  We present a formalism for calculating the absorption coefficient of a pair
of coaxial tubules. A spatially nonlocal, dynamical self-consistent field
theory is obtained by calculating the electrostatic potential produced by the
charge density fluctuations as well as the external electric field. There are
peaks in the absorption spectrum arising from plasma excitations corresponding
either to plasmon or particle-hole modes. In this paper, we numerically
calculate the plasmon contribution to the absorption spectrum when an external
electric field is applied. The number of peaks depends on the radius of the
inner as well as outer tubule. The height of each peak is determined by the
plasmon wavelength and energy. For a chosen wave number, the most energetic
plasmon has the highest peak corresponding to the largest oscillator strength
of the excited modes. Some of the low-frequency plasmon modes have such weak
coupling to an external electric field that they are not seen on the same scale
as the modes with larger energy of excitation. We plot the peak positions of
the plasmon excitations for a pair of coaxial tubules. The coupled modes on the
two tubules are split by the Coulomb interaction. The energies of the two
highest plasmon branches increase with the radius of the outer tubule. On the
contrary, the lowest modes decrease in energy as this radius is increased. No
effects due to inter-tubule hopping are included in these calculations.
