Interpretation of THz intensities of molecular crystals: the role of mixing between intermolecular and intramolecular vibrations

Through a quantitative discussion of THz modes in the racemic and enantiomeric crystals of alanine, we have examined the restriction on the THz selection rule imposed by the space-group symmetries. We have also revealed an essential role played by intramolecular vibrations in determining the IR intensities of THz modes in the low-frequency region. Although the intramolecular vibrations make marginal contributions to the vibrational energies of the low-frequency modes, they are crucial to the generation of transition moments.