 Welcome, so we are going to start a new topic today that is on advanced spectroscopic techniques. We know that nowadays spectroscopic techniques have taken a huge lead in material characterization. That is mainly because we need to know not only the compositions of the different materials we process but also we need to know the electronic states, vibrational states and many other features which we will discuss during the process of these lectures. The way I have outlined the different lectures for these part of the course is as follows. Very first I shall introduce you the electromagnetic spectroscopic theory in one lecture. Then I shall discuss different techniques which are used in the spectroscopic arena. First one is called UV visible spectroscopy. This will take us one lecture. This will be followed by photo luminescence spectroscopy for again one more lecture. Infrared spectroscopy which is used almost day to day life for the material processing will take about two lectures. Then Raman spectroscopy which was discovered in 1930s by a Nobel laureate C.V. Raman will be there in one lecture and lastly STEM the scanning transmission electron microscopic technique and yields energy loss spectroscopic techniques will be discussed in two lectures. I have already discussed about the basics of STEM in the microscopic techniques. I will concentrate the applications of this STEM and followed by obviously the yields which is used to obtain different kinds of spectroscopic information. So in a nutshell I will spend about nine lectures in this part of the course. So let us just begin first thing which I will do is some introduction to the electromagnetic spectroscopic theory. We know that in our curriculum metal science curriculum spectroscopic techniques are not taught extensively. So you might have got some introduction in some of the courses but the knowledge of the of this area in the present curriculum is very small. So and because this is an advanced course so we need to first know the basics as we know that the spectroscopic techniques actually deals with measurements based on light or any other forms of electromagnetic radiation per se and these are widely used in analytical chemistry for information. Nowadays people use even in other arena also. So basically it deals with interaction with this radiation and the matter and that is the subject of spectroscopy. So it basically I can tell that spectroscopy is at a very rudimentary level nothing but interaction of the radiations like light x-ray, gamma ray or any other part of the electromagnetic radiation with the material and we analyze whatever comes out after the interaction. EDS which is used in scanning electron microscopic technique is another spectroscopic technique where electron interacts with the material and we get signal out of that and then we analyze that and these techniques actually can give us all kinds of vital information regarding the structure of the material like molecular atomic species, their structure, electronic structure, band structure, many other things. So therefore we can classify these techniques very easily depending on the type of the region of the electromagnetic spectrum involved in the investigation and as you know the electromagnetic spectrum which is shown here it spans from radio frequency like very high wavelength 10 to the power 10 to the power 12 million nanometers are very low frequency obviously 10 to the power 4 to the power 8 to very high energy that is gamma ray with the frequency of 10 to the power 20 per second are harsh and so therefore this is if this is what is the electromagnetic spectrum we can utilize different part of the spectrum for the spectroscopic analysis starting from radio waves which are used for nuclear spin states measurements like NMR studies, nuclear magnetic resonance. Then we have IR where which can be used for understanding the molecular vibrations then we have a UV visible spectroscopic techniques which consist of both ultraviolet and the visible range which can be used to study the valence electronic excitation in the material and x-rays can be used for core electron excitation like x-ray spot election spectroscopy and many other techniques are there or just spectroscopies so that means depending on the kind of investigation we would like to do the uses of the electromagnetic radiation will depend on that. So basically it was down to the frequency of the radiations which should be used for excitation of the material at the beginning and so therefore if I go back so regions from gamma ray x-ray ultraviolet visible infrared microwave and radio frequencies all of these can be used for spectroscopic analysis was most notable so answer x-ray ultraviolet visible infrared and as well as microwave. So we normally for a beginner we differentiate the spectroscopic techniques or classifier spectroscopic technique according to the wavelength of radiations in this way. So chemical methods we know that it provides the information like a molecular structure both in quantitative and qualitative way for all kinds of material involved I think irrespective inorganic organic compounds well. So therefore you know that electromagnetic radiation is used in the spectroscopic technique so you must have some idea about the properties of electromagnetic radiation you have studied in your plus two or even in bachelor studies bachelor's degrees but I just like to reiterate this issues. Electromagnetic radiation is basically nothing but a form of energy that is transmitted to space at a very high velocity at the in fact velocity of light and they can be this obviously described by wave or like it has a wavelength it can be velocity can be related with wavelength and frequencies and obviously like any other way it has amplitude we know that in contrast to sound waves light requires no supporting medium we know that sunlight from sun travels such a long distance without any media so that means it can pass through vacuum so there is no problem and as far as the Planck theories of the concern we can always consider electromagnetic radiation to be consisting of discrete brackets of energy or particles called photons or quanta we know that the energy is equal to h nu of the photons where nu is the frequency and h is the Planck constant so that is again from Planck theory and also we know that that means what that means electromagnetic radiation is can be either treated as a wave or treated as a particle this wave dual particle duality is all dealt within the many of this case so by schematic diagram I can say that the light comes any electromagnetic comes at it you know an entity angle from the electric field and the magnetic field so if x and y sorry y and z are the two direction of the electrical magnetic fields then then the radiation comes in a different this direction z direction x direction which is the known as here directional propagation and we can always represents this with a wave so wave has a particular wavelength double lambda which is from the tough to tough or maybe the peak to peak positions distance and as amplitude a and this is the time of time or distance which way you can describe this are all very simple description of the wave now if you want to go into some more details we know the speed of light is fixed this is given by the this value 300 to the power 10 centimeter per second in a medium containing material or matter light travels with the velocity less than C because it interacts with the material and basically that electromagnetic interaction interacts with electrons in the atoms or molecules in the medium and that is how the velocity of the light get reduced since frequency of the radiation is constant so therefore wave length will decrease as the frequency increases and you know when light passes from back to a medium containing matter or wavelength will also decrease many times we represent wave number mu bar another way to describe the electromagnetic radiations and this is used in the spectroscopic technique that is why I like to know it is nothing but 1 by lambda so therefore it is a unit inverse of lambda lambda is unit of nanometer so this as unit of nanometer inverse or it can be have unit of centimeter inverse also that means it is a unit of distance inverse thus is what is shown here this is the amplitude this is the distance as you can see here the the wavelength and absolute in air is suppose given lambda s to the power 15 5 nanometers and when the same radiation enters the glass the wavelength decreases from 500 it becomes 330 and subsequently frequency because it remains same so that means the energy has changed basically energy sorry some part some of the interaction of the material has changed the wavelength and when it again comes out from the glass it retains the same wavelength so this is some of the basic features which we can see nowadays in all kinds of material and if you want to talk about particle nature as I discussed that they are photons so therefore energy of the any any kind of radiation can be represent by Frank's law that is equal to h nu which is nothing but hc by lambda but see the velocity of light or radiation and this can be again represent hc nu bar so this is what nowadays we normally use hc nu bar is energy so if you know nu bar that is the web number we can calculate this energy and you know that h is a plant constant with the value 6.6, 10 to the power minus 34 joule second so radiation power of any beam actually mean of radiation does not matter whether it is electromagnetic or the x-ray or gamma ray or even leather beam it directly proportional to the number of photons per second now as I told already discussed the what is called different spectrum electromagnetic spectrum region I am going to tell you the one exact values now normally UV ultraviolet comes in the range of 180 to 380 nanometers visible come 380 to 780 nanometers normally you study visible ray comes from 4400 to about 800 nanometers but it can be extended a little bit more near our infrared comes about 0.78 to 2.5 micron very large wavelength mean infrared comes 2.5 to 50 and rest is this called microwaves which are very high wavelengths may be of the order of centimeters with that is what I have shown you in this slide you can see microwaves can I have actually 10 to the power 8 almost nanometers that means about 10.1 meter well now if you will go to the molecular structure wise when a radiation falls on a material the sudden kind of changes happens and this changes will be depending on the type of radiation if the materials are submitted to that means the wavelength or the frequency of the of the radiations which is used to probe the material. So if I I am going to show you like these things in a two ways one the type of spectroscopic technique studying from NMR to microwave infrared alpha UV visible X ray and gamma ray and these are all the different what is called parameters this is the what is called the frequency you can clearly see 10 to the power 6 10 to the power 8 10 to the power 10 up to the power 8 18 and energy is starting from the minus 3 to 10 to the power 9 energy that is joule per mole and wavelength and wave numbers can be calculated very easily. Now if you have if you are basically focusing on very small energy that is 10 to the power minus 3 10 to the power minus 1 joule per second corresponding to very low frequency or rather high wavelength 10 meter to 100 centimeter which is used for the microwaves as a larger than microwaves even. So what you see is nothing but change of spin that is what normally happens quantum mechanically spin basically changes when you apply that so that means you can study nuclear magnetic resonance when as a spin of the both the particles like electron changes depending on the type of material so we can understand that. Now if you use microwaves then or even infrared which comes in the frequency 10 to the power 10 to the power 14 in this range harsh so you can have basically change of orientation of the molecule so that is what is shown here you can have the this kind of molecule suppose big at a small atom connected by a pond they can get differently oriented or you can have a geometrical change of the configuration. So this is what is normally happens when you use this in this range the any kind of addition whose frequency varies in this range and if you go to higher frequency level little higher 10 to the power 14 to 10 to the power 16 then you can have basically changes made in terms of electronic distribution in the material that is what is shown here you can see the electronic distribution can get modified and that can be studied and if you go to very energy like gamma rays 10 to the power 18 which corresponded to 100 picometers of wavelengths energy 10 to the power 9 joule per mole you can have even change in the configuration of nuclear configuration that is what you know if we bombard gamma rays into certain materials nuclear configuration gets changed you can get voids even in the material by remember the material this is all very well documented in the literature. So upon knowing this aspects let us now look at what actually spectroscopic technique does really because as I showed told you at the beginning and under for the last few slides that we use certain electromagnetic radiation and allow it to fall in the material and then see the changes happen in the material in terms of when the radiation comes out. So spectroscopics actually use this interactions so in this case sample is stimulated rather by applying energy in the form of either heat or you can apply electrical energy or even light actually or even you have a chemical reaction. So basically any kind of sample is stimulated by this energy sources and it is you know that at the beginning the we assume that analytical solutions of the material is in the ground state. So therefore Lewis energy state therefore as I stimulated with energy or with certain kind of energy source it will cause this material to undergo a transitions that is it is expected that energy will make the material to go from ground state to higher energy state or excited states. So if this is what actually perceived to be happening in the material then you can obtain information of the of the analytic material by measuring the electromagnetic radiation that will be emitted when it will return to the ground state because if we excite any materials by energy to higher energy level then obviously after certain time the material will come back to the ground state or the low energy state and extra energy will be emitted. So we can actually then measure the radiation emitted as a material comes back and by measuring the amount of the electromagnetic radiations and also type of radiations coming out absorbed we can do we can get a lot of information about the material. So that is in a natural very simple way any spectroscopist do does in the real in the laboratory. Now depends obviously then depending on that we can have different kinds of techniques you know in emission analytical basically sample is stimulated by heat or electrical energy or chemical reactions and this is called emission spectroscopy. So normally the stimulus is heat and electricity in this emission spectroscopy then you could have chemiluminescent spectroscopy that means in this case the sample is excited by chemical reactions not by heat or electric energy but a chemical reactions that is why it is called chemiluminescent spectroscopy. So measurement of the radiation power emitted when the material comes back to the ground state can be used both the cases to measure identify the concentrations and results of these measurements are basically has to be represented by a graph or spectrum which will see in few minutes time in some of the plots and this cases this is the y axis is can be any parameter as whatever radiation coming out and x axis will be either lambda or 1 by lambda. So lambda is wavelength 1 by lambda is wave number let us do the some see the some of the things suppose we have a sample like this and we have a energy source thermal chemical or electrical and we are excited the material once you excited the material obviously sample will absorb the energy certain part of energy and it will go to the high energy state as I said and then it will come back to the ground state after certain time and in that process it will emit the radiation that is what is known as PE here okay. So photo luminescence whatever you can say that it is PE now if I go back to this place this one there are any states here so suppose 0 corresponds to ground state and now I have energized the material excited the material so that it can go to the high energy state like 1 and 2 which are the excited states and as after certain time when it comes back to the ground state it can have different paths. So this is the this is the excitation to go to suppose state 1 is the excitation go to suppose states 2 now very fast thing can happen is basically you can come back from state 1 to the ground state and which correspond to energy emission of E1 or can be related to H new one or committed to AC lamp by lambda 1 okay our second thing can happen basically that from the energy states 2 if it excited and if you given so much of energy that equal to 2 and it comes back to the ground state and then this much of energy will be released E2 1 and which correspond to AC by lambda 2 1 sorry so if this case it will be E2 will be emitted not E2 1 E2 will correspond to AC by lambda 2 you can have a inner transition that is from 2 state to 1 state excited 2 to excited 1 in that case energy will be released like E2 1 with the wavelength given by 2 1. So as you can see here the energy E1 is obviously higher than E2 okay the amount of energy is sorry E2 is higher than the E1 and then E2 1 that you can see the length of the value and determine that also that is expected understandable. So if I plot P versus lambda which is done in this case that is what is the plot a spectrum we call and you can see we gets peaks corresponding to lambda 2 that means this is corresponding to transition from 2 to 0 then another peak correspond to lambda 1 this correspond to transition from 1 to 0 or we can have a very small peak corresponding to transition from 2 to 1 so these are all marked like lambda 2 lambda 1 lambda 2 1 so that means these are the characteristics wavelengths which will decide the type of material we are probing so these wavelengths once you know then we can actually probe we can say that what kind of material it is what kind of states the material has all these kinds of features can be done. So this is this is this are all very simplistic way of plotting the data or doing the measurements. Now we can actually have something like absorption instead of emission so when a sample is stimulated by application of external electromagnetic source several processes are obviously possible and you know how the instant radiation can be absorbed and then this absorbed radiation can promote the species to go to the excited states and in absorption spectroscopy which is just I have shown you in the last slide the amount of light absorbed is basically function of wavelengths and you can measure and then obviously we can use both these wavelengths a qualitative measurements to type a material and the area under these curves will give us the amount of energy released at that particular wavelength and that can be used for quantitative analysis. In a photoluminescent spectroscopy emission of photon is measured following this absorption the most important form of photoluminescent spectroscopy is fluorescence or phosphorescence spectroscopy which we will discuss later well this is what the absorptions looks like that the last one was emission absorption is like this so we have instant radiation P 0 ready of intensity it falls on a sample and then P is the transmitted radiations and we can assume that initially sample was at ground state once the radiation is absorbed in the material it goes you can go either to the state 1 or state 2 so this is the transition from 0 to 1 the transition from 0 to 2 and then we can basically measure a this is the absorption as a function of wavelength lambda and we are going to get two peaks again or two peaks correspond to lambda to a lambda 1 lambda 1 correspond into excitation from 0 to 1 lambda to correspond to excitation from 0 to 2 so this is absorption 1 something getting absorbed we can actually measure the absorb energy as a function of wavelength and find out the whole characteristics of the whole process well another one is luminescence as I said so you have suppose instant radiation P 0 falling on the sample okay and then some part of the radiation will be transmitted and some part with come as a luminescence well luminescence means some kind of inside transitions and which will just discuss and then you can measure this luminescence as a function of lambda so in a so I have discussed emission I have discussed absorption now I am discussing luminescence so in a luminescence again we can consider sample at the beginning to be at ground states and once you energize it can go absorb energy and go to state 1 or state 2 okay and then you can have different kind of transitions obviously first one transition can happen is basically form 1 to 0 when it comes back to ground state or your second one is 2 to 0 or even you can another way you can have is form 2 to 1 this dotted as basically talking about the transition suppose form 2 to 1 followed by 1 to 2 1 to 0 so and these all these things come as a specific wavelength and we can actually get this wavelength very easily that the area of the pics tells you the how much is the energy comes out as a function of in a as a luminescence so we will discuss more detail about now absorption process absorption is very important part of the spectroscopic we have I have shown you three different types of processes emission absorptions and the luminescence so you know absorption process always discuss in terms of beer slumber law or beer slumber law many times people call beer slumber law but both of these were involved this law if I look at it it tells us exactly how the attenuation of this energy depends on the concentration of the absorbing molecule and the path length or is the absorption takes place obviously you know in a solution or in a in a material if it is not 100% pure there will be concentrations of different kinds of pcs presence and this the attenuation of energy that means a much will be absorbed how much will be emitted how much will be having photo luminescence will depend on the what is called both the concentration of the absorbing molecule also the path length so how we can show this schematically this can be shown like this suppose you have parallel beam of monochromatic radiation lambda it forms and falls on a solutions and it passes through and the thickness of this absorbing solution is B and the concentration is C moles per liter B is terms of centimeter because of the interaction of this energy with this solutions okay that means interaction of the photons of the radiation with the molecules of the material inside absorbing the particles the radiation power of the beam will decrease from P0 to P so we can define the transmissions transmittance actually T like this ratio P by P0 and absorption is basically the log of P0 by P the opposite now so again I am doing that the transmittance is basically fraction of the incident radiation transmitted by the solution and it is often expressed as a percentage transmittance that is T equal to P by P0 into 100 and absorption is basically to the transmittance in a logarithmic scale or that is log of minus log of T that is log P0 by P remember this is P0 by P transmitted is P by P0 that is the difference so minus log T is absorption so that is the relationship between T and A so this is how they are related now ordinary transmittance and absorptions cannot be measured the way I have told you just showed you okay so that means what do I say they cannot be measured as shown because solution to be studied must be held in a container we cannot simply take the solution and float it in air or in a vacuum and then measure we need to keep a solution in certain container and that means if the whenever you keep in a container you have a you will have the problems of container coming to picture like your reflections or scattering losses from the cell walls of the container walls and this losses may be substantial then one has to take care of those losses you know light for example can be scattered in all directions from the surface of large molecules even particles such as dust or even solvent and this can cause further attenuation of the beam that means energy loss or energy absorb or whatever as it passes the solutions so one actually needs to take care of those how to take care of those or let me first show you different processes suppose you have incident beam PI then you can have reflection losses you have scattering losses you can have reflection losses at the even interface or you have a foreign particle inside or a big molecule inside you can have these processes taking place again so to compensate these effects how to take care of these power of the transmitted beam power of the beam transmitted to this cell or the container is compared with one that ever says an identical cell containing only solvent or a region blank so basically you are using a standard so an experimental absorption that closely approximate the two absorption of a solution can be obtained so that means you can write absorption equal to log p0 by p approximate equal to p solvent by p solution if you have this kind of container which contains only solvent of the reagent so that's how one can take this is very simple to describe I don't need to go into detail of that now by after knowing all this stuff let us know what is B as law actually the B as law or according to this law absorption a is directly proportional to the concentration of the absorbing pieces C and path length P of the absorbing medium so that means a is directly proportional to C that's a concentration of this absorbing pieces and a is directly proportional to B B is nothing but the path length okay so that's what I can write down and C is basically concentration so a can be written as equal to a into B into C where a B into C a is a proportionally constant and this is known as absorb bt because absorption is a unit less quantity a you know a has no unit is a logarithm logarithmic of any number has no units so therefore the the what's called C and B should have units which will cancel out C as unit of gamma liter B as unit of centimeter so therefore absorb it should have unit of liter per gamma per centimeter otherwise a capital A will never be unit less this is what is called B as law or B as Lambert's law whatever you can say in the literature this is very simple straightforward absorption proportional to C absorption to B path length so therefore absorption equal to a into B into C where a is a constant called absorptivity B is a path length centimeter and C is the concentration in come per liter so when we express the concentration in mole per liter B in centimeter the proportionally constant is sometime called molar absorptivity and then it can be reasoned given a symbol like epsilon and a equal to this formula and obviously epsilon will have units of liter per mole per centimeter now one can actually apply this is no to a mixture this is nothing but additive this law so beer law can be applies to any solution currently more than one kind of solving PCs are the substance obviously we need to consider the fact that there is no interaction between the PCs so total absorption is nothing but the summation of the individual absorption so that means I can write a total is equal to sigma i equal to 1 to n ai that's all I can say and this is very straightforward now so that means one can actually study the absorption process by using B as law and there are advantages of using this this is very simple and straightforward law but one also know what are the limitations that so that you can take care of there are few exception to this linear relationships between the absorption and the path length at a fixed constant as I said a equal to a to b into c therefore for c 1 a 1 a is equal to b this is the linear law we frequently observe the deviations deviations are seen and observed just like revision in Raoult's law all of you know Raoult's is also linear law but there are deviations here also and this some divisions can be either real deviations which are fundamentals and we need to talk about these limitations very clearly you can also have limitations as the coming consequence of the absorption measurements like as a result of the chemical changes to the concentration changes they are there can be instrumental deviations or chemical deviations so we will discuss one by one first and foremost one is the real deviations as I said the Bias law describe the absorption behavior of a dilute solutions only first for your kind information and it is a very little limiting law like Raoult's law as concentration exceeding about 0.01 molar ever a distance says between the ions and the molecules are diminished to the point where each particles affect the charge distribution and thus the extent of absorption of its neighbors so that means if you have a pcs in a solution whose concentration is more than 0.01 molar Bias law cannot be applied to that so that means Bias law can be applied only for solutions on the pcs in the solution whose concentration is very small the occurrence of this phenomenon causes deviations whenever you have concentration more than 0.1 you can cause deviations between the absorption and the concentration and when even ions are in close proximity the molar absorptivity of the solution of the analyte can be altered because of electrostatic interactions and this can also lead to departures but this is not widely absorbed this is very prominent things you can have chemical deviations as I said so deviations can of Bias law can also appear when absorbing pcs undergoes dissociations or associations or even reactions to the solvent that means if there is any reaction of the chemical pcs either it is a dissociation or dissociation or reaction does not matter it will give you different values of absorption than the if there is no has a kind of reactions and when this will lead to a departures and such a kind of departure can be predicted obviously by looking at the molar absorptivity is of the absorbing pcs and if you know that and obviously you should know also equilibrium constants. Unfortunately we are unusually unaware of such processes at the beginning okay we do not know that and so compensation is not very impossible typical equilibria that gives rise to this kind of things are actually monomer to dimer in polymers metal complexations or when more than one complex is presence acid base equilibria solvent analyte associations equilibria. So these are the things you should remember which can give rise to chemical reactions.