Graphene Raman G Scattering Kinematics

In this theoretical study, focusing on the single resonant Raman G mode, new details of Raman radiation from monolayer graphene are presented. The propagation direction of the emitted photon is discussed, explicitly considering the momenta of the incident photon, the scattered photon, and the phonon. It is demonstrated that graphene should be regarded as a quasi‐two‐dimensional system in light scattering phenomena, and the electron confined motion along the z‐direction, perpendicular to the atomic monolayer plane, must be taken into account. It is established that the out‐of‐plane nature of the emitted photon arises from the uncertainty of the electron momentum along the z‐direction. It is theoretically shown that at Raman G scattering, the wavenumber of the emitted iLO or iTO phonon is limited to a certain range, with boundaries determined by the crystal sizes and the wavenumber of the incident photon. Two‐sided limitation of the Raman‐active optical phonon wavenumber, in turn, restricts the propagation direction of the emitted photon. The developed approach is general and can also be applied to the analysis of other Raman lines of graphene.