Observing excitons with time-resolved ARPES

With the development and spread of time- and angle-resolved photoemission spectroscopy (tr-ARPES) comes the capability of observing electrons in the conduction bands of semiconductors after an ultrashort pump pulse. In systems with large Coulomb interactions, such as the two-dimensional dichalcogenide materials (e.g. MoS₂), the possibility exists of the excited electrons binding with the holes in the valence band and forming excitons, and these may be observed through tr-ARPES. We show what the signatures of these excitations are, what the relationship is between the dispersion of the observed electrons and the underlying material properties such as conductance/valence band effective masses and exciton radii, and discuss their time dependence.