Optical properties of Pd thin films exposed to hydrogen studied by transmittance and reflectance spectroscopy

The optical properties of hydrogen exposed palladium (Pd) and palladium oxide (PdO) thin films are calculated from reflectance and transmittance measurements in the visible and near infrared range (400-900 nm). The time evolution of the films' optical constants when exposed to hydrogen is shown. The real part of palladium's refractive index increases about 5%, whereas the imaginary part decreases nearly 15% after hydrogen absorption. The Pd films' resistance also increased upon hydrogen absorption, as expected. Pd oxide reduces to metallic palladium upon hydrogenation, showing a dramatic change in all the properties. The palladium films' resistance initially decreases after a first exposure to 5 Torr of hydrogen, and this is tentatively explained by the reaction of hydrogen with impurities trapped in the films.