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 297K by the difference FT-IR method using a cell with a BaF2/Al/PMMA (500nm)/Al layered structure. The observed temperature dependence suggests that the electric-field-induced spectra collected below 150K are due to the vibrational Stark effect (VSE), whereas those collected above 150K 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 2.8MV/cm at 77K. The slightly asymmetric infrared band with a peak at 1732cm−1 and its VSE spectrum were fitted with two bands with peaks at 1741 and 1731cm−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 0.0633Df−1 and −3.3Å3f −2, respectively, where f is a local field correction factor.