  We present a quantum analysis of the massless excitations in graphene with a
charge impurity. When the effective charge exceeds a certain critical value,
the spectrum is quantized and is unbounded from below. The corresponding
eigenstates are square-integrable at infinity and have a rapidly oscillatory
behaviour in the short distance, which can be interpreted as a fall to the
centre. Using a cutoff regularization, we show that the effective Coulomb
interaction strength is driven to its critical value under the renormalization
group flow. In the subcritical region, we find bound states with imaginary
values of the energy for certain range of the system parameters. The physical
significance of these bound states with imaginary eigenvalues is discussed.
