Optical Selection Rules and Temperature-Dependent Exciton-Phonon Coupling under Near-Valley Resonance in Monolayer WS ₂

Optical selection rules, endorsed by linear and angular momentum conservation, play critical roles in determining both optoelectronic and photonic properties of quantum materials, especially when complex degrees of freedom, such as valley and excitonic angular momenta, are involved. Herein, we systematically investigate the optical selection rules and temperature-dependent exciton-phonon coupling for Raman scattering in monolayer WS2 using helicity-resolved Raman spectroscopy (HRRS) under near-valley resonance. We propose a unified phenomenological model that explicitly incorporates the angular momentum conservation among photons, phonons, valleys, and excitons, elucidating the mechanisms for both helicity-conserved (σ+σ+) and helicity-changed (σ+σ−) Raman scattering, including a phonon-assisted excitonic intervalley process. Based on the temperature-controlled HRRS experiments and Raman tensor analysis, we quantitatively disentangle the relative contributions of deformation potential (DP) and Fröhlich interaction (FI) to the E2g1 phonon scattering from 10 to 290 K. We demonstrate that this phonon mode is governed by a coherent interference between DP and FI at room temperature, but transitions to an incoherent summation mediated by defect-assisted FI at 10 K. This work not only provides deeper insights into fundamental optical selection rules for helicity-resolved Raman scattering, but also offers an optical methodology to probe and distinguish complex exciton-phonon interactions in two-dimensional quantum materials.