 In this class today we are going to discuss about a very important spectroscopic techniques called Raman spectroscopy and as you know Raman was an Indian and lot of incontinent aspects are involved in relating this spectroscopic technique. So I will be able to do a little bit of justice to this in one hour time but I do not think it is possible to talk about the details of the spectroscopy in just a short span of time because there are a lot of aided theories are needed to be covered in such a class but I am going to give you some basic idea of the spectroscopic technique how it can be related to applications and obviously some amount of instrumentations will be dealt with. As you know the outline is like this obviously there will be some aspect of introduction going back to history and some basic features and then I will talk about some basics of the Raman spectroscopy where I will talk about theories and then some amount of scattering and vibrations and finally some instrumentation for Raman spectroscopy and obviously you know during the course of a lecture I am going to talk about some examples from the literature or from some of our own work. As far as the basics are concerned Raman spectroscopy basically probes the vibration modes of material it is more like an infrared spectroscopy I have already discussed with infrared spectroscopy in a few classes back and there I showed you by you know theoretical theoretical penitentiations that how this higher spectroscopy related to the vibration modes of the molecules but there is a distinct difference between higher and the Raman spectroscopy. In fact in higher spectroscopy the bands arises bands arise from the change in the dipole moment of the molecule the Raman's bands arise from a change of the polarizability not the change in the dipole moment remember so it is the polarizability change that makes Raman's so interesting so therefore there is a symmetric molecule it will not work that is the most important problem in Raman spectroscopy in many cases transitions that allow Raman's are forbidden in IR so there are techniques these two techniques are basically complementary to each other so you will see that we also you comparable spectra of from the Raman and from the IR to show you how these two techniques can be used for complimentary so therefore as I said at the very basic aspect of the Raman's bands arising during measurements is the change in polarizability of the molecule that is basically sets an exclusion rule what is the exclusion rule it says if the molecule has a center of symmetry then no modes are is active further is IR Raman so as you can look into is this slide you will be clear to you for a molecule with a center of symmetry as I said no IR transitions are Raman actives or vice versa what is it let us suppose this molecule which is COO that is CO2 and you know it is Raman active but IR inactive but on the other hand if the molecule has changed that means if this molecule is little bit of you know center of symmetry it will be this side right side then it becomes Raman active on the other and if the molecule become fully centrosymmetric then no bands no none of these techniques can apply as far the history is concerned that needs to be told because you Raman was the first normal or it in science obviously the first noble rise from this country is person for literature in 1928-2013 but 1930 Sibir Raman got noble prize in physics because of the discovery if I want to stress back the how this spectroscopic technique of the Raman effect came into picture first thing we need to understand is the Raman is basically related to in last is scattering so first time in last is scattering light scattering was predicted by Smickel in 1923 and then after five years 1928 Landsberg and metals them see the fast unexpected frequency change in a scattering from a quartz that is what this English is scattering means actually because skillet is scattering was known or even long long back from Rayleigh Lord Rayleigh's time but unless is scattering was not fully understood so it was in you know early part of the last century 20th century these things have started in 1928 Sibir Raman and one of his students very elasticity students case Kishnan first time while working on different solvents saw feeble flow sense and it was reported and after within two years it was become so you know intense activity Raman was awarded noble prize to tell you that case Kishnan this is the picture both of them are taken from study help line net where you can get bio data this is Raman's picture and this is Kishnan just to impress you upon that in those days when India was under British rule still there are scientists who are working on path making research and it was only possible because of these two gentlemen who worked hard to put the things into the world's perspective as per scientific content is concerned so and but you know normal price was given to Sibir Raman case Kishnan obviously was his PhD student who worked hard to prove that so his contributions were no nowhere less so that is why I have to show you and then 1961 laser came and found the measure actually after the second world war and 1977 first time using laser surface rare enhance Raman scattering was discovered is thanks to laser only it is possible to discover that and 1997 that is the big change happened in the Raman spectroscopy another big change when single molecule shows surface enhance Raman spectrum or scattering so therefore nowadays we can actually go all the way to the single molecule and get Raman scattering and then probe it using Raman spectroscopic technique so that is how it is actually possible as you as you understand talking all this surface and Raman spectroscopy and single molecule Raman spectrum Raman spectroscopy enhance Raman spectroscopy is beyond the scope of this course so those of you are really interested or maybe using this technique extensively they can look at different books which I will show you even some of the references while giving the lectures and then understand or try to understand maybe these are very advanced topics not even part of the this course well to show you how the force Raman spectra look like this is taken from this paper which is published long back okay you can see this is the spectrum Raman flows in spectrum obtained from still mercury arc lamp and this one is from benzene okay C686 so there are distinct bands you can see on a for the photographic plates and these bands actually are related to English scattering of molecule let us look into theory in detail that will tell you how either Raman actually scattering happens and how it can be to large if I have to tell you in one sentence Raman's effect is a two photon scattering process is not long as single photon scattering process and these process are all English this cutting type so first thing we need to talk about the stokes scattering in stokes scattering is always in less scattering because energy is lost by the photon now if I have photon in and obviously there is no vibration here and therefore this photon will be absorbed by the molecule the energy is sufficient enough for it to you know get excited to the excited states and some amount of photon with less frequency obviously will come out because some amount of has been observed that is what is the nature of English scattering you should know that and this will lead to vibrations and that is vibrations can lead to flu and is so scattering's energy is gained by the photon instead of loss so what happens you have a photon in and there is a vibration and then photon absorbs this this molecule actually absorbs a photon so some photon goes out but you know energy is gained by the photon in this way so there will be no vibrations so there are two kinds of scattering as you see in which one is energy is lost by the photon other energy is gained by the photon and both English is cutting type they are called stokes and anti stokes now there is obviously it is nominal scattering process which is elastic scattering elastic scattering means there is no energy loss when the photon is absorbed out and this is what is known by name of Rod Rayleigh long back and this is simple energy absorb energy goes out so in both the cases no vibrations so elastic scattering is cannot give this Raman scattering if instant photon as E and the vibration energy is V okay this is V so V then in terms of energy we can write E A minus V is a stokes scattering E plus V anti-stokes scattering and E is elastic scattering and these two gives Raman scattering not the last one you should know that this all been taught for the different set of particles now to talk about more of a less scattering this is again taken for huge in-heck optics books so if I have a molecule you can see nucleus and electron cloud there and certain photon energy comes h nu and then it is get excited ground states to the higher in this state it goes I can see that and then the excitation can always happen that means the molecule come back to the original ground state by emission of this photon this is fine and you know this can happen in the order of 10 to minus 8 seconds so therefore elastic scattering basically lambda doesn't change the wavelength of vibration doesn't change it can have random direction of emissions doesn't lead to flow sense and there will be little energy loss so therefore it's not and this is what is the basically the scattering energy where these are the different parameters the lambda is wavelength is the energy is the energy level downstate theta is the energy at which it is scattering the angle now if I want to put it the English is scattering into perspectives so I have I can actually make a schematic diagram and show you that so you know this is what is the ground state energy level E0 you can see and then if I have some photon in the wavelength of higher radiations energy will be the molecule be excited to the higher energy level E0 plus h nu and when it comes well this can be also explained using the the photon scattering in a energy landscape so as we see I have seen that there are two kinds of scattering one is elastic other one is stokes anti-stokes which are basically elastic scattering and they are related to Raman so if I consider E0 as my ground state for the molecule and this is the vertical excess energy so whenever certain infrared radiation is imposed on this molecule it will absorb and go to the higher energy band that is suppose the excited states and when it comes back it gives a radiation and that can be detected and that is what is the principle of higher spectroscopy now one can always think the realest scattering or elastic scattering in this way we have a higher energy h nu G o by which the molecule is goes to the virtual excited states not the nearest excited state and when similar amount of energy is released by the molecule it comes back to the ground state that is what I have shown you in the last slide here exactly same thing now in case of stokes suppose molecule is excited to the higher energy state of virtual state pi h nu 0 energy level and there is some elastic scattering happens some energy get lost so that it does not come back to the ground state at all that means it lost energy but the molecules can come back by losing this energy h nu 0 – h nu 1 2 then excited states and vibration states the above the ground state that is what is this one so this is obviously you can see it can flow easily when it comes back to the the the ground state another situation can happen in anti-store scattering is that molecule can get excited to the higher energy states than these previously virtual states okay and when it goes there it gets to this virtual states it gathered more energy maybe it can have h nu 1 extra energy okay so that total energy of the molecule of increased because it absorb another photon other than h nu 0 and when it comes back to the ground states it just have extra energy which is also coming out h nu plus h 2 0 this can also lead to flow sense. So therefore both scatter scattering anti-scattering can lead to Raman that is what I am saying but the catch is this the Raman's effect comprises a very small fraction about one in those seven of the incident photons so that means we need to have very good probe to detect the small number of photons which are actually after going this because most of these and the events will be happening in the realist scattering things now if I want to put it in perspectives in front of you if I have molecule energy goes out and energy comes goes in energy comes out so that means it get excited and is gets scattered so only you can see that Raman count is only one one that it the actually the laser actually which is used for the scattering is very high into the seven counts so that means one into the seven photons is scattered in last is killing so almost bulk of the photons get scattered by elastics and they can have you can have different kinds of Raman spectroscopy depending on what kind of you know scattering is what kind of things are happening like rotational states or vibrational states or electrical states in energy level to put it in terms of IR and the Raman spectroscopy in IR we have a laser beam certain intensity of the energy of the given frequencies impose fall put on the sample this one it absorbs and the ground spectra ground state releases the energy of different intensity and detector detector and you get the IR spectrum spectra in Raman it is basically not the transmission or absorption is the scattering so laser falls on energy falls on a sample is get scattered because of these is you know Raman scattering and Alice cutting this is Raman which is has changed because of the extra energy or loss of energy and then that is what is detected so you have this is the major difference between these two techniques if I want to put it even much better way Raman scattering sample version usually simpler liquid solid sample much if you have dust IR you cannot do that biological material which usually flows masking scattering is also possible so which is which are not possible in course IR spectral measurements on vibration modes in visible regions glass cells can be used and deep origin studies are easily made the laser radiation almost totally linearly polarized but in case of IR halide optics must be used as you have seen they are very expensive can be broken it can be water absorb also because they do not absorb they basically other than halides most of material absorb the IR radiation and it is not possible to get a good data IR spectrometers are not easily equipped with polarizers on the other hand now spectrometers metals are easily equipped with polarizers how does the Raman specter spectrum look like well this is what is shown you in terms in the picture this intensity this is the wave number again centimeter to the per inverse and you know a spectrum is nothing but a plot of intensity of the Raman scattered radiation is a function of a number of frequency difference from the radiations usually in units of a number so this difference is called Raman shift incident and the scatter radiation difference and this is for the carbon tetrachloride so you can see these are the frequency differences minus and these are the frequency differences plus for the anti-stokes and this is a rally so Lally will be the highest peak because this is the glasses cutting on the left of the rally but less wave numbers will be the stokes on the right side will be anti-stokes the elastic scatterings inertia scatterings where energy is lost energy is gained so let us look into the detailed theory of atomic vibration and Raman scattering which is can be just helpful as to helpful for us to understand the simple formalisms well as you know we can always consider diatomic molecule and with the spring attached to it one can be considered spring and they are all getting stretched the spring is getting stretched by mass M1 and M2 spring constant is K so if I stretch this molecule that is what is excitation basically by X1 amount this side X2 amount is on the right side of myself so I can always write down this is the Fuchs law force law basically M1 M2 divided by M1 plus M2 double derivative of space or the extension with respect to the time is nothing but displace sorry there is nothing but acceleration or deceleration this is the mass this basically reduce mass you can say and that will be related to the the the constant K K is a capital and X1 plus X2 so therefore I can write down this in terms of Mu dQ by d2 square Q by dt square equal to K into Q- this is Q displacement and this can be solved this where Vm Mu M basically is equal to 1 by pi root K by Mu this is nothing but the frequency vibrations so that is what is the you know whenever a molecule is stretched or excited this is what will happen it will X private with this and this will be the basically this placement can be related to the vibration this way Q equal to Q0 cos of twice pi rho Mu M into T now if once you look at it you know in different perspectives suppose this is the molecule which is vibrating as you see last time and this is the the change of the distance vibration Q and we are putting in certain radiations which is given by this equal to E0 cos of 2 pi nu 0 T so therefore this will lead to an dipole moment induced dipole moment that committed to alpha into E alpha is the polarizability and E is this E0 cos of this so for small amplitude of vibration the polarizability alpha is linear function of Q as you can see so therefore we can write down alpha is equal to alpha 0 the alpha by dQ into Q and ignore the higher order terms so P can be related to this way alpha 0 E0 you can see you can plug in this one and a big equation and finally after solving all these equations which I have done here in the slide you can get elastic scattering given by this and elastic scattering is given by this in the elastic scattering part you can always get mu 0- mu m or mu 1 mu 0 plus mu m so this is basically stokes this is anti-stokes these two are the factors which are responsible for the mass scattering so by just putting this is simple molecular structure and then one radiations we can always calculate the polarization or induced dipole moment and you can see dipole moment can calculation can lead to us this kind of mathematical theory so therefore if I want to give you some example let us suppose for carbon dioxide here you have CC COO molecule Q plus Q 0 Q okay so you can see that this is what is this I know there is no displacement here as point displacement as a negative displacement here Q you remember so if I like mu 1 but the skew space that is what we will give so there is this is the polarizability that there is a polarizability change this is bit amount active but on the other hand if there is no probability change this is the band in which it happens that this will be iractive which will not be amount active if the molecule is stretched or change this configuration this way this also be the iractive this can also show you the change in the frequency as function of Q so finally again I have put down this equations where the Rayleigh the elastic scattering and elastic scattering events are both are given one can do it for other molecules like water you can see all the modes are in water actually new one new two new three are active with R1 and this because this leads to change in polarizability and change in the polarizations both if you take CH2Cl2 there are stretching and bending both possible here so if I consider vs this is this is the stretching bands for this or VA pure vs here actually there are three A1 B1 A1 so that you know change in the stretching can lead to scattering or you can have a bending actually you can see this is the scissoring you can have scissoring this is all we discussed in in the higher spectroscopy or you can have a rocking or wagging or twisting this vibrations can lead to Raman active spectrum so to give you little more aspects of the polarizability if you have not understood much so that you can go back and look at the slides polarizability of molecule is it the mobility of the electrons when you apply a field basically apply field how the electrons move they move symmetrically then there is no change of polarizability if they move and distortion happens in the field direction of the field okay and then it is an isotropic but if suppose distortion is same this is isotropic so more molecules for many molecules probably depends on the direction of the applied fields direction of the applied fields like HH is here to distort along the bonds then perpendicular to the bond so plagiability is an isotropic variation of the alpha with direction is described a plagiability tensor tensor is another you know vector which can be used to detect to basically show the variation of alpha with the direction now vibration is active it is if it has a changing polarity any vibrations in the molecule if you put an energy will be active Raman active if there is a change in the plagiability alpha so basically I can tell this way plagiability is ease of distortion of the bond so therefore for Raman active vibrations incineration does not cause a change a dipole moment of the molecule but it change the polyability of the molecule that means it change the electronic distortion of the molecule it doesn't really change the basically dipole moment that the molecule the dipoles are not going apart the strengths are not changing but their plagiability means 10th of this electronic the you know mobility of the electrons are getting distorted changed so the starting vibration going the electric field of the radiation at time T is induces separation of charges as you at the beginning you put electric field it is just separation of charges and then this is basically dipole moment which can be related to this P into alpha and we remember alpha is basically the plagiability so we are talking about this change of alpha not change of P which is required for Raman change of P is required for the IR please do not get confused with the molecules dipole moment or the change in dipole moment because this is always often case to be 0 for the molecule of these molecules again I come back to CH2 CL2 okay so sorry I come back to this is wrong I come back to CO2 therefore most of CO2 and only out of four only new one is Raman active so new one is this one CCO so there is a change in you can see dipole moment is changes there is not 0 mu 3 and mu 4 basically 2 or 4 they are active so here means the dipole moment new actually alpha is possibility gives the vibration coordinate or is basically use this basically the displacement which we used that and this is what you can show this is new one so you can see project list changing like this this is mu 3 project list in a way small not active but polarization is the moment is changing extensively here dipole moment does not get changed to give another example of Benzene actually Benzene as you know this is as aromatic CH ring and the stage band at about close to 3000 actually and then you have a ring breathing mode which is about 992 and Raman this is the basically Raman ships you can see these are the Raman ships and this is the basically absolute CM Raman shift is calculated in by difference between the incidentation and the and the vibration which is coming up and this is what is the breathing mode and this is what is stretching mode can be calculated now to show you the difference between infrared and the Raman I take this complex molecule with NH2 groups so you can see that Raman ships gives you very nice spectra as compared to the higher and Raman ships comes at a higher delta mu than a mu actually this is Raman ship to exchange in frequency change in web number this is a mu and this is continuous here this is the because physically fingerprint regions here in IR part the most important things comes in this higher change in the wave number regions so this is again shown here this is the 50 and this IR Raman I can see that COCO is same but CC this one is highly increased for Raman and we can get other peaks also which is very similar in this case of molecule this again obtained from Macri's book to give you a list of the Raman and IR frequency for different functional groups alkanes this is very strong in IR Raman is also very strong CC stage obviously Raman is strong but IR is not at all available and then there are others C triple bonds CN you can see strong and very strong Raman scattering then you have very strong IR scattering for primary these bonds but weak Raman scattering and very strong Raman scattering also scum for CS test because there are a lot of change of projability of the molecule this is all obtained from Macri's book well there is another things which is know we should also know for Raman scattering is the radiation power or radiant power as you see the radiant power 5R is proportional to sigma VX VX to the power new X to sigma new X new X to the power 4 e0 ni e to the power – CI by KT where sigma new X is basically Raman scattering cross section which is in terms of centimeter square and new X the excitation frequency by which you are putting the energy there is a beam energy usually is the incident beam in radiance new ni is the number of density states in any state I and EI is the energy of the states and this is basically exponential term comes because of Boltzmann's so this sigma term is target area presented by a molecule for scattering that is what is this scattering cross section how much is that area target area by molecule for scattering event to occur so that can be also you know put it in it this way so as you see sigma is basically this sigma capital sigma at sigma by this way this is a scattering cross integration of the whole scattering for the molecule and this sigma by sigma this sigma means this is small sigma this capital sigma is nothing but scattered flux by unit solid angle divided by incident flux which is C incident flux by unit solid angle so scattering by incident and then you multiply the whole solid angle which is coming out and you get that so if I have to compare this with other techniques like UV IR flow sense Rayleigh Raman and other surface and as Raman you can see the scattering cross sections very high very high means not very high 10 to the minus 18 is pretty low but still it is high in case of UV and IR Rayleigh and Raman it is pretty low 10 to the minus 26 minus 29 that is the problem actually I want spectroscopy you need to have a very good detector to detect this scattering force again for excitations I am giving you some values for CH this CL okay you can see that these are the wavelengths excitational blends these are the sigma values all in the range of 10 to the minus 28 centimeter square this are all adopted from Akora's book of surface enhance vibrational spectroscopy now lastly which we are going to talk about is the typical Raman setup or five things how it is done in the lab scale as you can see this is a very complex first part of this most important part of this monochromatone spectrograph and then a laser beam resource which is basically passes to dielectric mirrors then put the sample then its radiation is collected or the scattering events are collected it is you can see that it is false and then doesn't grow like this but it comes back at a scattering angle collected by colleague collecting lanes focusing lanes analyzer then polarization scambler and then it goes to the slits and then monochromatose things in this part of this monochromatone you have several important aspects one is the controller second one is the photon counter photon can be counted by PMT photo multiplier tube and then you have a controller which will plot this display this data and then a display so that is what is the basically I think I have already discussed with you about monochromata monochromata are in case of photoluminescent spectroscopy so the spectrograph is where the data are plotted so scattering can be done in both ways for the laser light can falls and scattering can happen at 90 degree this way or you can have 180 degree coming and scattered and then falls scattering where falls on a lens and again this is focused and then they collected this is what is shown here collecting lanes focusing lanes analyzer and this then it enters the monochromata you can spectrograph spectrographs can be for almonds actually this is for this a 1877 tippled monochromata the whole thing is basically monochromata this is the specs 1303 14034 double monochromata let us look at first the double monochromata so you can clearly see that these are these beam splitters or mirrors actually it falls comes and falls on a mirror M5 sorry it falls on M1 then goes on to G1 then again M2 gets reflected finally comes out through this so entrance and this from the entrance and the exit there are truly one M1 to M5 there are five mirrors and there are this splitters G1 G2 and they are all actually and the by this way you can actually get these monochromatic radiations and either was you can do what you can do is this way you can have entrance then it passes through a lot of bands passes finally it goes to the subtractive dispersion and it falls on a three turret getting assembly and then comes out well we will not discuss so much of detail but there are two types of spectrographs used one is a double monochromata type one is triple monochrome depending on this has a better ability to collect data then then this one otherwise you can use photo detectors and actually nowadays people use photo detectors because these are easy to manipulate and easy to work with photo detector is nothing but basically you have you can see here you have photo cathode with a and then you have a within that you have electronic intensifier which will intensify and then these are actually cooler cold finger which will be cooling it and you have optical fiber coupler which will couple that and you can always use otherwise CCD camera so charged couple device where you can actually have pixel types in which you can detect radiations coming on each pixel so and what about others in this case water can be used as a solvent and very suitable for biological samples we know in IR we cannot use water as a solvent you can absorb and then because water can be used this is very easy by the surface can be done Raman spectra gels form a molecular vibration of fire frequencies spectrums are obtained using visible lights or you can a near IR radiations glass or quartz lenses cells optical fibers can be used standard it excess always use but nowadays no longer few intense water tones and combination bands like few spectral overlaps can be done you can totally symmetric vibrations are observable Raman's indices are basically proportional to concentration of the laser power this is the proportionality well as you have seen that this is much more simple and cheaper than the IR spectroscopy it has experimentation instrumentation is much less about the lower detection limit is the problem in case of Raman so background flow sense can always proper care problem for Raman which you will see how to take care and most suitable for vibration with bonds in low polarizability like chlorine carbon fluorine bonds so how to take care of this there are two backgrounds so if you suppose take with water and give you at this spectra and then separately you can take for water and give you it without sample for sample air is basically sodium sulfate and then you can basically subtract and get this the background removed that is what is can be done routinely nowadays for the computers applications Raman spectra can be used for many many such applications like molecular vibrations are quantity quality analysis also it can be done for all kinds of elements sort of many range solvents stress measurements high pressure and glasses also there are large number applications so with this I close this after Raman spectroscopy the only thing which is left over is the stem yields and the last spectroscopy which we will discuss in the next two classes