  The spectrum of large radius exciton in an individual semiconducting
single-walled carbon nanotube (SWCNT) is described within the framework of
elementary potential model, in which exciton is modeled as bound state of two
oppositely charged quasi-particles confined on the tube surface. Due to the
parity of the interaction potential the exciton states split into the odd and
even series. It is shown that for the bare and screened Coulomb electron-hole
(e-h) potentials the binding energy of even excitons in the ground state well
exceeds the energy gap. The factors preventing the collapse of single-electron
states in isolated semiconducting SWCNTs are discussed.
