We develop an approximate second quantization method for describing the many-particle systems in the presence of bound states of particles at low energies (the kinetic energy of particles is small in comparison to the binding energy of compound particles). In this approximation the compound and elementary particles are considered on an equal basis. This means that creation and annihilation operators of compound particles can be introduced. The Hamiltonians, which specify the interactions between compound and elementary particles and between compound particles themselves are found in terms of the interaction amplitudes for elementary particles. The nonrelativistic quantum electrodynamics is developed for systems containing both elementary and compound particles. Some applications of this theory are considered.

Recent infrared absorption studies of semi-insulating GaAs have revealed an electrically active, oxygen-related defect with remarkable spectroscopic and microscopic properties. This defect, t h e Structural analogue of the oxygen vacancy centre in Si, occurs in three cnarge states, the zero-, oneand two-electron states. The experimental fingerprint for each charge state is t h e local mode frequency which shows a characteristic charge-state-induced shift. Dependent on the Fermi potential, at thermal equilibrium only the local modes corresponding to the zero-or t h e two-electron state are experimentally observable. The metastable one-electron state disproportionates spontaneously into the zero-and t h e two-electron states. The energy positions of the associated gap levels are at E, -0.14 eV and at E, -0.58 eV for the first and t h e second electron, respectively. These assignments are derived from the thermally activated decay of t h e local mode lines and, independently, from t h e threshold energies of the optical transitions uE(1) and 032). Through a comparative deep-level transient spectroscopy and infrared absorption study on neutron-transmutation-doped, n-type samples the second electron level is identified as the well known EL3 level.

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