For MEX2 we have two standard methods of measuring the signal from the sample, drain current and fluorescence

Drain Current

In Drain Current mode, we measure the total emission of electrons from the sample as we scan the photon energy.  We measure this by measuring the electron current that flows back into the sample holder using a very sensitive current amplifier. 

Note this requires several prerequisites

1. The sample is measured in a vacuum, so it is not compatible with running in a Helium Atmosphere.

2. The sample must have some degree of electrical conductivity. 

Note we use double sided carbon tape that is very conductive even samples which are normally considered insulators if they are spread very thinly on this carbon tape will often have sufficient conductivity to measure a drain current.

If you have samples coated on to glass substrates if we make an electrical bridge from the surface to the stainless-steel ruler, we can often get sufficient conductivity to measure a drain current.  We can often use the double-sided carbon tape to make such a bridge.  If your samples are likely to be electrically insulating, please contact the beamline staff. 

3. There is a limit of around 1 atomic percent for the element of interest so if you have 1 S atom in a matrix of 99 other atoms, we can probably measure a XANES spectrum.  At 0.1 atomic percent we can probably measure a XANES signal.  At 0.01% we cannot measure a signal with drain current.

4. With samples of 1 Atomic percent and above, often the drain current signal is the best signal it has the advantage that the signal only comes from the first 10 nm or so into the surface of the material and in this region, it is over absorption free (over absorption is often also called self-absorption).

The drain current is quite surface sensitive if you are interested in the bulk properties of your sample you have to be attuned that the surface might be naturally different to the bulk.

Fluorescence Yield

In fluorescence mode, we measure the fluorescence photons produced when we excite the particular target atom. 

1. The sample can be measured in either a vacuum or in an atmosphere of helium. 

2. The sample is insensitive to electrical charging.

3. At present we do not know the ultimate minimum concentration of elements for which we can measure a XANES spectrum.  But we have had a measurement of 1 ppm of S from a NIST derived calibration standard.  A safe minimum concentration for which we would be able to get a recognisable XANES spectrum would be 10 ppm.  As with all such limits it is also subject to possible other elements giving interference signals which would increase the minimum detection limit.  It should be noted that the S measurement was made in a matrix of silicon containing glass, which did not affect the measurement.

4. Potentially we can measure samples in a liquid, though we have not made a suitable user cell, we are working on this, though if you wish to make your own, we will help with this.  Liquid cells can only be measured in a Helium Atmosphere. Please note that the standard 1 thou (1 thousandth of an inch = 25 micron) Kapton film strongly attenuates the X‑ray beam at the P and S edges we suggest using an 8-micron or less film.  We have some 8-micron film if required.  We cannot use the standard glued Kapton film as used at higher energies due to the absorption of both the Kapton and the glue.

5. We have never measured transmission on MEX2, potentially we could measure transmission but if you want to do this, please give a detailed explanation of how you will prepare such samples and indicate the thickness of your samples in microns.  Give some supporting transmission curves as to how these samples will operate at the very low photon energies that we have on MEX2.  If you do not give this, we will mark this part of your experiment as “technically infeasible” with a high likelihood that you will not get time.  If you think you can do this, we would like to help you achieve this.  

6. Diluting samples with cellulose, as is common on higher energy beamlines such as MEX1 and XAS is not normally effective due to the very short penetration depth of X‑rays in this photon energy range.  If you are going to do this, please then supply some rationale as to why you are doing this.  If you are doing this to allow for transmission measurements, then unless you can make a significant justification as to why this will work, we would mark this as Technically Infeasible.  Doing this to compensate for potential over absorption really requires that the element in the target matrix has to have a powder sample size of nominally less than ½ of the attenuation length which is often micron to sub-micron size. You might have the skill to make this, it will help your case if you mention the particle size that you will be aiming for to show you understand fully this requirement.  

7. Naturally dilute samples with concentrations in the 10,000 to 100 ppm level are well served by fluorescence measurements on the beamline and these measurements will likely not have over absorption issues.

Samples with concentrations above 10,000 ppm will probably suffer from some over absorption.  If possible if you can measure the sample with drain current at the same time as fluorescence yield, then the drain current can help to understand how the over absorption is manifesting with your samples.   You unfortunately cannot measure drain current through any sort of Kapton or polymer film. 

Even though the fluorescence yield will suffer from over absorption often in comparison between samples is enough to understand what might be happening.  Over absorption is a “fact of life” in this energy regime.  We implicitly assume that you recognise this, we will never mark your experiments as technically infeasible from measuring fluorescence from concentrated samples.  You do not have to have to detail a strategy for dealing with potential over absorption in your submission.