  Within the Dirac model for the electronic excitations of graphene, we
calculate the full polarization tensor with finite mass and chemical potential.
It has, besides the (00)-component, a second form factor, which must be
accounted for. We obtain explicit formulas for both form factors and for the
reflection coefficients. Using these, we discuss the regions in the
momentum-frequency plane where plasmons may exist and give numeric solutions
for the plasmon dispersion relations. It turns out that plasmons exist for
both, TE and TM polarizations over the whole range of the ratio of mass to
chemical potential, except for zero chemical potential, where only a TE plasmon
exists.
