  Monolayer MX$_2$ (M = Mo, W; X = S, Se) has drawn much attention recently for
its possible application possibilities for optoelectronics, spintronics, and
valleytronics. Its exotic optical and electronic properties include a direct
band gap, circular polarization dependent optical transitions, and valence band
(VB) spin band splitting at the $K$ and $-K$ points. These properties can be
described within a minimal model, called the massive Dirac fermion model for
which the parameters need to be experimentally determined. We propose that the
parameters can be obtained from angle resolved photoemission (ARPES) data from
bulk 2H-MX$_2$, instead of monolayer MX$_2$. Through tight binding
calculations, we show how the electronic structure at high symmetry points
evolves as the system changes from the monolayer to the three dimensional bulk
2H-MX$_2$ . We find vanishing $k_z$ dispersion and almost no change in the
direct band gap at the $K$ and $-K$ points, in sharp contrast to the strong
$k_z$ dispersion at the $\Gamma$ point. These facts allow us to extract the gap
and spin band splitting at the $K$ point as well as the hopping energy from
bulk ARPES data. We performed ARPES experiments on single crystals of MoS$_2$,
MoSe$_2$, WS$_2$, and WSe$_2$ at various photon energies and also with
potassium evaporation. From the data, we determined the parameters for the
massive Dirac fermion model for monolayer MoS$_2$, MoSe$_2$, WS$_2$, and
WSe$_2$.
