Perspective: Ultrafast imaging of molecular dynamics using ultrafast low-frequency lasers, x-ray free electron laser and electron pulses

The requirement of high space-time resolution and brightness is a great challenge for imaging atomic motion and making molecular movies. Important breakthroughs in ultrabright tabletop laser, x-ray and electron sources have enabled the direct imaging of evolving molecular structures in chemical processes.And recent experimental advances in preparing ultrafast laser and electron pulses equipped molecular imaging with femtosecond time resolution. This Perspectives present an overview of versatile imaging methods of molecular dynamics.High-order harmonic generation imaging maps the harmonic order of photons to the time delay for nuclear motion and can reach attosecond time resolution. Coulomb explosion imaging retrieves molecular structural information by detecting the momentum vectors of fragmented ions. Diffraction imaging encodes molecular structural and electronic information in reciprocal space, achieving high spatial resolution by using x-ray or electrons with short wavelengths. The enhancement of x-ray and electron source brightness pushes diffraction into single molecular limit without the restriction of crystalline materials. Laser-induced electron diffraction is a self-imaging method based on coherent electron scattering. We also present various applications of these ultrafast imaging methods in resolving laser-induced nuclear and electronic dynamics, as well as dynamic symmetry evolution of molecules.