The binding of two free excitons into a bi-exciton due to direct exciton-exciton interaction is studied. The process of bi-exciton formation via exciton-phonon interaction is not taken into account. By group theory methods the wave functions of two interacting excitons are constructed for a model applicable to CuCl and Cu,O crystals. Making use of these functions, the Fourier transform of exciton-exciton interaction energy is calculated. By the Green's function method the dissociation energy of the bi-exciton is obtained in the adiabatic approximation (the exciton-exciton interaction energy is much smaller than the ionization energy of the exciton). A numerical estimate and comparison with experimental values is made for CuCl crystals. M3yYeIIO 06pa30BaHHe 6H3KCHTOHOB Be3 YseTa KOCBeHHOrO B3aElMOAefiCTBHR Memny BKCHTOHBMH qepe3 @OHOHbI. MeTOAOM TeOpHEl I'pyIIII IlOCTpOeHbI BOJIHOBbIe @YHKUHll nByX B3aHMOnefiCTByEOUHX BKCHTOHOB AJIII MOAeJIH, IIpHMeHHMOfi K KPIIC-TaJIJlaM CUCl H cU,O. &kIlOJlL3yfI 3TH @YHK4HM, BblWiCJIeH @ypbe-O6pa3 3Heprkiki B3aHMOHefiCTBHR ABYX BKCHTOHOB . MeTOAOM @YHKUHfi rpHHa B a n u a 6 a~~s e c~o~ npH6JIHmeHHH (3HeprHR BBaHMOHefiCTBHH BKCHTOHOB 3HaWiTeJIbHO MeHLIUe 3HeprHil.i HOHH3aUMH 3KCHTOHa) BbIqHCJIeHa 3HeprHII nHCC0UHaUHH 6HBKCHTOHa. BbInon-HeHbI 4HCJIeHHbIe OUeHKH H CpaBHeHMe C 3KCIIepHMeHTaJILHbIMH pe3yJIbTaTaMl.3 EJIII EcpacTanna CuCI .

The intra-Landau-level excitations of the two-dimensional electron-hole liquid are characterized by two branches of the energy spectrum. The acoustical plasmon branch with in-phase oscillations of electrons and holes has a linear dispersion law in the range of small wavevectors, with a velocity which does not depend on the magnetic field strength, and monotonically increases with saturation at higher values of the wavevectors. The optical plasmon branch with oscillations of electrons and holes in opposite phases has a quadratic dependence in the range of long wavelength, a weak roton-type behaviour at the intermediary values of the wavevectors and monotonically increases with saturation similar to the case of the acoustical branch. The influence of the supplementary in-plane electric field leads to the drift of the charged particles in the crossed electric and magnetic fields and to the energy spectrum as in the reference frame, where the e-h system is moving with the drift velocity. A perturbation theory using the Green function method is developed on the basis of a small parameter v(2)(1-v(2)), where v(2) is the filling factor and (1-v(2)) displays the phase space filling effect.

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