Two-dimensional (2D) crystals of semiconducting transition metaldichalcogenides (TMD) absorb a large fraction of incident photons in thevisible frequencies despite being atomically thin. It has been suggested thatthe strong absorption is due to the parallel band or "band nesting" effect andcorresponding divergence in the joint density of states. Here, we show usingphotoluminescence excitation spectroscopy that the band nesting in mono- andbilayer MX$_2$ (M = Mo, W and X = S, Se) results in excitation-dependentcharacteristic relaxation pathways of the photoexcited carriers. Ourexperimental and simulation results reveal that photoexcited electron-holepairs in the nesting region spontaneously separate in the $k$-space, relaxingtowards immediate band extrema with opposite momentum. These effects imply thatthe loss of photocarriers due to direct exciton recombination is temporarilysuppressed for excitation in resonance with band nesting. Our findingshighlight the potential for efficient hot carrier collection using thesematerials as the absorbers in optoelectronic devices.
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