Low dose X-ray speckle visibility spectroscopy reveals nanoscale dynamics in radiation sensitive ionic liquids

X-ray radiation damage provides a serious bottle neck for investigating {\mu}s to s dynamics on nanometer length scales employing X-ray photon correlation spectroscopy. This limitation hinders the investigation of real time dynamics in most soft matter and biological materials which can tolerate only X-ray doses of kGy and below. Here, we show that this bottleneck can be overcome by low dose X-ray speckle visibility spectroscopy. Employing X-ray doses of 22 kGy to 438 kGy and analyzing the sparse speckle pattern of count rates as low as 6.7x10-3 per pixel we follow the slow nanoscale dynamics of an ionic liquid (IL) at the glass transition. At the pre-peak of nanoscale order in the IL we observe complex dynamics upon approaching the glass transition temperature TG with a freezing in of the alpha relaxation and a multitude of milli-second local relaxations existing well below TG. We identify this fast relaxation as being responsible for the increasing development of nanoscale order observed in ILs at temperatures below TG.Comment: 7 pages, 5 figure