Many of the unusual properties of liquid water are attributed to its unique structure, comprised of a random and fluctuating three-dimensional network of hydrogen bonds that link the highly polar water molecules. One of the most direct probes of the dynamics of this network is the infrared spectrum of the OH stretching vibration, which reflects the distribution of hydrogen-bonded structures and the intermolecular forces controlling the structural dynamics of the liquid. Indeed, water dynamics has been studied in detail, most recently using multi-dimensional nonlinear infrared spectroscopy for acquiring structural and dynamical information on femtosecond timescales. But owing to technical difficulties, only OH stretching vibrations in D2O or OD vibrations in H2O could be monitored. Here we show that using a specially designed, ultrathin sample cell allows us to observe OH stretching vibrations in H2O. Under these fully resonant conditions, we observe hydrogen bond network dynamics more than one order of magnitude faster than seen in earlier studies that include an extremely fast sweep in the OH frequencies on a 50-fs timescale and an equally fast disappearance of the initial inhomogeneous distribution of sites. Our results highlight the efficiency of energy redistribution within the hydrogen-bonded network, and that liquid water essentially loses the memory of persistent correlations in its structure within 50 fs.

We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser-induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5 picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

Time-resolved electron diffraction harbors great promise for resolving the fastest chemical processes with atomic level detail. The main obstacles to achieving this real-time view of a chemical reaction are associated with delivering short electron pulses with sufficient electron density to the sample. In this article, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model. It is found that space-charge effects can broaden the electron pulse to many times its original length and generate many eV of kinetic energy bandwidth in only a few nanoseconds. There is excellent agreement between the N-body simulation and the mean-field model for both space-charge induced temporal and kinetic energy distribution broadening. The numerical simulation also shows that the redistribution of electrons inside the packet results in changes to the pulse envelope and the development of a spatially linear axial velocity distribution. These results are important for (or have the potential to impact on) the interpretation of time-resolved electron diffraction experiments and can be used in the design of photoelectron guns and streak tubes with temporal resolution of several hundred femtoseconds.

Femtosecond electron diffraction (FED) has the potential to directly observe transition state processes. The relevant motions for this barrier-crossing event occur on the hundred femtosecond time-scale. Recent advances in the development of high-flux electron pulse sources with the required time resolution and sensitivity to capture barrier-crossing processes are described in the context of attaining atomic level details of such structural dynamics-seeing chemical events as they occur. Initial work focused on the ordered-to-disordered phase transition of Al under strong driving conditions for which melting takes on nm or molecular scale dimensions. This work has been extended to Au, which clearly shows a separation in time-scales for lattice heating and melting. It also demonstrates that superheated face-centred cubic (FCC) metals melt through thermal mechanisms involving homogeneous nucleation to propagate the disordering process. A new concept exploiting electron-electron correlation is introduced for pulse characterization and determination of t=0 to within 100fs as well as for spatial manipulation of the electron beam. Laser-based methods are shown to provide further improvements in time resolution with respect to pulse characterization, absolute t=0 determination, and the potential for electron acceleration to energies optimal for time-resolved diffraction.

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