The large-radius exciton spectrum in a linear crystal with two atoms in the
unit cell was obtained using the single-electron eigenfunctions and the band
structure, which were found by the zero-range potential (ZRP) method. The
ground-state exciton binding energies for the crystal in vacuum appeared to be
larger than the corresponding energy gaps for any set of the crystal
parameters.Comment: 9 pages, 1 figure, 1 tabl