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Fig. 16: a) Sketch of the typical experimental setup for TDI. The red lines indicate the direction of the incident and scattered X-ray beam. The black dashed line shows the direction of the transmitted beam. b) Example of time-domain interferometry data (squares with error bars) as a function of time in the supercooled state of 1-propanol.

Fig. 17: Sketch of the molecules undergoing the bJG relaxation (red spheres) at a given time. They are highly mobile, perform larger spatial excursions than the rest of the molecules (white spheres) and are spatially connected in a percolating cluster.

occurs, the structural relaxation is of the order of 100 seconds. This structural relaxation is, however, anticipated by the appearance of a faster process of molecular re-arrangements known as the Johari-Goldstein (bJG) relaxation [2], which remains active even below Tg and whose role in the glass transition whether that of an actor or of a spectator is still debated. Experimental difficulties in accessing the spatial and temporal range characteristic of this process, i.e., interatomic length-scales and times of the order of hundreds of nanoseconds, means this question has been left open.

Time-domain X-ray interferometry (TDI) was used at nuclear resonance beamline ID18 to probe and characterise the dynamical properties of the Johari- Goldstein relaxation in the glass-forming liquid 1-propanol close to its glass-transition temperature. TDI experiments are designed to detect X-ray scattering from the sample using an interferometer (Figure 16), providing information that is both time and space resolved and allowing researchers to probe molecular motions at the timescale where the bJG process takes