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5. The dipole matrix element between the X1 and X3 bands vanishes because of time-reversal symmetry. This follows from the method described in: Lax, M.: Int. Conf. of the Physics of Semiconductors, Exeter, 395 (1962), when applied to the selection rules between two X1 representations in a diamond-type crystal. The splitting in a III-V crystal of the X1 representation into X1 and X3 does not, to first order, alter the basic band states ,. We find for, f=2.