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[en] Following atomic photoionization, the abrupt change in potential can lead to secondary ionization of an outer-shell electron in a phenomenon known as shake-off, a process which gives rise to the asymmetric Kα profile and satellite lines. Investigation of chemical effects and relativistic quantum mechanics requires a theoretical determination of these satellite intensities; however, existing theoretical predictions are inconsistent with experimental results by up to an order of magnitude. Previously theoretical modeling required up to 12 fitting parameters to account for transition widths, energy corrections, spectator intensities, and spectator broadening. Using a multiconfiguration atomic model to account for electron-electron correlation, we provide here the first ab initio calculations of shake-off probabilities which are in agreement with experimental results (except for copper), an important step toward a complete theoretical profile.
[en] The copper Kα photoemission spectra is one of the most widely studied. Recent Dirac-Fock calculations have produced transition energies in good agreement with experiment, though they have relied on approximations that may not be transferable to other complex atoms in which uncertainties in theoretical results are dominated by poor convergence. Through a detailed examination of convergence issues in the copper spectrum, we consider the accuracy obtainable with the multiconfiguration Dirac-Fock (MCDF) method, provide the first determination of fine structure contributions to the spectrum, and demonstrate reliable techniques for modeling spectator states with vacancies in the 3p, 3d, and 4s shells.
[en] High resolution x-ray spectroscopy has revealed a complex structure in the spectrum of core-ionized elements. To date, theoretical reproductions must be fitted to experimental results using fitting parameters to account for transition widths, energy corrections, spectator intensities and spectator broadening - up to 12 or more parameters depending on complexity. We provide here the first accurate reconstruction of the Kα spectra in titanium using only instrumental broadening widths as free parameters. We also determine structural systematics in observed shake processes in transition metals for the first time.
[en] The 557.7 nm green line and the 297.2 nm ultraviolet line in oxygen have been studied extensively due to their importance in astrophysics and atmospheric science. Despite the enormous effort devoted to these two prominent transition lines over 30 years, and in fact going back to 1934, the ratio of their transition probabilities remains a subject of major discrepancies amongst various theoretical calculations for many decades. Moreover, theoretical results are inconsistent with available laboratory results, as well as recent spacecraft measurements of Earth's airglow. This work presents new relativistic theoretical calculations of the transition probabilities of these two photoemission lines from neutral oxygen using the multi-configuration Dirac-Hartree-Fock method. Our calculations were performed in both length and velocity gauges in order to check for accuracy and consistency, with agreement to 8%. Whilst remaining a challenging computation, these results directly bear upon interpretations of plasma processes and ionization regimes in the universe.
[en] We investigate the characteristic radiation and the complex asymmetric structure of photoemission lines of copper, which provides a benchmark for theoretical and experimental studies of x-ray calibration series in transition metals. Ab initio multi-configuration Dirac–Hartree–Fock (MCDHF) calculations have been performed to study the complex open-shell many-electron problem in copper. The biorthogonalization technique permits determination of transition intensities and Einstein A coefficients. The results from our MCDHF calculations demonstrate excellent convergence in transition energies and intensities, as well as gauge invariance to 0.6%. Shake processes caused by single and double spectator vacancies from 3d, 3p, 3s and 4s subshells have also been investigated extensively. MCDHF has been performed to calculate energies and relative intensities of 3d, 3d2, 3p, 3s and 4s satellites, resulting in the total number of configuration states exceeding 100 000 and more than 1500 transition components. Our theoretical calculations of shake-off probabilities using the multi-configuration method in the sudden limit have a high degree of internal consistency with the best available experimental data for copper . This supports the validity of relativistic atomic theory and sets a new benchmark even for poorly resolved characteristic spectra using current techniques of analysis. (paper)
[en] Climate models provide compelling evidence that if greenhouse gas emissions continue at present rates, then key global temperature thresholds (such as the European Union limit of two degrees of warming since pre-industrial times) are very likely to be crossed in the next few decades. However, there is relatively little attention paid to whether, should a dangerous temperature level be exceeded, it is feasible for the global temperature to then return to safer levels in a usefully short time. We focus on the timescales needed to reduce atmospheric greenhouse gases and associated temperatures back below potentially dangerous thresholds, using a state-of-the-art general circulation model. This analysis is extended with a simple climate model to provide uncertainty bounds. We find that even for very large reductions in emissions, temperature reduction is likely to occur at a low rate. Policy-makers need to consider such very long recovery timescales implicit in the Earth system when formulating future emission pathways that have the potential to 'overshoot' particular atmospheric concentrations of greenhouse gases and, more importantly, related temperature levels that might be considered dangerous.
[en] Natural carbon sinks currently absorb approximately half of the anthropogenic CO2 emitted by fossil fuel burning, cement production and land-use change. However, this airborne fraction may change in the future depending on the emissions scenario. An important issue in developing carbon budgets to achieve climate stabilisation targets is the behaviour of natural carbon sinks, particularly under low emissions mitigation scenarios as required to meet the goals of the Paris Agreement. A key requirement for low carbon pathways is to quantify the effectiveness of negative emissions technologies which will be strongly affected by carbon cycle feedbacks. Here we find that Earth system models suggest significant weakening, even potential reversal, of the ocean and land sinks under future low emission scenarios. For the RCP2.6 concentration pathway, models project land and ocean sinks to weaken to 0.8 ± 0.9 and 1.1 ± 0.3 GtC yr−1 respectively for the second half of the 21st century and to −0.4 ± 0.4 and 0.1 ± 0.2 GtC yr−1 respectively for the second half of the 23rd century. Weakening of natural carbon sinks will hinder the effectiveness of negative emissions technologies and therefore increase their required deployment to achieve a given climate stabilisation target. We introduce a new metric, the perturbation airborne fraction, to measure and assess the effectiveness of negative emissions. (letter)