Field-effect transistor configuration for the measurement of infrared Stark spectra

The infrared Stark spectrum of a thin film of poly(γ-benzyl-l-glutamate) (PBLG) has been measured in a field-effect transistor (FET) configuration, instead of the standard Stark cell with semitransparent metal electrodes. In the FET-type cell the PBLG film is sandwiched with an interdigital gold film and a SiO2/n-Si substrate; the gold and n-Si have been used as electrodes. The obtained Stark spectrum is in good agreement with that in the literature. Each of the observed Stark bands, which are assigned to the NH str, the CO str of the ester group, and the amide I, has been decomposed into the zeroth-, first-, and second-derivative components by the least-squares method. The effective electric field in the PBLG film has been estimated from the obtained second-derivative component for each vibrational mode on the basis of the reported difference electric dipole moment, following excitation to a vibrationally excited state. The effective electric field is 0.42 times as large as that of the corresponding capacitor with a plate electrode instead of the interdigital one. The FET-type cell can be used for the measurements of infrared Stark spectra on the basis of the obtained effective electric field.