light emission observed in ultrasmall CdSe nanocrystals is the result of many single CdSe nanocrystals, each emitting over the entire visible spectrum. These results indicate that each white-light-emitting CdSe nanocrystal contains all the trap states that give rise to the observed white-light emission.     

Experimental problems in Raman spectroscopy applied to pigment identification in mixtures

Raman spectroscopy provides useful information to detect and identify pictorial materials in artworks, although some problems are involved when the identification of individual pigments in mixtures is treated. With the hypothesis of the Principle of superposition, the mixture spectrum should be the direct sum of each pondered individual spectrum. In this work, we will show several mixtures where it can be noticed that the mixture spectrum is not qualitatively proportional to the sum of pondered individual spectra. Also there were some cases where the bands of one of the pigments could not be detected in the mixture spectrum. This non-linear behaviour could be attributed to specific proprieties of each material that are revealed when they interact with each other. We conjecture that, for instance, the different reflectances or the wavelength of the laser source could be determinant factors of the obtained results. In this paper an experimental method has been designed in order to characterize the quantitative behaviour of the Raman bands corresponding to each pigment in a mixture. Adequate coefficients are defined and calculated to facilitate the study of the spectral contribution of the different components of a mixture.     

Computer-aided interpretation of mass spectra. Information on substructural probabilities form stirs

The capability of the ‘Self-Training Interpretive and Retrieval System’ has been extended so that the compounds selected as providing the best match in each data class to the unknown mass spectrum are examined by the computer for the presence of each of 179 common substructural fragments. Stastical methods were used to evaluate the selectivity for identification of each substructure by each data class using a reference file of 18 806 spectra of different compounds. In tests using at least 373 unknown spectra for each of the substructures, with criteria that gave a mean reliability of 98.1%, the method averaged 49% recall, which corresponds to the identification of 2.55 substructures per unknown spectrum, as well as the normal “Self-Training Interpretative and Retrieval System” match-factor results, requires 68 s on a laboratory computer. The method is also available to outside users on an international computer network.     

The magnetically induced circular emission spectrum of Cr(III) in guanidinium aluminium sulphate

The magnetically induced, circular emission spectrum (MICE) of Cr(III) in hexagonal guanidinium aluminium sulphate hexahydrate has been measured together with the axial emission spectrum. Four lines are observed in the ²E region suggesting that these are the result of trigonal field and site symmetry splittings since each component exhibits an A term type dispersion. The MICE spectrum enables several of the vibrational features to be identified.     

AN ULTRAVIOLET RADIATION ACTION SPECTRUM FOR IMMEDIATE PIGMENT DARKENING

The wavelength dependence for immediate pigment darkening (IPD) was investigated by exposing the midback skin of volunteers to a series of incremental fluences of narrow waveband radiation isolated by band-pass filters in the310–400 nm region. The threshold IPD fluence for each waveband was determined by visual assessment of the skin responses immediately after each exposure. The action spectrum, constructed from the mean threshold fluences, was broad and extended from 320 nm to 400 nm with a peak at around 340 nm. No IPD could be evoked at 310 nm, even after erythemogenic fluences. The spectrum was similar in each of the three skin types investigated (III, IV, V). The broad nature of the action spectrum within the UVA region suggests that IPD may serve as an alternative endpoint for measuring photoprotection against these wavelengths.     

Sum frequency generation microscopy of microcontact-printed mixed self-assembled monolayers

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.     

Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts

A comparison of the triplet-minus-singlet (TmS) absorption spectrum of spinach chloroplasts, recorded some thirty years ago, with the more recently published TmS spectrum of isolated Chla/b LHCII (light-harvesting complexes associated with photosystem II of higher plants) shows that the two spectra are very similar, which is to be expected, since only the carotenoid pigments contribute to each spectrum. Be that as it may, the comparison also reveals a dissimilarity: photoexcitation of the sample does, or does not, affect the absorbance in the Qy region (650-700 nm), depending on whether the sample is a suspension of chloroplasts or of isolated LHCII. The Qy-signal in the TmS spectrum of LHCII decays, it should be noted, at the same rate as the rest of the difference spectrum, and its most prominent feature is a negative peak. As the carotenoids do not absorb in the Qy region, the presence of a signal in this region calls for an explanation: van der Vos, Carbonera and Hoff, the first to find as well as fathom the phenomenon, attributed the Qy-signal to a change, in the absorption spectrum of a chlorophyll a (Chla) molecule, brought about by the presence of triplet excitation on a neighbouring carotenoid (Car). The difference in the behaviours of chloroplasts and LHCII, if reproducible, would imply that the Car triplets which give rise to the TmS spectrum of chloroplasts do not influence the absorption spectra of their Chla neighbours. With a view to reaching a firm conclusion about this vexed issue, spinach chloroplasts and thylakoids have been examined with the aid of the same kinetic spectrometer as that used for investigating LHCII; the TmS spectra of both chloroplasts and thylakoids contain prominent bleaching signals centred at 680 nm, and the triplet decay time in each case is comparable to that of the Chla/b LHCII triplets. Results pertaining to other closely related systems are recalled, and it is concluded that, so far as the overall appearance of the TmS spectrum is concerned, spinach chloroplasts are by no means abnormal.     

The absorption spectrum of localized electrons in crystalline 1,8-octanediol, γ-irradiated at 4.2 K

Crystalline 1.8-octanediol has been γ-irradiated and its optical absorption spectrum has been measured at 4.2 K. The spectrum consists of three partially resolved peaks with maxima at 740, 590, and 490 nm. The peaks cannot be separated by photobleaching or annealing. It is suggested that they arise from transitions inherent in each localized electron.     

Raman spectroscopy as a probe of adsorption on rhodium particles

Raman spectroscopy has been used as a probe of the adsorption of pyridine onto small particles of rhodium. The band energies of the surface phase correspond with those found on roughened silver electrodes. Chemical experiments indicate that the pyridine is stronyly chemisorbed. The similarity with the spectrum of pyridine on Ag presumably means that the predominant surface species is the same on each metal. This work represents the first report of an enhanced Raman spectrum on metallic rhodium.     

AN ACTION SPECTRUM FOR ULTRAVIOLET INDUCED ELASTOSIS IN HAIRLESS MICE: QUANTIFICATION OF ELASTOSIS BY IMAGE ANALYSIS

To determine an action spectrum for ultraviolet (UV)-induced elastosis, four groups of 24 albino hairless mice each were exposed to four different spectra emitted by a xenon arc solar simulator fitted with cut-off filters (Schott WG 320, 335, 345, and 360). These filters progressively removed more of the shorter wavelengths until, in the final spectrum, only long wavelength UVA (greater than 335 nm) remained. Exposures continued up to 62 weeks. A fifth group of mice served as controls. Skin biopsies were taken at pre-determined dose points and were processed for light microscopy. Elastosis was quantified by computerized image analysis, yielding dose-response curves for each spectrum. The total energy required for a 50% increase in elastic tissue compared to controls was determined graphically for each spectrum. These were: WG 320, 65 J/cm2; WG 335, 865 J/cm2; WG 345, 1230 J/cm2; and WG 360, 2000 J/cm2. Our results were tested against published action spectra for erythema, photocarcinogenesis and elastosis. The erythema spectrum was the most predictive for elastosis except that the longer UVA wavelengths were less effective for elastosis than for erythema. Solar simulating radiation (WG 320 filter) with its UVB component was the most effective in inducing elastosis. Full spectrum UVA (WG 345) required 20 times more energy while long wavelength UVA (WG 360) required 30 times more energy to induce equivalent elastosis.     

Theory of covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval's theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.     

Multivariate statistical analysis of non‐mass‐selected ToF‐SIMS data

Cluster LMIGs are now regarded as the standard primary ion guns on time‐of‐flight secondary ion mass spectrometers (ToF‐SIMS). The ToF‐SIMS analyst typically selects a bombarding species (cluster size and charge) to be used for material analysis. Using standard data collection protocols where the analyst uses only a single primary bombarding species, only a fraction of the ion‐beam current generated by the LMIG is used. In this work, we demonstrate for the first time that it is possible to perform ToF‐SIMS analysis when all of the primary ion intensity (clusters) are used; we refer to this new data analysis mode as non‐mass‐selected (NMS) analysis. Since each of the bombarding species has a different mass‐to‐charge ratio, they strike the sample at different times, and as a result, each of the bombarding species generates a spectrum. The resulting NMS ToF‐SIMS spectrum contains contributions from each of the bombarding species that are shifted in time. NMS spectra are incredibly complicated and would be difficult, if not impossible, to analyze using univariate methodology. We will demonstrate that automated multivariate statistical analysis (MVSA) tools are capable of rapidly converting the complicated NMS data sets into a handful of chemical components (represented by both spectra and images) that are easier to interpret since each component spectrum represents a unique and simpler chemistry. Copyright © 2008 John Wiley & Sons, Ltd.     

EPR, optical and infrared absorption studies of Mn2+ ions doped in zinc malate trihydrate single crystal

Electron paramagnetic resonance (EPR) studies have been carried out on Mn2+ ions doped in zinc malate trihydrate single crystals in the temperature range 123-413 K on X-band frequency. The EPR spectrum at room temperature exhibits a group of five fine structure transitions each splits into six hyperfine components. Angular variation studies reveal that Mn2+ ions enter the lattice substitutionally. From the observed EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The variation of zero-field splitting parameter (D) with temperature is measured. From the optical absorption spectrum, the crystal field splitting parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The infrared spectrum exhibits bands characteristic of the carboxylic acid salts.     

Principal components analysis for the visualisation of multidimensional chemical data acquired by scanning Raman microspectroscopy

Raman microspectroscopy is ideally suited to surface analysis as it allows detailed chemical information to be acquired from surfaces at a relatively high spatial resolution (typically 1 microm). Using a motorised sample table or probe, it is possible to raster scan a surface to obtain spatially resolved chemical information. Visualisation of the acquired data is a problem, however, as the spectrum acquired at each point can contain several hundred individual intensity measurements. Existing visualisation methods are limited to plotting each scanned point with an intensity determined from the measured intensity at a single wavenumber, or the similarly between the point's spectrum and a reference spectrum. Such methods are wasteful as a lot of acquired information is discarded, and results are prone to misinterpretation due to background variance and instrumental noise. In this paper we introduce a new method that uses principal components analysis (PCA) to reduce the spectrum at each point to three factors that are then used to define the red, green and blue components of the corresponding point on a false colour map. To increase the effective resolution, interpolation is used to approximate the colours corresponding to points between those actually scanned. To demonstrate the technique, the internal surface of a beverage can, contaminated with a 40 microm diameter carbonised oven impurity, consisting mainly of sp2- and sp3-hybridised saturated carbon bonds, has been used as a case study.     

On the assessment of melanin in human skin in vivo

Abstract— A method for estimating the amount of pigment in normal human skin in uivo is presented. This method is based on remittance spectroscopy. The spectrum of normal skin is compared to amelanotic skin and the logarithm of the ratio is fitted with a straight line in the range 620-720 nm. The parameters obtained are strongly correlated for all the volunteers in this study. therefore each spectrum determines one parameter for each individual. When similar analysis was performed on DOPA-melanin we obtained the same strong correlation among different concentrations. We are thus able to determine a relation between the coefficient obtained from the remittance spectrum from normal skin and an equivalent concentration of DOPA-melanin in aqueous solution. We can thus estimate. to a first approximation, the total melanin mass in human skin non-invasively, and can determine a parameter that is uniquely correlated to the amount of melanin pigment in the skin.     

Using ultrafast infrared multidimensional correlation spectroscopy to aid in vibrational spectral peak assignments

Ultrafast infrared heterodyne detected vibrational stimulated echoes with full phase information are used to obtain the vibrational correlation spectrum from a mixture of metal-carbonyl compounds. The linear absorption spectrum displays four peaks in the carbonyl stretching region. In the absence of knowledge of the molecules that make up the mixture, the absorption spectrum could arise from four molecules that each produces a single peak to one molecule with four peaks. In contrast, the correlation spectrum displays four peaks on the diagonal and off-diagonal peaks that make it straightforward to determine which peaks belong to a particular molecule.     

Pinacol rearrangement of quinoline analogs of benzopinacol and evidence for rearrangement under the conditions of electron impact

The pinacols, 1,2-di(2-quinolyl)-1,2-diphenylethane-1,2-diol and 1,2-di(8-quinolyl)-1,2-diphenylethane-1,2-diol, were prepared and rearranged. The mass spectrum of the phenyl-2-quinolylpinacolone showed fragment peaks corresponding to the pinacolone resulting from 2-quinolyl migration. The mass spectrum of the phenyl-8-quinolylpinacolone showed fragment peaks corresponding to the pinacolone resulting from phenyl migration. Evidence for the rearrangement by electron impact was observed in the mass spectrum of each pinacol.     

同位素氧-18分子的He I紫外光电子能谱

HeⅠ photoelectron spectrum of isotopic 18O2 molecules has been firstly recorded on a double-chambers UPS machine Ⅱ which was built specifically to detect transient species. In comparison with UPS results of oxygen molecles 16O2 it is found that the reduction of adiabatic ionization potentials IPs, the decresae of vibrational intervals and the change of intensity of vibrational components on each ionic state of isotopic 18O2 molecules obviously appear on the UPS spectrum.     

A spectrum stripping method for the computer-coupled activation analysis of unknown samples by γ-ray spectrometry

A spectrum stripping method for the computer-coupled activation analysis of unknown samples using γ-ray spectrometry is discussed. Before the unscrambling of the γ-ray is performed using standard spectra of the components, each spectrum is smoothed, statistically weighted and corrected for background. The photopeaks of the standard spectra are then aligned with the respective peaks of the sample spectrum, one at a time after correcting for gain drift if necessary, and the undesired components are stripped out. After successive subtraction, the characteristic spectrum of the desired element is compared with that of the standard to determine its weight. The results obtained using the present method are as good as those obtained from least-squares analysis in ideal cases, but appear to be better when assumptions inherent in the least-squares method are no longer valid.     

Non-Newtonian behavior of polydisperse polymer melts as a consequence of the evolution of their relaxation spectra

The non-Newtonian flow of polydisperse polymer melts is shown to be described by a model according to which an increase in the shear rate leads to the suppression of the dissipative losses of the relaxation modes of each fraction. The higher the shear rate, the greater the suppression. The relaxation spectrum of each monodisperse fraction is represented by the Rouse distribution, and only this form of spectrum leads to a ??spurt?? effect at the critical shear stress. Hence, the physical content of the model that relates the non-Newtonian behavior of polymer melts to their molecular-mass distributions consists in the fact that the relaxation modes responsible for energy dissipation are gradually truncated from the side of high relaxation times. The higher the M of a given fraction, the greater the contribution of this part of the spectrum to the total viscous losses. In this case, the truncation of the spectrum from the side of high relaxation times is equivalent to the gradual ??elimination?? of high-molecular-mass fractions of the polydisperse polymer from the contribution to dissipation. The shear-rate-dependent evolution of the relaxation spectrum of the medium is the structural mechanism that causes the non-Newtonian flow of polymer melts. The efficiency of the proposed model is shown through calculation of the flow curves for polymers with known molecular-mass distributions. The calculation results are in agreement with the experimental data. The theoretical ideas developed with the use of the ?? function to describe molecular-mass distributions have made it possible to solve the inverse problem, i.e., to establish a quantitative relationship between the shape of the flow curve and the molecular-mass distribution and, thus, to calculate the molecular-mass distributions according to the shearrate dependence of the apparent viscosity.