  We study monolayer WSe2 using ultrafast electron diffraction. We introduce an
approach to quantitatively extract atomic-site-specific information, providing
an element-specific view of incoherent atomic vibrations following femtosecond
excitation. Via differences between W and Se vibrations, we identify stages in
the nonthermal evolution of the lattice. Combined with a calculated phonon
dispersion, this element specificity enables us to identify a long-lasting
overpopulation of specific optical phonons, and to interpret the stages as
energy transfer processes between specific phonon groups. These results
demonstrate the appeal of resolving element-specific vibrational information in
the ultrafast time domain.
