FTIR Introduction

What is synchrotron FTIR microscopy?

The principle of FT-IR spectroscopy is to promote the excitation of molecular vibrations by submitting a sample to an infrared beam. The vibrational energy (usually expressed as wave numbers) is directly sensitive to the molecular composition: atoms involved in the bound, nature of the bound (single, double…), surrounding atoms (hydrogen bounding…), structure (C=C cis or trans…), …. The technique is extensively used to characterize both organic and mineral samples. The principle of FTIR microscopy is to couple an FTIR spectrometer with a microscope. It enables on one hand to visualize the sample and to choose specifically the region for analysis, and on the other hand to carry out two dimensional acquisitions by raster scanning the sample. Infrared spectra are acquired at each pixel of 1D or 2D maps, and chemical maps can thus be derived. The principle of synchrotron FTIR microscopy is to use the synchrotron emission in the infrared domain as a source for FTIR microscopy. Compared to classical black body sources, the synchrotron radiation brightness is far greater and enables the beam size to be reduced below 10µm without a significant loss of photons.

Why an FTIR microscope at the ESRF ?

As with other microscopes, the performance of IR spectro-microscopes is limited by the source brightness. Despite a lower brightness compared to Infrared lasers, synchrotron radiation provides a broad spectral emission and wavelength tunability, as such required by advanced commercial FTIR-microscopes. In addition, the spatial resolution is no longer controlled by the geometrical aperture size, but rather by the numerical aperture of the optical system and the wavelength of the light. Therefore, the spot size is set to diffraction limit (a few microns). The advantages of synchrotron infrared radiation for micro-spectroscopy have been already demonstrated and exploited in most of the synchrotron facilities.

The development of a similar instrument at the ESRF was driven by two major considerations:

1. The number of infrared photons, emitted either from a dipole edge, or constant magnetic field, is essentially determined by the electron current in the storage ring. Therefore, despite being a high-energy machine, ESRF is a good infrared source as long as the appropriate collection geometry is fulfilled. Experimental measurements confirm the computations (obtained by using a SRW code developed at the ESRF) showing a photon flux compatible with a competitive scientific program. Most importantly the infrared spectral region is known to be very sensitive to the synchrotron source stability and consequently this project benefits highly from the ESRF machine performance. In addition, a particular attention was paid to avoid mechanical instabilities in the optical line, transfering the beam to the microscope.

2. The development of microanalysis and micro-spectroscopy methods, combining spatial and spectral resolutions has already attracted several very active users’ communities at the ESRF. It is worth noting that the different scientific communities that are interested in performing microanalysis, using either infrared or X-rays photons, are essentially the same. Most of the scientific cases require a multi-modal approach, consisting of a coupling of techniques providing chemical as well as structural information. The combination of micro-diffraction (ID13), micro-fluorescence (7-5keV on ID22 and 2-8keV on ID21) and micro-imaging (ID19, ID21 and ID22) and now FTIR microscopy (ID21) is a very potential micro-characterisation facility, unique in the world.