A Raman spectroscopic study of the carbon deposition mechanism on Ni/CGO electrodes during CO/CO2 electrolysis.

In situ and ex situ Raman analyses of porous Ni/CGO electrodes reveal differences in the amount, location and type of carbon formed during CO/CO2 electrolysis. The results demonstrate the limitations of optical in situ techniques applied to Solid Oxide Cells (SOCs) operated in electrolysis conditions. Increased carbon deposition close to the electrode-electrolyte interface is likely to be the result of high charge-transfer current in that area. The positive effect of a CGO interlayer on reducing carbon formation on the fuel electrode is demonstrated.

Towards a metrological determination of the performance of SERS media.

Enhancement factors under SERS conditions are characterised for two SERS active substrates with different geometries using a combination of optical pumping and photobleaching. A rigorous mathematical method of the photobleaching dynamics under SERS conditions is developed to allow the average enhancement factor to be investigated whilst maximal enhancement factors are studied using optical pumping. We show that both average and maximal enhancements are correlated for the surfaces with average enhancements of approximately 10(8).

Vibrational pumping in surface enhanced Raman scattering (SERS).

In this tutorial review, the underlying principles of vibrational pumping in surface enhanced Raman scattering (SERS) are summarized and explained within the framework of their historical development. Some state-of-the-art results in the field are also presented, with the aim of giving an overview on what has been established at this stage, as well as hinting at areas where future developments might take place.

Raman spectroscopy as a probe of temperature and oxidation state for gadolinium-doped ceria used in solid oxide fuel cells.

Raman spectroscopy is a noninvasive and highly sensitive analytical technique capable of identifying chemical compounds in environments that can mimic SOFC operating conditions. Here we demonstrate the use of Raman spectroscopy to perform local thermal and temporal measurements, both of which are essential if phase formation diagrams are to be mapped out and compared to thermodynamic phase stability predictions. We find that the time resolution of the Raman technique is more than sufficient to capture essential dynamic effects associated with a change of chemical composition.

Enhancement factor averaging and the photostability of probes in SERS vibrational pumping.

The technique of temperature dependent vibrational pumping in surface enhanced Raman scattering (SERS) has been recently demonstrated as a promising new tool to estimate SERS cross-sections. In this paper we expand on the previous developments and study several details around the implementation and physics of the vibrational pumping technique in SERS. Here we concentrate on two specific aspects related to: (i) the different averaging properties (over the distribution of enhancements) of the Stokes and anti-Stokes signals in the pumping regime; and (ii) the role of the finite photostability of the probes. The fact that the anti-Stokes signal is averaged differently from the Stokes counterpart leads to some unique phenomena in Raman spectroscopy that can only be observed under the conditions of vibrational pumping in SERS.

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping.

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.

A study of local heating of molecules under surface enhanced Raman scattering (SERS) conditions using the anti-Stokes/Stokes ratio.

We make systematic measurements of the anti-Stokes/Stokes (aS/S) ratios using low power lasers (0.5 mW at 514 and 633 nm) of rhodamine 6G (RH6G) on dried silver colloids over a wide range of temperatures from 140 to 350 K. We show that a scan in temperature allows the extraction of the contributions to the anti-Stokes/Stokes ratio from resonance effects and heating independently, thus decoupling the two aspects of the problem.

A conclusive demonstration of vibrational pumping under surface enhanced Raman scattering conditions.

We provide a conclusive demonstration of vibrational pumping under Surface Enhanced Raman Scattering (SERS) conditions by performing anti-Stokes/Stokes ratio measurements over a large spatial area and low power density, down to 10 K with dried silver colloids, the dye rhodamine 6G, and 676 nm laser excitation. The method we propose allows for the measurement of the cross sections for different modes and also provides the determination of the asymmetry between the anti-Stokes and Stokes SERS cross sections for each mode.

Temperature-dependent anti-stokes/stokes ratios under surface-enhanced Raman scattering conditions.

We make systematic measurements of Raman anti-Stokes/Stokes (aS/S) ratios using two different laser excitations (514 and 633 nm) of rhodamine 6G (RH6G) on dried Ag colloids over a wide range of temperatures (100 to 350 K). We show that a temperature scan allows the separation of the contributions to the aS/S ratios from resonance effects and heating/pumping, thus decoupling the two main aspects of the problem. The temperature rise is found to be larger when employing the 633 nm laser. In addition, we find evidence for mode specific vibrational pumping at higher laser power densities. We analyze our results in the framework of ongoing discussion on laser heating/pumping under surface-enhanced Raman scattering (SERS) conditions.

Resonance contributions to anti-Stokes/Stokes ratios under surface enhanced Raman scattering conditions.

Anti-Stokes/Stokes asymmetries under surface enhanced Raman scattering (SERS) conditions are studied for a wide variety of SERS-active media and different analytes. Evidence is provided for the existence of underlying resonances that create these asymmetries. We show here that these resonances are associated with the electromagnetic coupling between the analyte (probe) and the metal. The work demonstrates the use of the anti-Stokes/Stokes ratio as a tool to understand the hierarchy of resonances in the SERS problem, which is essential for quantification purposes.

Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids.

Surface enhanced Raman spectroscopy (SERS) of dyes in solution allows the study of the differences between ensemble averaged spectroscopic signals and single molecular events. We address several outstanding issues in single molecule detection via SERS; in particular, evidence for single molecule vibrational pumping and/or single molecule laser heating, the statistics of hotspots in the liquid, and anti-Stokes/Stokes anomalies. We demonstrate that anti-Stokes/Stokes ratios are a very unreliable measure of temperature, because the two processes are affected differently by the underlying frequency-dependent plasmon resonances. Subtle hints of vibrational pumping and/or heating in single molecules can only obtained via careful cross correlations between the parameters (frequency position, width, and intensity) of the Stokes signals for different excitation lasers. We introduce the use of single-peak parameter cross correlations for the study of these phenomena.

Stokes/anti-Stokes anomalies under surface enhanced Raman scattering conditions.

The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.

Electromagnetic optimization of light-harvesting proteins.

The problem of electromagnetic optimization of the structure of light-harvesting proteins is studied within a simplified diffraction scheme. It is suggested that evolutionary pressures for protein antennae has resulted in molecular structures that optimize the absorption in the light-sensitive pigments. Bacteriorhodopsin is shown to be an exceptional case in terms of optimization, a fact that can be connected to its extremely high sensitivity as a light detector down to a single photon level.

Subjective evaluation of four low-complexity audio coding schemes.

In this study the subjective performance of four low-complexity audio data compression methods are compared, operating at nominal bit rates of 2, 3, 4, and 5 bits per sample, applied to four 20-kHz bandwidth, 16-bits per sample digitized musical signals. The simple compression schemes compared were elementary differential pulse-code modulation (DPCM), noise feedback coding DPCM (NFC-DPCM), adaptive quantizer DPCM (DPCM-AQB), and a recently proposed method known as recursively indexed quantizer DPCM (RIQ-DPCM). Pairs consisting of a reconstructed signal and a reference signal were presented in a two-interval preference experiment. The reference signals were processed for specified levels of modulated noise reference unit (MNRU) in order to estimate the equality threshold rating (ETR) of the reconstructed audio stimuli. The subjective MNRU values were found to increase by 2-5 dB for each increment in bits per sample. The DPCM-AQB scores were found to be 8-10 dB higher than for DPCM and NFC-DPCM. RIQ-DPCM was rated highest, exceeding the DPCM-AQB results by 2-5 dB in all tests. Objective measurements of segmental signal-to-noise ratio (SNRSEG) for the reconstructed signals predicted a performance level 2-5 dB lower than was actually found in the subjective results, particularly for SNRSEG values below 25 dB.