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.

Figure 1: 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.

Figure2: 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.

Table 1: 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 (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.

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.

Figure 4: 100% reflectance lines 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.

Figure 5: Signal to Noise ratio obtained with the Globar source and with the 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.