ac calorimeter for measurements of adsorbed gases on metal films at 4He temperatures

An analysis is presented of the performance of an ideal ac calorimeter limited only by thermodynamic noise. A calorimeter based on this analysis is described which is designed for studies of the heat capacity of monolayers and multilayers of atoms and molecules adsorbed on a variety of surfaces. The calorimeter consists of Ge:Ga thermometers and NiCr heaters mounted on one side of a sapphire substrate, with an evaporated film deposited on the other side. The calorimeter is mounted on a 4He cold finger in a UHV system allowing heat capacity measurements down to 1.6 K. In order to verify the performance of the calorimeter, measurements have been made of the heat capacity of 25-μg samples of indium, and of submonolayer coverages of 4He on sapphire. The measured sensitivity corresponds to <10−2 monolayers of 4He adsorbed on the surface of the calorimeter in the gas phase.