Kaikai Zhang

Imaging ultrafast molecular dynamics with laser-induced electron diffraction

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.

High spatial frequency laser induced periodic surface structure formation in germanium by mid-IR femtosecond pulses

Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by femtosecond mid-IR pulses with wavelengths between

Laser-induced electron diffraction for probing rare gas atoms.

\lambda=2.0

Experimental investigation of strong-field-ionization theories for laser fields from visible to midinfrared frequencies

and

Universal pulse dependence of the low-energy structure in strong-field ionization

3.6 \; \mathrm{\mu m}

Ionization and fragmentation of methane with intense mid-infrared fields

was studied with varying angle of incidence and polarization. The period of these structures varied from

Publisher's Note: Universal pulse dependence of the low-energy structure in strong-field ionization [Phys. Rev. A93, 021403(R) (2016)]

\lambda/3

Diffraction using laser-driven broadband electron wave packets

to

Laser pulse duration dependence of the low-energy structure in strong field ionization

\lambda/8

OBSERVATION OF FEMTOSECOND, SUB-ANGSTROM MOLECULAR BOND RELAXATION USING LASER-INDUCED ELECTRON DIFFRACTION

. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy (TEM) shows the presence of a