Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves 76-79.

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We have observed the electronic transitions within the ('4)T(,1g) ground state of Co('2+) in MgO by Raman spectroscopy. We identified the (GAMMA)(,6g) (--->) (GAMMA)(,8g)('1) transition at 305 cm('-1) and the (GAMMA)(,6g )(--->)(, )(GAMMA)(,8g)('2) transition at 930 cm('-1); the (GAMMA)(,6g )(--->)(, )(GAMMA)(,7g) transition is not seen and is lost in the two-phonon band. We also observed an impurity induced vibration mode at 280 cm('-1), in resonance with the acoustic phonon branches at the surface of the Brillouin zone. The position of the electronic levels cannot be explained by crystal field theory, with the spin-orbit interaction to second-order included. We have calculated the effect of a dynamical Jahn-Teller interaction and have shown, for the first time, the necessity to include the coupling to the T(,2g) mode, in addition to the coupling to the E(,g) mode, although Co('2+) in MgO is a weak Jahn-Teller system. / We also used Raman spectroscopy to observe the vibrational modes of (beta)-In(,2)Se(,2). We observed, for the first time, the layer to layer shear mode(E(,2g)('2)), at 19 cm('-1). The position of this peak and the others, at 42(E(,1g)('1)), 117(A(,1g)('1)), 179(E(,2g)('1)), 181(E(,1g)('2)) and 231 cm('-1)(A(,1g)('2)), follows the trend of the vibrational spectra of (beta)-Ga(,2)S(,2) and of (beta)-Ga(,2)Se(,2) {65}. Using the linear chain model, we calculated the shear force constants and deduced the position of the doublet (E(,2g)('1) and E(,1g)('2)) with very good agreement with the experimental values.