Vibrational Stark effect (VSE) on the infrared spectrum of a poly(methyl methacrylate) thin film

Electric-field-induced infrared spectra of a poly(methyl methacrylate) (PMMA) thin film have been collected in the temperature range between 77 and 297 K by the difference FT-IR method using a cell with a BaF₂/Al/PMMA (500 nm)/Al layered structure. The observed temperature dependence suggests that the electric-field-induced spectra collected below 150 K are due to the vibrational Stark effect (VSE), whereas those collected above 150 K are caused by the VSE, electric-field-induced molecular orientation changes, etc. The dependence of the VSE spectrum on the electric field was also measured between 2.0 and 2.8 MV/cm at 77 K. The slightly asymmetric infrared band with a peak at 1732 cm^-1 and its VSE spectrum were fitted with two bands with peaks at 1741 and 1731 cm^-1. The 1731-cm^-1 band was assigned to CO stretching vibration. The values of the changes in the electric dipole moment Δμ and polarizability Δα between the ground and the first excited vibrational states of the CO stretching mode were determined to be 0.0633 Df^-1 and -3.3 ųf ^-2, respectively, where f is a local field correction factor.