Photothermal Spectroscopy of Surface Adsorbates.

Abstract

A new spectroscopic technique for obtaining directly the electronic spectra of thin absorbing films on the surface of a metal has been developed. The method, called Photothermal Spectroscopy, is based on the measurement of small temperature changes upon electronic excitation of surface adsorbed species. Extremely sensitive thin-film amorphous and extrinsic semiconductor bolometers are used as the temperature-sensing element. These devices, when cooled to cryogenic temperatures, are capable of detecting submonolayer coverages. Photothermal Spectroscopy offers sensitivity that is equivalent or superior to other methods currently in use for the study of surfaces, but with considerable reduction in instrumental cost and complexity. Under high-vacuum conditions and temperatures of 10 K, the adsorption of both multilayer and monolayer films of pyridine on evaporated films of gold and nickel were studied. Multilayer films were found to physisorb to the surface of the metal films, resulting in an electronic absorption b and with vibrational features similar to that found in the free molecule. Monolayer films of pyridine are very weakly chemisorbed to the metal surfaces. A broad, featureless electronic absorption b and , that was considerably blue-shifted, was recorded.