SERS background imaging – A versatile tool towards more reliable SERS analytics

Surface-enhanced Raman scattering (SERS) is a highly selective and sensitive straightforward analytical method, which is however not yet established in routine analysis due to a lack of reliability and reproducibility. To address this limitation, we show the distinct correlation of the ever-present but often neglected broad SERS background continuum with the SERS signal intensity of the analyte and how to exploit this correlation for an easy-to-handle, automatable and more reliable SERS measurement. First, fast and high-contrast imaging of the SERS substrate is performed for hot spot localisation utilizing the SERS background. Subsequently, highly enhanced SERS spectra are recorded at the centre of these spots. Furthermore, we correlate our SERS background imaging with other optical imaging modalities and electron microscopy to assess structure-property relationships. Monte Carlo simulations based on actual measurements illustrate the sampling error of a conventional SERS experiment and the advantages our method provides.

The hydroxyl satellite-line ‘flip’ as a tracer of expanding H ii regions

ABSTRACT Observations of the four 2Π3/2, J = 3/2 ground state transitions of the hydroxyl radical (OH) have emerged as an informative tracer of molecular gas in the Galactic interstellar medium (ISM). We discuss an OH spectral feature known as the ‘flip’, in which the satellite lines at 1612 and 1720 MHz flip – one from emission to absorption and the other the reverse – across a closely blended double feature. We highlight 30 examples of the flip from the literature, 27 of which exhibit the same orientation with respect to velocity: the 1720-MHz line is seen in emission at more negative velocities. These same examples are also observed towards bright background continuum, many (perhaps all) show stimulated emission, and 23 of these are coincident in on-sky position and velocity with H ii radio recombination lines. To explain these remarkable correlations, we propose that the 1720-MHz stimulated emission originates in heated and compressed post-shock gas expanding away from a central H ii region, which collides with cooler and more diffuse gas hosting the 1612-MHz stimulated emission. The foreground gas dominates the spectrum due to the bright central continuum; hence, the expanding post-shock gas is blue-shifted relative to the stationary pre-shock gas. We employ non-local thermodynamic equilibrium (LTE) excitation modelling to examine this scenario and find that indeed FIR emission from warm dust adjacent to the H ii region radiatively pumps the 1612-MHz line in the diffuse, cool gas ahead of the expanding shock front, while collisional pumping in the warm, dense shocked gas inverts the 1720-MHz line.

SERS background imaging – A versatile tool towards more reliable SERS analytics

Surface-enhanced Raman scattering (SERS) is a highly selective and sensitive straightforward analytical method, which is however not yet established in routine analysis due to a lack of reliability and reproducibility. To address this limitation, we show the distinct correlation of the ever-present but often neglected broad SERS background continuum with the SERS signal intensity of the analyte and how to exploit this correlation for an easy-to-handle, automatable and more reliable SERS measurement. First, fast and high-contrast imaging of the SERS substrate is performed for hot spot localisation utilizing the SERS background. Subsequently, highly enhanced SERS spectra are recorded at the centre of these spots. Furthermore, we correlate our SERS background imaging with other optical imaging modalities and electron microscopy to assess structure-property relationships. Monte Carlo simulations based on actual measurements illustrate the sampling error of a conventional SERS experiment and the advantages our method provides.

Understanding and modeling the scaling spectrum of climate

<p>Modeling climate dynamics in a comprehensive way and improving its predictability in a warming world requires a better understanding of climate variability across scales. However, fundamental mechanisms governing variability on long timescales are still poorly understood. <br>The temporal evolution of climate can be inferred from paleoclimate records, such as ice cores or marine sediments. Power spectra serve to quantify changes of variability over time and to identify timescales associated with periodic or quasi-periodic processes. The spectra of surface temperature not only comprise spectral peaks, but also reveal a continuous part. It was shown that the background continuum exhibits a scale break, following different power-laws on monthly to decadal versus millennial to longer periods. It is yet mostly unexplained, how these power-laws arise and whether a coupling between different timescales can be deduced from it. We study these questions by comparing and applying spectral analyses to paleoclimate records and climate model simulations for the Quaternary. The data is used to reconstruct the temperature spectrum on diurnal to astronomical timescales. We extend previous studies by including climate responses, such as δ<sup>18</sup>O and temperature records, and climate forcings, for example, insolation and volcanic forcing. The emergence of scaling in temperature variability is analyzed by successively accessing the background continuum. Higher order spectra test for correlations between forcings and responses. In particular, the bispectrum and bicoherence is computed for statistical processes and evaluated for temperature records in order to study whether the scaling properties are related to energy transfers between different states in time. We elaborate the potential of these methods to reveal dynamical processes governing the continuous spectrum of surface temperature.</p>

Imaging of polar cap patches with a low-cost airglow camera: pilot observations in Svalbard, Norway

We evaluate the capability of a low-cost all-sky imager (ASI), which has been operative in Longyearbyen (78.1° N, 15.5° E), Norway, to detect 630.0 nm airglow signatures of polar cap patches. The ASI is composed of a small camera, with a charge-coupled device (CCD), manufactured by Watec Co. Ltd., a fish-eye lens, and an optical filter whose central wavelength is 632.0 nm and full-width half maximum (FWHM) is 10 nm. In Longyearbyen, another ASI equipped with a cooled electron-multiplying charge-coupled device (EMCCD) camera has been operative for observations of polar cap patches. We compare the images from the two systems and investigate the performance of the low-cost ASI. On the night of December 4, 2013, a series of polar cap patches were observed by the EMCCD ASI. The low-cost ASI also detected regions of enhanced 630.0 nm airglow passing through the fields-of-view. The quality of the raw images from the low-cost ASI obtained every 4 s were visibly much worse than that of the EMCCD ASI. However, an integration of 7–15 consecutive images made it possible to capture the temporal evolution and spatial structure of the patches, for example, their anti-sunward propagation and finger-like structures along the trailing edge. The estimated values of the absolute optical intensity from the low-cost ASI were found to be consistent with those from the EMCCD ASI, whereby the offset was < 100 R. This offset can be explained by the contribution of the background continuum emission to the low-cost ASI images, because the band width of the optical filter used for the low-cost ASI is ~ 3 times wider than that used for the EMCCD ASI. The results indicate that the airglow measurement with the low-cost ASI is feasible even for quantitative studies of F-region phenomena such as the dynamics of polar cap patches.

A degenerate polar lattice for cloaking in full two-dimensional elastodynamics and statics

A lattice design of a cloak for full two-dimensional elasticity is suggested when the background continuum is isotropic with Lamé parameters (λ, μ ) satisfying μ ≤ λ. The lattice is polar in the sense that it elastically resists rotations; and is degenerate meaning it admits a stressless collapse mechanism. These characteristics are attained through the use of appropriately distributed restoring torques in conjunction with hinge-like spring-mass contacts. Thus, the lattice is proven to exhibit a rank-3 elasticity tensor lacking the minor symmetries. Accordingly, it rigorously adheres to the form-invariance requirements of the transformation method under the Brun–Guenneau–Movchan gauge. The cloak is numerically tested in statics and in dynamics under pressure and shear incident waves and shows satisfactory performance. Finally, a theoretical generalization extends the design to three dimensions and to arbitrarily anisotropic backgrounds so as to enable cloaking as well as other transformation-based static and dynamic field manipulation techniques in these cases.

Agency and Co-production: A Multimodal Perspective

Building on multimodal (inter)action analysis as a theoretical and methodological framework, this article introduces and develops the theoretical/methodological tool called primary agency. Taking the mediated action as a unit of analysis, agency can be analysed as a feature of action. However, there is a lack of empirical approaches for the study of agency, and an overemphasis on language as the most important site for identifying agentive action. I develop primary agency through an analysis of three co-produced higher-level actions from a research project into high school tutoring. These are the higher-level actions of conducting research, tutoring and reading a text. Applying co-production and the modal density foreground/background continuum I explore how the researcher, the tutor and the student co-produce these higher-level actions. Through this analysis, I identify the most significant mediational means for each higher-level action, and the social actor with ownership or agency over these mediational means. I define this social actor as the one with primary agency over the co-produced higher-level action. Finally, my analysis outlines the implications of primary agency for co-produced higher-level actions, including the role of the researcher, the attention/awareness participants pay to overarching research projects, and links between primary agency and successful learning.

ENSO-Like Variability: 1900–2013*

ENSO-like variability is examined using a set of univariate indices based on unfiltered monthly global sea surface temperature (SST), sea level pressure (SLP), outgoing longwave radiation (OLR), sea level, and the three-dimensional ocean temperature (OT) fields. These indices, many of which correspond to the leading principal components (PCs) of the respective global fields, are highly correlated with each other. In combination with their spatial regression patterns, they provide a comprehensive description of ENSO-like variability in the atmosphere and ocean across time scales ranging from months to decades, from 1950 onward. The SLP and SST indices are highly correlated with one another back to the late nineteenth century. The interdecadal-scale shifts in the prevailing polarity of ENSO that occurred in the 1940s, the 1970s, and around the year 2000 are clearly evident in low-pass-filtered time series of these indices. On the basis of empirical mode decomposition, ENSO-like variability is partitioned into an interannual “ENSO cycle,” to which equatorial ocean wave dynamics imparts a distinctive equatorial signature, and a red noise background continuum, most prominent on the interdecadal time scale, which resembles the ENSO-like variability in some models in which the atmosphere is coupled to a slab ocean. The background continuum bears the imprint of the Pacific–North American (PNA) pattern, the leading mode of the Northern Hemisphere wintertime variability of the atmospheric circulation over the Pacific sector. The superposition of the ENSO cycle and the background continuum imparts a distinctive frequency dependence to the patterns of ENSO-like climate variability.

Test spectra experimental construction for evaluating gamma-spectrometry computer codes for the 235U determination

The determination of 235U in environmental samples from its 185.72 keV photons may require the deconvolution of the multiplet photopeak at ~186 keV, due to the co-existence of the 186.25 keV photons of 226Ra in the spectrum. Successful deconvolution depends on many parameters, such as the detector characteristics, the activity concentration of the 235U and 226Ra in the sample, the background continuum in the 186 keV energy region and the gamma-spectrometry computer code used. In this work two sets of experimental test spectra were constructed for examining the deconvolution of the multiplet photopeak performed by different codes. For the construction of the test spectra, a high-resolution low energy germanium detector was used. The first series consists of 140 spectra and simulates environmental samples containing various activity concentration levels of 235U and 226Ra. The second series consists of 280 spectra and has been derived by adding 137Cs, corresponding to various activity concentration levels, to specific first series test spectra. As the 137Cs backscatter edge is detected in the energy region of the multiplet photopeak at ~186 keV, this second series of test spectra tests the analysis of the multiplet photopeak in high background continuum conditions. The analysis of the test spectra is performed by two different g-spectrometry analysis codes: (a) spectrum unix analysis code, a computer code developed in-house and (b) analysis of germanium detector spectra, a program freely available from the IAEA. The results obtained by the two programs are compared in terms of photopeak detection and photopeak area determination.