Picosecond infrared laser driven sample delivery for simultaneous liquid-phase and gas-phase electron diffraction studies

Abstract

Here, we report on a new approach based on laser driven molecular beams that provides simultaneously nanoscale liquid droplets and gas-phase sample delivery for femtosecond electron diffraction studies. The method relies on Picosecond InfraRed Laser (PIRL) excitation of vibrational modes to strongly drive phase transitions under energy confinement by a mechanism referred to as Desorption by Impulsive Vibrational Excitation (DIVE). This approach is demonstrated using glycerol as the medium with selective excitation of the OH stretch region for energy deposition. The resulting plume was imaged with both an ultrafast electron gun and a pulsed bright-field optical microscope to characterize the sample source simultaneously under the same conditions with time synchronization equivalent to sub-micrometer spatial resolution in imaging the plume dynamics. The ablation front gives the expected isolated gas phase, whereas the trailing edge of the plume is found to consist of nanoscale liquid droplets to thin films depending on the excitation conditions. Thus, it is possible by adjusting the timing to go continuously from probing gas phase to solution phase dynamics in a single experiment with 100% hit rates and very low sample consumption (<100 nl per diffraction image). This approach will be particularly interesting for biomolecules that are susceptible to denaturation in turbulent flow, whereas PIRL-DIVE has been shown to inject molecules as large as proteins into the gas phase fully intact. This method opens the door as a general approach to atomically resolving solution phase chemistry as well as conformational dynamics of large molecular systems and allow separation of the solvent coordinate on the dynamics of interest.

FIG. 1.

Schematic drawing of the experimental setup for coupling the PIRL–DIVE plume with a femtosecond electron gun and an optical bright-field microscope. (a) Schematic diagram of the side view of the experimental setup. (b) Schematic diagram of the top view of the experimental setup. See text for further details.

FIG. 2.

Upper row: 50 shots averaged optical images of PIRL-driven gas-phase glycerol plumes at different delays after the PIRL ablation pulse. The white scale bar corresponds to 200 μm. Middle row: 550 shots averaged electron images of PIRL-driven gas-phase glycerol plumes in real space at different delays after the PIRL pulse. The white scale bar corresponds to 200 μm. Bottom row: 2750 shots averaged electron diffraction images (I_total × s²) of PIRL-driven gas-phase glycerol plumes at different delays after the PIRL ablation pulse. The white scale bar corresponds to 20 nm⁻¹. The PIRL fluence is at 450 mJ/cm². See text for details.

FIG. 3.

Upper row: single-shot optical images of the PIRL-driven glycerol thin liquid film (bubble) at different delays after the PIRL ablation pulse. The white scale bar corresponds to 200 μm. Middle row: 550 shots averaged electron images of the PIRL-driven glycerol thin liquid film (bubble) in real space at different delays after the PIRL pulse. The white scale bar corresponds to 200 μm. Bottom row: 2750 shots averaged electron diffraction images (I_total × s²) of glycerol thin liquid film (bubble) at different delays after the PIRL ablation pulse. The white scale bar corresponds to 20 nm⁻¹. The PIRL fluence is at 220 mJ/cm². See text for details.

FIG. 4.

Simulated (a) and (c) and experimental (b) and (d) electron diffraction results (I_total × s²) on gas-phase glycerol (a) and (b) and liquid-phase glycerol (c) and (d). The white scale bar corresponds to 20 nm⁻¹. See texts for details.

FIG. 5.

Radial average curves of two-dimension electron diffraction patterns from PIRL-DIVE plumes prepared with a PIRL fluence of 450 mJ/cm² (a) and 220 mJ/cm² (b). The dash lines are the experimental results. The solid curves are the fitted results. The panels (c) and (d) are the corresponding radial distribution curves f(r) obtained by sine transforming the radial average curves of (a) and (b), respectively. See text for details.