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The dispersion relation for the normal modes of vibration propagating along the 00, , and 0 directions in a single crystal of pure gallium arsenide at 296^ has been determined by neutron spectrometry. The frequencies (units 10^12 cps) of specific modes are (a) (0, 0, 0): TP 8. 020. 08, LO 8. 550. 20; (b) (1, 0, 0): TO 7. 560. 08, TA 2. 360. 015, LO 7. 220. 15, LA 6. 800. 06; (c) (0. 5, 0. 5, 0. 5): TO 7. 840. 12, TA 1. 860. 02, LO 7. 150. 07, LA 6. 260. 10. The results are analysed in terms of the dipole approximation model. Although a fairly complex version of this model provides a satisfactory description of most of the data, certain significant discrepancies remain. These indicate that the neglect of quadrupole interactions is a serious weakness of the dipole approximation model.
Waugh et al. (Sun,) studied this question.