Junliang Xu

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.

Effect of orbital symmetry on the orientation dependence of strong field tunnelling ionization of nonlinear polyatomic molecules

In the strong field molecular tunnelling ionization theory (Tong X M 2002 Phys. Rev. A 66 033402), the ionization rate depends on structure parameters of molecules which can be extracted from molecular wavefunctions in the asymptotic region. By using molecular orbitals obtained from standard quantum chemistry packages, we extract these parameters for several selected nonlinear polyatomic molecules. We show that the symmetry properties of the molecular orbital are reflected vividly in the angle-dependent tunnelling ionization rates. The structure parameters for 17 nonlinear molecules have been calculated and tabulated for future applications.

Time-resolved molecular imaging

Time-resolved molecular imaging is a frontier of ultrafast optical science and physical chemistry. In this article, we review present and future key spectroscopic and microscopic techniques for ultrafast imaging of molecular dynamics and show their differences and connections. The advent of femtosecond lasers and free electron x-ray lasers bring us closer to this goal, which eventually will extend our knowledge about molecular dynamics to the attosecond time domain.

Self-imaging of molecules from diffraction spectra by laser-induced rescattering electrons

We study high-energy angle-resolved photoelectron spectra of molecules in strong fields. In an oscillating laser electric field, electrons released earlier in the pulse may return to recollide with the target ion, in a process similar to scattering by laboratory prepared electrons. If midinfrared lasers are used, we show that the images generated by the returning electrons are similar to images observed in typical gas-phase electron diffraction (GED). These spectra can be used to retrieve the positions of atoms in a molecule as in GED. Since infrared laser pulses of durations of a few femtoseconds are already available today, the study of these high-energy photoelectrons offers the opportunity of imaging the structure of transient molecules with temporal resolution of a few femtoseconds.

The roles of photo-carrier doping and driving wavelength in high harmonic generation from a semiconductor

High-harmonic generation from gases produces attosecond bursts and enables high-harmonic spectroscopy to explore electron dynamics in atoms and molecules. Recently, high-harmonic generation from solids has been reported, resulting in novel phenomena and unique control of the emission, absent in gas-phase media. Here we investigate high harmonics from semiconductors with controllable induced photo-carrier densities, as well as the driving wavelengths. We demonstrate that the dominant generation mechanism can be identified by monitoring the variation of the harmonic spectra with the carrier density. Moreover, the harmonic spectral dependence on the driving wavelength is reported and a different dependence from the well-known one in gas-phase media is observed. Our study provides distinct control of the harmonic process from semiconductors, sheds light on the underlying mechanism and helps optimize the harmonic properties for future solid-state attosecond light sources.The properties of high harmonic generation from solids are not fully understood. Here, Wang et al. control the photo-carriers injected in a semiconductor to distinguish interband and intraband contributions to different high harmonics, and investigate the wavelength dependence of the harmonics.

Imaging ultrafast dynamics of molecules with laser-induced electron diffraction

We introduce a laser-induced electron diffraction method (LIED) for imaging ultrafast dynamics of small molecules with femtosecond mid-infrared lasers. When molecules are placed in an intense laser field, both low- and high-energy photoelectrons are generated. According to quantitative rescattering (QRS) theory, high-energy electrons are produced by a rescattering process where electrons born at the early phase of the laser pulse are driven back to rescatter with the parent ion. From the high-energy electron momentum spectra, field-free elastic electron-ion scattering differential cross sections (DCS), or diffraction images, can be extracted. With mid-infrared lasers as the driving pulses, it is further shown that the DCS can be used to extract atomic positions in a molecule with sub-angstrom spatial resolution, in close analogy to the standard electron diffraction method. Since infrared lasers with pulse duration of a few to several tens of femtoseconds are already available, LIED can be used for imaging dynamics of molecules with sub-angstrom spatial and a few-femtosecond temporal resolution. The first experiment with LIED has shown that the bond length of oxygen molecules shortens by 0.1 Å in five femtoseconds after single ionization. The principle behind LIED and its future outlook as a tool for dynamic imaging of molecules are presented.

Imaging Ultrafast Dynamics Using Laser-Induced Electron Diffraction

A new imaging technique, laser-induced electron diffraction is applied to study the bond relaxation dynamics of oxygen molecule. The retrieved bond lengths at three wavelengths all shows a bond contraction of about 0.1 A within 4-6 fs after tunnel ionization.

Elastic scattering and impact ionization by returning electrons induced in a strong laser field

We study elastic scattering and impact ionization by returning electrons induced in a strong laser field based on the recently developed quantitative rescattering theory. The high-energy angle-resolved photoelectron spectra of molecules in strong fields are attributed to elastic scattering of the returning electrons with the parent ion. We demonstrate that the high-energy photoelectron spectra can be used to retrieve the position of atoms in a molecule. We also investigate the nonsequential double ionization of an atom in intense laser fields which is partly due to the impact ionization of the parent ion by the returning electron.

Genetic-algorithm implementation of atomic potential reconstruction from differential electron scattering cross sections

We demonstrate the successful implementation of genetic algorithm for the retrieval of atomic potentials using elastic differential cross sections (DCSs) between free electrons and atomic ions for electron energies from a few to several tens of electron volts. Since the DCSs over this energy region can be extracted from laser-generated high-energy photoelectron momentum spectra, the results suggest that infrared lasers can be used to image the target structure. Extending to molecular targets, in particular, to transient molecules created by an earlier pump pulse, our results suggest that few-cycle infrared probe lasers can be used for dynamic chemical imaging with temporal resolution of a few femtoseconds.