The present work presents LO and TO functions in the mid-infrared region for thin films deposited from
glow discharge plasmas of tetramethylsilane (TMS) diluted either in Ar or O2 or in mixtures of these two
gases. These functions were calculated through the Kramers−Krönig analysis of transmittance spectra of the
films supported on KBr disks. To correlate structural aspects of the films with the observed LO−TO splittings,
a group frequency analysis based on the literature was made. Such an analysis indicated that the films deposited
from the TMS−Ar mixture were formed mainly by a polycarbosilane skeleton, whereas those deposited from
TMS−O2 and TMS−O2−Ar were formed by a random network of four types of distorted tetrahedra: (CH3)3SiO0.5, (CH3)2SiO, (CH3SiO1.5), and SiO2. From the LO−TO splitting for the asymmetrical stretching mode
of Si−O−Si groups, the density and the presence of defects in samples obtained from TMS−O2 and TMS−O2−Ar mixtures were evaluated. The number of defects increased as the Ar-to-O2 flow rate decreased. We
also report for the first time LO−TO splittings for bands related to the bending of CH3 and to the stretching
of the Si−C bond in Si(CH3)
x
groups. The knowledge of such splittings is very important for a correct evaluation
of the infrared reflection−absorption spectra taken at oblique incidence of thin films containing Si−O bonds
and Si(CH3)
x
groups deposited on metals.