Notice of infrared beamlines : AILES et SMIS

Infrared spectroscopy covers a tremendous number of studies including for chemical characterisation of materials, the optical studies of solids or pure vibrational and rotational molecular spectroscopy. Fourier Transform Infrared spectroscopy (FTIR) is an easy-to-use and non destructive method which belongs to the main analytical techniques in chemistry, physics and biology. Although, the commercial IR spectrometers are equipped with conventional thermal (globar) sources providing power comparable to the IR radiation emitted from a synchrotron, the synchrotron IR light because of its brightness (100-1000 times greater) has allowed important breakthroughs in two area : extension to the lowest energy range (Far infrared) and study of extremely small samples. Nowadays, extraction of infrared radiation is part of almost all developing programs in existing or planned facilities.

This project concerns two IR stations at SOLEIL, one dedicated to far-IR spectroscopy, and one dedicated to IR microspectroscopy and imaging. These two beamlines will be internationally competitive, and will make possible state-of-the-art IR investigations for several ( national and international) groups.

The spectral domain of far infrared (l @ 10-200 m m, or w= 1/l @ 50-1000 cm^-1) remains one of the last borders in vibrational spectroscopy. It is an experimentally challenging domain due to the limited intensity of the "classical" thermal sources which is little over the emission of the room temperature blackbody of the optical elements. This difficulty can be overcome by the use of high brightness sources, which are equivalent to several thousands degree-blackbody.
The absorption in the far infrared corresponds to transitions from an initial state to a low energy final excited state. For isolated molecules, this excitation corresponds to rotational transitions, vibrational excitations of large molecules or roto-vibrational transitions. Their measurements at high resolution will provide a better description and understanding of species or chemical reactions. The very high brightness of the synchrotron radiation allows to achieve absorption lengths of 100 meters. The association of these three elements (Synchrotron source, Interferometer and multiple path cell) constitutes an ensemble of very high detectivity. One can then extend measurements to absorption of species produced under low concentration such as van der Waals molecules produced in cooled cells and neutral or charged radical species created in the plasmas generated by electric discharges.
In the case of condensed matter, the intermolecular motions have characteristic resonant frequencies in the far-IR. The study of intra- and inter-molecular vibrations will yield information on bonding properties in crystals, glasses, liquids and melts, thereby providing a microscopic description of thermochemical properties.

In solids, the infrared absorption, also referred to as optical studies, provides information on electronic and structural properties of condensed materials through the understanding of electronic excitations, such as crystal field, charge transfer and excited states of insulators, intraband transitions (Drude band or plasmon), and interband electronic transitions. It also allows studies of transitions evidenced in the phase diagram such as those occurring as the doping level, temperature, pressure or confining geometry are modified.

In surface and interface studies, adsorbates which are bonded to a substrate are characterized by adsorbate-substrate absorption bands in the 250-500 microns region (except for hydrogen, which is a light atom). As these vibrational motions have a very poor dynamic dipole moment (one of the atom involved in the motion is a substrate atom, almost " rigid"), detection and identification of such bands were precluded until the use of a synchrotron radiation source. Additionally, thanks to the strict proportionality of the IR radiated intensity with the current of the storage ring, unrivalled information have been obtained, which are of paramount importance for understanding reaction dynamics at surfaces (more particularly the large change of electronic properties of metallic substrates by adsorbates).

Infrared microscopy has benefited a lot from the high brightness of the synchrotron source. The lateral resolution, which is achievable, has become diffraction- limited ( half of the probing wavelength in a confocal configuration). This analytical technique is mainly used in the mid-IR region (2.5 to 50 microns), which is the domain where almost all of the internal motions of individual molecular groups show resonant frequencies. It is used for identification of compounds, as each molecule has its own vibrational spectrum . Thanks to its improved lateral resolution (about one order of magnitude) synchrotron IR microscopy has became an important application field, in microspectrometry and in chemically-selective imaging technique, opening up new investigations, for example in Geology, Cellular Biology Forensic science and Environment.

Geological applications :

The Earth's deep interior may contain up to four times the amount of water present today in the hydrosphere. These estimates have been made by measuring the water content either of the mantle xenoliths that reach the surface, or of the lavas emitted at mid-oceanic ridges. To determine the importance of nominally anhydrous minerals as water reservoirs in the mantle, numerous studies have been undertaken to measure the solubility of water. Deep investigations in petrography during the last decades have demonstrated that most of the rock formations and their successive transformations in earth evolution are dealing with any kind of fluid, whether the fluid is a silicate magma, a water solution, and/or a mixture of gas and hydrocarbon. The identification of the organic matter included in fluid inclusions inside mineral is of prime importance for petroleum exploration. The search for life in extraterrestrial materials is directly linked with search of water and organic molecules in meteorites. FTIR is known as having the best detection limit for all these fluid characterizations. Therefore, it appears challenging for petrologists to detect and analyse very locally within representative minerals the chemical composition of the fluids at each step of the evolution at a few micron scale range with a non-destructive method.

Studies of interstellar durst particles :
Much of what we know about the interstellar medium and other stars comes from spectroscopy. Infrared spectroscopy generally provides identification of solid compounds and minerals in the dust. Recently, the Infrared Space Observatory (ISO) has greatly expanded the infrared spectroscopy of interstellar and circumstellar regions. Features are seen in emission from hot regions, or, absorption in cold regions illuminated from behind by a continuum source. To associate a specific absorption feature with a particular compound or mineral the infrared signature must have been measured in the laboratory for comparison. Interstellar grains generally show broad 10 and 20 mm absorption features that suggest a disordered silicate. A variety of grains, including crystalline silicate, glassy silicate, oxides, and other phases, many yet unidentified, occur in the circumstellar environment. A comprehensive project to characterize the components of IDPs (interplanetary dust particles), meteorites and terrestrial minerals, has begun in 1999 in an effort to identify the compounds and minerals in interstellar and circumstellar grains. The IDPs, which are believed to be fragments of comets and asteroids, consist of micron to sub-micron aggregates of different minerals and compounds. Because of the small size of these individual subunits, the high intensity of synchrotron-based FTIR is required to obtain spectra of the subunits of the IDPs.

Biologically relevant studies:

Although conventional IR microspectroscopy has proven extremely valuable for resolving the chemical components in biological samples, the low brightness of the internal source ( globar) has been the limiting factor in the achievable lateral resolution (typically > 15 mm). This constrains the analysis of biological specimens to the tissue level only. Individual biological cells are typically 5-30 m m in diameter, making them too small to probe with a conventional IR source. The high spatial resolution of a synchrotron IR source permits the chemical mapping of single living cells for the first time. With the ability to probe smaller and smaller areas with the synchrotron IR microscope, new techniques are currently being applied to aid in sample visualization with fluorescence microscopy and probed with the IR microscope. Once identified, the IR microscope can be used to analyse the chemical environment in and around that region of interest. It should be noted that fluorescent labels are generally present in extremely low (i.e.nanomolar) concentrations, so they do not interfere with the IR technique. From cellular biology, at single cell level, to high contrast study in tissues, this field will emulate new studies, and necessitate nearby facilities and equipment for biological manipulations. The study of human tissues ( such as skin and hair) is a topic of importance not only for medical application, but also for cosmetic purpose. Synchrotron IR microscopy is currently exploited for the study of human hair composition and structure. Several studies are undertaken to identify tumoral tissues and cells, using synchrotron IR microscopy. This has potential application in diagnosis of such a disease. In addition, it has been recently shown that IR microscopy could be used as a diagnostic tool in the case of Alzheimer disease.

Moreover, combined fluorescence analysis as sample visualization benefits from the use of fluorescence illumination. Fluorescent compounds absorb and subsequently light at wavelengths longer than the absorbed wavelength. Primary (natural) fluorescence is known to occur in plant cell walls, wool, as well as many pharmaceutical products. Secondary fluorescence arises from the use of fluorescent dyes or fluorochromes to illuminate samples that do not exhibit native fluorescence. The dyes are typically bound to the compounds of interest.

BLP 21 : Infrared Spectroscopy and Microscopy.The SAC recommends the construction of this beamline.

The project is very good and is supported by a strong French community in IR spectroscopy. The SAC advises the promoters of the project:

• To pay much attention to the possible noise problem which may occur with the edge emission source,
  
  
• To consider including an active feedback system to cope with possible mechanical instabilities in the optics,
  
  
• To evaluate the possibility of extending the IR wavelength beyond 200 microns without significantly degrading the technical specifications of the beamline,
  
  
• To encourage contacts with additional partners concerning the research programme envisaged on heterogeneous catalysis (industrials, IFP, IRC, etc.).

Les techniques IR se développent très rapidement et concernent des applications variées. Compte tenu du dynamisme du domaine, d’un coût relativement modeste et de la bonne maîtrise des problèmes techniques, la ligne avec ses deux stations devrait être opérationnelle dès la phase I.Il serait souhaitable qu’une seconde ligne de microscopie IR soit construite ultérieurement sur financement industriel.