RESUMEN
In this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced CT polaron involves local electronic and structural reorganizations. Femtosecond X-ray and optical absorption spectroscopies show that the local structural trapping process occurs on similar timescale for particles with 11 nm and 70 nm sizes. The local photoinduced spin transition, elongating the Co-N bonds and driving the CoIIIFeII â CoIIFeIII CT, activates coherent lattice torsion modes. The elastic deformation waves, launched by these bond elongations, drive macroscopic volume expansion and breathing of the particles. The timescale of this macroscopic deformation depends strongly on the size of the particle, which is more evidence of the multiscale nature of photoinduced phenomena in molecular materials.
RESUMEN
One of the main challenges in ultrafast material science is to trigger phase transitions with short pulses of light. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a coherent macroscopic transformation pathway for the semiconducting-to-metal transition in bistable Ti3O5 nanocrystals. Employing femtosecond powder X-ray diffraction, we measure the lattice deformation in the phase transition as a function of time. We monitor the early intra-cell distortion around the light absorbing metal dimer and the long range deformations governed by acoustic waves propagating from the laser-exposed Ti3O5 surface. We developed a simplified elastic model demonstrating that picosecond switching in nanocrystals happens concomitantly with the propagating acoustic wavefront, several decades faster than thermal processes governed by heat diffusion.
RESUMEN
We use femtosecond optical pump-probe spectroscopy to study the Light Induced Excited Spin State Trapping (LIESST) dynamics in an FeII spin-crossover material. In these systems, LIESST derives from fast molecular switching induced by light from low (LS, S = 0) to high spin (HS, S = 2) states, as reported for molecules in solution as well as in the solid state. Since the direct LS-to-HS conversion is forbidden by selection rules, the switching dynamics involves intermediate electronic states such as metal-to-ligand charge transfer (MLCT) or ligand-field excited states of singlet or triplet nature. In addition, the HS state is structurally trapped by the elongation of the metal-ligand bond, which is accompanied by the coherent activation and damping of the molecular breathing mode. The ultrafast LIESST dynamics was mainly investigated in FeN6 ligand field systems with almost octahedral symmetry, under MLCT excitation. Our recent study on the FeII(pap-5NO2)2 spin-crossover material, with a FeIIN4O2 ligand field of C2 symmetry, has shown that in addition to MLCT bands, optical excitation, through quite intense and low-energy shifted d-d bands, can also drive LIESST. Compared to MLCT, d-d excitation involves shorter-lived intermediates, drives faster LS-to-HS switching, and enhances the coherent structural dynamics. In this paper, we present an ultrafast study of the pump wavelength dependence of LIESST and we evidence a photoselective crossover from the MLCT to the d-d pathways.
RESUMEN
The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using â¼3 keV X-ray pulses.
RESUMEN
Light-induced excited spin state trapping (LIESST) in FeII spin-crossover systems is a process that involves the switching of molecules from low (LS, S = 0) to high spin (HS, S = 2) states. The direct LS-to-HS conversion is forbidden by selection rules, and LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in an HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process has been investigated in FeN6 ligand field systems, under MLCT excitation. Herein, we studied LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allowed for quite intense and low-energy shifted d-d bands. By combining ab initio DFT and TD-DFT calculations and fs optical absorption measurements, we demonstrated that shorter intermediates enhanced coherent structural dynamics, and d-d excitation induced faster LS-to-HS switching, compared to MLCT.
RESUMEN
Aperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature.
