High-resolution movies of molecular rotational dynamics captured with ultrafast electron diffraction

Abstract

Imaging the structure of molecules during a photoinduced reaction is essential for elucidating reaction mechanisms. This requires high spatiotemporal resolution to capture nuclear motions on the femtosecond and subangstrom scale, and a sufficiently high signal level to sample their continuous evolution with high fidelity. Here we show that, using high-repetition-rate ultrafast electron diffraction, we can accurately reconstruct a movie of the coherent rotational motion of laser-aligned nitrogen molecules. We have used a tabletop 90-keV photoelectron gun to simultaneously achieve high temporal resolution of 240 fs full width at half maximum and an electron beam current that is more than an order of magnitude above the previous state of the art in gas-phase ultrafast electron diffraction. With this, we have made an essentially continuous real-space experimental movie of the rotational motion of the molecular wave packet as it evolves from initial alignment and past multiple revivals.

Quantum state tomography of molecules by ultrafast diffraction

“…We then apply this iterative QT method to the ultrafast electron diffraction (UED) experiment to extract the quantum density matrix of N 2 rotational wavepacket, prepared at a temperature of 45 K. The experimental parameters are described in detail in a previous publication 39 . We use a tabletop kilo-electron-volt (keV) gas-phase UED setup to record the diffraction patterns of nitrogen molecules that are impulsively aligned by a femtosecond laser pulse.…”

“…We use a tabletop kilo-electron-volt (keV) gas-phase UED setup to record the diffraction patterns of nitrogen molecules that are impulsively aligned by a femtosecond laser pulse. The details of the keV UED setup has been introduced in 39 , 40 , which is schematically shown in Fig. 1 .…”

“…The temporal evolution of diffraction patterns can be characterized by the anisotropy, defined as ( S H − S V )/( S H + S V ), where S H and S V are the sum of the counts in horizontal and vertical cones in the diffraction patterns at 3.0 < s < 4.5 Å −1 , with an opening angle of 60 degrees. The temporal evolution of angular probability distribution Pr( θ , ϕ , t ) can be retrieved using the method described in 39 , followed by a deconvolution using a point spread function with FWHM width of 280 fs to remove the blurring effect due to the limited temporal resolution of the setup. Data is recorded from before excitation of the laser up to 6.1 ps after excitation.…”

THz-Enhanced DC Ultrafast Electron Diffractometer

“…The major challenge for generating short electron bunches is to overcome the inherent space-charge broadening effect. The main approaches have been based on sacrificing bunch density to reduce spacecharge forces [9], shrinking the propagation distance, increasing the electron energy [5,10], or recompressing the electron bunches with rebunching cavities [8,11,12]. Although operating in the single-electron or lowelectron-density regime avoids space-charge limits to the time resolution [13,14], it demands high repetition rates, long exposure times, and high system stability to build a statistically meaningful diffraction pattern.…”

Quantum state tomography of molecules by ultrafast diffraction

“…We use a tabletop kilo-electron-volt (keV) gas-phase UED setup to record the diffraction patterns of nitrogen molecules that are impulsively aligned by a femtosecond laser pulse. The details of the keV UED setup has been introduced in [39,40], which is schematically shown in Pr(θ, φ, t) can be retrieved using the method described in [39], followed by a deconvolution using a point spread function with FWHM width of 280 fs to remove the blurring effect due to the limited temporal resolution of the setup. Data is recorded from before excitation of the laser up to 6.1 ps after excitation.…”