  Two-dimensional (2D) crystals of semiconducting transition metal
dichalcogenides (TMD) absorb a large fraction of incident photons in the
visible frequencies despite being atomically thin. It has been suggested that
the strong absorption is due to the parallel band or "band nesting" effect and
corresponding divergence in the joint density of states. Here, we show using
photoluminescence excitation spectroscopy that the band nesting in mono- and
bilayer MX$_2$ (M = Mo, W and X = S, Se) results in excitation-dependent
characteristic relaxation pathways of the photoexcited carriers. Our
experimental and simulation results reveal that photoexcited electron-hole
pairs in the nesting region spontaneously separate in the $k$-space, relaxing
towards immediate band extrema with opposite momentum. These effects imply that
the loss of photocarriers due to direct exciton recombination is temporarily
suppressed for excitation in resonance with band nesting. Our findings
highlight the potential for efficient hot carrier collection using these
materials as the absorbers in optoelectronic devices.
