Abstract
The detection of longitudinal optic (LO) components of the vibrational modes has been demonstrated for different types of glasses, through the use of infrared specular reflectivity at oblique incidence. In this work, both natural and polarized infrared radiation have been used in order to measure the reflectance of ZBLAN (ZrF 4–BaF 2–LaF 3–AlF 3–NaF) glasses at oblique incidence. The series of glasses analyzed contained between 0–25 mol% NaF. IR reflectance was measured for incidence angles between 10° and 70° off-normal and the near-normal incidence data using natural light was transformed by Kramers–Kronig analysis, in order to yield the LO (and also the transverse, or TO) spectra. These results are compared with the experimental polarized reflectivity curves, as a function of the Na content of the glasses and with calculated reflectance spectra. The experimental detection of LO modes by infrared reflection spectroscopy at oblique incidence appears to be quite general and it has been confirmed for all types of glasses or amorphous thin films studied so far. The p-polarized component of the radiation is primarily responsible for the LO mode detection.
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