  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 as
pseudo-spins. The corresponding optical interband transitions are governed by
robust excitons. In double resonant Raman spectroscopy, we uncover that the 2s
exciton state energy differs from the 1s state energy by exactly the energy of
the combination of several prominent phonons. Superimposed on the exciton
photoluminescence (PL) we observe the double resonant Raman signal. This
spectrally narrow peak shifts with the excitation laser energy as incoming
photons match the 2s and outgoing photons the 1s exciton transition. The
multi-phonon resonance has important consequences: Following linearly polarized
excitation of the 2s exciton a superposition of valley states is generated
which can relax fast via phonon emission and with minimal loss of coherence
from the 2s to 1s state. This explains the high degree of valley coherence
measured for the 1s exciton PL.
