Peak-Shapes (FWHM & BE) Distorted due to Poor Flood Gun Alignment
Peak-Shape Tails can be due to Poor Flood Gun Alignment – a Differential Charging Effect
Peak-shapes distorted due to differential charging appear to have tails on the low BE side of high energy resolution chemical state spectra.
These tails look like they require more Lorentzian peak-shape in the peak-fit. See the C (1s) spectra in the Poly-propylene example below.
This is a charging problem. If all high resolution spectra have a similar tail on the low BE side, that is proof of differential charging which means the flood gun was not setup correctly or the beam alignment was wrong. In some cases, as shown in the Teflon example below the tail becomes a peak. These extra peaks move and are not real.
This form of “Differential Charging” is usually due to poor alignment of the Flood Gun, but may also occur if the acceleration voltage is too low, or the current is too low. Due to poor alignment, the FWHM is larger than normal and the BE is larger than normal. Proper alignment of the flood gun may depend on the roughness of the sample, the size of the sample, the position being analyzed, the size of the X-ray beam, and the dielectric nature of the sample. Before trying to analyze your new sample, it may be useful to analyze a known material, such as Polypropylene of HDPE which have single peaks.
These two examples use Teflon and Polypropylene to demonstrate the effects of poor flood gun alignment. Poor alignment normally causes the FWHM to increase, and causes the Peak Energy to move to higher BEs.
Proper alignment of the Flood Gun beam occurs when the FWHM is ∼1.5 eV or smaller, and the BE of the peak is as small as possible.
In each example you can see the X:Y positions of the Flood Gun beam and the effect that position has on the shape of the peak, which should appear as a single narrow peak. The ideal shape is shown at the center of each slide.
At the bottom of this page, we show a charge-control mesh-screen apparatus that is easy to make and might be useful to control charging.
Charge-Control Mesh-Screen Apparatus and Spectrum Examples
This mesh is made of Nickel metal and is ~90% transparent to electrons. The mesh is sitting on a 3 cm x 3 cm square of aluminum metal. The mesh-screen is sitting < 1 mm above the surface of the sample. The mesh is held down using aluminum tape. The holes can be used to trap the mesh-screen with screws. The mesh screen needs to be connected to ground. If you use a magnetic lens, then you should use a non-magnetic metal, not Nickel. (Nickel works, but it moves up and down.) If your X-ray beam is small, then the mesh needs to be closer to the sample, maybe ~0.5 mm. If the mesh touches the sample, you will see the Nickel signal in your survey spectrum, but not your chemical state spectra.