Dynamics of soft nanoparticle suspensions at hard X-ray FEL sources below the radiation-damage threshold.

Felix Lehmkühler,Tetsuya Ishikawa,Martin A Schroer,Joana Valerio,Makina Yabashi,Dina Sheyfer,Birgit Fischer,Gerhard Grübel,Kensuke Tono,Wojciech Roseker

doi:10.1107/s2052252518013696

Abstract

The application of X-ray photon correlation spectroscopy (XPCS) at free-electron laser (FEL) facilities enables, for the first time, the study of dynamics on a (sub-)nanometre scale in an unreached time range between femtoseconds and seconds. For soft-matter materials, radiation damage is a major limitation when going beyond single-shot applications. Here, an XPCS study is presented at a hard X-ray FEL on radiation-sensitive polymeric poly(N-isopropylacrylamide) (PNIPAM) nanoparticles. The dynamics of aqueous suspensions of densely packed silica-PNIPAM core-shell particles and a PNIPAM nanogel below the radiation-damage threshold are determined. The XPCS data indicate non-diffusive behaviour, suggesting ballistic and stress-dominated heterogeneous particle motions. These results demonstrate the feasibility of XPCS experiments on radiation-sensitive soft-matter materials at FEL sources and pave the way for future applications at MHz repetition rates as well as ultrafast modes using split-pulse devices.

Highlights

The study of the structure and dynamics of matter is one of the key applications of hard X-ray free-electron lasers (FEL)
The X-ray photon correlation spectroscopy (XPCS) data indicate nondiffusive behaviour, suggesting ballistic and stress-dominated heterogeneous particle motions. These results demonstrate the feasibility of XPCS experiments on radiation-sensitive soft-matter materials at freeelectron laser (FEL) sources and pave the way for future applications at MHz repetition rates as well as ultrafast modes using splitpulse devices
This is covered by X-ray photon correlation spectroscopy (XPCS) based on coherent X-ray scattering

Summary

Introduction

The study of the structure and dynamics of matter is one of the key applications of hard X-ray free-electron lasers (FEL). One example is the study of dynamics on time scales between femtoseconds and seconds This is covered by X-ray photon correlation spectroscopy (XPCS) based on coherent X-ray scattering. The timescales accessible in sequential-mode XPCS are typically limited by the repetition rate of the FEL, i.e. at minimum 8.3 ms (LCLS; 120 Hz) at currently operating FEL sources (Carnis et al, 2014; Lehmkuhler et al, 2015) This will be extended to the (sub)ms regime by the European XFEL (Tschentscher et al, 2017). In our recent study at SACLA we demonstrated that XPCS can be performed on a prototypical soft-matter sample, i.e. the diffusion dynamics of silica nanoparticles in suspension (Lehmkuhler et al, 2015). As samples we chose two different dense colloidal PNIPAM [poly(N-isopropylacrylamide)] systems Since these materials scatter X-rays weakly, experiments below the radiation-damage threshold at storage-ring sources are challenging. The q dependence of c / qÀp and the shape parameter provide information of the sample dynamics, such as diffusion (p 1⁄4 2; 1⁄4 1), ballistic motion (p 1⁄4 1) or cooperative dynamics ( 1⁄4 2) found in soft-matter glasses (Caronna et al., 2008; Conrad et al, 2015)

Samples

Experimental setup at SACLA

Results and discussion