  Interatomic forces that bind materials are largely determined by an often
complex interplay between the electronic band-structure and the atomic
arrangements to form its equilibrium structure and dynamics. As these forces
also determine the phonon dispersion, lattice dynamics measurements are often
crucial tools for understanding how materials transform between different
structures. This is the case for the mono-chalcogenides which feature a number
of lattice instabilities associated with their network of resonant bonds and a
large tunability in their functional properties. SnSe hosts a novel lattice
instability upon above-bandgap photoexcitation that is distinct from the
distortions associated with its high temperature phase transition,
demonstrating that photoexcitation can alter the interatomic forces
significantly different than thermal excitation. Here we report decisive
time-resolved X-ray scattering-based measurements of the nonequlibrium lattice
dynamics in SnSe. By fitting interatomic force models to the excited-state
dispersion, we determine this instability as being primarily due to changes in
the fourth-nearest neighbor bonds that connect bilayers, with relatively little
change to the intralayer resonant bonds. In addition to providing critical
insight into the nonthermal bonding origin of the instability in SnSe, such
measurements will be crucial for understanding and controlling materials
properties under non-equilibrium conditions.
