Making the Molecular Movie

“Making the Molecular Movie” – First Frames and First Surprises

One of the long sought objectives in science has been to directly watch atomic motions during structural changes. This objective requires extremely high spatial resolution (<.1 nm) and temporal resolution (100 femtosecond time scale) with sufficient source brightness to fully resolve the dynamics of interest. In this space-time limit, it is possible to directly observe chemistry as it happens, to follow the primary events driving biology, and to separate electronic and nuclear factors in studying strongly correlated electron- lattice dynamics. This level of acuity makes it possible to directly determine the anharmonic couplings, common to all dynamical problems, in a single measurement. The importance of this problem is one of the drivers for CFEL and has been emphasized throughout this report. The Miller group was the first to achieve atomically resolved structural dynamics in the required subpicosecond domain through the development of what can be considered “ultrabright” nonrelativistic electron sources to light up the atomic motions. At the atomic level of observation, we have now reached what can be considered the fundamental limit to following structural dynamics.

This level of acuity has revealed a number of surprises. The first report focused on the simplest structural transition possible, that of melting, albeit under rather unusual conditions of a strongly laser-driven phase transition [Science 302 (2003) 1382]. The first surprise was that the material melted from the “inside-out” in a process known as homogeneous nucleation rather than the conventional melting from the “outside-in” as for example in the everyday experience of melting ice. In the process, it was possible to literally watch bonds break and the lattice shake itself apart, forming nucleation sites or liquid domains that remained on the nm or molecular level.

  As much of a pure curiosity that this observation may seem, this simple finding demonstrated the physics needed to selectively excite and remove material without the formation of macroscale nucleation sites and associated cavitation with shock wave induced damage. A new concept in laser surgery was developed along with a novel solid state laser concept that made it possible to selectively energize water in biological tissues and remove soft tissue without damage to surrounding tissue [Opt. Express 17 (2009) 22937; Phys. Chem. Chem. Phys. 12 (2010) 5225]. This laser concept has now achieved the fundamental limit to surgery and most important without apparent scar tissue formation [PLoS 5 (2010) e13053]. This discovery could be a significant breakthrough as scar tissue leads to complications in all surgeries and loss of function to some degree. In some cases, full function is not recovered. This latest advance opens up the prospect of surgery with near perfect healing at the single cell level of precision. This advance came from the basic research program but has far reaching implications for both surgeries and biodiagnostics. A unique collaboration with University Medical Center Hamburg-Eppendorf (UKE) medical researchers has been formed to fully explore the medical applications with a new laboratory built at the UKE to facilitate translation of these finding to clinical applications.

Since this first report, the group has continued to increase the source “brightness” by increasing the bunch charge and minimize momentum spread. They have achieved single shot structure determinations with time resolution better than 200 fs. On this timescale, the lattice is essentially frozen. The use of femtosecond laser excitation makes it possible to change the electron distribution and directly observe the changes in the potential energy landscape of the lattice, i.e. the affect on the fundamental bonding in the lattice. Under extremely high excited electron distributions, it was possible to track the formation of Warm Dense Matter and discover that a nascent liquid state is involved as well as evidence for bond hardening in Au [Science 323 (2009) 1033].

This latter observation is surprising as almost all materials become softer when excited (e.g. metals become more malleable upon heating). Here the lattice was becoming harder not softer. This process is only observable under the incredibly fast time scale of these observations and is in agreement with recent time dependent density functional calculations. Studies of the Peierls distorted lattice of Bi also gave some surprises. It was found that when up to 10% of the valence electrons were excited, the lattice was transformed into a true liquid in less than ½ period of a lattice vibration [Nature 458 (2009) 56]). This work constitutes the first observation of ballistic melting, melting without collisions, and one of the fastest structural changes ever measured. The motions involved are larger in amplitude and faster than the cis to trans isomerization step of vision that was the fastest structural change known until this finding. This work was further extended to even more strongly coupled electron-lattice systems in following the highly cooperative response of Charge Density Waves to perturbations in the electron distribution as exemplified by TaS2 [Nature 468 (2010) 799]. Here again there as a surprise. It was possible to directly observe the structure order parameter in which the supermodulation of the lattice (Charge Density Wave) collectively responded to the electron perturbation. The laser excitation literally becalmed the Charge Density Waves, and as soon as the excitation was transduced to acoustics not matching the order parameter, the system immediately latched back to the supermodulation in an incredibly cooperative response. The quality of data, and full in-plane reconstitution of the atomic motions along the key coordinates made possible with electron diffraction, illustrates the importance of this new tool for studyinghighly cooperative effects that in turn could lead to new material properties.

For the full story please refer to the review:
“Making the Molecular Movie: First Frames”
by R. J. D. Miller, R. Ernstorfer, M. Harb, M. Gao , C. T. Hebeisen, H. Jean-Ruel, C. Lu, G. Moriena, G. Sciaini
[Acta Cryst. A 66 (2010) 137].