  The interband pi and pi+sigma plasmons in pristine graphene and the Dirac
plasmon in doped graphene are not applicable, since they are broad or weak, and
weakly couple to an external longitudinal or electromagnetic probe. Therefore,
the ab initio Density Function Theory is used to demonstrate that the chemical
doping of the graphene by the alkali or alkaline earth atoms dramatically
changes the poor graphene excitation spectrum in the ultra-violet frequency
range (4 - 10 eV). Four prominent modes are detected. Two of them are the
intra-layer plasmons with the square-root dispersion, characteristic for the
two-dimensional modes. The remaining two are the inter-layer plasmons, very
strong in the long-wavelength limit but damped for larger wave-vectors. The
optical absorption calculations show that the inter-layer plasmons are both
optically active, which makes these materials suitable for small organic
molecule sensing. This is particularly intriguing because the optically active
two-dimensional plasmons have not been detected in other materials.
