The photoinduced ring opening of 1,3-cyclohexadiene is a prototypical photo-allowed, ultrafast reaction in agreement with the Woodward-Hoffmann rules. Furthermore, it is a model reaction for the biosynthesis of vitamin D. The reaction mechanism involves coupled dynamics of the electrons and nuclei of the molecule in the vicinity of a conical intersection between the excited state and the ground state. It has been intensively investigated by spectroscopic methods, which were primarily sensitive to transient changes in the electronic structure during internal conversion through a conical intersection between the excited state and the ground state. The corresponding structural dynamics have so far only been investigated by a ultrafast x-ray diffraction study. However, spatial resolution down to bond lengths has not been achieved so far. We have investigated the ring opening by gas phase MeV ultrafast electron diffraction with an unprecedented combination of femtosecond temporal and sub Angstrom spatial resolution. The obtained momentum transfer range allows us to follow transient bond length changes by real space transformation of the experimental dataset. Thus, the experimentally observed structural dynamics can be directly compared to quantum molecular dynamics simulations. We, furthermore, can follow the further structural relaxation of the molecule for several hundred femtoseconds beyond relaxation through the conical intersection with the ground state.
Ultrafast electron diffraction (UED) has the potential to capture changes in the structure of isolated molecules on the natural spatial and temporal scale of chemical reactions, that is, sub-Angstrom changes in the atomic positions that happen on femtosecond time scales. UED has the advantage that electron sources can easily reach sub-Angstrom spatial resolution, but so far femtosecond resolution had not been available for gas phase experiments due to the challenges in delivering short enough electron pulses on a gas target and the velocity mismatch between laser and electron pulses. Recently, we have used relativistic electron pulses at MeV energy to solve these challenges and reach femtosecond resolution. We have, for the first time, imaged coherent nuclear motion in a molecule with UED. In a proof-of-principle experiment, we captured the motion of a laser-excited vibrational wavepacket in iodine molecules. We are currently performing experiments in more complex molecules to capture laser-induced dissociation and conformational changes. We have also developed a table top 100 keV source that relies on a pulse compressor to deliver femtosecond electron pulses on a target and uses a tilted laser pulse to compensate for the velocity mismatch between the laser and the electrons. This source has a high repetition rate that will complement the high temporal resolution of the relativistic source.
SLAC recently launched the ultrafast electron diffraction and microscopy (UED/UEM) Initiative, with the goal to develop the world’s leading ultrafast electron scattering instruments, which are complementary with x-ray free-electron lasers such as LCLS and LCLS-II. The first step of the Initiative is a MeV UED system which is now actively supporting an ultrafast science program, and at the same time serving as a testbed for instrumentation development.
In this talk, design of the SLAC MeV UED system will be briefly introduced. Key machine performance parameters will be reviewed, including machine stability and reproducibility, as well as reciprocal-space and temporal resolution. Ultrafast dynamics from a variety of samples, including 2D materials, thin nanofilms, nanoparticles, and gas-phase molecules have been studied using this machine. Selected ultrafast science experiment results will be presented. In the meantime, much R&D efforts have been devoted for novel machine capabilities to enable new science opportunities. For example, we have experimentally demonstrated a femtosecond MeV electron microdiffraction, which is capable to resolve local structure from single crystal of μm lateral size with 100 fs root-mean-square temporal resolution. Future developments include 10-fs temporal resolution UED, THz pumping capability, etc. We will also discuss R&D towards the next step of the Initiative, which is to develop key technologies for future UEM with unprecedented combined spatial-temporal resolution. This R&D will focus on a superconducting radio-frequency photocathode gun, which features high accelerating field hence high beam brightness, excellent energy stability, and outstanding flexibility in bunch length from picosecond to a hundred picoseconds.
Ultrafast electron diffraction (UED) is a powerful technique that can be used to resolve structural changes of gas molecules during a photochemical reaction. However, the temporal resolution in pump-probe experiments has been limited to the few-ps level by the space-charge effect that broadens the electron pulse duration and by velocity mismatch between the pump laser pulses and the probe electron pulses, making only long-lived intermediate states accessible. Taking advantage of relativistic effects, Mega-electron-volt (MeV) electrons can be used to suppress both the space-charge effect and the velocity mismatch, and hence to achieve a temporal resolution that is fast enough to follow coherent nuclear motion in the target molecules. In this presentation, we show the first MeV UED experiments on gas phase targets. These experiments not only demonstrate that femtosecond temporal resolution is achieved, but also show that the spatial resolution is not compromised. This unprecedented combination of spatiotemporal resolution is sufficient to image coherent nuclear motions, and opens the door to a new class of experiments where the structural changes can be followed simultaneously in both space and time.