  The coupling of light to electrical charge carriers in semiconductors is the
foundation of many technological applications. Attosecond transient absorption
spectroscopy measures simultaneously how excited electrons and the vacancies
they leave behind dynamically react to the applied optical fields. In compound
semiconductors, these dynamics can be probed via any of their atomic
constituents. Often, the atomic species forming the compound contribute
comparably to the relevant electronic properties of the material. One therefore
expects to observe similar dynamics, irrespective of the choice of atomic
species via which it is probed. Here, we show in the two-dimensional transition
metal dichalcogenide semiconductor $MoSe_2$, that through a selenium-based
transition we observe charge carriers acting independently from each other,
while when probed through molybdenum, the collective, many-body motion of the
carriers dominates. Such unexpectedly contrasting behavior can be traced back
to a strong localization of electrons around molybdenum atoms following
absorption of light, which modifies the local fields acting on the carriers. We
show that similar behavior in elemental titanium metal carries over to
transition metal-containing compounds and is expected to play an essential role
for a wide range of such materials. Knowledge of independent particle and
collective response is essential for fully understanding these materials.
