Optical Spectroscopy

The optical spectroscopy of polymers yields information on the chemical structure and orientation of molecular units, electronic and vibronic excitations and relaxations, and optical materials constants such as absorption coefficient and refractive index. Experienced staff technicians keep the ‘Know How’ of the instrumentation and technical developments. They are responsible for running condition and maintenance of the sensitive spectroscopic equipment. The activities of the optical spectroscopy group involve two major points:

1. Methods like absorption- and reflection-spectroscopy from the ultraviolet to the infrared (IR) spectral range, fluorescence- and Raman-spectroscopy are applied as a routine service to provide optical spectra to everyone who is not familiar with the details of the complicated and valuable instruments. Special training is provided to those scientists who need the optical techniques very frequently that they can perform the measurements themselves.
 Supplementary techniques are microscopes for the IR- and Raman-spectrometers, cryo- and thermostats to achieve variable sample temperatures from 10 K to 500 K, and high-intensity lamps for photochemical reactions.

2. The development of measurement techniques is frequently required for special projects in the Institute. Examples of such developments are:
- IR reflection spectroscopy under grazing incidence conditions can be used to improve the measurement sensitivity of monolayers. This gives valuable information on the orientation and molecular order of ultrathin films.
- Reflection and absorption spectroscopy in the visible and near-infrared range is used to determine the refractive index of thin films. The method is based on solving the Fresnel equations without any fit parameters and yields the dispersion of the refractive index.
- A prism-coupler is used to study planar waveguides. The refractive index, thickness, and attenuation loss of thin film waveguides can be determined at various laser wavelengths which are available from the Raman experiment.

Principle of reflection spectroscopy of a monolayer on a metallic substrate under grazing incidence conditions of infrared radiation. The resulting electrical field E is oriented perpendicular to the metal surface. Only the molecular vibration a) can be excited. Vibration b) is oriented parallel to the surface. It cannot interact with the IR beam