  Intercalation offers a promising way to alter the physical properties of
two-dimensional (2D) layered materials. Here we investigate the electronic and
vibrational properties of 2D layered MoSe$_2$ intercalated with atomic
manganese at ambient and high pressure up to 7 GPa by Raman scattering and
electronic structure calculations. The behavior of optical phonons is studied
experimentally with a diamond anvil cell and computationally through density
functional theory calculations. Experiment and theory show excellent agreement
in optical phonon behavior. The previously Raman inactive A$_{2u}$ mode is
activated and enhanced with intercalation and pressure, and a new Raman mode
appears upon decompression, indicating a possible onset of a localized
structural transition, involving the bonding or trapping of intercalant in 2D
layered materials. Density functional theory calculations reveal a shift of
Fermi level into the conduction band and spin polarization in Mn$_x$MoSe$_2$
that increases at low Mn concentration and low pressure. Our results suggest
that intercalation and pressurization of van der Waals materials may allow one
to obtain dilute magnetic semiconductors with controllable properties,
providing a viable route for the development of new materials for spintronic
applications.
