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International audience; Recent chlorophyll-a fluorescence yield measurements, using single-turnover saturating flashes (STSFs), have revealed the involvement of a rate-limiting step in the reactions following the charge separation induced by the first flash (Magyar et al. 2018). As also shown here, in diuron-inhibited PSII core complexes isolated from Thermosynechococcus vulcanus the fluorescence maximum could only be reached by a train of STSFs. In order to elucidate the origin of the fluorescence yield increments in STSF series, we performed transient absorption measurements at 819 nm, reflecting the photooxidation and re-reduction kinetics of the primary electron donor P680. Upon single flash excitation of the dark-adapted sample, the decay kinetics could be described with lifetimes of 17 ns (~50%) and 167 ns (~30%), and a longer-lived component (~20%). This kinetics are attributed to re-reduction of P680•+ by the donor side of PSII. In contrast, upon second-flash (with Δt between 5 μs and 100 ms) or repetitive excitation, the 819 nm absorption changes decayed with lifetimes of about 2 ns (~60%) and 10 ns (~30%), attributed to recombination of the primary radical pair P680•+ Pheo•- , and a small longer-lived component (~10%). These data confirm that only the first STSF is capable of generating stable charge separation - leading to the reduction of QA ; and thus, the fluorescence yield increments elicited by the consecutive flashes must have a different physical origin. Our double-flash experiments indicate that the rate-limiting steps, detected by chlorophyll-a fluorescence, are not correlated with the turnover of P680. This article is protected by copyright. All rights reserved.

In benzophenone, intersystem crossing occurs efficiently between the S-1(n pi(star)) state and the T-1 state of dominant n pi(star) character, leading to excited triplet states after photoexcitation. The transition mechanism between S-1(n pi(star)) and T-1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S-1 -> T-1 and indirect S-1 -> T-2(pi pi(star)) -> T-1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T-1 state has a mixed n pi(star)/pi pi(star) character and that the T-2(pi pi(star)) state acts as an intermediate state between the S-1 and T-1 states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloise et al., J. Phys. Chem. A, 2008, 112, 224-231].

Soil carbon dioxide (CO2) efflux is an important component of the carbon (C) cycle but the biological and physical processes involved in soil CO2 production and transport are not fully understood. To improve our knowledge, we present a new approach to measure simultaneously soil CO2 concentrations and efflux, and their respective isotopic signatures (C-13-CO2). To quantify soil air (CO2)-C-13 and (CO2)-C-12 concentrations, we adapted a method based on CO2 diffusion from soil pores into tubes with a highly gas-permeable membrane wall. These tubes were placed horizontally at different depths in the soil. Air was sampled automatically from the tubes and injected through a diluting system into a tuneable diode laser absorption spectrometer. The CO2 and C-13-CO2 vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for C-13-CO2. Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO2 concentrations and C-13-CO2 were not affected significantly by the measurement system. While C-13-CO2 in air-filled pores below 5cm was constant over 3days, we observed large diurnal variations in C-13-CO2 efflux. However, the average difference between the two measurements was close to -4.4 parts per thousand, which supports steady-state diffusion over this 3-day period. This new method seems to be a very effective way to measure the C-13-CO2 profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the C-13-CO2 of the soil CO2 efflux on a daily timescale while advection may account for within-day variations.

International audience; The interaction of acetic acid (AA, CH3COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H2O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H2O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H2O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.

We report X-ray emission spectra of Fe(III), Fe(II), and Co(II) spin-crossover compounds in their high-spin and low-spin forms. It is shown that all X-ray emission features are sensitive to the spin state. Variations of the Kbeta and the Kalpha emission line shapes, which are in agreement with theory, can be used as quantitative probes of the spin state; it is suggested that with appropriate reference experiments one can extract the spin momentum for a general case. Resonant X-ray emission spectra unveil details of the redistribution of electrons on the 3d levels associated with the spin-state change by revealing features at the X-ray absorption preedge not accessible through standard absorption measurements.

Novel x-ray spectroscopies such as high-resolution x-ray absorption (XAS) performed in the partial fluorescence yield mode (PFY), and resonant emission (RXES) offer new opportunities to probe the bulk electronic configuration in strongly correlated systems. We performed an Yb L-3 PFY-XAS and RXES investigation of the intermediate valence compound YbAl2 under external pressure. We found that the Yb valence increases from 2.25 at ambient pressure to 2.9 at 385 kbar. This experiment has extended the range of pressures explored until now, in order to cover the whole valence transition, which we find to be completed around 200 kbar. Moreover, the new data considerably reduce the experimental error bars on previous estimates and provide the most accurate description to date of this transition. We discuss the relative merits of various approaches to extract the 4f occupancy from the spectroscopic data.

Two experimental coherent Raman techniques, stimulated Raman and coherent anti-Stokes Raman spectroscopy (CARS), were employed to study the Raman spectrum of CF 4 in the 900 cm −1 region. The stimulated Raman and CARS spectra of the ν 1 band of CF 4 were recorded at high resolution. The analysis of these spectra enabled some spectroscopic constants of this band to be refined and higher order constants to be determined for the first time. The first high-resolution CARS spectrum of the 2ν 2 (A 1 ) harmonic band is also presented. The rotational transitions of this band up to J = 40 were assigned and the first set of spectroscopic constants for this sub-level were determined. These results improve our knowledge about the Raman-allowed vibrational bands of CF 4