  The nature of a phase transition is inherently connected to the changes in
the crystalline symmtry, which is typically probed by elastic or inelastic
scattering with neutrons, electrons or photons. When such a phase transition is
stimulated by light or other sudden perturbations the solid evolves along a
non-equilibrium pathway of which the underlying physics is poorly understood.
Here we use picosecond Raman scattering to study the photo-induced ultrafast
dynamics in Peierls distorted Antimony. We find evidence for an ultrafast
non-thermal reversible structural phase transition. Most surprisingly, we find
evidence that this transition evolves toward a lower symmetry, in contrast to
the commonly accepted rhombohedral-to-simple cubic transition path. Our study
demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of
photo-induced non-thermal transient phases.
