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The electronic states at the direct band gap of monolayer WSe2 at the $K^+$and $K^-$ valleys are related by time reversal and may be viewed aspseudo-spins. The corresponding optical interband transitions are governed byrobust excitons. In double resonant Raman spectroscopy, we uncover that the 2sexciton state energy differs from the 1s state energy by exactly the energy ofthe combination of several prominent phonons. Superimposed on the excitonphotoluminescence (PL) we observe the double resonant Raman signal. Thisspectrally narrow peak shifts with the excitation laser energy as incomingphotons match the 2s and outgoing photons the 1s exciton transition. Themulti-phonon resonance has important consequences: Following linearly polarizedexcitation of the 2s exciton a superposition of valley states is generatedwhich can relax fast via phonon emission and with minimal loss of coherencefrom the 2s to 1s state. This explains the high degree of valley coherencemeasured for the 1s exciton PL.

Double resonant Raman scattering and valley coherence generation in monolayer WSe2