 In this lecture, we will learn about electron energy loss spectroscopy. So, the overall thing what we can see is electrons, energy loss and spectroscopy. So, in this particular case, we incident a particular electron beam of known energies of known kinetic energy and we let it interact with the material. And depending on how much energy is being lost, from that we obtain a particular spectrum to comment on. So, may be the chemical nature or the electronic state or straight or the dielectric state of a particular material. So, in this particular characterizing techniques, eels if we call it electron energy loss spectroscopy, we allow a particular known side of electron energies known kinetic energy of electron. We let it interact with the material and we see how what is the overall loss, energy loss is occurring because of certain vibrational modes or some elastic scattering to attain a particular spectrum. Since the overall term is energy loss, it means that the electrons are interacting in elastically. So, there are certain losses. So, it is not an elastic interaction, but more or less like an inelastic interaction with the material to result certain scattering. And in this particular case, we allow a known narrow range of kinetic energies. So, in this particular case, we take a particular known spectrum. So, we use, we can utilize say such as cylindrical mirror analyzer. So, we can allow only a certain range or narrow range of kinetic energies to pass through the channel. And then, once we have that known narrow range of kinetic energy of electrons, we can let it interact, we can bombard those electrons on a sample surface. And then, we can measure the energy loss of a beam via this electron spectrometer. So, once we attain, this is the incident energy of the electron and later on after it has interacted with the material, we can attain a overall electron spectrum from the interaction of electron with the sample. And then, we can analyze that which all processes have resulted this particular energy loss. So, the known energy of electrons bombarded on a sample, then we measure the output energy. So, the difference in the energy or the loss of energy is now being analyzed from the processes which are responsible for that particular loss. So, then those particular processes can let us, they can let us know what all interactions are occurring and what is the characteristic of a particular atom or an adsorbate or the dielectric nature of the material, which are causing this particular losses. And from all those information, we can comment on many of the structures, it can be compositional, it can be dielectric, it can be ionization. So, we can comment on those particular aspects of a particular material via using electron energy loss spectroscopy. So, in this particular case, we are measuring the kinetic energy after they have interacted with the material and it is definitely based on the inelastic scattering, because we are worried about the loss, the electron energy loss which is occurring after the interaction. And that interaction of measuring via supplying an only narrow range of kinetic energy of electron and that energy range is between 1 to 10 electron volt. And then again, we are analyzing those kinetic energy and that will provide us the energy transfer, which is happening because of certain surface vibrational modes. So, depending on that, we can attain different kind of scattering with the material. So, in this particular case, what we can get is we have a sample, we are applying certain set of energies to interact with the sample. And again, those interactions can be a specular or off specular and then once we get a particular spectrum, particular scattering. In certain cases, so we have incident energy and this is a scattered electron. So, what we can get finally is, scattered electron is equal to E i minus certain losses which have occurred H mu. So, we have the overall energy of the electron which is coming out after interacting with the sample. So, we have sample here, this is the incident. So, incident energy minus certain energy which is now been absorbed by the material and those are nothing but the energy losses. So, that is what we are interested in. This particular term, how much energy is being lost? So, we can define them that how much energy is now lost because of the elastic scattering and that energy is responsible for the surface vibrational modes and that can give us much more information about the overall loss spectrum. So, in this particular case, we have the energy loss spectrum which is ranging between 0 to 5 to 200 electron volts and typically the resolution is approximately 20 to 30 inverse of centimeter. Whereas in certain special cases, the resolution can be as better as 8 centimeter inverse. So, that is what we can get from the yield spectrum. And again, the yield spectrum, we see a typical spectrum which appears more like this. Initially, we have a 0 loss peak then it is followed by a certain valley and we have the first peak and then we have high loss region. So, in this case, we can see energy loss along this side. This is the intensity of the frequency. So, first what we see is nothing but the 0 loss peak. So, if we can come to the 0 loss peak, so this is the 0 loss peak and in this case, we are achieving the interaction of electron which are elastically scattered. So, there is no loss of energy. So, in this case, the overall electron energy is same as that of incident electron. So, in this case, this is called a 0 loss peak because electrons are coming back after their elastic scattering, after the elastic interaction with the material and there is a 0 loss peak. So, this is the width of this particular 0 loss peak will tell us about the resolution of this particular yield spectrum. And then later on, we have low loss region which is consist comprising of this particular region and this case, we have energy loss less than 100 electron volts and this is basically dominated by the plasmon oscillations. So, once we have some plasmon or plasmon oscillations or those of interactions which are occurring because of the polarization of electrons in a particular area. So, those particular interactions, they are very rapid and those can be captured easily via yield spectrum. Whereas, 0 loss peak comprises mostly the phonon interactions. So, those basically tend to heat up the material and those energy losses are very fine less than 0.2, 0.3 electron volts. So, those can be detected by the yield spectrum. But this plasmon oscillations, they can they appear in the low loss region and from that we can detect much more of the dielectric nature of the composition of a particular material. So, the first peak the 0 loss peak provides like the resolution of the yield spectrum, the low loss region provides more of the composition or the dielectric nature of a particular material. And in the high loss region, we have greater than we can call it greater than 100 electron volts or greater than 50 electron volts, we can get a spectrum which tells much about the bonding or the ionization structure of a particular material. So, in this case we can see inertial ionizations which is rare because energy is low, but we can see those spectrum very easily in the high loss region around this side. So, we see high loss region, we see a low loss region and we see a 0 loss peak in the yield spectrum. And generally the resolution spectral resolution is generally approximately 1 electron volt for this particular spectrum. So, we cannot really measure which energy losses when they are lesser than 1 electron volt such as phonon losses they comprise between 0.2, 0.3 electron volts. So, those can be related detected by the yield spectrum and there were a variety of main manners in which energy can be lost and those all have to be in elastic interaction because some energy is being absorbed by the material. So, we can have phonon excitations, we can have inter-entraband transitions, we can also have plasmon excitations, we can also have inertial ionizations and at the same time we can also have some negative ion resonance. So, phonon excitation it means that we are allowing some lattice vibrations to occur and those tend to heat up the material and the energy losses are very low in this particular case. We can also have inter and intra band transitions. So, in that particular case we can have electron jumping from one shell to another shell or from higher shell to another shell and then even between 2 different atoms. So, that is nothing but the inter and intra band transitions and within a particular shell there can be some transitions between the 2 p 1 and 2 p 2 3. So, that part can also happen with as a inter band transition, we can also have some plasmon excitations that means the localization or the polarization of the electrons in a particular entity and then that can also lead to the absorption of energy. We can also have some inertial ionization, we can allow electron to get ionized an atom to get ionized via release of an electron. At the same time we can also see certain negative ion resonance in that particular case, we can have certain different transitions which can allow electron to stay much for much longer time in a certain molecule orbital. So, electron will stay trapped for certain time. So, it might appear that energy is being absorbed for a certain duration of time. So, those are certain energy losses which can occur and those can be incorporated in the field spectrum. But these are the losses which are basically contributing to the energy loss spectrum, but it does not mean that all the losses can be easily visualized such as plenon losses, they can cannot be really resolved in the field spectrum. And again there can be variety of scattering which is which can occur. In this case in the dipole scattering we have scattering to a long range hundreds of angstroms via strong columbic field. So, if an electron is coming interacting with the surface and it goes off via certain loss in the momentum in the horizontal direction. In this particular case the perpendicular momentum is now conserved. So, we can see that the perpendicular motion is now conserved. So, we have a similar perpendicular line, but the horizontal part is basically being lost. So, horizontal momentum is basically being lost. So, that is what we see here. So, much energy is being absorbed by the electron in the horizontal direction. So, in this particular case in the dipole scattering we can achieve scattering in case of because of columbic fields. And in this particular case we have the retention of the momentum in the perpendicular direction, but there is loss of momentum in the horizontal direction. It is happening because of the columbic field of the surface. It can also attain certain out of plane scattering can also occur. And in this particular case we have kinematic scattering and that is only for a shorter range in couple of angstroms. But in this particular case when an electron is basically interacting with the material it is losing both perpendicular as well as the horizontal momentum. So, in this particular case whatever the distances we had momentum we had in the horizontal and vertical direction those are basically being lost in both the cases. So, we have some extra loss in this part as well as some extra loss in the vertical direction. So, we have loss in vertical direction as well as some loss in the horizontal direction. And that occurs because of certain strong nuclear motions or vibrations and that results some potential on the surface. And because of that electron gets scattered over wide range of angles. So, angles are much wider, but they are at short ranges few angstroms. So, they go in and out of the plane of incidence. And we are not able to retain the electron momentum in both the directions which are both parallel in the perpendicular direction. So, we had dipole scattering in that we were able to retain the momentum in the perpendicular direction. But because of impact scattering those are occurring because of the nuclear motions or vibrations. And in that particular case we achieve very wide range of angles, but for a shorter distances. And in that particular case we destroy both the perpendicular and the perpendicular and the parallel motion or the momentum of the electron. And again we can also have some negative ion resonance. And in that particular case we can induce some impurities and those basically induce some molecular orbitals that can come because of the adsorbate. So, we have incident electron beam it interacts with the sample, but since there are some adsorbate adsorbates which are present on the on the surface of a particular sample they generate some extra molecular orbital orbitals. And because of that the electron can stay trapped out there or the. So, our electron can stay transiently trapped on those particular empty or high lying molecular orbitals. And because of that some energy is being being basically utilized out there. And that vibrational feature intensity it strongly depends on the incident energy or the which causes the resonance. So, that is also happening we have dipole scattering, we had impact scattering and we can also have negative ion resonance scattering. In that particular case we are able to have the electron being transiently trapped in certain molecular orbitals for certain duration of time. But though these are some loss features most of the interaction is occurring elastically. So, we get the strongest peak is coming out because of the elastic interaction and that basically takes care of the 0 loss peak. So, basically our overall spectrum of the loss spectrum or the overall loss spectrum is generally very very weak because major of the phenomena they are occurring elastically. So, that is the reason about the spectrum which is coming out as a loss spectrum it is generally very very weak. So, eventually our detection system has to be strong enough to be able to detect that weak spectrum and comment on the overall nature or the structure or the composition of the sample. So, that is what the overall feel of the is spectroscopy is and depending on what kind of information we have. So, we can see that we can see that we can have collective oscillations they can come out either from plasmons or they can come out of phonons. So, plasmons are nothing but the collective oscillations of free electrons. So, in this particular case we have polarization of electrons and electrons are generally very free entities. So, they tend to be dispersed like in a metal matrix. So, we have it is basically predominant in metals when we have high density of electrons which are floating without any bound bound bound without any bounding to the bounding to a particular atom. So, they are flowing freely on a metal surface and they cause basically the most common in elastic interactions, but the problem with them is they die out very quickly in 10 to power minus 15 seconds and they are also localized to less than 10 nanometer and they can they basically are scattered to less than 0.1 milli milli rats and their energy is approximately 5 to 25 electron volt. So, once they have energy much greater than 1 electron volt these particular signals can be easily captured by the air spectrum. So, we can see that the plasmon losses they arise because of the localized polarization of the electrons because they arise from the collective oscillations of electrons and they are predominant in the metals because they are very high electron density and because their energy losses are in the order of 5 to 25 electron volt they can be easily captured by the air spectrum. On the other hand we can also have some phonon phonon oscillations those are the collective oscillations of atoms or they also arise because of the lattice vibrations and because of that they generate heat and they tend to heat up the specimen and they can also be generated by certain other other elastic processes such as OJ energy or the X-ray energy and those can also cause the lattice vibrations or the oscillations of the atoms. But the problem with them is they generally tend to have a very low energy loss which is to the order of less than 0.1 electron volt and since it is less than the resolution limit of eels which is approximately 1 electron volt we are not able to detect all these losses in the eels spectrum and the one more thing is that the phonon can get can get scattered to a very large angle to the order of 1 5 to 15 milli rods and because of that they can generally give a very high or the diffused background in the case of phonon. So, the collector oscillations they are limited to plasmons and phonons and plasmons they are collective oscillations of electrons whereas, phonons tend to be the collective oscillations of atoms and depending on what kind of energy losses they have we can get a certain spectrum which can basically grab the particular picture or the density of electrons can be easily captured by the plasmon peak or the low loss energy low loss regime of the eels spectrum. So, overall we can see that that our phonon excitations they are limited to 0.2 electron volt and they give out as a very diffused background and they basically come out in the 0 loss peak and since they are very low the overall energy loss is very very low they cannot result and because of that it can create certain problem in the resolution of a particular eels spectrum at the same time they tend to heat up the specimen. So, anyway those particular things appear in the in the 0 loss peak and they tend to heat up the specimen then later on we can also have some intramend transitions and that can provide as a signature. So, in the low loss region we can get some signals which are basically between the 5 to 25 electron volt and those transitions can be easily captured which can provide as a structure of a particular material and plasmon excitations they can be limited either as a surface plasmons or as a bulk plasmons and their energy again comes in the low loss region and we can in the surface plasmons we have basically the transverse waves which can provide they are basically half the energy of the bulk plasmons and the bulk plasmons we generally have interaction with the interaction as the longitudinal wave and this particular spectrum can be easily attained in the low loss region and later on once we have once we have much higher loss. So, that is basically being created by the inertial ionization and that generally is greater than 100 electron volt and from that we can always attain a elemental information because we know what are the transitions or the ionizations which are responsible for causing this much loss of the energy. So, overall we can see that we have zone distributed in three regimes we have zero loss peak zero loss peak basically it defines the overall energy resolution higher the basically energy loss the poor is the resolution because the broader the particular peak is the poor the resolution will be and higher the k v the poor will be the resolution and again this thing is arising it this thing is arising because of forward scattering which is to the order of a few millirides and again as we saw that the overall broadening is because of the phonons which basically give out a diffuse pattern and again this corresponds to more like the zero zero spot of the diffraction pattern and again the bright diffraction peak which are to the order of 20 millirides they really enter the spectrometer. So, we have this much particular resolution which is now being the concentrating factor and this also includes the energy loss of 0.3 electron volt which comes out from the phonon interaction and so though it is including the phonon losses it is not able to resolve those phonon losses. So, we can see that zero loss peak it is responsible for the overall defining the overall resolution or the energy resolution of the spectrometer and later we have a plasmon peak and then we have a high energy loss peak. So, so again in the this is again the kind of a valley before the plasmon peak and the information what we can get from the plasmon peak is basically the overall dielectric nature of a material we can get the overall dielectric nature of a particular material and also we can find the composition whereas, the high loss regime it can provide us much more information on the overall ionization structure or the bonding because those ionizations are occurring in this particular high loss regime and then again we have energy loss which is near the edge structures this is the edge and we have energy loss near the edge structure and whereas, this regime is the extended energy loss fine structure. So, we have either fine structure or near edge structure and again it is in the high loss region and from this we can get much more of the bonding effects how the how is the overall bonding which is happening and we can also attain overall diffraction effect which can occur from the atoms surrounding the ionized atom. So, we can get much more information as either energy loss fine structure or as energy loss near the edge structure. So, we have regime which we which is nothing but the 0 loss peak and then we have low loss low loss peak which is limited to 50 to 100 electron volt and then we have high energy loss regime which is basically the either it can be a fine structure or the near edge structure and from that we can get much more bonding or the ionization effects. So, again there are different parts of the spectrum. So, again coming to the initial part we already saw that this is nothing but the 0 loss peak. So, we have incident beam energy it is interacting with the material and it is getting elastically interacted and that energy is being captured back as the 0 loss peak because the incident electron they have not lost any energy while interacting with the specimen. So, the result out the 0 loss peak and later on they have a plasmon peak in this case we are we are observing plasmon losses those are rather because of the polarization of the electrons and because of that material is losing certain energy via certain oscillations and we can we have one regime of value before the plasmon peak and then again the peak the plasmon peak. So, in this case we tend to see this particular peak because of the free electron density and that can tell us much more about the dielectric constraint of the particular specimen as well as the composition of the specimen. So, that part we can attain from the plasmon peak or the low loss regime that we can comment because the distribution of electrons or the polarization of electrons will tell about the dielectric nature of the material how easily the electrons can flow and how easily they can reduce the energy loss. So, that part can be easily obtained and again we can also from that we can also attain the overall composition of the specimen from the free electron density. Second from the high energy loss regime we have near edge structure and the fine structure which is the extended region and it can arise as more than the critical energy for the ionization is imparted to the core electron. So, from that we can attain what is the overall ionization which can which is happening out there and it is basically the excess energy which is approximately few electron volts. We can also achieve attain some plural elastic scattering and that can give us the bonding structure between the two atoms and the excess energy is greater and that can be correlated to the single scattering event and that can provide as the local atomic arrangement. So, we can achieve the bonding between the atoms what is happening in terms of the arrangement of the local atoms. So, we can get overall structure either the bonding of the ionized atom how that ionized atom is coordinated to the other atoms from the overall arrangement and that can also provide as the density of states of the solid as well as the radial distribution function for all these entities. So, that part we can attain from the extended regimes of the which is near edge and the extended fine structure. So, that information we can get from the high energy loss spectrum either from the energy loss near edge structure which is the ELNES or from the extended energy loss fine structure that is EXELFS. So, we can attain that much information from out here. So, later on we can also see that the overall information what we can get from here is if we get an intensity. So, we are getting certain intensity out here and then in this case we have energy loss which is an EV. So, we have energy loss. So, we are seeing one first peak which is coming out exactly at 0 and then we have a very fine plasmon losses and that is it and then we have high loss period. So, if we can if we start basically extending it or increasing magnifying it this is nothing but a 0 loss peak 0 loss peak and then we can go back then we can go back and see this particular regime. So, this is nothing but the plasmon peak if we start magnifying it then we can we can see that we are seeing certain peaks. So, at certain higher peaks we can see that there is some information which can be available certain information which can be available and these regimes occur basically at around 100 electron volt. So, this is the low loss region and we can start seeing something at much at the high loss regime high loss again this is high loss. So, we can start magnifying it. So, in this particular case we are seeing 0 loss peak those are nothing but the arising from the elastic scattering and that can tell us more about the resolution part of this particular e spectrum and then once we once we are going and seeing the plasmon losses we can tell much more about the dielectric nature of the composition and as we see here we can have some certain spectrum. So, in this case it can be silicon L L shell. So, we can get certain information about the bonding that what kind of energy is responsible for creating this particular ionization it can also be arising from some other material say K of carbon. So, this particular part tells us two things first of all what is the overall bonding because this is nothing but the ionization energy. So, that is telling directly about the bonding and then it can also tell us about the oxidation state whether how much energy is basically being provided here. So, from that we can also get the oxidation state and we can also know the overall concentration because depending on the height of this particular peak we can know how much carbons are really interacting to give out this particular intensity or giving out this particular regime. And we can also see that there can be certain secondary basically peaks out here. So, we can further magnify it. So, we can see the extension of this one. So, if this can be 50 x this can be 500 x and this can again be 1000 x. So, at later on we can also again see some more peaks which can be same if they belong to oxygen say K from the K shell. So, from one spectrum we can keep magnifying it. So, the blue one what we saw earlier is the initial spectrum. Then we magnify it by 50 times and then we see certain spectrum that tells much more information about whether there is some silicon in it. It will tell us about the bonding or the oxidation state. We magnify it further to say 500 x and then what we get we can see some carbon presence of some carbon and that will tell us about the overall concentration of carbon. We can magnify it further and what do see is some presence of some oxygen. So, we can get a couple of information from here the bonding, the oxidation state and the concentration even the electronic structure of this particular entities. Those all information we can get very easily from the yield spectrum. So, that is what we can see in this particular case. Further we can also do some yield micro analysis and in this particular case we can even detect a single atom of thorium even on a carbon film. So, if we see we can get a particular spectrum, we can get a particular spectrum and in the low loss regime we can see some presence of some thorium if I had a particular film. So, I can get some signals which are basically for carbon. This peak is coming from a thorium with a cluster of thorium, but if I only have one single atom of thorium this is the energy loss the intensity. I can also compare it with say if I had certain peaks of carbon and then if I allow only one thorium atom to basically get absorbed on the surface. I have do see some bump out here and with a similar kind of a carbon peak. So, I can even detect even when I have single thorium atom which is now absorbed on the carbon surface I still will be able to detect that particular part. So, that tells how sensitive that yield spectrum is that I can detect even a single thorium atom. So, I can detect that because the thorium atom if it forms a cluster I can still see the similar peak at the similar energy loss regime and that might be at either at lower energy or may be less than 100 electron volt. So, that part I can see from the yield micro analysis and I can detect even a single atom of thorium on a carbon film. So, that is the overall sensitivity of the yield spectrum. So, in this particular case I have carbon I had a carbon film. So, I do see the peaks which are arising because of carbon and those are generally in the high loss regime. So, they can tell me about the overall excitation nature or the bonding nature of the electrons in the ionization state. So, I can get much more information of what are the transitions which are occurring in the carbon alone. So, depending on what kind of carbon it is I can get the pi interactions or the sigma interactions which are occurring for the electrons and for the thorium it is now absorbed on the carbon surface. So, I am getting a much bulk or the peak on the carbon film and that is because of the cluster of thorium atoms, but even when I have single thorium atom sitting on the surface of carbon as an adsorbate I can still detect that. So, that is the overall sensitivity of the yield spectrum. So, again in certain cases we have diamond, graphite and fullerene and again they only have carbon in it nothing else. So, we are considering three systems diamond, graphite and fullerene and they all consist basically carbon, but they have absorption peaks of around 284 electron only in the in the yield spectrum which correspond to the existence of carbon atom. So, we see 284 we have certain regime of carbon which is present. So, we do see that the carbon is present out there and from the fine structure of the absorption peak the difference in the bonding state and the local electronic state can be detected. So, you can find the bonding and the local electronic state from the fine structure of the yield spectrum and again if you go further we can see further graphite we have this particular bonding which is one as or the pi bonding one as to the pi bonding we have certain energy absorption. So, we are able to see this particular peak in the graphite and the similar peak we can also see in the C 60, but that peak is totally absent in the diamond structure. So, we can see that because the diamond we have sp3 hydro hybridization. So, we do not have a transition which is which can really see one as to going to the empty pi bond. So, we can see that carbon k shell electron from one as it is not able to bond to yield a pi bonding orbital in case of diamond. So, that can be very nicely being that is very nicely being observed out here with the sharp peak at the absorption h which corresponds to the excitation of the carbon k shell electron or the one as electron to the empty anti bonding pi orbital that is present only in the graphite as well as in the C 60 because they both have sp2 type of hybridization. So, they still have one orbital in p which is empty. So, there can be a transition from the as shell or the one as shell to the p bond and that particular energy regime is following in the regime out here and from this we can say that the diamond we have sp3 hybridization and there is no empty pi electron basically which can come out here. So, there is no one as electron which can jump to the empty pi electron and that is the reason we have this particular energy level vacant out there. So, that is that is the kind of information we can get that from difference between the graphite C 60 and the diamond where everything is now composed of carbon only and that thing is now being captured from the yield spectrum. So, in the yield spectrum we can see that if we have large number of particular electrons at say 284 e v that is what we saw. So, we have overall spectrum and then from 280 we are seeing very large number of spectrum and this thing is at 284 electron volt. So, from this we directly know that that we have energy loss happening much more at the 284 and it means that there is some entity some ionization which is happening at this particular level only because some particular atom is present and that is leading to this particular energy loss. So, this happens to be in electron energy loss which equals to the ionization of carbon from k shell. So, if a k shell electron present in the carbon atom and that equals to the energy of that particular electron equals to 284 e v. So, directly means that if we have loss which is happening at 284 electron volt. So, we have energy loss out here in e v and we have intensity out here. So, once we have once we see a very high intensity exactly 284 electron volt we can correlate it to the ionization of carbon from the k shell. So, that directly tells us that we have some atom present out here and that is leading to the ionization of this particular atom and incident beam is somehow interacting with this particular atom and it is ionizing the carbon. So, that is the first information what we can get from here that this particular energy is equal to the energy which is required to remove an electron from the k shell of the carbon. So, this tells we have presence of carbon and the intensity of this also tells us what is approximate composition or the approximate content of this particular carbon because the more the carbon the higher the intensity of this intensity of this peak at 284 electron volt. So, we can also say that there is a significant amount amount of carbon which is present in the material and from that from the interaction as we saw if we have certain bonding or bonding present. So, from as shell to the p from the pie shell if you can see if we see that particular peak it means that type of a bonding can be present if not that it means it is not there. So, we can also find what is the type of atom its overall its overall amount and number of atoms of each type. So, in case we have certain hybridization which is happening we can know whether the atom which have sp2 hybridization or sp3 hybridization which is present in a particular case and also we can find the scattering angles. So, depending on the from the width we can also find the overall scattering angles and that can provide us much more of the dispersion relations for a particular atom. So, from this overall information from the yield spectrum first of all we can identify where the particular peak is getting we are where we are getting the exact peak and that will tell us how much energy losses are occurring and that from that we can correlate it to the ionization of a particular entity say carbon in this case that will tell us the amount of carbon type of bonding which is happening the overall scattering angle from the dispersion for a for a particular entity. So, all this information we can get easily from a yield spectrum and how do we get that is we initially we have electron source and first of all we are providing a narrow range of kinetic energy. So, we have narrow range of kinetic energy and that thing is being achieved from a cylindrical mirror analyzer or it can also we have certain two cylinders out there and we apply certain bias to it. So, we have energy less than E naught it gets basically gets entrapped out there if I have E greater than E naught then it gets entrapped out here. So, what we can get out is a narrow range of energy which has limited E naught and that particular energy of known E naught value will now interact with the sample and once it interacts with the sample after the interaction how much energy is being lost by the electron that thing is again sent back to the analyzer. An analyzer we can again do the same operation and we can select water energies are coming out. So, by putting the bias certain biases to the particular analyzer we can somehow detect the overall spectrum from a detector. So, we can get the overall spectrum of what is the overall energy loss which is happening because once we know the incident energy minus the energy which is being observed later on that is nothing but equal to the energy loss which is occurring for the for a particular electron. So, that is what we are getting it from the overall spectrum that we are sending certain energy we have electron source we are sending it through a monochromator to get a particular set of known narrow range of kinetic energy for electron and those are interacting with the sample and upon interaction with the sample it is now sent to an analyzer with those will separate out the energies. Once we are able to separate out the energies we know the in incident energy we know the final energy. So, the subtraction of that E i minus E s will tell us how much energy loss is now occurring and that energy loss is now being correlated to the overall structure of a particular for a particular material or even from for a even we can also find out the overall dielectric nature or the ionization state or the bonding state for a particular material and that thing we are getting from this eels spectrum. And again the advantages of the eels spectrum it basically can be applied to materials which are normally unstable under electron beam because in this particular case our energies are very very low. So, because our energies are very very low we can also apply it to the materials which are basically unstable under the electron beam. And again in this particular case we can have multiple scattering occurring because as we saw we have a dipole scattering we can we can also have a specular scattering we have impact scattering dipole scattering and we can also have some negative ion resonance. So, all those mechanisms have behave very differently in this in one case we can retain the momentum in the perpendicular state in a second case the momentum was not at all conserved both parallel as well as perpendicular and all those basically scattering mechanisms are allowed and that will allow us to observe the overall energy loss. So, we can observe all the modes which are parallel and perpendicular to the surface. So, that can reveal much more information at the same time we can observe modes between 0 to 4000 inverse of centimeter and also we can attain spectrums which are whichever lower energy or the lower frequency modes we can also attain the spectrum for that and that basically we can also observe the molecule surface. So, that part gives us the information much more about the what is happening at the surface level or the low energy low energy losses also we can we can attain. And again we can perform vibrational and electronic loss spectroscopy we can also induce and probe current induce interactions with the variable incident energy and this case we do not need any background subtraction because we know what is the incident beam and what is the beam intensity which is coming out. So, we do not need to do any background subtraction and it is very very common technique for the surface and bulk phonon measurements because that will create the initial broadening of the zero loss peak. So, that part we can tell very nicely what is the overall surface and bulk phonon interaction which is happening with the material and in this particular case we can find the atomic composition. So, we can also attain what is the atomic composition which is out there what is the overall chemical bonding which is happening in the material because from the intensity of a particular intensity of a particular is peak we can find what is the overall how much is the overall presence of a particular entity which is causing this particular increase in the peak intensity. So, that can tell us about the overall composition and it can tell us about the overall bonding nature because depending on where the particular peak is appearing. So, like in the case of carbon from 1 s to pi shell we saw that the peak was operating at 284 electron volt it is not present we can tell much more about the bonding nature of a bonding nature of carbon itself. So, we know whether it is sp2 or sp3 hybridization and again it can tell us about the valence and electronic band electron profile. So, that part can also be obtained from the ionization part we can also get much more of surface properties because we saw even when a single thorium atom is present on a carbon surface it can be basically detected. So, the this particular instrument is highly sensitive. So, we can get much more information even from the yield spectrum and again we can find the element specific peer distribution function as we know as we had seen earlier from the from the from the distribution of the specular specular scattering we can find the correlation between the radial profile. So, that can also provide us much more information about how the specific peer distance is now being distributed. So, that part we can also obtain in this particular yield spectrum. So, overall advantages are that it is basically we can find atomic composition we can also find the chemical bonding we can also find the valence and conduction band profiles we can get surface properties we can also get some element specific properties and it can also give us the modes which are lower and lower frequency mode less than 400 centimeter inverse and we can perform many vibrational electronic loss spectroscopy and we can also find some phonon measurements we can also do some phonon measurements and also we do not need to do any background corrections for in this particular spectrum. So, these are certain advantages of the yield spectrum what we can get we can get very fine detailed information, but but there are couple of challenges out here that it has a poor resolution the best it can achieve is approximately 6 to 8 inverse of centimeter and since we have very low energy electrons which are utilized in this particular case we can work only in the ultra high vacuum or to the order of 10 to power minus 10 to 12 torque. So, that is the overall regime we need to work in because the electrons need to travel very large distances. So, that they can reach the detector. So, we need to perform all these operations under ultra high vacuum that is the most problem with it and that makes it very expensive it is very complex because we need to generate a first of all a monochromatic beam and then let it pass through analyzer and then again get it detected. So, again it is very complex and it requires a delicate instrumentation because we also need to detect what is the beam energy kinetic energy and what is the energy we are getting after it has interacted with the sample at the same time electron should be maintained well within the ultra high vacuum. So, that makes it very delicate it requires very delicate instrumentation also we also detect the some spectral information in terms of the dispersion or the diffused spectrum what we get. So, that tells more about the phonon interactions and that makes the instrumentation much delicate and thus attaining the spectrum is little slower 15 to 60 minutes per spectrum and again it can also result some surface charging because we are dealing with the flow of electrons on the sample surface some plasma on oscillations. So, it can also lead to surface charging. So, generally conducting samples are much more preferred in this particular case. So, again it also has a very difficult theory because depending on what all ionizations are occurring how the losses are occurring they require exact identification of how those how those processes are leading to the loss in energy and that makes the theory little difficult to grasp. So, the overall disadvantage is the advantages include that it needs to work in ultra high ultra high vacuum it requires very sophisticated instrumentation it is very complex and it is very expensive and it is a little slower 15 to 60 minutes and it has a poor resolution in comparison to the reflection at the reflection at the adsorption infrared spectroscopy. So, that makes it much more poorer resolution, but despite that it is still very very appealing and in comparison to the edacs in the yields we have we can just basically have a correlation between the idea yields and the edacs. Then like an edacs we have sensitivity for higher atomic number elements whereas for the yields we have a high detection efficiency for the low atomic number elements. So, that is the that is one part out here and in edacs we can get only elemental information, but in this case yields we can get elemental we can get chemical as well as we can get the dielectric information. So, that in that part we can get the composition we can also get the ionization state and those all information we can get easily from the yields. In the edacs we have energy resolution limited to 100 electron volts whereas in this particular case we have approximately 1 electron volt. So, in this particular case it energy resolution is much higher as compared to edacs and we can also get we can we generally we tends to give less number of peak overlaps it can also provide us very fine structure for the ionization edge ionization edge. So, this part also we can get from the yields and in case in case of edacs we have in efficient signal collection and that makes it very time consuming that elemental mapping generally becomes very time consuming in this particular case, but in yields we have very high efficiently very high or efficient signal collection because it is highly sensitive and it can it can detect even a single thorium atom which is if it is sitting on a carbon film and that makes it very very efficient in terms of mapping. So, it can provide efficient elemental mapping on a particular surface at the same time since it is very efficient it becomes very fast technique it is fast technique, but it requires very complex theory though edacs is little slow it is much more simpler and we can get a spectrum very very quick, but again this is a slow process slow technique, but it can have very simpler processing is required in the edacs. So, that is that is the overall thing about yields that we can we are inciting known electron energy and we are letting it interact with the material and from the detection of the loss which has occurred upon interaction of electron with the material and from that energy energy we can distinguish the overall spectrum either as 0 loss, low loss and the high loss. In the 0 loss we have mostly the elastic scattering and that tells us much about the full on losses which are occurring in the material then later on we have the plasma losses and then plasma losses basically are the main thing which we which which can tell us more about the overall composition of the information, composition or the dielectric nature of the material. Whereas, in the in the high loss in the high loss regime we can see the near edge structure or the fine structure which is near the extended which is called the extended energy loss fine structure. So, we can get some information out from there which tells us more about the ionization state or what is this ionization of a particular material or also about the bonding which is predominant in that particular location and that those all those information we can get from the high loss regime in the yield spectrum and as we see it is a very sensitive technique and it can detect a very fine adsorbate which are there on the surface of a particular entity or a particular surface and this can serve as a very fine tool in terms of resolution in terms of resolving any adsorbate which are there on the sample surface. So, basically with that I will end my lecture here. Thank you.