 So far we have learnt about absorption and emission spectroscopy in the sense that we know that absorbance is epsilon Cl and absorbance is actually defined as log of I0 by I t where I0 is the intensity of the incident light, I t is the intensity of the transmitted light and we also learnt that for a non-reflecting absorbing sample I t is simply I0-I abs since I abs is the important quantity for us let me write it like that. We have also studied that for emission spectroscopy we define something called phi em emission quantum yield which is given by I em divided by I abs and little bit of mathematical manipulation for this expression gave us I em divided by I0 into 1-10 to the power-absorbance. And then we said if we now have an instrument by which we can record absorbance we have an instrument by which we can record emitter intensity, intensity of emitted light then we should be able to work out the emission quantum yield also. Now why we want to know emission quantum yield that we will come to later but let us first see how it is recorded. These are recorded by using instruments that are called spectrophotometers. If you are working with absorption spectrum it is called an absorption spectrophotometer. If you are working with emission spectrum then it is called an emission spectrophotometer. Emission spectrophotometer sometimes can be more specialized some of them might be such that you they allow you to look at fluorescence they are called photometer so on and so forth. What is fluorescence we have not discussed that yet we will do it later okay. Now let us see what is there inside a typical spectrophotometer what do you need to do in order what do we expect from a spectrophotometer. What is the output as I said in one of the previous module the output is this plot where x axis is energy or some measure of it y axis is intensity or some measure of it. So if you want say an absorption spectrum now we know that here we are going to write absorbance. If you are talking about emission spectrum in the y axis we are going to write intensity of emission okay and then we expect to see a plot that looks maybe something like this. To get this I should be able to record absorbance in this case and in the case of emission spectrophotometer emission intensity at different wavelengths or different energies okay since most of our discussion will be on electronic spectroscopy and it is conventional to use either centimeter inverse or more commonly wavelength I am going to write wavelength for now but one thing that we should not forget is that wavelength is a reciprocal scale 1 by lambda is actually energy. The direction of increase in wavelength is actually direction of decrease in energy let us not forget that. So I need to be able to record the intensity of light at different wavelengths. So how do I do that what I had drawn earlier is we have a sample and right now we are talking about absorption some light falls on it part of the light gets through suppose I put some kind of a detector here detector in this case is something like an electronic eye will we see what will we see the detector is going to sense the entire amount of light that falls on it right and it will not be wavelength resolved or energy resolved or anything okay it can tell you what is the total intensity. So in this case what is total intensity total intensity is the area under the curve okay so it is very easy if you put a detector to tell you what is the area under the curve is but if you want to know what the curve looks like then we need to put in something more in between something that will be able to differentiate colors from one another something that will be able to differentiate the energies of wavelength of light from one another okay how do we do it so we have to use a phenomenon of what is called dispersion right we have to break down a polychromatic light not necessarily white light but polychromatic light into its component. So for that we have to use some kind of a dispersing element has anybody seen any dispersing element once again outside the lab yes so everybody has seen a rainbow and the reason why you see it is that you have this droplets of water in the air which serve as a dispersing element and I think all of us have read of this famous experiment by Newton when Newton had collimated sunlight and made it fall on a prism and sunlight broke down into colors of different into components of different colors okay so this is a way of preparing rainbow. What we essentially have to do is from this light we have to prepare a rainbow and then we have to make different parts of the rainbow fall on the detector at different times right that is our job how do you do you can use a prism nobody uses a prism anymore why because in a prism what happens is the light actually travels through some amount of glass or sodium chloride or whatever it is and there invariably there is some absorption. So you lose light you lose signal instead the dispersing element of choice is a grating what is the grating it is some solid object in which you have lines that are parallel to each other at regular intervals okay has anybody seen a grating outside the lab CD is just that the groups are circular that is why if you look at the shiny surface of the CD you can see a rainbow and the principle on which this grating works is that is called what Bragg's law what is it in lambda equal to 2D sin theta what it essentially says is that different wavelengths travel in different directions so polychromatic light falling on this lambda 1 will go in this direction lambda 2 will go in this direction okay what is the deser spacing between the groups right. So you have created a rainbow now what happens one option is I take my detector here and here and here that is not so easy so what is usually done is first of all you put in a slit what is the slit a slit is like 2 solid plates with a very narrow opening between them. So if I use slit here so the way I have drawn it here is that this is these are solid plates so no light will go through them but there is a gap between so whatever light is here will go through and you can understand that this light that goes through will have some particular wavelength of course it will be some lambda n plus minus delta lambda okay. So now if I do something I put a slit I have a grating and I put the grating on a revolving something revolving bench so when the grating is in this position some wavelength lambda n goes through if I turn the grating like this then some other wavelength will go through so this way I can select which color goes through this slit and I do not have to move my detector the detector can sit nicely here okay. So this is a dispersing element and a slit the combination is called a monochromator we do not wish to go into more detail of this in this series of lectures once again wherever is interested please go through our lecture series on spectroscopy there we have spoken in a little more detail about this and if you want to know more I would suggest that you read the book by Skook it is about chemical instrumentation it is Skook and somebody but I have forgotten the second name Skook is usually enough to find the book right. So you need a monochromator okay so let us clean up this a little bit what did we say we have light we have sample and maybe I will put the monochromator here I want monochromator I want light of particular wavelength to go through some lambda lambda 1 so here I can put in a monochromator since I am making a mess of writing I will just write monolambda okay and of course the lamp is somewhere here the lamp is the source of light and remember we are talking about absorption spectrophotometer here and here we put a detector if this is the sample then this detector gives you an idea of what I0 or IT or what intensity of the light that hits this detector is it transmitted light or incident light or reflected light or what transmitted light right so this detector gives you IT but I need I0 as well so the most common there are 2 ways of doing it first is do the measurement twice have a sample and do not have a sample so spectrophotometers that work in that way are called single beam spectrophotometers but split beam spectrophotometer is much more common or sometimes what is called dual beam spectrophotometer what you do is you put in a beam splitter you can think of beam splitter as a partially reflecting mirror typically in absorption spectrophotometers you want to use a mirror with 50% reflectivity 50% of light goes through 50% is reflected and here nothing is absorbed so here what will happen is I0 will be IT in this direction plus IR I reflected but that IT will actually be I0 for this so it goes here and then you have a completely reflecting mirror from there you put in a reference what is the reference suppose you are doing the experiment in solution phase there is some solvent it is usual that you put in that same solvent in the reference side so light goes through there is another detector here and that gives me an idea of I0 okay and then all instruments are run by computer and computer is good at doing math so this gives you the computer gives you A equal to log of I0 by IT okay now if we go back to the previous diagram see you have a grating and you have a slit right what happens if you open the slit more light gets through so intensity is higher but what do you compromise on in that case resolution definitely right so in a typical absorption spectrophotometer you do not really care about the intensity that is going through because in any case you are going to take a ratio so you want to measure you want to record a small slit width so the delta lambda is talking about the width of wavelength the range of wavelength that gets through that is typically called bandwidth the most usual value for an absorption spectrophotometer of bandwidth is 2 nanometer but sometimes you might need more accuracy so some spectrophotometers have variable bandwidth can go from 0.5 nanometer to higher that is one thing the second thing is what is the lamp here again I will not go into detail because the lectures already exist in that spectroscopic course but let me just say that for electronic spectroscopy UV visible range you use a combination of lamps deuterium lamp and tungsten lamp deuterium gives you ultraviolet tungsten gives you visible in some spectrophotometers they also use xenon lamp but it is not very popular because first of all xenon lamp is more costly more intense and in absorbance kind of experiment you do not want an intense lamp because what you are recording is a difference right if it is too intense the difference will not be too much so deuterium and tungsten combination is the most popular choice for absorption spectrophotometer. Now let us move over and talk about an emission spectrophotometer emission spectrophotometer remember let me draw 2 spectra together let us say x axis is lambda let us say this is the absorption spectrum and let us say this is the emission spectrum emission typically occurs at lower energy than absorption for reason is not very difficult to understand now the thing is in order to get emission remember I will draw that first diagram that I drew you are exciting somewhere and then emission takes place and especially for electronic levels it is not this simple you have associated vibrational levels and all that emission always takes place from the lowest vibration level so the question is why do I excite I might want to excite here I might want to excite here and for different samples different molecules I might need different excitation wavelength. So in an emission spectrophotometer instead of one monochromator there are 2 first of all you have this light source and the light source for UV visible range is typically xenon lamp when you go to the lab we will actually show you what a xenon lamp looks like but xenon lamp gives you white light in fact you have actually seen xenon lamp on the road may be sometimes you see these cars with very very bright headlight so those are expensive cars that are fitted with xenon lamp headlights so it is very bright regular headlights in car are all tungsten lamps they are not so bright right but not so bright so usually use xenon lamp for 2 reasons first of all the output spectrum of xenon goes from ultraviolet to IR so huge width so you can get whatever wavelength you want secondly it is a source of intense light okay now unlike absorption spectroscopy when you do emission you want an intense source of light why remember IEM is equal to phi M emission quantum yield multiplied by I ABS and I ABS what is I ABS I 0 into 1-10 to the power – absorbance right so how much is the absorbance depends on the sample but the other control you have is I 0 if you use a more intense source in emission it is actually beneficial because I 0 is more and consequently intensity of emission light is also more that is why xenon is a source of choice for emission spectrophotometer in UV visible range but then it gives you light so you have to break it down into its components you have to use a monochromator here what is a monochromator monochromator is a combination of a dispersing element and slits next you have the sample and the sample limits if you are talking about spontaneous emission then emission is in all directions like what you see in this light so you can in principle collect in any direction but it is most usual to collect at 90 degrees unless there is some problem in doing that for some reason typically you want to collect 90 degrees why because in addition to the emission you also transmitted light right so if you put your detector here then there is a very strong chance that your spectrum will be contaminated by transmitted light of course you will use a monochromator but monochromators also have their own limitations so that problem is usually avoided by recording at 90 degrees so 90 degrees you have another monochromator and then you put a detector in when you come to the lab will show you one detector the kind of discussion we are doing in UV visible range the detector of choice is photomultiplier tube you can have more expensive detectors you can have multiple detectors you can have diode arrays but this is the most usual thing will actually show you an example of a multiple detector as well okay now in absorption as well as emission how is the measurement done we said that we have some kind of light falling on the monochromator and the output is some particular wavelength lambda n but then you want your lambda n to go from here to here or from here to here you want to span a range how do you do that we said that by changing the grating turning the grating so here now if you think of this spectrum the spectrum in this arrangement does not come all at the same time it is built point by point right so you turn the grating at a particular angle while the grating is moving no measurement is done then it stops then you make a measurement right now for how long does the grating have to stop for how long do you have to make measurement at every point that depends on what kind of intensity you are dealing with if the intensity is large then perhaps it is enough if you spend say one-tenth of a second per point but if intensity is low then it is essential that you spend more time because for the time that is the detector gather signal computer averaging goes on right so suppose you record the signal for some 10 seconds this is a signal that you get you record the signal for the next 10 well one-tenth of a second this is a signal you get so there will always be a fluctuation and the point that you get here is actually the average of all these points that is there now if you spend one second instead of one-tenth second for collecting this ensemble of points and averaging then as you know in statistics larger the sample size the better it is so if you have a greater integration time or greater residence time at every point then you actually get a better spectrum if you spend lesser time you get a more what is called noise spectrum so there you have to use a judgment there is no golden rule if you look at your spectrum may be run it using a small integration time see whether you get a good spectrum or a bad spectrum and then increase your acquisition time accordingly or decrease if the spectrum is too good you do not need such a good spectrum but it is important that when you record a spectrum it should be noise free nobody likes to see a spectrum that looks like this stubbles may be fashionable but spectrum with stubble is not cool you want your spectrum to be absolutely clean shaven and for that you have to decide what is the minimum amount of acquisition time you need okay there is no golden rule you have to think on your feet and decide on the spot how much time every measurement is required okay there is another kind of measurement that one can do and I am saying this only because we show you an example of this let us say you have this grating polychromatic light has fallen on it and it has been broken up into different components in the discussion we have had so far we said we put a slit and at a time only one wavelength will go up and we will have one detector but now it is possible to have not one but hundreds of detectors so each point here is a detector okay and you are actually familiar with this also your cell phone camera is basically made up of a detector like this it is just it is more complicated here I have drawn a one dimensional array of detectors the cell phone camera has a 2 dimensional array right so if you have something like this then you can actually get the spectrum in one shot then you do not have to do it point by point but even then do not forget that what has happened is that the entire spectrum has come at one time because this every detector has actually recorded something there also acquisition time is important there also even though everything is recorded at the same time the spectrum will be noisy if your acquisition time is not enough in fact in the example that we are going to show you we do get a noisy spectrum because we do not need an accurate measurement there so it does not matter whether you do whether you use a point detector or an array detector it is important to decide what kind of acquisition time what kind of integration time you require to get a good spectrum we stop on this note there is something else that we want to talk about in terms of excitation spectrum but that can wait we can talk about it when we actually come to a problem but let us stop here today next day we are going to actually show you the instruments in the lab and then maybe we will even get an opportunity to talk about excitation spectrum.