Raman Signal Intensity Research Articles

Surface Enhancement Factors for Ag and Au Surfaces Relative to Pt Surfaces for Monolayers of Thiophenol

Raman signal intensities from the 997 cm−1 ring breathing mode of thiophenol monolayers adsorbed at Ag, Au, and Pt surfaces were employed for determination of absolute surface enhancement factors (SEFs). Unlike previous estimations of SEFs, these SEFs are determined by referencing the surface-enhanced Raman scattering (SERS) intensities to the unenhanced Raman scattering at Pt surfaces. The surfaces studied include those commonly prepared in a laboratory ambient and those prepared in vacuum. Surfaces prepared in ambient include polycrystalline Ag electrochemically roughened by an oxidation-reduction cycle (ORC), mechanically polished (MP) polycrystalline Ag, chemically polished (CP) polycrystalline Ag, Ag (111), MP polycrystalline Au, and MP polycrystalline Pt. Vacuum environment surfaces include coldly deposited Ag films (cold Ag) and room temperature-deposited (RT) “thick” Ag films. Each of these thiophenol/metal systems was sampled with an excitation wavelength (λex) of 514.5 nm; MP Au surfaces were also studied with λex of 720 nm. SEFs of 2.0 × 104 for ORC Ag, 5.3 × 103 for MP Ag, 160 for cold Ag, 64 for MP Au720 (λex = 720 nm), 69 for Ag (111), 39 for CP Ag, 7.9 for RT “thick” Ag, and 2.2 for MP Au514.5 (λex 514.5 nm) are observed relative to the SEF of MP Pt, which is assigned as 1. For practical purposes, the significance of the magnitudes of these SEFs is discussed in terms of estimated surface Raman limits of detection (LODs).

Small-Volume Raman Spectroscopy with a Liquid Core Waveguide

A liquid-core waveguide based on a new fluoropolymer tube is proposed as the basis of a novel, small probe-volume Raman-scattering-based detector. Illumination is transverse to the axis of the tube, with end-on detection through a transparent window at the tube terminus. Using this approach and a ∼6-nL probe volume, we have measured benzene, carbon tetrachloride, and aqueous solutions of 2-propanol (in 5−100% concentration) as test compounds. We show successful collection of Raman spectra to shifts of ∼200 cm-1 collected without a holographic notch filter. Additionally, the apparatus naturally discriminates against Raman signals from the capillary material. This permits simplified detection of the liquid sample without the need to subtract the otherwise omnipresent contribution of the capillary material. A simple, multiple-pass, laser illumination scheme is found to enhance the Raman signal intensities; the gain in S/N increases slightly less than linearly with the number of passes. In the case of 2-propanol, experiments were conducted in both static and in a flow injection mode. With a correction applied for RI change due to the solute, the Raman intensity was linearly dependent on the analyte concentration.

Near-Infrared Surface-Enhanced Raman Scattering of Ultrathin Films of CnAzoNaph(1,4)CmN+Br- (n = 7, 10, 12; m = 4, 6)

Near-infrared (NIR) Fourier transform (FT) Raman spectra have been measured for a series of 4-[4‘-[[(4‘ ‘-alkyloxy)phenyl]azo]naphthyloxy] alkyltrimethylammonium compounds [CnAzoNaph(1,4)CmN+Br-] in a solid state, in a cast film on a CaF2 plate, and in dipping films on etching silver foils and for one- and three-layer Langmuir−Blodgett (LB) films of C10AzoNaph(1,4)C4N+−sodium dextran sulfate (SDS) on a CaF2 plate and etching silver foils. The one-monolayer LB film on the silver foil shows much more intense Raman signals than the 75-layer film on the CaF2 plate, showing a great surface-enhanced Raman scattering (SERS) effect for the LB film on the silver foil. Since SDS plays a role as a spacer, C10AzoNaph(1,4)C4N+ is not directly adsorbed on the silver surface. Therefore, the strong SERS effect for the LB films of C10AzoNaph(1,4)C4N+−SDS may be explained by an electromagnetic mechanism. The one-monolayer dipping film of C10AzoNaph(1,4)C4N+Br- on the silver foil shows about 10 times stronger Raman signals ...

Next generation V 2O 5 cathode materials for Li rechargeable batteries

We report on investigations of vanadium oxide thin film cathodes prepared by three different synthesis techniques. Our experimental results on PLD-grown, textured V 2O 5 crystalline films concur with reports in the literature that there is a voltage threshold above which, cycling appears to be completely reversible and below which, cycling appears to be irreversible. Crystalline films discharged beyond the threshold to 2.0 V exhibited an immediate and continuous fade in capacity as well as a nearly 90% decrease in XRD peak intensity and a similar decrease in Raman signal intensity in as few as ten cycles. PLD-grown amorphous films show capacity loss of <2% over 200 cycles. Amorphous plasma-enhanced chemical vapor deposition (PECVD) films have capacities as high as 1.5 Li/V with excellent stability over 3000 cycles. Solution-grown nanoparticles (<100 nm) of VO 2 were spray-deposited and sintered at relatively low temperatures to produce nanoporous films. Cycling properties along with structural investigations by XRD and Raman scattering will be presented.

Comparison of fibre-optic SERS sensors with differently prepared tips

Fibre-optic sensors using surface-enhanced Raman scattering (SERS) with a single fibre for both the excitation and the signal collection require the preparation of properly roughened metal films at the tip of the fibres. Suitable techniques are slow evaporation of metals forming island-films, vacuum deposition of metal films on alumina or diamond particles, and evaporation of metals on sandblast-roughened fibre tips. All of these preparations result in SERS-active sensor tips with similar enhancements of the Raman signal intensities. The newly proposed sandblast method may be superior in practical applications because it promises the highest durability and the easiest regeneration of the thus produced sensors.

Silver Colloids Impregnating or Coating Bacteria

We produce silver nanocolloids selectively within and on Escherichia coli bacteria. Silver preferentially concentrates in the bacteria, distributes uniformly within them, or, alternatively, forms a rough coating over them. Oscillations in polarized light scattering vs scattering angle are more pronounced and shift to larger angles, compared to a control that has no silver. Very intense surface-enhanced Raman signals are observed for E. coli with a “wall colloid”. The main bands can be associated with peptides and polysaccharides in the cell wall and its membrane. This work was extended to Gram-positive Bacillus megaterium.

Structural Phase Transformations in V2O5 Thin Film Cathode Material for Li Rechargeable Batteries

ABSTRACTWe report on investigations of V2O5 thin film cathodes prepared by pulsed laser deposition and the phase transformations which occur during electrochemical cycling. Our experimental results on PLD-grown, textured V2O5 crystalline films concur with reports in the literature that there is a voltage threshold above which, cycling appears to be completely reversible and below which, cycling appears to be irreversible. Crystalline films discharged beyond the threshold to 2.0 V vs. Li exhibited an immediate and continuous fade in capacity as well as a ∼90% decrease in XRD peak intensity and a similar decrease in Raman signal intensity in as few as 10 cycles. We have made co-phase material by both electrochemically discharging virgin, crystalline V2O5 and by further discharging previously cycled films which showed irreversible structural changes.

The dependence of Raman signal intensity on particle size for crystal powders

Abstract The dependence of the intensity of the Raman radiation emitted by powdered inorganic samples has been studied with FT Raman spectroscopy. Using potassium chromate and barium nitrate of different particle sizes of the order 0.1 mm it is shown that the Raman signal intensity increases as the particle size decreases. This is contrary to previous reports in the literature which were not supported by experimental results. The equation describing the dependence of backscattered Raman signal intensity on particle size, developed from the theory put forward by Schrader and D'Orazio in 1976, has been fitted to the experimental data, obtaining values for the scattering and absorption parameters for the materials studied.

Photoluminescence spectroscopy of CVD diamond films

Photoluminescence spectra from diamond films deposited from CHO and ArCHO gas mixtures were measured using the 488 nm excitation wavelength of an Ar laser in the energy range 1.6–2.5 eV (7587-328 cm −1). With this excitation energy only a fraction of the diamond band gap of 5.45 eV is detectable. However, our results suggest that even with this limitation, fundamental information about the defect structure, for example the incorporation of non-carbon atoms into the diamond films, can be obtained. The luminescence spectra are dominated by peaks from Raman excitations at 512 cm −1 (Si substrate), 1332 cm −1 (diamond), 1360–1600 cm −1 (graphite), 2170 cm −1 (second-order peak from graphite) and 6952 cm −1 (1.68 eV, luminescence peak). The 1.68 eV peak is related to the incorporation of silicon (from the quartz wall of our CVD reactor) into the diamond films. The peak intensity for films prepared under different gas compositions correlates well with plasma characterization by optical emission spectroscopy (OES) showing the existence of silicon in the plasma during diamond synthesis of which the concentration increases with increasing concentration of atomic hydrogen. As a consequence we find a correlation of the 1.68 eV peak in the luminescence spectrum with the concentration of atomic hydrogen in the plasma gas (determined using OES). A further correlation was detected between the broad luminescence emission and the quality of the diamond film (as determined from Raman peak intensities related to diamond and amorphous carbon). For example, for diamond films deposited from ArCH 4O z; gas mixtures the luminescence signal increases with step-like behaviour by about 30% when the molar fraction of carbon X c c+o is increased to greater than 0.5. This change reflects the change of carbon deposition from amorphous to diamond, which can be revealed by the Raman signal intensity of these species.

Surface-Enhanced Fourier Transform Raman Scattering from a DNA Triple Helix Poly[dA]·2Poly[dT] at a Silver Electrode: Beyond the Short-Range Mechanism

Surface-enhanced Fourier transform Raman scattering (FT-SERS) spectroscopy was used to characterize the adsorption structure of a poly[dA]·2poly[dT] DNA triplex at an ex-situ roughened silver electrode. We found that the triplex molecules have a favorable tendency to be adsorbed without any obvious destabilization at higher positive-charged surfaces via a phosphate-moiety-directed mechanism. However, the observed enhancement of intense Raman signals exhibits long-range rather than short-range character, as evidenced by the appearance of several strong SERS bands involving relevant vibrations of the ribose−phosphate backbone and the dT residues. Therefore, this phenomenon cannot be satisfactorily interpreted by the short-range enhancement mechanism that is usually believed to be the main contribution of the Raman enhancement of double-helical nucleic acids at a charged silver surface.

Laser raman spectroscopy of lead-zirconate-titanate ceramics during exposure to atomic hydrogen

Raman scattering spectra are measured for lead-zirconate-titanate piezoceramics with measurements performed at varying temperatures and with exposure of the surface to atomic hydrogen. In order to investigate the kinetics of interaction of the ceramic surface with atomic hydrogen, the fixed-frequency time dependences of Raman scattering intensities are obtained as well. These dependences were analyzed and the activation energy of heterogeneous reduction reaction at the surface both for kinetics-controlled and for the diffusion-limited modes of the process were derived. The possibility of studying the kinetics of heterogeneous reduction reactions by an analysis of the time dependence for a fixed-frequency Raman signal intensity is demonstrated.

Femtosecond inverse Raman spectrum of molecular J‐aggregates

AbstractThe femtosecond inverse Raman spectrum of J‐aggregates of a cyanine dye was studied theoretically and experimentally by the pump—probe experiment. The time dependence of the Raman signal intensity of the aggregates was fitted with an instantaneously responding component due to the Stark effect together with 200 fs and 1 ps lifetime components due to the excitons. The instantaneous response in the time‐resolved difference transmission spectrum including the inverse Raman, induced and saturated absorption was successfully reproduced by the calculation under sub‐resonant excitation based on the density matrix formalism.

Performance Analysis of an Integrated Process Raman Analyzer Using a Multiplexed Transmission Holographic Grating, CCD Detection, and Confocal Fiber-Optic Sampling

The performance of an integrated turn-key on-line Raman spectrometer based upon a multiplexed transmission holographic grating, holographic laser rejection filters, and a charge-coupled-device (CCD) detector is described and compared with a laboratory-based research-grade FT-Raman spectrometer. Data for the dispersive system were acquired with a noncontacting, confocal dual-fiber probe of up to 100-m length, with Raman shifts down to 50 cm−1 and no apparent fiber background features. Despite the fact that the FT-Raman system was directly coupled to the sample (i.e., not through fibers), the sensitivity of the dispersive system was almost three orders of magnitude greater for equivalent incident laser powers and accumulation times. The potential for performing multivariate calibrations with the dispersive equipment was also investigated. With samples of known density, it was possible to produce a partial least-squares calibration for poly(thylene terephthalate) (PET) density with a precision of 0.002 g cm−3, with the use of a two-factor model. This precision compared favorably with previous calibrations using FT-Raman data. The effect of changing the f-number of the collection optics of the fiber probe head (to allow variation in the working distance) was also examined. It was found that, for transparent samples, the f-number could be changed by a factor of three without greatly affecting the Raman signal intensity, whereas for an opaque solid increasing the f-number greatly reduced the detected intensity. The reasons for this difference are discussed

Optical, Raman and resonance Raman spectra and lattice dynamics calculations of the misfit layer compounds, (SnS) 1.17NbS 2 and (PbS) 1.18TiS 2

The UV—visible (340–800 nm), Raman and resonance Raman spectra (10–850 cm −1) of powdered samples of two misfit layer compounds, (SnS) 1.17NbS 2 and (PbS) 1.18TiS 2, at 300 K were investigated. In agreement with X-ray data, vibrational assignments are proposed for most of the internal and external modes of the “TS 2” and “MS” sublattices and the band wavenumbers at Γ ( k=0) are compared to those in the 2H-NbS 2 and 1T-TiS 2 polytypes in order to appreciate the interactions in which “MS” acts as the donor part of the structure. In addition, owing to their incommensurate modulated structures along the a direction, new intense Raman signals are observed and explained by the k=0, ± q selection rules ( q is the modulation vector); such effects are more dominant in the (PbS) 1.18TiS 2 compound in which a stronger structural distortion is effective and a weak but non-negligible charge-transfer mechanism is occurring. New information about the lattice dynamics properties of the “TS 2” sublattices are thus obtained and confirmed by valence force field calculations on the stoichiometric model compounds (SnS) 1.0NbS 2 and (PbS) 1.0TiS 2, which reproduce satisfactorily the essential features of experimental data.

New approach for non-destructive sensing of fruit taste

A new method for the non-destructive sensing of fruit taste has been proposed for Japanese orange, one of the products of the Kumamoto area. Oranges contain sugars and acids, which are easily analysed by HPLC using the juice of the fruit, and the taste of oranges can be given primarily by the sugars/acids ratio. On the other hand, the intensity of the non-destructive Raman signal due to carotenoids around 1155 and/or 1525 cm −1 obtained from the surface of orange peel using Ar + laser light (e.g., 514.5 nm) increase during ripening of the orange, reaches a maximum at harvest time and then decreases. Interestingly, the Raman signal intensity obtained from the peel is found to show a good correlation with the data on sugars/acids ratios of the orange juice, suggesting that the Raman intensity from the peel of oranges cna be used as a measure of the taste for screening the sweetness of oranges. This technique, together with some other methods, has also been applied to muskmelon to evaluate the sweetness non-destructively.

Wavelength selection for fiber optic Raman spectroscopy Part 1

Blue wavelength excitation is usually preferred for analytical applications of Raman spectroscopy because of the lambda(-4) dependency of the Raman signal intensity on excitation wavelength. However, for remote Raman measurements using long optical fibers, the transmission spectrum of the fiber should be considered in determining the optimal excitation wavelength. In this note, a quick, approximate approach is developed to determine the optimal excitation wavelength for Raman spectroscopy over optical fibers.

The Determination of Resonance Raman Intensities with Respect to the Absorption of Incident and Scattered Light

Raman spectra are usually corrected according to the highly nonlinear wavelength-dependent functions of the monochromator transmission and the photomultiplier cathode response. In the case of resonance in which the exciting frequency is almost or exactly identical to one of the eigen frequencies of the molecule, the absorption of the incident laser light and the reabsorption of the scattered light falsify the Raman intensities. One method for correcting Raman intensities is the application of difference Raman spectroscopy. Several authors corrected resonance Raman signal intensities with theoretically derived functions which required the knowledge of the exact pathlength of the laser and the scattered light through the sample cell. The functions were related to the spectral internal absorbance of the sample and did not consider sample cell specific light intensity attenuations. To avoid this and to exclude imperfections in the pathlength determination, we used experimentally derived correction functions. The Raman signal intensity Is recorded at right angle to the incident beam is a function of the excitation light intensity I0, the number N of the exposed molecules, the fourth power of the absolute scattering frequency νs, and the sum σ of the squared tensor elements due to linearly polarized light: Is = σνs4NI0. (1)

Resonant inverse Raman scattering in red-HgI 2

Inverse Raman scattering in red-HgI 2 single crystals at 1.6 K is observed with the use of two tunable nanosecond pulse lasers. The A 1g Raman scattering cross section is estimated from the inverse Raman signal intensity. It is enhanced toward the exciton level and amounts to 0.1 (cm sr) −1 at 1.6 meV lower than the level. In contrast with the case of dye solution reported by Haushalter and Morris, inversion of the spectral shape is not observed even in the large absorption region.

Spatial Dependence of Inverse Raman Signals

The spatial dependence of inverse Raman signal intensities was investigated. Aqueous 0.5 M potassium nitrate in cells ranging from 1 to 50 mm was used as the test system. Three focusing lenses were used. The data do not fit the arctan dependence derived for Gaussian beams. The deviations are ascribed to the non-Gaussian profile of the pulsed dye laser beam.

Oil film thickness measurement using airborne laser-induced water Raman backscatter

The use of laser-induced water Raman backscatter for remote thin oil film detection and thickness measurement is reported here for the first time. A 337.1-nm nitrogen laser was used to excite the 3400-cm(-1) OH stretch band of natural ocean water beneath the oil slick from an altitude of 150 m. The signal strength of the 381-nm water Raman backscatter was always observed to depress when the oil was encountered and then return to its original undepressed value after complete aircraft traversal of the floating slick. After removal of background and oil fluorescence contributions, the ratio of the depressed-to-undepressed airborne water Raman signal intensities, together with laboratory measured oil extinction coefficients, is used to calculate the oil film thickness.