Infrared Spectroscopy

Basic Principles

Infrared absorption spectroscopy can be used to probe the vibrational modes of molecules via resonant excitation. Since the energy of the absorbed light is characteristic for the chemical bonds and masses of the atoms, infrared spectroscopy is a relatively simple method to identify unknown substances or check the purity of a given sample. Further details can be found in this introduction to infrared spectroscopy (in german).

Experimental Setup

Michelson interferometer

For our measurements we use a Vertex70 and IFS48 FTIR spectrometer from Bruker Instruments with a wide spectral range (4400 - 250 cm-1), high sensitivity and a spectral resolution better than 0.5 cm-1.
For better signal-to-noise ratio and faster scanning Fourier transform IR spectroscopy (FTIR) has become popular. Using a white light source and a Michelson interferometer setup with a periodically moving mirror spectra are obtained from the measured interference pattern by Fourier transformation.

Polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS)

For surface sensitive IR spectroscopy - both at solid/gas and liquid/solid interfaces - we employ the polarization modulation technique which provides a practical method of measuring and subtracting the background for thin film samples on conducting substrates.
PM-IRRAS instruments as the PMA50 shown here effectively achive the necessary subtraction by rapidly modulating the polarization and using a lock-in amplifier to extract the signal. IRRAS spectra with a very low signal-to-noise ratio can thus be obtained.


Recently, we studied the protein-resistivity of organic monolayer (SAMs) on gold-surfaces using in situ FTIR measurements. The understanding of this resistivity is regarded as essential for the construction of implants for the human body and sensory elements for the detection of specific proteins.

For our previous work on IR spectroscopy, see our list of publications.


[1] H. Günzler, H.-U. Gremlich, IR-Spektroskopie, Wiley-VCH 2003
[2] Griffith, de Haseth, Fourier Transfom Infrared Spectroscopy, John Wiley & Sons, New York, 1986