Low Dose X-Ray Speckle Visibility Spectroscopy Reveals Nanoscale Dynamics in Radiation Sensitive Ionic Liquids

X-ray radiation damage provides a serious bottleneck for investigating microsecond to second 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 22–438 kGy and analyzing the sparse speckle pattern of count rates as low as $6.7\times {10}^{-3}\mathrm{per}$  pixel, we follow the slow nanoscale dynamics of an ionic liquid (IL) at the glass transition. At the prepeak of nanoscale order in the IL, we observe complex dynamics upon approaching the glass transition temperature $T_G$ with a freezing in of the alpha relaxation and a multitude of millisecond local relaxations existing well below $T_G$. We identify this fast relaxation as being responsible for the increasing development of nanoscale order observed in ILs at temperatures below $T_G$.

Figure 1

(a) X-ray diffraction pattern from the supercooled ionic liquid C8mimCl associated with the nanoscale order. The inset shows the diffraction pattern as recorded on the area detector. The red box indicates the $Q$ range used for the XSVS analysis. (b) The integrated x-ray signal from the area of the red box in (a) plotted as a function of the absorbed x-ray dose or exposure time. The green area indicates the range of doses used in the experiment.

Figure 2

Blue data: The difference between the actual measured zero-photon probability and the zero-photon probability calculated assuming no dynamics but a simple increase of the mean count rate according to $\overline{k}=r{t}_e$. The decreasing number of zero-photon events indicates the decreasing speckle contrast as a function of the exposure time caused by the sample dynamics. Red data: The same plot as in blue but here for the one-photon probabilities.

Figure 3

Speckle contrast of the ionic liquid as a function of the exposure time for temperatures of 220 and 200 K and $Q=2.6{\mathrm{nm}}^{-1}$, respectively. The solid lines are assignments to three different processes labeled $P1$, $P2$, and $P3$. The calorimetric glass transition temperature is 214 K.

Figure 4

(a) Speckle contrast as a function of the temperature and delay time for the ionic liquid C8mimCl at $Q=2.6{\mathrm{nm}}^{-1}$. The dynamics is progressively slowing down upon approaching $T_G$ with a separation in a fast and slow frozen-in dynamics visible at the lowest temperatures. (b) Contour plot of the pattern shown in (a).

Figure 5

(a) The x-ray speckle contrast at $t_e=1\mathrm{s}$ exposure time as a function of the temperature. (b) Temperature dependence of the averaged decorrelation time of the nonfrozen part in the IL (blue dots). The red squares represent the appearance of the frozen-in components (i.e., dynamics slower than 1 s). The black line represents an Arrhenius fit with an activation energy of $25\mathrm{kJ}/\mathrm{mol}$.