frontier-orbital interactions with atom specificity. We anticipate that the method will be broadly applicable in the chemical sciences, and complement approaches that probe structural dynamics in ultrafast processes.In our experimental set-up (Figure 1a), the valence electronic structure of Fe(CO) 5 is probed with femtosecond-resolution resonant inelastic x-ray scattering (RIXS) at the Fe L 3 -edge (Fe experiments. This triplet arises from a singlet state with a time constant of 300 fs, consolidating the notion 6 that sub-ps intersystem crossing appears to be common in the excited-state dynamics of transition-metal complexes 7,[22][23][24] . The persistence of the triplet Fe(CO) 4 ( 3 B 2 ) up to our maximum time delay of 3 ps is consistent with it undergoing a slow, spin-forbidden reaction with intersystem crossing to a solvent-complexed singlet state on the 50-100 ps time scale 4,5, 25 . However, the observed branching on a sub-ps time scale into the competing and simultaneous reaction channels of spin crossover and ligation to form coordinatively saturated species introduces an efficient pathway circumventing this spin barrier. It also supports the idea that the high density of electronic excited states and the relatively large amount of excess energy available in the system determine the course of the excited-state dynamics, rather than spin selection rules alone 5,6 . Fast ligation could be facilitated along the singlet pathway, confirming the general notion that solvent-stabilized metal centers form fast 3, 4, 11 and consistent with the observation of unsaturated carbonyl Cr(CO) 5 forming a solvent complex in alcohol solution within 1.6 ps 26 . An alternative proposal 20 for Fe(CO) 5 involves concerted exchange of CO and EtOH on the time scale of ligand dissociation of 100-150 fs. This would also proceed along a singlet pathway and in agreement with our results, as the temporal resolution of our measurements is not sufficient to distinguish between this concerted and the alternative sequential process. Revealing in detail 8 the influence of solvent-solute interactions will have to be the subject of future studies, which could also explore whether the structure of the solute prior to dissociation 20 influences the excited-state branching ratio between the different pathways.We find that the ligation capability of Fe(CO) 4 is mostly determined by its d σ * LUMO, which receives σ donation from occupied CO or ethanol ligand orbitals. Population of the antibonding d σ * orbital in excited singlet ( 1 B 2 ) and triplet ( 3 B 2 ) Fe(CO) 4 impedes σ donation from ligands (see sketches in Figure 3), explaining the inertness of these species against ligation; this problem is absent in the ligation channel that produces coordinately saturated species. Establishing this correlation of orbital symmetry with spin multiplicity and reactivity 27 is enabled by the atom specificity with which x-ray laser based femtosecondresolution spectroscopy can explore frontier-orbital interactions. This ability gives unique access t...

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs 2 AgBiBr 6 , in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs 2 AgBiBr 6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO 2 . Femtosecond UV−vis−NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron−phonon coupling via Froḧlich interactions is observed in terms of pronounced coherent oscillation of a strong A 1g optical phonon mode of the double perovskite at 177 cm −1 . Similar behavior is also found for the related vacancy-ordered perovskite Cs 3 Bi 2 Br 9 and the parent compound BiBr 3 . Excitonic effects and electron−phonon coupling are known to induce unwanted electron−hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.

The so-called S* state has been suggested to play an important role in the photophysics of beta-carotene and other carotenoids in solution and photosynthetic light-harvesting complexes, yet its origin has remained elusive. The present experiments employing temperature-dependent steady-state absorption spectroscopy and ultrafast pump-supercontinuum probe (PSCP) transient absorption measurements of beta-carotene in solution demonstrate that the spectral features of S* are due to vibrationally excited molecules in the ground electronic state S(0). Characteristic spectral signatures, such as a highly structured bleach below 500 nm and absorption in the range 500-660 nm result from the superposition of hot S(0) absorption ("S(0)*") on top of the ground-state bleach of room-temperature molecules. Appearance and disappearance of the S(0)* molecules can be completely described by a global kinetic analysis employing time-dependent species-associated spectra without the need to invoke the population of an intermediate electronically excited state.