(color online) (a) Pump beam with circular polarization excites K valley only, probe beam with the same or the opposite circular polarization measures the population in K or K′ valley respectively. (b) Illustration of valley relaxation of initially excited holes by exchanging spin and momentum with an electron in the other valley. (c) Illustration of valley relaxation of holes by scattering through the Γ valley. (d) Transient absorption spectra of monolayer MoS₂ at 74 K with delay time varying from 0 ps to 15 ps between pump and probe beams. The red lines show SCP and blue lines show OCP. The photo-induced absorption (PIE), stimulated emission (SA) and ground state bleaching (GSB) peaks are labeled in the figure. (e) Optical selectivity (η) decays with disorder (W). Lines with different colors represent different optical transition energies off the A exciton resonance of 1.860 eV. The numbers near lines show the transition energy relative to the direct band gap. (f) Schematic illustration of A and B excitonic transitions in the K and K′ valleys. The gray arrows show the exciton spin orientations. Coulomb-induced intervalley coupling in TMDC materials leads to a decay of the optically generated valley polarization. The impurity assisted exchange term X leads to a carrier transfer between the resonant states in the two valleys (A–A, B–B coupling). The Coulomb-induced renormalization term Y couples A and B excitons with the same spin at different valleys. It does not cause carrier exchange between the valleys but leads to energetic shifts of the excitonic states in both valleys. Panels (a)–(d) are reproduced from Ref. [28]; panels (e) is reproduced from Ref. [26]; and panels (f) is reproduced from Ref. [31].