Speaker
Description
Photoexcitation by ultrashort laser pulses plays a crucial role in controlling reaction pathways, creating
nonequilibrium material properties, and offering a microscopic view of complex dynamics at the molecular
level. The photo-response following a laser pulse is, in general, non-identical between multiple exposures due to
spatiotemporal fluctuations in a material or the stochastic nature of dynamical pathways. However, most
ultrafast experiments using a stroboscopic pump-probe scheme struggle to distinguish intrinsic sample
fluctuations from extrinsic apparatus noise, often missing seemingly random deviations from the averaged shot-
to-shot response. Leveraging the stability and high photon-flux of time-resolved x-ray micro-diffraction at
Beamline 7-ID-C at the Advanced Photon Source, we employed some established statistical tools to
quantitatively characterize the stochastic behavior of the photoinduced dynamics in a solid-state lithium-based
ionic conductor. By analyzing temporal evolutions of the lattice parameter of a single grain in a powder
ensemble, we found that the sample responses after different shots contain random fluctuations that are,
however, not independent. Instead, there is a correlation between the nonequilibrium lattice trajectories
following adjacent laser shots with a characteristic “correlation length” of approximately 1,500 shots, which
represents an energy barrier of ~0.4 eV for switching the photoinduced pathway, a value that is close to the
activation energy of lithium ion diffusion [1]. I will conclude the talk by discussing new opportunities brought
by this type of analysis to study fluctuations and explore photoinduced metastable states buried in oft-presumed
random, uncorrelated stochastic dynamics.
[1] J. McClellan, A. Zong, et al., “Photoinduced correlations in stochastic dynamics of a solid-state ionic
conductor,” Nature Communications, in press (2026)