 So, we start with steady state spectroscopy what is the meaning of steady state spectroscopy those who have used this would know that it essentially means that we are recording spectra but why is recording spectra called steady state to do that again we go back to something we learnt in class 11 what is the meaning of steady state what is the difference between equilibrium and steady state open system. Steady state means you do not see an apparent change but the system is open equilibrium happens in a closed system. So for the sake of simplicity let us consider a molecule with 2 energy levels lower one and a higher one okay you shine it with light that causes some upward transition this is called absorption okay. And then this molecule comes back to the ground state as well either by a radiative pathway or by a non radiative pathway well the names are self-explanatory but tell me what is the meaning of radiative pathway what is the meaning of non radiative pathway yeah radiative pathway means light is emitted non radiative pathway means light is not emitted so molecule comes down from excited state to ground state light is not emitted where does the energy go it gets redistributed in the various modes of motion of the surroundings if it is a solution then it gets redistributed in various kinds of motion in the solvent or in whatever medium it might be. So now if you keep the light on then what happens you can think like this you turn the switch on there is no molecule in the excited state excited state population is 0 okay. If you have kept the light on for some time upward transition absorption is taking place but downward transition is also started in this scenario what is the population of the excited state I mean I do not know what the population of the excited state is but does the population of the excited state remain the same or does it change in the initial period population of excited state is 0. So when the light is switched on the population increases for some time right and then what happens is it starts getting depopulated as well so eventually what happens is this rate of excitation and rate of de-excitation they become equal. So in that state in that situation dpx where px is the population of the excited state dt is equal to 0 okay this when this condition is satisfied then we say that a steady state has been achieved have you encountered the term steady state sometime again in class 11 or maybe first year college chemical kinetics we use a steady state approximation for what if you have a reaction like reactant goes to intermediate intermediate forms product then we say that after an initial induction period population of the intermediate does not change so you get this kind of an equation there that is called steady state approximation okay I will not allow myself to digress and talk more about steady state approximation that is a favorite topic but so that is what happens when you shine molecule with light as well if you keep the light on then after a very very short time and very short time means because I can do something steady state is achieved alright now if you record an absorption spectrum or an emission spectrum provided the sample does not degrade the spectrum will not change alright so the question is what is the meaning of a spectrum I just use the term spectrum what is the spectrum range of wavelengths in English language a spectrum means a varied collection right a spectrum of events or something like that but the spectroscopist a spectrum is a plot where y axis is intensity or some measure of intensity and x axis is energy or some measure of energy okay it can be energy it can be wave number it can be frequency it can be wavelength something like that okay so if I draw some kind of a spectrum absorption or emission under steady state provided your sample is stable the spectrum does not change okay and of course you can do 2 things you can focus on this and try to record absorption spectrum or you can focus on this try to record emission spectrum so before we go on to the actual measurement what we would like to learn is how do you what is it that you measure and how does say intensity depend on various factors so we start with something that many of us might actually know we start with Lambert-Bierce law what is Lambert-Bierce law yeah equal to epsilon cl I just write absorbance this is called absorbance right not absorption and I will write its expression log of what is it I 0 by I t so let us say this here is your sample light with intensity I 0 falls on it and the intensity of light that goes out or is transmitted is I t so I t is usually less than I 0 so what is I 0- I t where is that intensity gone it must have been absorbed or it might have been reflected so for now we talk about samples that absorb and do not reflect so then for those I 0- I t will be equal to I abs right so this is something that we are going to need in a manager so see if I want to measure absorbance I need to know I 0 as well as I t so I must have a way of measuring both and in one of in the next module actually we will go to the lab and we are going to see how an absorption spectrum is actually recorded one of you is going to record it and then we will see it with our own eyes but let us not forget that we need to know I 0 as well as I t we are going to draw the schematic very soon so this is the important thing for absorption spectroscopy now before we leave this tell me what is the unit of C here mole per liter molar what is the unit of L length this is L right length of the sample through which the light goes that is in centimeter and C is in mole per liter what is the unit of absorbance there is no unit please remember absorbance does not have a unit sometimes in literature we see absorbance written in arbitrary units it is wrong the unit is not there so it is arbitrariness and the question does not arise you can have normalized absorbance sometimes you want to make all the spectra of the same size that is a different issue and you write normalized absorbance okay so then what is the unit of epsilon yeah liter per mole per centimeter and what is epsilon called well I used to call it molar extinction coefficient but then about 5 years ago I had a student who told me that this extinction coefficient is apparently outdated and we must call it molar absorption coefficient I actually like the term extinction because it tells you how the light slowly gets extinct as it passes through the sample but then if it is outdated it is outdated what can you do but I think everybody understands the meaning of molar extinction coefficient so now let me ask another thing again going back to something that we learnt at the beginning of thermodynamics we all know what extrinsic and intrinsic quantities are right so in this expression for absorbance can you tell me which quantities are extrinsic which quantities are intrinsic absorbance itself is extrinsic no doubt about that L is extrinsic concentration you can change well for a given solution concentration is intrinsic but for the substance if I think of the substance no matter what the concentration is no matter what the length of sample is epsilon is an intrinsic quantity okay so what is epsilon what does it tell us about what does epsilon tell us about we are talking about 2 states and absorption is an upward transition between the 2 states and what we are saying is that it does not depend on the number of molecules okay epsilon gives us an idea about the probability of transition in fact it is proportional to the square of what is called transition moment integral transition moment integral is the probability of transition the major probability of transition that we calculate using time dependent perturbation theory epsilon is something that we get experimentally okay they are related we are not going to deal upon this further whoever is interested to know more about this you can go through our lectures in the NPTEL course on molecular spectroscopy alright so much for absorption the take home message from the point of view of spectroscopies what are the take home messages first of all absorbance has no unit epsilon does have a unit and if you want to record absorbance you must know I0 as well as I and that is what determines what kind of instrument we have to use to measure absorbance alright and before leaving this topic one more question what are the things that epsilon depend upon epsilon is probability of transition so of course it depends on which 2 states it is right so if you take if you are thinking of a didi everybody knows right didi for transition is La Porte forbidden so if you are talking about transition between 2 d orbitals of course epsilon is going to be small if you are talking about n pi star transition if you are talking about pi pi star transition in which will epsilon be larger pi pi star right n pi star transition is actually forbidden right once again for more discussion of this please refer to our molecular spectroscopy lectures that are available on NPTEL and YouTube so that is first thing which states are involved experimentally well actually this is what it leads to does it depend on concentration epsilon usually know actually know but does it depend on wavelength yes otherwise your spectrum would not be a spectrum it would have been flat of course it depends on wavelength and wavelength dependences sort of comes from what we have stated already that it depends on which levels are involved in the transition ok transition moment integral does it depend on temperature not unless one of the states get populated by change in temperature otherwise it is not an activated process generally only activated processes depend on temperature but here it may be that we are talking about transition from one state to another one and one of the states is not formed at a particular temperature then it might but that is not something that usually happens ok so that is what it is now let us remember this and let us also remember this I0 – I t is equal to I abs for a non-reflecting sample let us do a quick discussion on emission we will talk about electronic state later let us say that once again we go back to those levels and there whatever I am not saying they are electronic levels some level so you have populated the excited state and now it comes down draw the same picture once again ok so you have some intensity of emission right and we have already discussed intensity of the light that is absorbed right that is I0 – I t so now see what kind of intensity I will get depends on 2 things first is once again what is the probability of downward transition between these 2 levels right so if you go back to Einstein's treatment which you are going to do a few classes later here we are talking about spontaneous emission so that would depend on something called Einstein's a coefficient what else suppose the intensity of absorption is very high will that affect the intensity of emission intensity of absorption what is the definition of intensity number of photons per unit area per second per unit time right so if that is high for absorption that means what what do photons do they promote molecules to the higher state right so if I bombard the molecule with more number of photons per unit area per second then naturally the excited state will be formed to a greater extent and if the excited state is formed to a greater extent it is not very difficult to understand that IEM should be proportional to the population of the excited state and population of the excited state depends on IEMs so IEM also depends on IEMs so I can write it like this IEM by IEMs is a constant since we are doing a generalized discussion here we will call it phi em and for now we will just write emission quantum yield anybody want to say anything about this quantity emission quantum yield is it related to something that we have discussed already yes so what is that what have we said about intrinsic property and for emission what is the quantity that we mentioned few minutes ago which determines the intrinsic probability of transition Einstein a coefficient is it not. So this quantum yield you can say is actually this Einstein a coefficient so now suppose I experimentally want to determine quantum yield what will I do is it enough if I decode the emission spectrum or do I need the absorption spectrum as well remember it is not so easy to get absolute values of intensity from these experiments you only get relative values and even if you get absolute values is it enough to record the emission spectrum if I want the quantum yield or do I need to measure absorption spectrum as well both are required because quantum yield is IEM by IABS I need to know IABS as well okay and remember something we said IABS what is it I0-IT okay so that is what will come in here I0-IT now what is what would be the relationship between IABS and your absorbance will there be some relationship A is equal to log of I0 by instead of IT I will now write I0-IABS right so it is not very difficult to see I hope that rearrangement of this can let me actually get rid of well so I already got rid of IT what happens if I rearrange this log right so I will write 10 to the power minus A on the left hand side what will the right hand side be I0-IABS divided by I0 so that turns out to be 1-IABS-I0 I can perhaps get rid of all this now so now IABS if I make that the subject of formula what do I get 1-10 to the power minus A multiplied by I0 right so in this expression I can write Phi EM is equal to IEM divided by I0 1-10 to the power minus A well there is one factor that we have not written here it becomes important in solution sometimes and that factor is your refractive index but generally the error induced by it is not so much but it is important to know this 10 to the power minus A okay this is what Phi EM is right so we have all the working formulae we know what we need to record we know that if you want to measure absorbance then it is important that I know not only IT but also the transmitted intensity of the transmitted light but also I0 the intensity of the incident light so I should have some way of measuring both and we now know what emission quantum yield is and we learn that in order to determine emission quantum yield it is not enough to do an emission experiment only you must perform an absorption experiment as well in fact you must perform the absorption experiment first so the right sequence of doing things is record absorption spectrum first and then record the emission spectrum many times we see that students who want to work on emission spectroscopy just record an emission spectrum it is not enough if you are going to talk about emission quantum yield so always it is a good practice to record an absorption spectrum first then record an emission spectrum then you record something called an excitation spectrum but we will come to that in the next module so the next module let us discuss rudimentary structure of the instruments that are required to record absorption and emission spectra they are called absorption and emission respectively spectrophotometer that is what we will do in the next module and then we are going to go to the lab and we will have a look at an actual absorption spectrophotometer an actual emission spectrophotometer