Liquid-phase mega-electron-volt ultrafast electron diffraction

Abstract

The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure–function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.

Photodissociation of aqueous I3− observed with liquid-phase ultrafast mega-electron-volt electron diffraction

“…1(d) , red line]. The independently measured is consistent with XSS measurements of water heating 24,29 as well as molecular dynamics simulations, 9 as shown in supplementary material Fig. S6.…”

“…Optical laser pump/MeV electron probe experiments were carried out at the SLAC MeV-UED instrument in the liquid phase endstation described in Ref. 9 . The sample was prepared by combining 130 mM I 2 and excess KI salt (500 mM) in aqueous solution, with pH adjusted to 4 with perchloric acid.…”

“…In a low electron bunch charge mode (∼2 fC/pulse), ultrafast temporal resolution is achieved and is estimated to be 180 fs full width half maximum (FWHM). 9 In a high bunch charge mode (∼100 fC/pulse), picosecond temporal resolution is achieved and is typically between 0.5 and 1 ps FWHM. The pump laser spot size was 160 × 270 μ m FWHM; pulse energy for high bunch charge data was 40 μ J (120 mJ/cm 2 ), while for low bunch charge data, the laser pulse energy was 60 μ J (180 mJ/cm 2 ).…”

“…Planned improvements to the LUED setup will help address these issues, including a near-term upgrade of the repetition rate by a factor of three, from 360 to 1080 Hz, that will decrease data collection times. 9 Additionally, replacement of the integrating-mode detector with a direct electron detector with single-electron and shot-by-shot capabilities 46 will allow for more sophisticated data treatment, including shot-to-shot differencing, a technique which has been shown to improve signal-to-noise in transmission-mode x-ray studies. 47,48 Further development of the liquid jet system, including more stable gas flow and improved nozzles, could improve jet stability and therefore data quality.…”

“…Electron scattering in solution environments, however, has been historically challenging due to the sub-micron penetration depth of electrons in liquids, the complications multiple scattering introduces to data analysis, and the need for high vacuum. Recently, the development of mega-electron-volt (MeV) UED and gas-accelerated liquid sheet jets 8 that produce sub-micron, in-vacuum liquid samples made possible the development of a liquid phase UED (LUED) instrument 9 with sub-200 femtosecond time resolution. Here, we pioneer the use of LUED to study the reaction dynamics of a molecule in solution.…”