Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 179
molecule’s vibration can resist the electromagnetic field by changing its dipole moment
[3]. The molecule does not need to have a permanent dipole for a vibration to be
stimulated, just a change in dipole moment needs to occur. Some vibrations will not be
affected by the incident radiation, and are known as infrared inactive [3].
Vibrations of polyatomic molecules
For diatomic molecules, there is only one mode of vibration, the bond stretch. In
polyatomic molecules, a number of modes exist, stretching and bending [3]. Nonlinear
molecules with N atoms have (3N – 6) modes, and linear molecules with N atoms have
(3N – 5) modes. Water (H2O), for example, has [(3 × 3) – 6] = 3 modes: (i) symmetric
stretch, (ii) asymmetric stretch, and (iii) bending modes. Fundamental and overtone
frequencies of these modes make up vibrational infrared spectra of molecules such as
H2O [3]. The symmetric stretch of CO2 leaves the net dipole moment unchanged,
therefore this mode in infrared inactive. However, the asymmetric stretch is infrared
active, as the dipole moment changes on vibration [3]. For a more detailed treatment of
infrared transmission, and multiphonon absorption in glass, see section 2.4.2.