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00:16 Photoelectric effect, which was discovered by Albert Einstein in 1905
Photoelectric Effect says that “When electromagnetic radiation, such as light, hits a material, electrons from the surface are emitted out.”
01:10 The basic equation for the ejection of electrons from the surface by electromagnetic radiation becomes, h𝑣 = BE + KE + 𝜙
02:23 Instrument: First thing is source of radiation Mg ka and Al Ka. Actually only one source can emit both the radiation.
05:56 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.
06:20 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.
08:38 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.
09:19 One important aspect of the instrument is presence of gas molecules in air in the path of X-ray. The gases present in air can also absorb the x ray and also the ejected electrons will collide with the gas molecules in their path, this will reduce the KE of ejected electrons. To avoid this complete instrument is maintained under Ultra high vacuum which is around 1 x10 -10 mbar pressure.
With simple rotary pump we can reach unto 10 -3 mbar, in addition if we can combine turbo molecular pump it can generate 10-8 mbar. But at this level of vacuum also water molecules sticking to the chamber are not removed, so we can do baking of the chamber. During Baking, chamber is heated upto 120C for about 24 hrs, at this temperature water molecules sticking on the surface gets vaporised and can be pumped with turbo molecular pump. After doing all, still hydrogen molecules remain inside the chamber, as the hydrogen molecules are very small they can not be removed by turbo molecular pump. So we need titanium sublimation pump. Titanium sublimation pump evaporates metallic titanium inside the chamber which is very reactive and the residual hydrogen reacts with the evaporated titanium and stick to the walls of the chamber. In this way we can reach unto the required vacuums of 1 x 10-10 mbar.
So that was all about the basic principle and instrumentation of XPS, In next video we will discuss the spectrum in more details, till than Thanks for watching this video and if you have any doubt, you can ask me in comment box.
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