X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT-->3MLCT-->5T cascade from the initially excited state. The time scale of the 3MLCT-->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

Multiferroics have attracted strong interest for potential applications where electric fields control magnetic order. The ultimate speed of control via magnetoelectric coupling, however, remains largely unexplored. Here, we report an experiment in which we drove spin dynamics in multiferroic TbMnO3 with an intense few-cycle terahertz (THz) light pulse tuned to resonance with an electromagnon, an electric-dipole active spin excitation. We observed the resulting spin motion using time-resolved resonant soft x-ray diffraction. Our results show that it is possible to directly manipulate atomic-scale magnetic structures with the electric field of light on a sub-picosecond time scale.

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum 1, 2 . Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation 3-5 . The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these femtosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a majority of the angular momentum lost from the spin system on the laser-induced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system. 2Broadly speaking, proposed mechanisms for ultrafast demagnetisation fall into two categories: spin-flip scattering mechanisms and spin transport mechanisms. The first category explains the demagnetisation process as a sudden increase in scattering processes that ultimately result in a decrease of spin order. These scattering processes can include electron-electron, electron-phonon, electron-magnon and even direct spin-light interactions. On average, such scattering must necessarily involve a transfer of angular momentum from the electronic spins to some other subsystem(s). Candidates include the lattice, the electromagnetic field, and the orbital angular momentum of the electrons. Numerical estimates and experiments using circularly polarised light strongly suggest that the amount of angular momentum given to the electromagnetic field interaction is negligible 6 , and experiments using femtosecond x-ray magnetic dichroism (XMCD) indicate that the angular momentum of both electronic spins and orbitals decrease in magnitude nearly simultaneously 7-9 . The only remaining possibility for a spin-flip induced change in angular momentum therefore appears to be a transfer to the lattice via spin-orbit coupling, but this remains to be experimentally verified.The second category of proposed mechanisms relies on the idea that laser excitation causes a ...