Microspectroscopic observation of vibrational linear dichroism using polarization-modulated infrared synchrotron radiation

We report on a polarization modulation apparatus on the infrared microscope of the IRIS beamline at BESSY II for dichroitic measurements with high spatial and temporal resolution. It is comprised of a beam compressor in the Fourier transform infrared spectrometer and a linear polarizer and a photoelastic modulator between the spectrometer and the microscope. We show that the spatial resolution of the instrument on thin films of polypropylene is diffraction-limited (10 × 10 μm2) in the mid infrared wavelength range. This new instrumentation makes possible polarization-dependent microspectroscopic studies of the vibrational linear dichroism and hence the preferred molecular orientation in a variety of samples, ranging from synthetic polymers, wood fibres to living cells. The high spatial resolution permits mapping of small and heterogeneous sample areas while the time resolution can be exploited to probe dynamic processes such as the response of a sample to mechanical stress.