X- Ray Photoelectron Spectroscopy (XPS) Instrumentation

So the basic principle of photoemission spectroscopy also known as XPS is photoelectric effect, which was discovered by Albert Einstein in 1905 for which he got the Noble prize also.

Photoelectric Effect says that “When electromagnetic radiation, such as light, hits a material, electrons from the surface are emitted out.”

That means electrons can be ejected from the surface of materials by just bombarding them with electromagnetic radiation or light. Important thing here is that the energy of photons of light should be higher than that of the binding energy of electron. Because if photons of light are not having enough energy to remove electrons from the bind state to free state. It won’t work.

So the basic equation for the ejection of electrons from the surface by electromagnetic radiation becomes,

h𝑣 = BE + KE + 𝜙

Where, h𝑣 is Photon Energy of Source,  BE is the binding energy of the emitted electron and KE is the kinetic energy of the electron as the electron is moving away with some velocity. And phi is a constant called as work function which depends on instrument.

That means when electron is removed some of the energy of electromagnetic radiation is used to remove the electron and remaining energy is used to accelerate electron away from the surface. Which is kinetic energy of electron.

Our analyser of XPS instrument measures the Kinetic energy of electron, and if we know the photon energy of incident electromagnetic radiation, we can easily find the binding energy of the emitted electron.

Now lets try to see how the instrument is made of, 

Source of radiation: As valence shell electron of any element is bonded very weekly with the nucleus, they can be ejected with low energy radiation like Ultra violet light, and the instruments which use UV light as source are known as UPS, We will learn more about UPS in upcoming videos in this series. 

For core level electrons or the inner shell electrons we need high energy radiation, which is only available in x rays. In commercial instruments two types of X-rays are very common, they are Mg ka and Al Ka. Actually only one source can emit both the radiation. 

The X-ray source is made-up of a metal tube and the end of the tube are having two surfaces, one is coated with Mg and other is with Al, and very near to these coatings two filaments are arranged. When current in these filaments are supplied they gets heated and starts emitting electrons, these electrons when collide with the coated surface, which is made cathode by applying very high voltage of about 20KV on metal tube. they produce x ray. So when we need the radiation of Mg ka, we heat the filament of Mg side and when we need radiation of Al ka we heat the filament of Al side. Cooling water is used to cool the cathode else it gets very hot.

Please note that the energy of Mg ka is 1253.6eV and Al ka is 1486.6eV that means the electron with BE higher than this can not be ejected with these sources. And the line width of Mg ka is 0.7eV and Al Ka is 0.85eV that means if two electrons having energy difference less than these values, they can be separated, This is also termed as resolution of electronic signals.

So if we need higher resolution, we need to find a source which have narrow line width.

But most of the common sample can be easily analysed with these two X rays.

When x ray hits the sample, it ejects the electrons, and there is partial positive charge is induced on the surface, this is called charging of sample, to nullify this effect sample need to be always grounded. This is important as due to induced charge, electrons environment changes thus changing the actual binding energy of electron, which shifts the position of the peak in xps spectra.

Lenses: we use electrostatic lenses to collect the emitted electrons which are focussed to the entrance slit of the analyser. Lens and the slit system decides the area of the simple from where we want to collect the electrons. Or area of the sample to be analysed.

Analyzer: most of the commercial xps instruments are having hemispherical analyser, which is made up of two hollow hemicylindrical electrodes. Outer sphere is negatively charges and inner sphere is positively charged. Generally ejected electrons travel in straight line but when they enter in hemispherical analyser, they feel attraction from positive electrode, and repulsion from negative electrode. Because of this electric field the straight line path of electron is now bend in the direction of electrodes. Now it is important to note that if constant voltage difference is applied across two hemispherical electrodes, the electron with high velocity of kinetic energy will be bended to a lesser extent and will collide with the outer wall of the path. And if the ejected electron have low velocity or kinetic energy they will be bended to a larger extent and they will hit the inner wall of analyser.

That means only certain electrons with a fix velocity or kinetic energy will travel exactly through the path and will be allowed to pass through the exit slit. Rest of the electron with lower or higher kinetic energy will be lost in hemispherical path. This is quite important because by varying the applied voltage across two electrodes we can pass the electrons of certain kinetic energy, this means now we can separate electrons as per their kinetic energy.

In commercial instruments the negative voltage on the outer electrode is kept constant which is also known as pass energy and the positive voltage on the inner electrode is changed to scan the electrons by their kinetic energy. 

Electrons coming out of the exit slit are counted with electron multiplier tube or channeltron.

So finally the number of electrons counted and the kinetic energy of electron from the analyser is send to the computer, which plot a graph of Kinetic energy in x axis and number of electrons counted in y axis.

In modern instruments, KE is converted into BE by computer, so the graph or spectrum becomes BE Vs Electron counts.

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