Lecture 15

Added: 2026-05-09

[00:00:03][music] [music] Hello everyone, welcome back to the MOS's course on advanced analytical technique. Today we are going to start the second unit of this course and the title of this unit is molecular luminous spectroscopy.

[00:00:35]This is the first lecture of the fourth week. Let us now begin the concept of molecular luminous spectroscopy.

[00:00:43]The molecular luminance spectroscopy or in short ML spectroscopy refers to the study of emission spectrum of excited molecules. So in one sense we can say that it is also a type of spectrum that is related to our atomic emission spectroscopy. But the difference is that in case of emission spectroscopy or the atomic emission spectroscopy the sample emits light by absorbing of heat. But in this particular case there is no heat source used. The substance emit light based on its cold body conditions. Means the sample is not heated by any source neither by flame or by a plasma or an electrical heating. We instead we use a light source to excite the sample means the absorption is also taking place and then the excited sample returns back from the excited state back to its ground state releasing light radiations which are being captured by the detector. So an emission spectrum is generated. In this in one sense we can say that both the phenomena of absorbance as well as emission. In one sense we can say that both the phenomena of absorption as well as emission is taking place in case of molecular luminous spectroscopy and that is why I have covered the absorption and emission spectroscopy in unit one. So that when you got an idea about the concept of absorption and emission spectroscopy it would be very easy for you to understand the ML spectroscopy.

[00:02:27]So here we have first of all we have for example we have an atom present in a sample. This atom is excited by a light source. We generally used UV light in this particular case. The absorbance of light causes excitation of electrons from the ground level E not to the excited level and when these electrons come back they emit light which creates an emission spectrum. So fluoresence spectroscopy is basically a type of emission spectroscopy but it involves the phenomena of absorption as well as emission both.

[00:03:06]So the sample molecule absorb a light photon causing excitation. So this excitation it is due to absorption of light photons.

[00:03:22]Sometimes this ex excitation can also occur by a chemical reaction if light absorption is not taking place. In some cases we have excitation of electrons due to chemical reaction only based on its chemical energy. In that case also we see some light radiations coming out from a sample. That particular type of uh emission that particular type of emission is called a schem that we'll discuss later. so far uh but till now it is also a type of but it is also a type of molecular luminous which is created by the action of chemical reaction by the energy provided by a chemical reaction. So in broader sense we can say that ML spectroscopy.

[00:04:09]So in broader sense we can say that molecular luminous refers to the emission of light from a sample either by absorption of light radiation or by a chemical reaction.

[00:04:22]This emission of light is called as luminescence and since it is given by a molecule because we are not automizing a molecule into individual atoms like we do in case of atomic emission or atomic absorption spectroscopy. There we use the word atomic before absorption and emission.

[00:04:38]which sh which says that our focus in that particular case is on the individual atoms or elements I should say. Therefore we automize the sample completely dissociate it into individual atoms and then excite those atom while in this case we use the term molecular luminous. It means that we are not going to automize these molecules. We are not going to dissociate these molecules into individual elements. We directly take the sample and the molecules present in the sample are excited either by and the molecules present in the sample are excited by a light radiation incident light radiation which causes excitation of electron upon absorption and when the electrons come back light of longer wavelength is emitted creating a fluoresence spectrum which is a type of emission spectrum. This can also be observed by chemical energy from a chemical reaction.

[00:05:29]So this emission is called as luminescence.

[00:05:32]However, this luminescence is quite different from incandescent which is the emission of light by heating. I hope you all have seen the incandescent bulb the yellow color bulb which we use few years back maybe a decade back where where we have these types of bulbs where we have these types of bulbs that gives yellow light. These bulbs are called as incandescent bulbs where we have a filament made up of tungsten and that filament is heated at very high temperature by passing current current between the two electrodes and at very high temperature it starts emitting light radiations. So that is called as incandescent and that is called as incandesence and incandesence means the emission of light by absorbing heat while luminous means emission of light by absorbing incident light radiation of lower wavelengths. So emission. So in conditions means light emission by absorbing heat light emission by absorbing heat.

[00:07:00]Light emission by absorbing heat.

[00:07:13]It's called as light emission by absorbing heat.

[00:07:24]While luminous means light emission by absorbing light of lower wavelength and emitting light of longer wavelength. So this is the difference. So this is the difference between incandesence and luminous.

[00:07:54]Now these molecular luminance phenomena are divided into three groups. First is called as the fluoresence. Then we have phosphorusence and chemuminescence.

[00:08:05]Fluoresence is defined as the emission of light of longer wavelength from an excited species after it absorbs light photons.

[00:08:16]While phosphorus is also the emission of light of longer wavelength from an excited species after it absorbs a photon. Now then what is the difference between the two? The difference between the these two phenomena of flloresence and phosphorus is that their existence.

[00:08:36]While fllorisence exist for a very short time it is called as a short-lived phenomena. It because it involves high energy singlet state. On the other hand, phosphorusence is a little bit longived phenomena. It exists for a comparatively longer period of time because it involves the low energy triplet state.

[00:08:56]Here we have a triplet state while in this case we have a singlet state.

[00:09:04]While in this case we have a single state.

[00:09:10]So which means that fluoresence is the emission of light of longer wavelength upon absorption of light of shorter wavelength and as soon as we stop the incident light the emission of light from the fluoresing molecule is stopped.

[00:09:23]On the other hand, phosphorusence refers to the emission of light of longer wavelength upon absorption of light of shorter wavelength. But the phenomena of emission of light continues for a little bit longer period of time even after cutting off the incident light radiation that is being irradiating the sample. So this is the difference between these two. The concept is same but the emission the duration of the emission of light basically keep this process apart.

[00:09:56]Then we have a third photooluminence phenomena called as the chemolum luminous which refers to the emission of light from a molecule due to chemical reaction. Here we do not need an excitation source.

[00:10:12]No light source is needed for excitation for no light source is needed for exciting the molecule. A chemical en chemical energy is released by a chemical reaction that that excites the molecule resulting into the emission of light from that molecule. And this phenomena you might have observed from some insects as well. And even especially during night time you observe this phenomena near sea beaches where aquatic organism showcence and you can see light radiations from the water.

[00:10:48]History of these two phenomena.

[00:10:51]Although both history of these two phenomena because both these phenomena are natural phenomena. They have been discovered long back but their name like but their name was given a bit later.

[00:11:04]They both have a very rich and fascinating history. Even in the 15th century in the year 1565 there was a scientist Nicholas Monards. He was a botonist as well as a phys physician. He was a botonist as well.

[00:11:21]In the past, people may have more than one skill. In the present time, we have one person specializing in one particular area who does not have any knowledge of some allied areas. But in the past you might have heard about some physicist who got Nobel prize in chemistry. Some chemist who got Nobel prize in biology. People many of the uh people of yesterday years I can name who who were mathematicians as well as astronomers and they have knowledge of chemistry as well. So at that time we have a person with more than one skills.

[00:12:00]So at that time we have person with who is expert of more than one area. So this so Nicholas Monards was also a B was a botist as well as a physician and he observed and reported that infusion of the extract from the wood of lignum nephriticum into the patient vein and when the veins are being seen in sunlight they are exposed to sunlight they show blue color. This was the first report of the phenomena of fluoresence.

[00:12:29]This lignum nephriticum this tree is also known as kidney tree.

[00:12:39]The extract of the wood of this tree ligignicum is used as a diuretic. Do you know what is diuretic?

[00:12:49]Diuretics are the medicines or the drugs that are used to increase the urination of a person. In case if the person is having some sort of infection or it is having high blood pressure hypertension then in order to reduce the salt level in order to maintain the salt level as well as the hydration level these diuretics are being given to the patient which ultimately helps the patient to maintain the body fluids the salt level and in this way the patient can maintain its blood pressure especially in the past these wood extract or other natural products are being given to the patient as a diuretics. So this was the first report of the phenomena of fllororesence or I should say it is the first document of the examples of flloresence. Then in the years 16003 means in the 16th century there's a Italian scientist there's an Italian scientist Vincenzo Cascalerio there's then in the year603 means in the 16th century there is an Italian scientist Vincenzo Cascariolo Vincenzo was working on bologna stone and when this stone was exposed to sunlight he observed that the stone is emitting light radication even after when the stone is placed inside means it is not directly exposed even after putting it inside the stone continues to emit light radiations.

[00:14:32]So this was the first report of the phenomena of phosphorusence.

[00:14:36]While the reports of Nicholas Monarch's was relating to the phenomena of fluoresence where the extract of the wood of lignum nephriticum into the veins through infusion show and shows light shows blue color of the vein when they are exposed to sunlight. But in this case Vincenzo Cascariolo he showed that even the phil he reported the phenomena of phosphorusence when bologna stone was exposed to sunlight it started emitting light radiation and even after keeping it inside the react the emission of light continues for a certain period of time. So these were the cons the so these reports are considered as the first report of these phenomena of photooluminescence.

[00:15:21]Then in the year 1852 sir George Gabriel Stokes he was a British physicist he gave the first explanation of the phenomena of fluoresence. He was working on some fluorite minerals and there he observed that these minerals emit blue light when illuminated with ultraviolet radiation. So he took those minerals and he irradiate them with UV light.

[00:15:53]And then he observed that the molecule that and then he observed that these fluorite minerals emit blue light means visible light.

[00:16:09]And based on the name of the mineral fluorite, he gave this phenomena, he gave the name to this. And based on the name of this mineral fluorite, he gave this phenomena the name fluoresence.

[00:16:26]And based on the name of these mineral fluorite, he gave this phenomena the name of fluesence.

[00:16:33]So he was Sir George Gabriel Stokes who coined the term fluoresence based on the flourishing based on the fluoresence properties of fluoride minerals.

[00:16:49]Clear? He also showed that the light radiation that is being absorbed by these fluoride minerals was of lower lambda and higher energy.

[00:17:05]While the light that is being emitted by these minerals was of longer lambda and lower energy. This is the light that is being absorbed and that is why this because this is UV light which is having because this is UV light he radiated the samples with is having high energy and lower wavelength as compared to the visible light that is being emitted and later the difference between the wavelength of the light absorbed and the wavelength of the light emitted is called as the sto shift. If in case the wavelength of light absorbed and the wavelength of light emitted is exactly same then the difference between the peaks will then there will be no difference between these two peaks and we can say that the stoke's shift is zero but this is not the case with fluoresence but this is not common in fluesence. In case of fluesence, the light which is being emitted by the sample is always of longer wavelength and lower energy than the light which is being absorbed. We'll discuss these stroke shift in the later parts of this lecture.

[00:18:27]So his pointering work the work of s Gabriel Stokes. So sir jaj Gabriel Stokes laid the foundation of the understanding of the photooluminence phenomena. There people understood that fluoresence consist of excitation or I should say absorption as well as emission means both the phenomena the absorption of light as well as emission of light they are taking place sequentially one by one. First absorption or excitation is taking place then we are having emission of light. These two phenomena collectively form the phenomena of these two phenomena collectively creates the phenomena of fluesence.

[00:19:13]Now the phosphorusence was studied in detail in the 19th and early 20th century particularly after after the development of quantum theory the phenomena of phosphorus was understood in detail.

[00:19:32]Scientists discovered later that the difference between these two phenomena.

[00:19:37]As I said the only difference between those two phenomena is the the period for which the light is being emitted. In case of fluoresence the light is emitted for a very short period. Once we have uh once we cut off the incident light radiation in case of fluoresence the light emission is immediately stopped.

[00:19:56]While in case of phosphoresence even after cutting off the supply of incident radiation the sample continues to emit light radiation for a certain period of time and therefore the only difference is the time period for which the emission of light exist in these two sample and scientists later discover that this difference is due to the electronic in spin states of these two phenomena which means that fluoresence involves some other spin state while fos phosphoresence involves some other spin state which means that fluoresence involve one spin state while phosphorusence involves the other spin state which I I'll be showing you in the next slide they they further reported that in case of false they further reported that in case of fluoresence the transition of electrons involves a singlet to singlet transition means the transition is uh possible. The transition involves from ground state. Of course, it is having from ground state to even or sometimes it is from or sometimes from ground state it is going to E2. But in these two cases we have singlet state. We have singlet spin state in the ground state as well as singlet spin state in the excited state. And when these electron from singlet state come back they emit light.

[00:21:24]And since singlet state is short-lived state we immediately the light immediately disappears as soon as the incident light is is stopped. If this is the instant light the moment we stop this instant light instantaneously there will be no light emission from the sample. But in case of phosphorusence since the excitation involves a triplet state as well. How that triplet state is involved that I'm going to discuss in the next that I'll tell you in the next slide. But phosphorusence involves a triplet state as well. And since triplet state means actually the transition starts from singlet to singlet and then from singlet is it the electron goes to triplet state and then finally it comes back.

[00:22:13]So singlet to triplet and then finally it comes back to singlet. So therefore and since so from singlet to triplet and then finally it comes back to singlet and in this case you can see the triplet state is involved which is long lived state. Therefore even therefore in case of phosphorus the substance continues to emit light even after the incident light is cut off.

[00:22:41]This is the this is the famous Jablonsky diagram. This is the very basic form of Jablonsky diagram. The advanced form showing detailed phenomena relating to the photooluminescence will be shown in the next lecture.

[00:22:58]But here this basic but here this simple figure helps you to understand the phenomena of fluoresence as well as false floresence. This famous Jabloski diagram was given by Alexander Jablski in the year 1933. And that is why this diagram has been given the name Jabloski on the name of the person who discovered this diagram. So in this figure you can see that we have a molecule in the ground state and this molecule receives light radiation means light photon H new. Once these light radiation are being absorbed by the molecule, it causes excitation of its electron. The electrons are promoted from ground level to the excited level. Now there are two possibilities.

[00:23:55]In the ground state, the spin is paired.

[00:23:58]Is spin is paired.

[00:24:03]Therefore, it is singulated state.

[00:24:06]sometimes referred to as S not because it is the ground state. So we put a zero and the excited state is written as S1 because it is the excited state but the molecules but the electrons are still paired because even in UV spectroscopy there is a selection rule which sh which says that transition of electron always occurs with retention of spin. very famous rule which very famous selection rule of ultraviolet invisible spectroscopy or the other name is or the alternative name is electronic spectroscopy that transition of electrons always occur with the retention of a spin which means that since ground in ground state the spin is paired the transition always occurs with the retention of spin means in even in the excited state the spin remains paired so the transition is occurring from S not to subs.

[00:25:06]Now from S1 directly the electron returns back emitting light radiation.

[00:25:12]This is called as fluesence. But in some cases at this stage because singulated state is high energy state so it is less stable. Why? Because in this state we have two electrons in the same orbital.

[00:25:27]And since electrons carry negative charge. So these two negative charge have some repulsion causing the this state to be little bit causing this state to be less stable and therefore they emit some light radi. They emit some energy in the form of nonradiative energy which means they lose energy but not by emitting light but by emitting heat. So not radiative relaxation means the electrons release some of their light and changes their spin from singlet to triplet. Now they come to triplet which is a little bit lower energy state and that's why it is more stable here because in this particular case we have electrons in different orbital because the spin is parallel and therefore repulsion is lower and repulsion is lower. Therefore we have more stability. So electrons from high energy state reaches to low energy triplet state from high energy singlet state by a non-radiative loss of heat by a non-radiative loss of energy which is in the form of heat not in the form of light. So when they come from S1 to T1 and then from T1 they come back. So from S1 to T1 they are coming and then from T1 they are coming back directly to the F not. This three means it is tripled state. One means it is singled state. So we can say that um so I can show you that uh so I can represent the phenomena of phosphoresence like this s not it is going to s1 from s_sub_1 it is going to t1 and from t1 it is coming back to s not so this is your phosphorusence while fluoresence is s to s1 and subs to s not coming back and why these are called a singlet and triplet state this is by calculating the multiplicity which is I I equ= 2 S + 1 and is spin. In case of paired we have plus half and minus half making it zero.

[00:27:32]So we put the value of S. We have 2 * 0 + 1 giving one. So it is singulated state.

[00:27:41]While in case of triplet state we have unpaired or parallel spin. So we have 2 s + 1 and s = 1 by 2 again 1 by 2 both are having plus charge so it comes out to be 1. If we put this value of 1 2 into 1 + 1 so if we put this value 2 into 1 + 1 giving it three. So that's why it is called as stupated state while it is called as singlet state. Going to the next slide.

[00:28:16]The definitions of fluoresence as and phosphorusence again. So the definition of fluoresence and phosphorusence.

[00:28:22]Flloresense is defined as the emission of light from a substance upon fluoresence is defined as the emission of light by a substance upon absorption of light of shorter wavelength. And this light is stopped and this light emission is stopped as soon as we stop the incident light radiation. And the phenomena of fluoresence involves singlet state. While phosphorusence is defined as the emission of light by a substance upon absorption of light of shorter wavelength and the emission of light is continued even after the light radiation the incident light radiation is stopped and this phosphoresence phenomena involves the triplet state.

[00:29:04]The mechanism as I just mentioned for example if we have a molecule where electrons are present in the ground state means singlet state when it absorbs light radiation ex shows it is the excitation means the incident light radiation the electrons may be excited to the first excited level or even to the second excited level and when the electrons come back from the excited level S1 or even from S2 directly to the S not the light emitted is called as the fluesence light. On the other hand, if from S not on the other hand if from S1 the substance goes to T1 by emission of non-radiative energy means and then from T1 they are coming to S not by releasing night by releasing light radiation then it is called as phosphorusence.

[00:30:09]In this slide, let us discuss about the poly's exclusion principle which you all must be aware of. According to the poly's exclusion principle, no two electrons in an orbital can have the according to this according to Paul's exclusion principle no two electrons can have the same value for all the four quantum numbers. Which means that for two electrons that are present in the same orbital since for these electrons the value of principle isomethyl as well as magnetic quantum number is same.

[00:30:42]Therefore they have a different value of spin quantum number. One is having a plus half spin which is clockwise while the other is having minus half anticlockwise.

[00:30:54]So in the so these two electrons they can only remain in an orbital if their spin is paired.

[00:31:04]If their spin is not paired they cannot remain in single orbital. They have to be in two different orbital to have their spin parallel.

[00:31:15]So the same is being written here.

[00:31:16]Orbital can contain a maximum of two electrons. And these two electrons must have opposing spin. They cannot have same spin. If they have same spin they must be present in different orbital but if they are present in same orbital their spin must be paired.

[00:31:32]Here you can see in this particular case we have here you can see we this particular case we have the electrons paired one is having clock the it is going the one is showing the direction upward while the other is downward. So it is clockwise it is anticlockwise. For this one we have plus half value. For this we have in minus half. Here also it is paired. But if they are unpaired then they must be in different orbital.

[00:32:03]They cannot remain in the same orbital.

[00:32:05]If they are having same spin. So it cannot exist like that. If the two electrons have parallel spin they have to be in different orbital. It is not possible that two orbitals that are present in the same orbital. It is not possible that two electrons that are present in the same orbital they have parallel spin. It is not possible. If this has to be the case the electrons should be in different orbitals.

[00:32:31]Singate interpolate state as I said ground state always in as I said in ground state electrons are always paired means in ground state we have two electrons in an orbital in which one is having clockwise or plus half value of spin while the other is anticlockwise spin. Now in singlet state also, now in singlet excited state also we have now now in singlet excited state also we have a spin of the two electrons paired which means that one is they have same spin as they have in case of the ground state. Again it is paired spin.

[00:33:08]So it is a singlet state. Now triplet state refers to the state in which the two electrons are present in different orbital and that and that is why only they have parallel spin which means that they both have a clockwise spin. They both have the value of spin quantum number as plus half. While in singlet it is always zero because one is having plus half other is having minus half. It is zero in the singlet in the singlet excited state. in the single excited state. While in case of ground state also one is having plus half other is having minus half to give zero. But singlet states are less stable because they are more energy state. Why? Since electrons having same charge and they are present in the same orbital so they are very close to each other causing repulsion making the state less stable.

[00:34:00]On the other hand triplet in the triplet state electrons are having parallel spin which means that they are placed in the different orbitals. So they are far away and therefore they have no repulsion or I can say very little repulsion and that's why this state is more stable.

[00:34:17]This is the theory of fluoresence and phosphorusence. Here the time lapse of the time required here the time lapse or the time required for each phenomena is presented. The absorption of light of the molecule is a very very quick phenomena which happens in 10 ^ -14 to 10 raised ^ -15 seconds means even less than a picoscond this absorption of the light taking place causing excitation and when the electrons are excited to the next level now they can come back they either come back directly from the excited singlet to the excited they can come directly from excited singlet to the ground singlet in that case it's called as flow for which the time need the time lapse is 10 ^ 9 to 10 raised ^ -7 seconds or I can say from 1 to 100 nanoconds while the ex absorption or excitation it is even lesser than one picoscond very fast phenomena on the other hand Phosphoresence the phenomena as it is a long live phenomena the light is being emitted even for a longer period of time it ranges the time ranges from 10 raised to the power minus3 to 10 raised to the power + 2 seconds means for phosphoresence we can have means for means for phosphoresence the time lapse is from 1 millisecond to 100 seconds. So we can witness phosphoresence even up to 100 seconds after the incident light is removed and then as I said in case of when we have from single to triplet uh transition by a non-radiative energy loss in the form of heat for this the time lapse is time to the power minus 8 to 10 raised to ^ 9 means 1 to 10 nanconds is required means 1 to 10 nconds are required for non-radiative heat transfer.

[00:36:27]So based on this theory we can say that the quickest phenomena is the excitation of electron by absorption of light. Then in terms of time lapse in terms of the existence we have fluesence which exists for from 1 to 100 nconds and the non-radiative heat uh and the non-radiative energy loss in the form of heat which also exist for a very short period of 1 to 10 ncond but the phenomena of phosphoresence can be lasted from 1 millisecond up to 100 seconds. So this is the time duration of this phenomena.

[00:37:10]This both these phenomena of fluesence as well as phosphorusence can be used for qualitative and quantitative analysis just like we can use atomic absorption and atomic emission spectroscopy for both qualitative and quantitative analysis. In this slide fluoresence is compared with UV visible spectroscopy. Here you can see that with UV spectroscopy we can go up to 10 we can go up to very low detection limit.

[00:37:33]This UV visible spectroscopy helps us to identify moles even if they are in very little in number 10 raised to the power -3 to 10^ -16 moles are present although mole is quite although moles contain quite large number of molecule it is equal to gather's number 6.023 0 to 3 into 10^ 23 molecules are present in a single mole.

[00:37:58]So UV visible can be used for the detection of moles from 10^ -3 to 10^ -6 while fluoresence can be with fluence we can go even lower values from 10 ^ -15 to 10^ -17 uh moles we can go up to means even it is 10 times more sensitive as compared to our UV visible technique. On the other hand, in terms of concentration, here we have in terms of mole, in terms of concentration, we can go from 10 raised to ^ -5 to minus 8 means from 10 micro molar we can go up to 10 nanomar concentration. While in case of fluoresence again it is more sensitive.

[00:38:47]The detection limit is even lower. We can have the detection of sample from 100 nano molar to 1 nanomar. We can here also we can go up to 10 times higher.

[00:39:07]So in this case also we can have detection limit even 10 times better than UV visible spectroscopy. Therefore, although UV visible is a universal technique which can be applied to nearly all molecules because all the molecule absorb light radiation, fluorosins is quite sensitive. It can not be applied to all the molecules. It can only be limited to the molecule that flues or if the molecule do not fluise, we can make them fluise by artificial fluoresence to make them a to make them fluise.

[00:39:45]So flloricence is a sensitive phenomena.

[00:39:47]It cannot be applied to all molecule. It can only be applied to molecule that show fluoresence or if they are not showing fluesence, we make them show fluoresence by adding some fluoresence dye which is known as artificial fluesence phenomena that we'll discuss later. So UV visible quite universal while while our fluoresence it quite is while our floresence is quite sensitive. This is being shown here. A sample is being radiated with light and the transmitted light is passing through the sample but we are concerned with the light that is being emitted by the sample upon absorption of light. While in this case in case of UV visible we measure the incident light and the transmitted light and notice the difference because this difference of the intensity of the incident and transmitted light basically gives us an idea about the light intensity that is being absorbed. But in case of fluoresence, we do not focus on transmitted light radiation. We have we have nothing to do with it. We let it pass through the sample. We focus on the light that is being emitted when the electrons returns back. Let's move to the next slide. This is this is the phenomen. So these figures shows the phenomena of fluence and phosphoruses.

[00:40:58]In fluence we have these molecule showing fluesence or a substance showing fluesence looks like this. While in case of phosphorusence we have it completely dark and very little uh portion is being being eliminated. While in case of phosphorusence the whole while in case of phosphorusence it is dark and very little portion is being eliminated it exists for 0 seconds. Even when the light is removed while it exists for certain period of time even in some cases it can go up to hundreds of seconds means we can go up to 1 minute in case of phosphoresence. So this is the difference between the appearance of these two phenomena with this I'm going to uh end this lecture and in the next next lecture we'll discuss about the Jablonski diagram in detail and the factors affecting the phenomena of florosins and phosphoruses. Till then, thank you very much. [music]