2 Synchrotron Instrumentation
2.3 Detection Modes
The absorption of synchrotron light with matters follows the Beer-Lambert law (Equation 1-4): It/I0 = exp (-μt), where I0 and It are the intensity of incident and
transmitted X-rays, respectively; μ is the absorption coefficient and t is the sample thickness. Clearly, absorption measurement with transmission mode is the most
straightforward way as demonstrated in Figure 2-8. Nevertheless, the attenuation of light in probing materials constitutes a major limitation of sample thickness in order to use the transmission mode. Because if the sample is too thick, all incident photons will be absorbed. On the contrary, if the sample is too thin, the homogeneity of the sample (i.e., the sample thickness varies from region to region) will greatly affect the transmitted signal, resulting in inconsistent results. Hence, in order to collect synchrotron data with a good signal-to-noise ratio using the transmission mode, the ideal sample thickness should be close to the one-absorption-length, i.e., t = 1/μ (or μt = 1). In this case, the incident X- rays has already been attenuated by ~63.2 % as It = I0/e ≈ 0.368 I0. Therefore, the
transmission mode is typically used in hard X-ray experiments for measuring sample with reasonable thickness, where the high energy of incident hard X-rays with its deeper penetration in materials compared to soft X-rays, allows the more practical sample thickness control. Alternatively, in most cases where the transmission mode is not feasible, total electron yield (TEY) and/or fluorescence yield (FY) as well as
in Figure 2-8 are used instead during synchrotron measurements. Specifically, as described in Chapter 1.3, when the energy of X-ray reaches the absorption threshold of the core level of an element of interest, a core electron can be excited and then a core– hole is created. Then de-excitation processes take place to annihilate the core–hole, resulting in the ejection of photoelectrons, Auger electrons and secondary electrons in the non-radiative way, or X-ray fluorescence emission in the radiative way. Since all these secondary processes are related to the probability of the presence of core–hole created by X-ray absorption, so that they are proportional to the absorption coefficient μ(E) of the element of measured.
Figure 2-8 Illustration of synchrotron detection modes.
2.3.1
Total Electron Yield (TEY)
Total electron yield (TEY) is one of the detection modes in collecting XANES spectra, TEY collects all the ejected electrons (via core-hole excitation) from the sample and can be detected via monitoring the sample neutralization current (to ground, Figure 2-8) according to the Equation 2-1.
0(1 e ) t TEY
I fcI (2-1)
Where ITEY is the intensity of the TEY signal, f is the transmission efficiency for the
detection of the electrons resulting from excitation and subsequent decay, c is a function proportional to photon energy, and I0(1–e-μt) is the photons absorbed. TEY is a surface
electron escape depth as shown in Figure 2-9. TEY is most desirable for soft X-ray measurement since soft X-ray has a shallow penetration depth and TEY does not suffer from thickness effect.
Figure 2-9 Recent electron escape depth via synchrotron radiation (SR) [13].
2.3.2
Fluorescence Yield (FY)
Fluorescence yield (FY), as another detection mode for measuring synchrotron spectra, however, is bulk sensitive compared with TEY because the attenuation length of
fluorescence X-ray is comparatively larger; it can reach hundreds of nanometers or even microns depth. For example, the X-ray attenuation length of TiO2 is shown in Figure 2- 10, of which the attenuation length before Ti L-edge is ~1 μm while a lower value of ~200 nm is for the O K-edge, and the attenuation length before Ti K-edge can reach as high as ~12 μm. Similarly, the intensity of FY (If, Figure 2-8) follows the Equation 2-1
as shown above whereas f is related to the probability of fluorescence detection instead of electrons in TEY. Normally, microchannel plates (MCPs) are used for FY detection, meanwhile, the prevention of electrons from impinging on the MCPs can be carried out by applying a large negative voltage to the first channel plate so that only fluorescence X- rays will be collected. An electron cascade will be created when X-ray photons enter the MCPs which can be detected as a current. FY can be recorded as a total-yield and/or
partial-yield using a monochromator for the energy window selection of fluorescence X- ray.
Figure 2-10 X-ray attenuation length of TiO2 (density = 3.893 g cm-3) at the Ti L-, O K, and Ti K-edges with an incident angle of 90 degree [14].
2.3.3
Photoluminescence Yield (PLY)
If the material is light-emitting, then a charge-coupled device (CCD) detector can be employed to collect the photoluminescence yield (PLY). Since PLY measures the yield of ejecting optical photons resulting from fluorescence X-ray with its favorable energy transfer in optical channels, thus the detection depth of PLY and FY are more or less comparable. Likewise, the intensity of optical luminescence (Iop, Figure 2-8) follows the
Equation 2-1 as indicated above whereas f is related to the probability of optical photons detection instead of electrons in TEY and fluorescence in FY. As the measurement of PLY accords with the simultaneous collection of energy-dependent XEOL spectra with a continuous energy scan across the absorption edge of interest, hence, PLY is also a reflection of XEOL intensity variation as a function of photon energy. In addition, like the case of FY detection mentioned above, PLY also can be measured as a total-yield and/or partial-yield using a monochromator or spectrophotometer for the wavelength window selection of photoluminescence.
2.4
References
[1] Adapted from: http://www.lightsource.ca/beamlines.html.
[2] Adapted from: https://www-als.lbl.gov/index.php/beamlines/beamlines- directory.html.
[3] Adapted from: http://sgm.lightsource.ca/.
[4] T. Regier, J. Paulsen, G. Wright, I. Coulthard, K. Tan, T. K. Sham, and R. I. R. Blyth, AIP Conf. Proc. 879, 473 (2007).
[5] K. V. Kaznatcheev, C. Karunakaran, U. D. Lanke, S. G. Urquhart, M. Obst, and A. P. Hitchcock, Nucl. Inst. Meth. 582, 96 (2007).
[6] Adapted from: http://exshare.lightsource.ca/sm/Pages/SM-Home.aspx. [7] Adapted from: http://sxrmb.lightsource.ca/.
[8] Y. F. Hu et al., AIP Conf. Proc. 1234, 343 (2010).
[9] Adapted from: http://exshare.lightsource.ca/hxma/Pages/HXMAHome.aspx. [10] D. T. Jiang, N. Chen, L. Zhang, K. Malgorzata, G. Wright, R. Igarashi, D.
Beauregard, M. Kirkham, and M. McKibben, AIP Conf. Proc. 882, 893 (2007). [11] J. J. Jia et al., Rev. Sci. Instrum. 66, 1394 (1995).
[12] Adapted from: http://www-als.lbl.gov/als/als_users_bl/8.0.1-Overview.pdf. [13] J. Zegenhagen, Eur. Phys. J. Appl. Phys. 70, 20701 (2015).