Reflectance spectroscopy: critical remarks on the evaluation of diffuse reflectance spectra

Die Reflexionsspektroskopie wird normalerweise zur Bestimmung der Eigenschaften von Oberflächen wie Farbe oder Anteile von diffuser und direkter Reflexion verwen-det. Darüber hinaus können pulverförmige Materialien, die häufig in der Katalyse verwendet werden, in vielen Fällen nur in diffuser Reflexion vermessen werden. In der Literatur geben verschiedene Autoren die unterschiedlichsten Verfahren an, mit denen ein Quasi – Absorptionsspektrum aus der Reflexionsmessung erzeugt werden kann, z.B. durch die Berechnung von optischer Dichte, 1 – R oder F(R). Diese Arbeit befaßt sich mit einigen grundsätzlichen Fragestellungen, die bei solchen Reflexionsmessungen auftreten können: • Welche Informationen sind im Reflexionsspektrum enthalten? • Welche Darstellung der y-Achse kommt einer Absorptionsmessung am nächsten? • Sind Verdünnungsreihen, wie sie in der Transmissionsspektroskopie oft ver-wendet werden, auch in der Reflexionsspektroskopie sinnvoll? Um den Zusammenhang zwischen Absorptions- und Reflexionsspektren zu klären, sind Holmiumoxidfiltergläser zunächt in Transmission bzw. Absorption ver-messen worden; danach wurden die Filtergläser gemörsert, verschiedene Korngrös-sen ausgesiebt und anschließend sowohl unverdünnt wie auch verdünnt mit SiO2-Pulver Reflexionsspektren aufgenommen. Ergebnisse und Schlußfolgerungen: • Die Reflexionsspektren ändern sich in Abhängigkeit von der Korngröße in ähnlicher Weise wie sich Transmissionsspektren in Abhängikeit von der Schichtdicke ändern. • Die Darstellung als F(R)–Spektrum ohne irgendwelche Einschränkungen kommt in ihren Charakteristika (Maxima etc.) dem Absorptionsspektrum am nächsten. • Verdünnungen bringen keine Verbesserung der spektroskopischen Charakte-ristika, sondern nur eine Schwächung des Signalanteils des Probenmaterials

A newly developed attachment to the Lambda 9 spectrometer for in situ reflectance measurements of catalysts at temperatures up to 623 K

A newly developed attachment to the Lambda 9 spectrometer for in situ reflectance measurements of catalysts at temperatures up to 623 K M.Thiede and J.Melsheimer Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin, Germany Abstract The spectroscopic investigation of catalyst powders under reaction conditions is generally possible only in reflectance. Two problems occur: 1. Low signal level Normally the sample is positioned directly at the integrating sphere and all diffusely scattered light from the sample is reflected back into the integrating sphere. A hot reactor cell cannot be attached directly, it must be positioned as far as possible from the integrating sphere to avoid its heating. If the reactor cell is attached at some distance from the measurement window of the integrating sphere only a portion of the diffusely scattered light will be reflected back into the sphere. At a distance of 12 mm, for example, this will only be 20%. In a first setup we bridged the distance with highly reflecting ceramics, to increase the part of light reflected into the sphere (Fig.1). 2. Thermal radiation The thermal radiation of the hot reactor cell also enters the integrating sphere.At higher temperatures it is significantly stronger than the measurement light from the spectrometer itself. This leads to increased noise and saturation of the detector. In the newly developed setup we tried a. to make the distance between integrating sphere and reactor cell with oven as large as possible; b. to increase the signal level c. to decrease the surface which causes the thermal radiation, and d. to bring reactor cell and oven into a vertical position which facilitates the work with powder samples. These requirements could be fulfilled by the application of a specially formed light conductor made of quartz, in which the light is conducted to the sample and back into the integrating sphere by total reflectance. At the same time, the surface which causes the thermal radiation was reduced (Fig.2). In this way, we succeeded in improving the signal-to-noise ratio by a factor of 4-5 compared with the first setup

Pyrene-DNA Conjugates: Influence of sticky Ends on the Supramolecular Self-Assembly

The supramolecular assembly of DNA conjugates has caught attention in supramolecular chemistry. DNA bears some unique features enabling the design of complex nanostructures. The DNA framework offers a defined spatial arrangement of modifications. In preceding work DNA was modified with phenanthrene at the 3’-ends of DNA forming vesicular supramolecular assemblies with unique light-harvesting properties.[1] Interestingly, the modification of DNA with E-tetraphenylethylenes sticky ends led to aggregation-induced emission (AIE) active assemblies.[2] In this work, we modified a DNA strand at the 3’- and 5’-end with 1,6-pyrene (Figure 1A). We varied the length of the sticky ends to compare their self-assembly properties (1, 2, and 3 pyrene units on either side). For example, AFM measurements of the 1,6-pyrene-DNA conjugates with a total of 6 pyrene units revealed their self-assembly into vesicles (Figure 1B). In addition to AFM studies, fluorescence and UV-vis spectroscopy measurements will be presented and discussed

Rôle of the Jahn-Teller Coupling in the Luminescence Spectra of Fe²⁺ in Zinc-Blende Compounds

The luminescence spectra of Fe2+ in zinc-blende-type II—VI and III—V compounds do not show an equally spaced set of emission lines as predicted by spin orbit interaction in a plain crystalline field. The unequal separation between these lines is a signature of the Jahn-Teller effect in these systems. Attention is focused here on the general trend of the 5 E-derived energy levels providing the end states for the emission transitions. The intervall between the second and third energy levels (γ4 and γ3) is employed as a sensitive test based on the two following characteristics: First, this is the spacing that varies the most; second, the emissions to these levels are usually quite sharp as they involve energies not overlapping with phonon-assisted transitions. This property is studied in the plane [hω,EJT] (energy of the coupling phonon and the Jahn-Teller energy which is directly related to the coupling strength). The general behaviour is then studied under different theoretical conditions, in particular those that maximize the effect. Application of this theory to each real compound is thus possible by choosing the right combination of the two variables. To this end, the examples of luminescent substitutional Fe2+ ions in ZnS, ZnTe, and CdTe are discussed based on published spectra. The main emphasis is placed on new precise measurements of the ZnSe : Fe2+ emission. With crystals containing different iron concentrations, changing line shapes, including self-inversion of several emission lines, have been obtained in the 2600 to 2800 cm-1 spectral range. The properties of the four host/impurity systems are satisfactorily explained while an overall description emerges for the whole family of these compounds from a compilation of the derived coupling parameters

Rethinking the economic costs of malaria at the household level: Evidence from applying a new analytical framework in rural Kenya

BACKGROUND: Malaria imposes significant costs on households and the poor are disproportionately affected. However, cost data are often from quantitative surveys with a fixed recall period. They do not capture costs that unfold slowly over time, or seasonal variations. Few studies investigate the different pathways through which malaria contributes towards poverty. In this paper, a framework indicating the complex links between malaria, poverty and vulnerability at the household level is developed and applied using data from rural Kenya. METHODS: Cross-sectional surveys in a wet and dry season provide data on treatment-seeking, cost-burdens and coping strategies (n = 294 and n = 285 households respectively). 15 case study households purposively selected from the survey and followed for one year provide in-depth qualitative information on the links between malaria, vulnerability and poverty. RESULTS: Mean direct cost burdens were 7.1% and 5.9% of total household expenditure in the wet and dry seasons respectively. Case study data revealed no clear relationship between cost burdens and vulnerability status at the end of the year. Most important was household vulnerability status at the outset. Households reporting major malaria episodes and other shocks prior to the study descended further into poverty over the year. Wealthier households were better able to cope. CONCLUSION: The impacts of malaria on household economic status unfold slowly over time. Coping strategies adopted can have negative implications, influencing household ability to withstand malaria and other contingencies in future. To protect the poor and vulnerable, malaria control policies need to be integrated into development and poverty reduction programmes

Curiosity in exploring chemical spaces: Intrinsic rewards for deep molecular reinforcement learning

Computer-aided design of molecules has the potential to disrupt the field of drug and material discovery. Machine learning, and deep learning, in particular, have been topics where the field has been developing at a rapid pace. Reinforcement learning is a particularly promising approach since it allows for molecular design without prior knowledge. However, the search space is vast and efficient exploration is desirable when using reinforcement learning agents. In this study, we propose an algorithm to aid efficient exploration. The algorithm is inspired by a concept known in the literature as curiosity. We show on three benchmarks that a curious agent finds better performing molecules. This indicates an exciting new research direction for reinforcement learning agents that can explore the chemical space out of their own motivation. This has the potential to eventually lead to unexpected new molecules that no human has thought about so far