 So today we are going to discuss a new advanced surface characterization technique, the name of the technique is Auger Electron Spectroscopy or in short it is known as AES, we have discussed about the SPS or XA photoelectron spectroscopy in details, some of the concepts I will borrow from the XPS to the EES, but I will have to discuss many other new things in the advanced spectroscopic technique. So therefore I need to constrain myself so that time does not go out of order. So in Auger spectroscopy basically the materials which I am going to use here are taken from different sources, but most notably this book is the source of the material for all the spectroscopic techniques in the encyclopedia of materials characterization edited by Bundle Evans Wilson published by Butterworth-Hinman, Boston in 1992. The schematic representation or the outline of the particular lecture series on AES is as follows. First I will talk about some historical perspectives of Auger Electron Spectroscopy, how it has come about it, then I need to discuss in detail about the basic principles and you must know very well that, then there are certain nomenclatures which are used in AES like any other techniques, then I will go to AES spectra, what any spectra of AES, how does it look like, what are the information we can obtain from there, this will be followed by discussion on the other effects, there are many other effects and obviously some amount of instrumentation we need to discuss and in all this many examples will be given to show you how relevant this technique is for different applications in material science. So let me just start with the historical perspectives, as you know Auger Electron Spectroscopy is a very wild use technique for surface characterization of materials. It is used to know the electronic states of different elements present in the surface, it can be used to measure compositions of the different elements on the surface. This is a non radiative deacceleration process, unlike XPS where you are using an X-ray source to eject out a core level electron from an atom and that is how and then the electrons comes out from the material and measure the candidancy of the core level electron and do remaining other things, this is not like that, this is a non radiative deacceleration process remember this is not deacceleration process but this is also non radiative, that is we are not radiating, we are not getting this phenomena because of irradiation like in XPS. This deacceleration occurs obviously by Coulomb interaction where an atom loses energy by emission of one or more electrons and then ejected electron to one continuum state is what is known as Augia, obviously at the beginning definition will not be clear to you, it needs to be discussed, it needs to be you know talked about it, I am going to do that in the basic principle stuff. But first is what discovered in 1923 by a lady named Leis Metner or Lee Metner basically her picture is shown here, this is all taken from different books and after two years in 1925 this was independently discovered it seems, this was independently discovered by Pierre Augia and after his name the spectroscopy is quant, this is Pierre Victor Augia picture you can see them, this paper by Augia was published in you know journal of physics radium in 1925 and he even this process is named after him to give you even how the developments took place later on let me just talk little detail. So that by 1923-25 the effect was discovered independently by Metner and Augia, Metner published his paper journal of jet script for physics and Augia published in journal of radium, their pictures are shown here but then there was a big flaw nothing happened because of obviously because of instrumentation problem, 1953 Jens Lander uses electrons to excited to create excited Augia electrons and those Augia electrons are used to study the surface impurities, the problem are basically the vacuum system because you know these electrons which are created by Augia process are very low energy so therefore vacuum has to be very good to detect them and also obviously detection system needs to be developed. Then 1968 so that was 1953 by Lander and 1968 L. Harish demonstrates the usefulness of this technique when he differentiates the energy distribution of Augia electrons emitted from a bombarded surface at the same time Weber and Peria employed lead optics as Augia spectrometers. Lead I have taught actually low energy electron diffraction this will be taught by my fellow colleague so lead actually is was called lead can optics can be used for Augia for detection system that was done by Weber and Peria. Then 1969 plumbers et al invent the cylindrical mirror analyzer as known as CMA which greatly improved the speed and sensitivity of the technique. So you can see from 1925 to 1970 almost like 35 years took to develop this technique to be useful and then in mint 80s saw the real implementation of the this technique because of the short key filimeters as electron source short key filimeters as a very high brightness this we discussed in the electron microscopy. So these allow is actually to analyze remember surface features size of 29 meters very small size and then in fact afterwards 1990s even beginning of 1990s lot of implement happen in analyzing analyzers actually and so says this limit is now pushed to 10 nanometer in fact to be frank this limit is pushed below 10 nanometer is recently. So that's how actually the whole the technique has been so nicely developed but we must give credit to Liz Medna and PA Augia for discovering this technique because this was a very fundamental technique which is used extensively in material science even till today before I actually talk about Augia spectroscopy in detail let me just tell you you know we know that any spectroscopic technique whether it's electron spectroscopy or except spectroscopy or Augia spectroscopy it actually basically depends on the interaction of the radiation or the electron or x-ray or any other radiation with the material and material means atoms atom means nucleus and the new electrons. So an ions electrons are photons they are all actually source of radiation this falls on the surface material any atom actually in the sense which is kept inside a vacuum it can cannot generate ions electrons and photons and if we analyze this which is generated because of this interaction process we can get a lot of information about the atom that is the basic principle of any spectroscopic whether it's a electromagnetic spectroscopy or XPS or Augia. So that means the interaction between the surfaces of a material with the incoming high energy radiation source generating other radiation like ions electrons or photons is what is the basic thing of spectroscopy well to you know talk about the Augia spectroscopy we need to know little bit about the electronic structure as you know electrons rotates around the nucleus in an atom in an atom. So and these electrons have different energy levels and all we know that the energy level depends on their quantum numbers the principal quantum numbers the that is SPDF then you have magnetic quantum number L then you have azimuthal quantum number say azimuthal quantum number L magnetic quantum number M has been quantum number S. So depending on the energy levels we can actually split the electron energy levels in different cells like this one is shown K L and M K cell has 1 s electrons that is the inner cell or core electron cell L cell has 2 s 2 p electrons M cell has 3 s 3 p electrons. So the binding energy of electron will increase as you go closer and closer to the core level. So that means K level case electrons will have more binding energies than M and so forth and then you have a basically we have a basically at the above the acquired cells you have what is known as permeable and then you have a now we can actually demarcate these energy levels in 2 s as you know in the x-ray nomenclature this one L basically K is K so K is K this 1 s L 1 is known as 2 s L 2 p is known as L 2 3 M 1 is known as 2 s L 2 p is known as a 3 actually 3 s corresponds to M 2 3 correspond to 3 p and then you have 3 d 3 e 3 4 s and 4 p levels and above that is why actually x-ray I know people use the nomenclature and this is atomic nomenclature XPS we have used another kind of nomenclature. Now if I consider the electronic structure of a metal like sodium what it has sodium as 2 electrons in 1 s level 2 electrons in in the 2 s level then you have 6 electrons in 2 p levels correct. So there that means K L 1 L 2 3 these are the energy levels created. So in a solid state the core levels of atoms are little perturbed and essentially remain as a discrete localized levels that we know and this is this the balance in a orbital cyber hall up sometimes significantly with those of the neighboring atoms genetic bands of specially decolized delocalized energy levels. So after knowing this now let us talk about the physical basis of Auger spectroscopy what is the physical basis of this particular technique. Well I think I talked about XPS let me just bring in the XPS concept and what happens in a XPS concept is we have X-ray source which is a high energy X-ray source it is allowed to follow the sample and this high energy X-rays then basically interact with the electrons core electrons of the atom and eject the core electrons of the atoms living behind a hole there. So then once it leaves behind a hole and the ejected electron goes out and we are analyze the ejected electron energy levels or kinetic energies and that is how we actually do the analysis. So if I have to show it very nicely I can just show it on the board. Let us suppose we have K level this is K this is L and this is M right and then you have EF is a Fermi level and you have then vacuum E back that is how the energy diagram of a atom can be represented. Now if I have certain radiation like X-ray photon falls on this K level electrons and what will happen this is because of this this electrons some amount of this electron because of this X-ray photon this electron the K level will be ejected out and then this passes through because this is a very high kinetic energy normally the incident beam will have very high energy energy higher than almost 10 times higher than the kinetic the binding energy of this electron in the K level because of this excess energy of the incident X-ray the ejected electron will have lot of kinetic energy and because of this high kinetic energy this electron will go out and that is what we do measure in the XPS. So this is what is used in the XPS as you know I have discussed that. This is irreative process and this is also kind of leads to create a hole in the K level now let us discuss what happens in OGM. So I draw the same thing here again so I have K level electrons L cells and then you have M cells and then you have so K, L, M and then you have right. So now I have already created a hole in the K level electron because of this radiation. So what will happen this an atom is now in a excited state because ion is basically is an ionized atom because one electron has gone out. So because of that it has one you know because of this high energy state so what is what this atom can to come you know to a lower is energy some higher level electrons can jump suppose this level electron L1 here can jump into the this hole or can jump to the K level hole and fill this space and if it does so what will happen if it does so this electron has jump and fill the space. So now what will happen because of this difference in that range is between the L and K some amount of x-rays will be coming out and this x-rays which will come out can eject out another electrons from the higher levels like M or L, M level whatever or maybe higher level and this is what is called OGR electrons and this is what is known as OGR process. So that means basically what is done here is as a result of this follows which can be seen from this picture very carefully what I have drawn here and what I am showing in the slide is similar. So because you have a hole created in the K level as the electron has gone out so one electron from the L1 level will jump and fill the hole and because of this there will be some energy released and this energy can then eject out electron from suppose L2, 3 cell and this is what is known as OGR electrons. So in a nutshell I can say anise the atom that remains after removal of the core hole electron is of course a highly excited state and will therefore will like to rapidly relax back to the lower energy state and it can do by two roots as you say one is known as X-ray flow sense root otherwise OGR emission root. Let us consider because we are not talking about X-ray flow sense here that is a separate thing we will consider only OGR. So as you say here the this is what is the OGR source so therefore a rough estimate of the kinetic energy of this OGR electron can be done by knowing the binding energies of the various levels. So kinetic energy is basically EK minus L1 that is what is the jump minus L2, 3 this is what is the difference which is created the energy and then this is the binding energy of the electron L2, 3 cell. So therefore basically EK minus EL1 plus E2, 3 the kinetic energy of the electron is independent of the mechanism of initial hole creation this is very important initial hole creation can be anything basically OGR process uses electrons electron beams to create the hole not the X-rays. So basically it is very clear that the kinetic energy of this OGR electron is independent of whatever the processes used to create the hole in the inner shell or the core level whole cells. So normally in XPS normally what is done is that you know to practice we use a very high energy incident beam and the electrons like of the order of 5 to 10 kilo electron volts so holes will be produced by these electrons or it can produce by even backstri-electron also. So that how I hope now that these two processes the difference between these two processes are clear to you. So because of the creation of this OGR electrons is totally different from the XPS the energy of the OGR electron is also very small because it is very small energy is much small as compared to the XPS electrons so the information which we can gather will be from very small depth of the surface of the material. So great to give you much little perspectives let us do it again so how the electron beam comes suppose this is the ground state of the electron k level L1 L2 3 and then above how the electrons it can also photons but normally you do not use any photons you see electrons and it rejects the electron from the core level or k level leaves behind a hole this is what is ionized electron. So one electrons then falls form the L1 level to fill this k1 level because of this some energy released and this energy then eject out L2 3 level electrons and the electron gets transported this is what is in our shell whole OGR process and this is what is shown here at z plus h nu this is z is the atomina of the material suppose h omega is the energy of the electron z a plus electron level and z a plus electron level produces whenever OGR electron it will be JLBC plus electron plus electron a that is what is the electron a that is OGR electron. So if you use a conservation law EBA EBA is energy of the material is basically N1 minus N of the two final state in the initial state N1 is N is number of electrons EKA that is the OGR ABC ABC is the three processes a is the beginning process b actually intermediate process and c minus u is basically the error g to remove the electron this is there. So OGR electron and g is independent of the excitation as I have told you u is known as the OGR parameter OGR parameter is the machine dependent parameter. So what are the nomenclatures this is what is shown here in the whole process well nomenclatures are very complex in a space I need to discuss in detail about that the space normal pressures are basically like 2s 2p half 2p 3 by 2 you know half is basically 1 minus half and 3 by 2 is 1 plus half and one transfer P SS nothing is basically stands for 0 and then if you use L L will be cost 1 to 2 so 2 minus half is 3 by 2 or 2 plus half is 5 by 2 this we have seen. So what happens in SP in OGR as you see here in this case source comes keeps a whole this comes down and then electron goes up that is what is shown. So in XA in OGR we use X-ray nomenclature so what is this X-ray nomenclature conventionally used nomenclature is X-ray type in OGR suppose for example KL1 L23 that is what is here happened KL1 L23 there is a whole transition when electronic levels are energetically well distinguishable it is common to use most sub indices like L123 M23 M45 this also one can use very well distinguish atoms can be done that for a group of transition sub indices are in many times omitted like KLL KMM MVB or for transitions involving levels in the valence band commonly used V instead LM and OP for example M4 by 5 V so but normally we use this or this other things are not extensively used let me just discuss in detail about a table and to show you so that you do not forget so basic nomenclature which is used in XPS can be written like this suppose this is quantum number this is quantum number and this is suppose notation so we are going to notations both XPS and X-ray and that is what is used in AES right. So quantum number means N, L and J right so this is what we are going to do so let us suppose for quantum number N equal to 1, L equal to 0 and J equal to half L is the azimuthal quantum number and J is the spin quantum number here you can use instead of J let us use S spin quantum number so and then we use basically what we use 1S half this is just 1S there is no need of writing in X-ray we use this as a K so if it is 2 quantum number this becomes 0 and this become half then what is this XPS we use this we use 2S half right and this is corresponding in X-rays L1 so if you suppose 2 again quantum number 2 and 1 and half here so this will be 2S 2S not 2P half basically because 2L is 1 2P half and this will be L2 I guess yes L2 and then if we use 2, 1, 3 by 2, 3 by 2 is 1 plus half 3 by 2 this will be 2P 3 by 2 this is what we have discussed how 2P half and 2P 3 by 2 comes in XPS and this will be L3 right now let us do something for 3 actually when pencil quantum I come 3 this is 0 this is half this will be what this will be 3P 3S sorry 3S half and this will be M1 so if it become 3, 1 and half so this will be 3, 1, 1, 2, 3, 1, 2, 3, 1, 2, 3P 5 by 2 half 3 half so 3P half and this will be M 2 and last one let me see 3,1, 3 by 2, 3, 1, 3 by 2 P3 by 2 3P 3 by 2 this will be M3 right and so on so as you see at k by 2 this will be m3 right and so on. So as you see here k stands for 1 s half, l 1 stands for 2 s half, l 2 stands for 2 p half, l t stands for 2 p 3 by 2, m 1 stands for 3 s half, m 2 stands for 3 p 2 by 2 and m 3 stands for 3 p 3 by 3, 3 by 2. And this 3 by 2 basically comes because of spin, j pass, l, l is suppose 1 here so it becomes 1 plus half is 3 by 2. So that is how it comes. So basically in XPS we use these notations to demarcate different peaks on the other hand in OGR we use these notations. This is funny because these two techniques are same spectroscopic stem is actually but we use different different notations for this you know transitions. So that means if there is a transition form suppose like this one k, l 1, l 2, 3, l 2, 3 is 2, 3 levels so it will be k, l 1, l 2, 3 or k, l l both are possible. If there is transition form k suppose l 1, m 1 then it will be k, l 1, m 1 or k, l m or k, m m is also possible. All these things are possible different kinds of commissions are possible. Well to give you in a better perspective so what is the difference between this photoelectron or just spectroscopy will be clear from here. Well as you see here 1 s, 2 s, 2 p this is energy level of an atom or the different electron energy level basically then you have a permeable quantum band, quantum bands and the vacuum and the free electrons. You have a instant x ray with energy k nu falls on it, it creates it removes and core level electrons this core level electron goes out. So that means the current energy of electron will be h nu minus binding energy of this electron minus phi specification is basically basically because of the machine but in a augia what is happening you have created a hole because of this hole the electrons from the l 2, 3 jumps and then emitted x ray comes out this x ray is then generate or ejects out another electron from this l 2, l 3 cells and this electrons is known as augia. So the kind of energy is obviously will be depending on k, l 1 and l 2, 3 and that is how we have calculated in the earlier also. Now next thing is you have discussed is what is the augia signal or what is basically augia how does the augia spectrum look like. In general initial ionization is non selective because initial ionization means how the hole in the k level is created is non selective and initial hole may therefore be in various shells like k cell, l cell, m cell, initials because that depends on the how the initial ionization has taken place. And then there will be many, many augias transition possible for a given element some will be very weak some will be strong. So augia specters could be basically based upon the measurement of the kinetic energy of the emitted electrons of the augia, augia electrons. So each element in a sample being studied will give rise to characteristic spectrum peaks of various kinetic energies. So that is a augia spectrum is basically will contain large number of peaks and you know that creates many times a problem. To give an example let us suppose let us take the augia spectrum of palladium metal palladium has a very large number of electrons in the outer cells or even inner cells and this is generated by 2.5 kV electron beam to produce initial core electron vacancies and hence to stimulate the augia emission process. The main peaks of the palladium occurs between 220 to 340 electron volts. The peaks are situated between you know between basically on a high background that is why the problem in augia. This mainly because of the secondary electrons are generated by multitude of inelastic scattering processes. So augia spectra often shown in differentiate form that is why and the reason for this is partially historical partially because it is possible to actually measure the spectra directly in this form and doing so get a better sensitivity of a detector detection actually. The plot actually shows a spectrum this is the signal versus ke you can see here there is a high background because of the secondary electron and these are the peaks which are coming out which are between 220 and 340 but whenever we take a differentiate of the signal with respect to the energy level dn by d we get a smoother background and the peaks can be seen very nicely that is what is normally used in augia this one is not used in augia. Well to give you much better idea this is a plot between dne versus de with respect to electron and this is basically from the silver as you see here this is ne of silver this is energy level electrons this is the ne for 10 and then this is dne by e signals are much stronger when you plot dne by de passes and that is why we use all this. Now to tell let you give you some examples of these augia transition lines I have already discussed you different transitions let us do it for I think certain example this is taken from msc thesis of university of campinas long back if you look at that on this plot you have intensity versus kinetic energy of indium element indium you have both your xps peaks which are here and augia peaks which are here augias will have less energy than the xps less than 3400 or less than 3500 actually is these energies of the of these augia and higher than 3600 basically from 3 s3p and 3d electron levels of the this was done with a excitation with a titanium kk alpha which has energy of 4511 electron volts. So what do you see we see all the tension like LMM and LMM transition indium as a large number of electrons so therefore there will be large number of augia transitions possible the ones which are basically very you know routine is L1 m45 m45 m45 is m45 m45 and all else L2 m45 m45 again L3 L3 also they are from L3 to m transitions mm transition then these are actually called all LMM transitions the different different element transition L1 to L4 m45 m45 and then L1 to m45 m45 L3 L2 and so on. So basically these are all LMM transitions as you see here and there are many and there is only two this is the one element this is another one element this is also LMM transitions and these transitions actually can be detected very nicely in a space spectrum here already is plotting DNA intensity or whatever energy coming out from the electrons but many times if you plot DNA versus DE versus kinetic energy DNA by DA is what is plot the slope of this curve basically that will give you much better picks so for a given elements several lines of augia will be observed was seen that is what is observed. So as a transition lines for different elements can be plotted like this this is atomic number buses electron energy and the different elements starting from lithium to uranium correct. So what you see are basically KLL transition KMM transition mm transitions and so on red dots actually indicates the most intense lines which are seen in the spectrum. So I could see here we can observe KLL transition till sulphur as you can see here because energy levels are small the atomic numbers are small so therefore the same as you go on we can observe seeing LMM transitions from atomic number about 12 to atomic number about 40 and then mmn transitions since at higher at me number starting from 37 to over 84 85 and then so on you can observe actually m and a higher level transitions like this ones for very high at minimum elements. So this is this is very important because this table tells you what all transition you can expect when you are doing the xp a auger measurements I think last one today's class today's lecture I am going to show you is the analysis volume as you so know that when a certain energy source is falling on a simple material it interacts with the material and it creates a volume of interaction or interaction volume rather. So the x-rays comes out from a large volume then comes out the cat this actually category x-rays as you see here which had an atomic number 4 a backscatter electron also comes out second electron comes out for a Mars level all the electrons comes out only form 4 to 5 50 Armstrong this is the depth from which it can come out very small depending on the spot size of the electron gun it is possible to have special resolution very small spatial resolution in the direction perpendicular to the surface the analysis volume depends on the electron mean fee path so mean fee path was electronic if you plot if you see a mean fee path basically first it decreases from gold to you know bottom to beryllium then it increases till sodium so this is what is the this was the main factor was the beam size rather than the mean fee path so if you use a perpendicular measurement system here like that then a mean fee path becomes very important.