Null ellipsometer for the studies of thin films at gas-water interface

Ellipsometry is a sensitive nondestructive optical technique which can be successfully used for determining the thickness of an optically transparent thin film deposited on a dielectric substrate. In this laboratory we have constructed a vertical null-type ellipsometer which has been used with a homemade aluminum Teflon-coated Langmuir trough, to study thin films at gas-water interface. Surface pressure, surface potential, and ellipsometric measurements have been performed simultaneously. Ellipsometric measurements have been done directly on the aqueous substrate contained in the trough, with and without the film, with reproducibility (± 0.02°) and good signal stability (± 0.1 mV) on the 10-mV scale. The performance of the homemade instrument had been checked against the known ellipsometric properties of spread arachidic acid on acidic water solution. In this paper, details of construction and performance of the ellipsometer as well as characteristics of the trough and ellipsometric study of β-palmitoyl-α-oleoyl-L-α- phosphatidylcholine is presented. Stability of the photometric signal as a function of the structure of the film-forming molecules is also discussed.