*

Carrier dynamics in a semiconductor following pulsed optical excitation is relevant to ultrafast optoelectronic applications. We report herein a comparative study of bulk WSe₂ and single-layer (SL) WSe₂ grown on bilayer-graphene-terminated SiC. Subtle dimensional effects lead to substantial differences in the transient response as revealed by time-resolved angle-resolved photoemission spectroscopy (TRARPES) measurements. The conduction band minimum in bulk WSe₂, populated by optical pumping, decays promptly. The corresponding decay for SL WSe₂ is more complex but overall much slower due to spin filtering arising from the lack of space inversion symmetry. Concurrent measurements of the valence bands show longer but different response times arising from lattice excitation, carrier transport, and dissipation. These results illustrate the various time scales and processes at work for carrier and band structure evolution.