INFRARED EXTERNAL REFLECTION SPECTROSCOPY OF ADSORBATES ON DIELECTRIC SUBSTRATES - DETERMINING ADSORBATE ORIENTATION IN LANGMUIR MONOLAYERS

Infrared external reflection spectroscopy has enjoyed enormous success in the characterization of adsorbates on metal substrates, but has been used only rarely to study adsorbates on nonmetallic substrates. In this report we present the theory of infrared external reflection spectroscopy as applied to the study of adsorbates on dielectric substrates. We begin by reviewing the well-established general macroscopic theory in which Maxwell's equations are applied to the-model system ambient-phase/adsorbate/substrate. Then we explicitly consider the case of external reflection from an arbitrary dielectric substrate and derive simple, intuitive equations for the infrared external reflection intensity as a function of macroscopic optical parameters. Our interest is in formulating a theory which will allow us to extract from experimental data information about the microscopic structure of the monolayer, such as the orientation of adsorbates on dielectric substrates, without the necessity for concomitant use of other spectroscopic techniques, such as infrared transmission or internal reflection spectroscopies, which may be impossible or inconvenient to implement; such is the case for Langmuir monolayers of long chain amphiphile molecules at the air-water interface. Unfortunately, the use of only external reflection spectroscopy to study a system results in more unknown optical constants than experimental data. To overcome this difficulty we propose for Langmuir monolayers a microscopic model which relates the optical parameters of the adsorbate monolayer to select molecular parameters, those which will allow us to extract the adsorbate orientation from experimental data. To illustrate the method we model the external reflection spectroscopy of long chain amphiphile CH2 stretching modes of typical fatty acids and alcohols. We also derive for this system a very useful dichroic ratio which facilitates the direct interpretation of experimental data. We demonstrate the behavior of the reflection spectrum as a function of adsorbate orientation using simulated spectra.