SR-FTIR Endstation

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PLEASE NOTE THAT THE FTIR END-STATION WILL SHUTDOWN ON THE 4TH OF OCTOBER 2017. IT IS NO LONGER POSSIBLE TO SUBMIT PROPOSALS FOR THIS END-STATION.

Introduction

    General Scheme End-station

 

Overview of the SR-FTIR endstation, including the transfer line, the spectrometer and the microscope.

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 project of an infrared end-station at the ESRF, on the ID21-beamline, was initiated in December 2003. The infrared microscope operates with synchrotron radiation since April 2005.

The design and the construction of the infrared end-station is the result of the collaborative work of lots of people: both from the ESRF (J. Susini, K. Scheidt, M. Cotte, E. Gagliardini, R. Baker, S. Bohic, M. Salomé) and from SOLEIL (O. Chubar, P. Dumas, F. Polack).

The ID21 FTIR end station aims at providing the X-ray microscopy community with complementary analytical tools, in particular, micro-imaging techniques with high lateral resolution, high chemical sensitivity and, low detection limit. Both IR and X-ray scanning microscopes probe different chemical scales and enable identification and location of atoms, by µ- X-ray fluorescence, atom environments (oxidation state, geometry), by µ-XANES, molecular groups and structures, by µ-FTIR.

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FTIR transfer line

The synchrotron emission is collected on BM22. The transfer line, which links the ring to the spectrometer is a complex succession of 12 mirrors, whose function is to collect, collimate, transfer, direct the beam and finally provide a parallel source. The optical concept aims at reaching the best matching of the IR beam divergence with the microscope Schwarzschild lens aperture.

M1

 The IR extraction mirror

The extraction mirror was one of the most critical technical challenges. A new dipole chamber with enlarged vertical opening had to be installed in the cell 22. Edge geometry, i.e. the edge radiation emitted from the short straight section (focusing electron lenses) upstream of the bending magnet, was preferred. The extraction mirror located at 3.2m from the dipole entrance is a flat un-cooled aluminium mirror, with a horizontal slot. This 5mm slotted-aperture lets the energetic part of the synchrotron light go through for absorption 2.5m further down. The mirror assembly is vertically movable, and by the use of thermo-probes, located on its backside and separated by 2.5mm, the slot is kept vertically centered on the heart of the X-ray beam in a slow feed-back loop. The edge radiation from up- and down-stream dipoles as well as constant field radiation are all collected and transferred to the spectrometer and microscope via a series of 12 mirrors.

 

Transfer Line

The optical pathway

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FTIR microscope

The FT-IR spectro-microscope is composed of the Thermo Nicolet Nexus infrared bench associated with an infrared Thermo Nicolet Continuum microscope. The latter has several features customised for various synchrotron applications: visualization of the sample even while acquiring data, infinity corrected optics which allows additional optical systems to be inserted such as visual and infrared polarizers, Nomarski differential interference contrast (DIC) optics, and UV fluorescence.

 

End-station

The infrared microscope at ID21

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FTIR beam flux

Comparison of the flux intensity obtained with the synchrotron source and with the internal source (Globar)

For the characterization of the beam, all the measurements were performed by reflection on a gold mirror, changing either the SR beam intensity or the beam size and comparing with the internal source. The spectral distribution of the intensity is not constant. Figure 1 shows the single beam signal obtained by reflection on a gold mirror, with an aperture of 8x8µm2, using the synchrotron source (200mA) and the internal source. The scale for internal source was magnified by a factor of 40.

Single Beam

Single beam reflected on a gold mirror, using the synchrotron source (at 200mA) and the Globar source, with an aperture of 8x8µm2 (64 scans, resolution: 8cm-1).

To estimate the flux intensity, the peak-to-peak value (ptp) of the interferogram is measured. Figure 2 and Table 1 show the variation of ptp (in V) for an increasing aperture size (from 5x5 to 15x15µm2), using i/ the internal source (Globar), ii/ the Synchrotron source, whit a current intensity going from 10 to 200mA.

ptp_sr_g

Peak-to-peak value of the interferogram, obtained by reflection on a gold mirror, with different apertures, and using the Globar source and the synchrotron source, at different current intensities.

Tableau

Table with peak-to-peak values of the interferogram, obtained by reflection on a gold mirror, with different apertures, and using the Globar source and the synchrotron source, at different current intensities (normalised with a gain of 1).

In practice, different operating modes are available at the ESRF, with a current of 16mA in single bunch mode, 90mA in 16 bunch mode and 200mA in uniform, 2/3 and hybrid modes (http://www.esrf.fr/Accelerators/Operation/Modes). Most of the experiments are carried out with a beam current of 200mA. Figure 2 shows the ratio of the peak-to-peak value with SR at 200mA vs. the Globar source.

radioptp

Figure 3: ratio of the peak-to-peak value of the interferogram synchrotron radiation (at 200mA) vs. Globar, signal measured by reflection on a gold mirror, with different apertures.

The advantage of synchrotron radiation vs. classical source is all the more important as the aperture decreases (with a factor of ~100 at the smallest apertures, in term of ptp values). This is linked to the high brightness and corollarily the low divergence of the synchrotron beam. The use of synchrotron radiation is essential when working with aperture size smaller than 15x15µm2. To calculate the signal to noise ratio, 100% reflectance lines were acquired on a gold mirror, by measuring consecutively the background and the sample on the same position. These spectra were acquired for different apertures, using the internal source (Figure 4, left) and the synchrotron radiation (Figure 4, right).The RMS noise value is determined between 2450 and 2550 cm-1 and indicated in each spectrum. These graphs confirm the better RMS noise offered by the synchrotron source, and also show that the focused spot size of the synchrotron source is diffraction limited: for apertures inferior to ~7x7µm2, the signal at low energy (below 1000 cm-1), is strongly affected by diffraction. In practice, a balance must be found to conciliate the beam spot size and the energetic range required for analysis.

Reflectance

Reflectance signal obtained for different apertures from 5x5 to 10x10µm2, using the Globar source (left) and the Synchrotron source (at 200mA, right). The RMS noise is indicated in percent.

The signal to noise value at 2500cm-1 was calculated by dividing the single beam intensity at 2500 cm-1 by the previous RMS noise value.


Signaltonoise

Signal to Noise ratio obtained with the Globar and Synchrotron Source (at 200mA) for different aperture sizes.

The Signal to Noise ratio is strongly affected by the aperture size when using the internal Globar source. On the contrary, it is quite constant with the synchrotron source, for aperture sizes above 10x10µm2. The synchrotron source is around 1000 times better than the internal source in terms of signal to noise ratio, for apertures of 10x10µm2 and smaller.

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FTIR users corner

How to apply for beam-time?

Please note that the FTIR end-station will shutdown on the 4th of October 2017. It is no longer possible to submit proposals for this end-station.

How to prepare the samples?

To get more details about the sample preparation, please download the following file: sample preparation guide.

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