Context. The infrared emission features observed in the mid-infrared wavelength range in astronomical objects, often called the Aromatic Infrared Bands, exhibit differences in shape and position. Three astrophysical spectral classes have been proposed based on the spectral characteristics. The band positions in most sources are similar to those of aromatic materials, however, the exact nature of the emitters is still unknown. Aims. The spectral diversity of the bands provides a clue to the nature of the materials. An evolutionary scenario for the nature of the emitters can be inferred by comparison with laboratory analogues. Methods. The laboratory spectra of a wide range of soot material samples were recorded and a global analysis of the infrared absorption spectra performed. This spectral analysis, allied to the band shape and position variations, were then used to interpret the diversity and evolution of the features in the astronomical spectra. Results. We find correlations between the spectral regions characteristic of the CC and CH modes and use these to shed light on the origin of the infrared emission features. In particular, the observed shift in the position of the 6.2-6.3 μm band is shown to be a key tracer of the evolution of the aliphatic to aromatic component of carbonaceous dust.

Development of low-cost and high-performance oxygen evolution reaction catalysts is keyto implementing polymer electrolyte membrane water electrolyzers for hydrogen production. Iridiumbased oxides are the state-of-the-art acidic oxygen evolution reactio catalysts but still suffer from inadequate activity and stability, and iridium's scarcity motivates the discovery of catalysts with lower iridium loadings. Here we report a mass-selected iridium-tantalum oxide catalyst prepared by a magnetron-based cluster source with considerably reduced noble-metal loadings beyond a commercial IrO2 catalyst. A sensitive electrochemistry/mass-spectrometry instrument coupled with isotope labelling was employed to investigate the oxygen production rate under dynamic operating conditions to account for the occurrence of side reactions and quantify the number of surface active sites. Iridium-tantalum oxide nanoparticles smaller than 2 nm exhibit a mass activity of 1.2 ± 0.5 kA g Ir -1 and a turnover frequency of 2.3 ± 0.9 s -1 at 320 mV overpotential, which are two and four times higher than those of mass-selected IrO2, respectively. Density functional theory calculations reveal that special iridium coordinations and the lowered aqueous decomposition free energy might be responsible for the enhanced performance.Water electrolysis (2H2O → 2H2 + O2) driven by renewable power sources (for example, solar and wind) offers a sustainable strategy to store energy in the form of hydrogen fuel 1,2 . The polymer electrolyte membrane water electrolyzer (PEM-WE) operating in acidic media serves as a promising technology for such energy conversion and is preferable to alkaline conditions for hydrogen production because of its high current density, fast response, stable operation performance and low cross-over under pressurized

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