Tey background treatment

The original measured spectra have several features folded into the signal: i) The gradually diminishing transmission of the beamline towards higher photon ener-gies. ii) Presently unavoidable at BESSY II is the continuously decreasing ring current of the storage ring⁵, which evokes a falling background. The unwanted signal components can be removed to a large extent by normalizing the signal to the current measured at the last refocussing mirror.

In case of the CoPt nanoparticles the tey signal is strong (signal to background ratio 2:1), comparable with a several layer thick thin lm, that support the mass selected clusters. After normalization to the last mirror current remains only a slightly falling background that represents the background due to the silicon wafer (respectably the copper crystal) and will be tted by a straight line and then removed. The proper treatment for it is the linear tting in the pre-L3-edge region of the spectrum and subtraction of this linear function from the entire spectrum. All processing steps are illustrated in gure3.20.

The magnetic properties of the3delements are almost entirely due to the not com-pletely lled d-states. For the application of the XMCD sum rules (formula 2.3 and 2.4), which only consider excitations from 2pto3dstates, the spectrum has to be stripped of all2pto continuum and2ptos-state contributions (section2.1).

This is typically done by subtracting a hyperbolic step function with two steps, one at the L3 and one at the L2-edge with a step height ratio of two to one (section 2.1.1) resulting in a spectrum consisting only of the contributions from 2p1/2 and 2p3/2 to empty d-state transitions. All spectra taken for the mass se-lected cluster substrates and the wetchemically prepared nanoparticles have been equally treated in described way. Errors assumed for the sum rules are typically 10% [25].

5. This eect is due to the injections of new electrons into the storage ring every 8h. The most recently build storage rings use the so called top up mode, which allows an almost continuos injection of new charge carriers. The injection of new particles every minutes is much more demanding, but if it is achieved allows the beam to be used at a constant intensity and almost completely removes the heat load uctuations within the beamline due to ring current changes.

795 790 785 780

775 800

Photon energy (eV) measured sample current

slope normalized to last mirror current

stepfunction

slope corrected step function removed

Total electron yield (arb. units)

795 790 785 780

775 800

795 790 785 780

775 800

795 790 785 780

775 800

Photon energy (eV)

Photon energy (eV) Photon energy (eV)

Total electron yield (arb. units)Total electron yield (arb. units)

Total electron yield (arb. units)

a b

c d

Figure 3.20: An illustration of the main data evaluation steps: The total electron yield raw data measured at the Co L2,3-edges (a) has to be normalized to the photon ux, using the last mirror current, then removing the slight slope (b) of the falling background and the step function (c) originating in non 2p to 3d

excitations, one receives the pure resonant 2p to 3dabsorption signal (d).

The coverage of the mass selected clusters being only 3% of an atomic layer on the surface gives a very weak signal, which makes the background treatment slightly more challenging. First of all it is crucial to measure background. A spectra, that records the substrate features in the region of the deposited clusters, without the clusters present (gure 3.21).

Photon energy (eV) 810 800 790 780 760 770

Co L Co L

Co L

Co L

a b

3 3

2

2

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Photon energy (eV)

Total electron yield (arb. units) Total electron yield (arb. units)

Figure 3.21: The background for the mass selected clusters is recorded in the region of the cobalt L-edges, for a nickel substrate (a) and an iron substrate (b). Very evident is the dierent background structure, while in (a) there are absorption features only visible at the Co L-edges (a beamline contamination) in (b) the beamline is clean, but there are strong features due to the iron substrate Obviously the two backgrounds are very dierent for the nickel and the iron thin lm. First of all there are absorption features at the Co L-edges in case of the nickel thin lm and none when using the iron layer. Between the last beamtime with iron thin lm as substrate layer and the beamtime with nickel as substrate, the switching mirror unit was plasma cleaned to remove carbon from the mirrors.

The antenna to produce the high frequency electromagnetic eld needed to ignite the plasma had a brass thread. The plasma cleaned o the carbon, but some of the brass remained in the vacuum chamber, producing new absorption features in the x-ray beam emitted by that beamline. One absorption feature was at the cobalt L-edges. Otherwise the nickel background looks rather smooth, but not so for iron. The strong wiggling features there are iron EXAFS oscillation from the iron L-edges at h·ν = 706.8 eV and 719.9 eV.

The background spectra taken will be used to be subtracted from the measured cluster spectrum, if the background is really smooth. If the background has structure, it is better to divide the cluster spectrum by the background. This procedure has thoroughly been discussed by J.Stöhr [132] and successfully been applied for deposited mass selected clusters by J.T.Lau [77] and M.Reif [111].

measured signal (CoPt₃)

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Photon energy (eV)

normalized signal (CoPt₃)

820 810 800 790 780 770 760

Photon energy (eV)

a b a

Total electron yield (arb. units) Total electron yield (arb. units)

Figure 3.22: The raw data has to be normalized to the photon ux using the last mirror current. In the case of the deposited clusters, the necessity of this procedure becomes more obvious than in the earlier example of the wetchemi-cally synthesized nanoparticles (gure3.20a+b). Without the normalization the

recorded data is completely dominated by the characteristic of the beamline.

The necessity to normalize the measured spectrum to the current of the last mirror to eliminate some eects (including contaminations of the last refocusing mirror) become more apparent, when measuring very small coverage of particles.

Figure 3.22 displays on the left side the actual data measured for mass selected clusters and on the right, the signal after normalizing to the last mirror current.

The basic procedure is the same for both experiments, only with the additional step of the division by the measured background, that follows right after the normalization to the last mirror current.

The treatment for the background of mass selected deposited clusters was per-formed as described above for all clusters deposited on nickel. In the case of the iron substrate a slight variation had to be used that will be discussed in the section of the results of the mass selected cluster measurements (section 5.1).

Chapter 4

Wet Chemical Nanoparticles

In this chapter the measured data of the wet chemically prepared CoxPt100−x

nanoparticles will be presented and discussed. First the line shape and integrated areas of the white line spectra will be analyzed to estimate the oxidation state and the branching ratio (BR). Then the circular scans and the dichroism spectra will be investigated and the thereby derived magnetic properties will be presented.