  The gravitational field is usually neglected in the calculation of atomic
energy levels as its effect is much weaker than the electromagnetic field, but
that is not the case for a particle orbiting a black hole. In this work, the
canonical quantization of a massive and massless particles under gravitational
field exerted by this tiny but very massive object, both neutral and charged,
is carried out. By using this method, a very rare exact result of the
particle's quantized energy can be discovered. The presence of a very strong
attractive field and also the horizon make the energy complex valued and force
the corresponding wave function to be a quasibound state. Moreover, by taking
the small scale limit, the system becomes a gravitational atom in the sense of
Hydrogenic atoms energy levels and its wave function can be rediscovered.
Moreover, analogous to electronic transitions, the transition of the particle
in this case emits a graviton which carries a unique fingerprint of the black
hole such as black hole's mass and charge.
