Molecular overtones and two-phonon combination bands in the near-infrared spectra of talc, brucite and lizardite

Abstract. The near-infrared (NIR) spectra of hydrous minerals display absorption bands
involving multiple excitations of vibrational modes. They usually involve OH
stretching modes, but their interpretation is not straightforward due to the
combined effects of bond anharmonicity and vibrational coupling. In the
present study, the mid-infrared (MIR) and near-infrared spectra of
well-ordered samples of trioctahedral layered hydrous minerals, talc,
brucite and lizardite, have been measured on a spectral range extending
from the fundamental vibrational modes to the second OH stretching
overtones. The bands corresponding to molecular overtones are interpreted
using an effective approach allowing us to infer the anharmonicity and coupling
parameters controlling the OH stretching frequencies from spectroscopic data.
They follow the usual relation between transition energy and quantum number
of the excited state, which facilitates the comparison of NIR and MIR
spectra. The results support the assignment of the main overtone bands to
specific environments of OH groups and bring new constraints for the
identification of the vibrational bands related to Fe and Al substitutions
at octahedral sites in serpentines. The two-phonon absorption bands are
theoretically analyzed at the density functional theory level by computing
the absorption arising from the self-energy of the IR-active vibrational
modes. The characteristics of the two-phonon OH stretching continuum between
7300 and 7400 cm−1 and of the combination bands between 4000 and 4800 cm−1 are related to the specificities of the one-phonon and two-phonon
densities of states of the three minerals.
