case of indirect excitons 9 this interaction is not forbidden by translational symmetry. It may be realized in the form of simultaneous scattering of electrons and holes between different X points by either Coulomb or electron-phonon interaction.A quantitative determination of the matrix elements in question and the deformation potentials is difficult because of uncertainties in the decomposition of overlapping bands A, B, and C. At the present time we can only give a rough estimate of these quantities. Using the hydrostatic deformation potential 10 to eliminate A x we find that A 3 »7 eV per unit shear deformation \e ee -?(e xx + e yy ). This rather large value again indicates that the exciton transition does not take place at the center of the Brillouin zone but that a star degeneracy is involved. Preliminary analysis shows that E X
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