Raman spectroscopy of kaolinites using different excitation wavelengths

The Raman spectra of both low‐ and high‐defect kaolinites in the hydroxyl stretching and low‐wavenumber region were obtained with excitation at three visible wavelengths of 633, 514 and 442 nm and a UV wavelength of 325 nm. The UV‐excited spectra were comparable to those excited by the visible wavelengths. The Raman spectra show hydroxyl stretching bands at 3621 cm −1 attributed to the inner hydroxyl, at 3692 and 3684 cm −1 attributed to the longitudinal and transverse optic modes of the inner surface hydroxyls and at 3668 and 3653 cm −1 assigned to the out‐of phase vibrations of the inner surface hydroxyls. Two bands were observed in the spectral profile at 3695 cm −1 for the high‐defect kaolinite at 3698 and 3691 cm −1 and were assigned to TO/LO splitting. An increase in relative intensity of the transverse optic mode is observed with decrease in laser wavelength. The intensity of the out‐of‐phase vibrations at 3668 and 3653 cm −1 of the inner surface hydroxyls shows a linear relationship with the longitudinal and transverse optic modes. In the low‐wavenumber region excellent correlation was found between the experimentally determined and the calculated band positions.