Imaging ultrafast molecular dynamics with laser-induced electron diffraction

Abstract

Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-ångström spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-ångström precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3 μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1 Å displacement in the oxygen bond length occurring in a time interval of ∼5 fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution.

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

“…S ince the introduction of femto-chemistry [1][2][3][4][5] , electron or X-ray diffraction have been the most commonly used techniques to track nuclear rearrangement in molecules [6][7][8][9] . Unfortunately, these techniques are largely insensitive to the more subtle and irregular structural changes that can occur within a single small molecule undergoing a chemical reaction.…”

Axis-dependence of molecular high harmonic emission in three dimensions

“…Most molecules are asymmetric tops and even reactions with linear molecules generally proceed via an asymmetric-top transition state. Thus, an alignment decomposition into three dimensions is a solution that could be employed by much of the ultrafast molecular science community, enhancing or augmenting techniques such as time-resolved X-ray 13,14 and electron diffraction 15,16 , photoelectron spectroscopy 17,18 , as well as HHG 19,20 .…”

Electron gun for diffraction experiments off controlled molecules

“…The use of ultra cold atoms as electron sources increases the coherence [36,37]. Other possible sources for time-resolved electron diffraction are low energy electron setups [38] or laser-induced electron diffraction [39,40].…”

“…The use of ultra cold atoms as electron sources increases the coherence [36,37]. Other possible sources for time-resolved electron diffraction are low energy electron setups [38] or laser-induced electron diffraction [39,40].If the molecular samples are prepared in the necessary strongly-controlled fashion, their densities are typically on the order of some 10 8 molecules/cm 3 [21,41]. Assuming Rutherford scattering, for the prototypical molecule 2,5-diiodobenzonitrile an effective cross section on the order of 10 −15 cm 2 can be derived for our experimental geometry and a beam stop blocking a solid angle of 1.3 × 10 −3 sr. To align or orient the molecules, they are typically exposed to laser fields with intensities of 1 TW/cm 2 [25], which can be achieved by focusing the ps-duration mJ-pulse-energy laser beam to 100 µm [42].…”