The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here we gain experimental insights, beyond the Born–Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy)₃]²⁺ compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

The characterization of carbon nitride films with stoichiometry ${\mathrm{C}}_3{\mathrm{N}}_4$ is heavily restricted by the problem of getting pure crystalline samples with the right C/N ratio. However, thin films with lower nitrogen concentration (5–25 %) have been found relatively easier to deposit, for example, with reactive magnetron sputtering. It is also in this range of nitrogen content that the recently discovered “graphiticlike $\to$ fullerenelike” phase transition has been suggested to take place. Therefore, in order to add more information to the above experimental evidence, it is important to use theoretical methods to obtain further characterization of carbon nitride models with a high C/N ratio such as that of ${\mathrm{C}}_{11}{\mathrm{N}}_4.$ It is relevant to propose a cross checking on the role played by the nitrogen concentration in determining the stability, hardness, and electronic properties of ${\mathrm{CN}}_x$ compounds with different stoichiometries. For the sake of simplicity we have here compared the ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{C}}_{11}{\mathrm{N}}_4$ systems, which are isoelectronic to each other. For this purpose two ${\mathrm{C}}_{11}{\mathrm{N}}_4$ phases, namely, $\alpha$ and $\beta ,$ are presented and investigated with density-functional-theory methods within the local density approximation. These phases contain less than $\sim 30\%$ of nitrogen than the well-known ${\mathrm{C}}_3{\mathrm{N}}_4$ and are formally derived from the so-called pseudocubic ${\mathrm{C}}_3{\mathrm{N}}_4.$ Cohesive properties, heats of formation, bulk and elastic moduli have been calculated and a full detailed analysis of the density-of-states and energy-loss-near-edge-structure spectra is presented. We propose that the lowering of the nitrogen concentration does not prevent the finding of new ultrahard materials and indeed brings a significant increase in the cohesive energy of carbon nitrides. However, the computed enthalpies of formation have shown values that are positive and generally larger than the analog carbon-deficient phases.

The electronic and magnetic structure of the antiperovskite-structure transition-metal nitrides ${\mathrm{NiFe}}_3$N and ${\mathrm{PdFe}}_3$N are calculated employing the augmented-spherical-wave method. From these calculations the binding character is found to be mainly covalent for the nitrogen-iron bonds and metallic between the iron and nickel atoms, as stated in the earlier literature. The magnetic structure exhibits itinerant moments for the iron atoms and more localized moments at the Ni (Pd) atoms. Effects of external pressure are studied by varying the lattice constant. Such calculations yield the pressure dependence of the magnetic moments and the hyperfine fields. The total energy is computed as a function of magnetic moment and volume, yielding total-energy surfaces, that provide the basis for a spin-fluctuation model at finite temperatures. The possibility of an Invar-like behavior is found.