An automated ir ellipsometer for measurements of the optical properties of thin adsorbed films is described. The instrument makes use of two stationary polarizers that bracket a rotating polarizer and the reflecting surface. This combination introduces modulation in the transmitted light intensity at twice and four times the frequency of rotation of the polarizer. The phases of these two frequency components are then measured in digital form relative to signals derived from the rotating device, the entire instrument being under automatic data acquisition control. A result for an ir ellipsometric study of formic acid chemisorbed on silver is presented.

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Gevirvnan and Kozirovski observed that the G O stretching band for CO adsorbed on evaporated alkali metal halide films at 77 K frequently appears as a doublet, with a splitting of about 10 cm-'. It is shown here that their data may be accounted for quantitatively using the optical equations derived by Dignam, Rao and Roth. On this basis, the splitting of the C-0 band arises predominantly as a result of a local field effect, the planar array of adsorbed CO molecules providing a local unaxiai symmetry.

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This paper presents a detailed analysis of the application of ellipsometry to obtaining the optical spectra (principally infrared (i.r.)) of molecules adsorbed on reflecting surfaces. Both external and total internal reflections are considered and the conditions for optimum sensitivity examined. A new empirical quantity, the relative complex optical density, is defined which exhibits thin film properties well, particularly in the case of multiple reflection measurements. An explicit expression is derived for this density function (relating it to the optical constants of the media and other system parameters), which is both reasonably simple and correct to second order terms in the film thickness. It is shown that for thin films, no higher order terms need be included, but that in general the second order term must be retained. Various limiting cases are examined to gain insight into the optical behavior of thin films, and to the same end, model calculations performed for CCIL physically adsorbed on Ag, Ni, Sb, and Ge. In relation to conventional reflection spectroscopy, ellipsometric spectroscopy is shown to have three major advantages: (I) in general, higher sensitivity to adsorbate properties; (2) very much lower sensitivity to absorption of radiation by the adjacent gas phase; (3) more information, permitting the optical constants and film thickness to be determined. Finally, the practicability of the technique is demonstrated by presenting preliminary results for C H 3 0 H reversibly adsorbed on Ag, showing clearly the C-H stretching bands.Cette publication presente une analyse detaillee de I'application de l'ellipsomttrie pour obtenir le spectre optique (principalement i.r.) de molCcules absorbees sur des surfaces rkflCchissantes. Les rkflexions externes ainsi que les reflexions totales internes sont considerees et les conditions de sensibilitk optimum sont examinees. Une nouvelle quantite empirique, la densitk optique complexe relative, est dCfinie, elle montre bien les propriCtCs des films minces, particulierement dans le cas des mesures de reflexions multiples. Une expression explicite est derivee pour cette fonction de densite (la reliant a des constantes optiques du milieu et d'autres parametres du systeme), qui est raisonnablement simple et correcte jusqu'aux termes du second ordre pour 1'6paisseur du film. I1 est dkmontrk que pour les films minces, des termes d'ordres superieurs ne sont pas necessaires, mais qu'en gkneral, le terme du second ordre doit &tre retenu. Plusieurs cas limites sont examines afin de mieux comprendre le comportement optique des films minces et dans le mCme but, des calculs modeles ont kt6 effectuks pour CCI4 physiquement absorb6 sur Ag, Ni, Sb et Ge. Par rapport a la spectroscopie de reflexion conventionnelle, la spectroscopie ellipsometrique possede trois avantages majeurs: (I) en gknkral, une sensibilitk supkrieure aux propriktts d'un adsorbate; (2) beaucoup moins de sensibilitk a I'absorption de radiation par la phase de gaz adjacent; (3) plus d'information, permettant de ...