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Infrared Spectrosopy of Surfaces and Interfaces

Infrared spectroscopy is a very powerful method for the investigation of surfaces and adsorbate films with a number of distinct advatanges over other surface analytical methods:

  • It is based on a fairly cheap and straightforward instrumentation
  • It provides highly specific, molecular and structural information
  • It is sensitive to monolayer and submonolayer coverages
  • It is not restricted to a gas-phase ambient medium

Classical surface studies at solid or liquid surfaces in contact with air or vacuum can be performed as well as interface studies at solid/liquid or solid/solid phase boundaries. Our group focuses on the development and optimization of surface-sensitive techniques for the IR spectroscopic characterization of ultrathin adsorbate films on flat solid substrates. Apart from instrumental developments, this project relies heavily on computer simulations of surface infrared spectra, for which a comprehensive software package PARTS(TM) has been developed in our group. It allows not only a prediction of the optimum experimental parameters for each sampling technique, but is also an indispensable tool for the correct interpretation and quantitative evaluation of thin film IR spectra and provides access to structural details such as packing densities or surface orientation of the adsorbate molecules. Depending on the particular sample system and the optical properties of substrate and ambient phase, the following techniques are used :

1. External Reflection Infrared Spectroscopy (ERIRS, IRRAS)

Left: beam path
Right: IRRAS spectrum of a bilayer film of hydroxyundexanethiol (HS-(CH2)16-OH, A) and octadecyl-siloxane (OxSi-(CH)2-CH3, B) on gold

This technique is normally used for the characterization of thin films on highly reflecting metal substrates, but can also be applied for non-metals and even liquid surfaces. Whereas thin films on metals give rise to conventional absorption spectra, from which the composition and structure of the film is easily derived ("Surface Selection Rules"), nonmetal substrates cause very complex, derivative-shaped absorption profiles, which can be interpreted only with the aid of spectral simulations.

2. Internal Reflection Infrared Spectroscopy (ATR)

Left: beam path
Right: Time resolved ATR-spectra monitoring the formation of an octadecylsiloxan monolayer on silicon from a benzene solution

This method is particularily useful for studies of solid/liquid interfaces. In conjunction with a commercial variable incidence angle unit we have set up a continuous flow system, which allows in-situ monitoring of surface adsorption and reactions on IR transparent solids like silicon, germanium, zinc selenide etc. in direct contact with an adsorbate solution.

3. Brewster Angle Transmission Spectroscopy

Left: beam path
Right: IR Transmission spectrum of a monolayer film of octadecylsiloxan on mica

For IR transparent substrates with a low refractive index the classical transmission geometry is the most sensitive set-up. Unwanted interference fringes from multiple reflection within the substrate can be avoided by using p-polarized light at an oblique incidence angle equal to the Brewster angle.

4. Internal Transmission Spectroscopy

This is one of the few methods suitable to probe buried solid/solid interfaces, which is currently developed and adapted in our group for investigations of solid/solid bonding processes - see Monolayer Glues