Reflectivity changes of optically-thin nickel films exposed to oxygen

Optically-thin nickel films evaporated on the end of optical fibers show a reflectivity change when exposed to oxygen. This effect has been measured as a function of metal thickness and wavelength. Growth of NiO on the nickel surface reduces the thickness of the metal and thus the phase shift of the light passing through it. This phase change causes a change in the reflectivity. A simple classical model using Fresnel reflection coefficients fits all the data. The 4.2 Å reduction in nickel thickness deduced from the optical measurements agrees well with measurements made using modern surface analytical techniques. This new optical method for monitoring chemical interactions between adsorbates and metals may prove useful for gas sensing and corrosion monitoring. In addition, it may provide information on how chemisorption modifies the local electron density of states in metals.