  Time- and angle-resolved photoemission spectroscopy accesses the ultrafast
evolution of quasiparticles and many-body interactions in solid-state systems.
However, the momentum- and energy-resolved transient photoemission intensity
may not be unambiguously related to the intrinsic relaxation dynamics of
photoexcited electrons. In fact, interpretation of the time-dependent
photoemission signal can be affected by the transient evolution of both the
one-electron removal spectral function as well as the photoemission dipole
matrix elements. Here we investigate the topological insulator
Bi$_{1.1}$Sb$_{0.9}$Te$_2$S to demonstrate, by means of a careful
probe-polarization study, the transient contribution of matrix elements to the
time-resolved photoemission signal.
