 In the previous module we started talking about solvation dynamics and then we went on to talk about solvation dynamics of water that is where we stopped and we did not go into a lot of detail but we said that if you take bulk water then the ultrafast solvation dynamics that you see there is not single exponential it is tri exponential or bi-exponential with the Gaussian component and in Fleming's work they have actually been able to assign each of these components to different kinds of motion of water including rotational motion liberational motion and so on and so forth. So and your homework was to read that 1994 nature paper and understand we also said that in confined medium the solvation by water becomes slower by orders of magnitude and that is explained once again we did not go into much detail but it is explained mainly by Bakchi's model one of which is exchange between bound and free water. So today we will discuss a different aspect of water what we will do is we will show you the vibrational spectrum of water and then we will discuss the different normal modes of water not isolated water but water as water liquid and there we will try to see what happens when one of the vibrational modes is excited how does the energy get dissipated how does it get redistributed in different normal modes of vibration and liberation. And this work is mainly by Elsie's group and our present discussion will focus on these three papers that are listed here there has been more work so it is up to you to go and read more recent literature in this topic so to start with let me show you a vibrational spectrum of liquid water and here the only difference between this and the vibrational spectrum that you usually record is that y axis is absorbance not percentage transmittance as it usually is with IR spectrum but this is a regular steady state FTIR spectrum. Now here you see three major features 1, 2, 3 and you see a broad feature here and a broad feature here what are they this one I think everybody knows the one between 3000 and 3600 700 centimeter inverse that is the OH stretch of water that is the strongest signal and that is what predominates whenever there is anything that contains an OH group you always see this broad absorption that is very well known the next one also I think is well known it is OH band. The third broad absorption here along with this even broader and less prominent feature is ascribed to liberation as we had perhaps talked about in the last module liberations are restricted rotation so your water molecules that want to rotate but then since the hydrogen bonded they are brought back so it is sort of wagging motion and you are all familiar with wagging motion in a molecule for IR. So here this is not within one molecule but it is in the intermolecular coupled system a particular water molecule wants to rotate but is brought back because it is hydrogen bonded this kind of motion is called liberational motion of course energy of this liberation motion would be much lesser than your stretch or bend and here it is this is actually called the L2 band the prominent liberational motion low frequency liberation of motion here you have some high frequency liberation of motion which is not very well defined but will play an important role in our discussions and what are these A, B and C we will come back to that later in a couple of slides. So the question that we are trying to adjust trying to understand is this ultrafast dynamics involved in coupling of these modes generally what is the meaning of normal modes that these modes of vibrations are independent of each other but in an in a correlated system associated system when you make one vibration happen it does affect the other vibration so you can think that what happens when hydrogen bonding is stronger what vibrations will be affected what happens when this bending takes place it affects the hydrogen bond dynamics right the moment it affects the hydrogen bond dynamics that is going to affect liberation of motions as well liberation of motion will be more or less depending on what happens to the hydrogen bond. So this is basically what we want to study and the reason why you have coupling between normal modes is if you look at the energy diagram this is what it is this one is OH stretch the biggest energy gap this is OH band and this is liberational motion they might remind you of some asymmetric vibration but the energy gaps are really really small as you can see this is the energy gap between 0 and 1 for OH stretch this is the energy gap between V equal to 0 and V equal to 1 for liberational motion much smaller and the notable feature here is if you look at the energy of V equal to 1 of the OH stretch manifold that has more or less the same energy as V equal to 2 of OH band right. So what happens when they are close what is it called what are the symmetries of vibration of water now C2 is the point group so what is the symmetry of this OH stretch symmetric stretch asymmetric stretch all that thing is there right the symmetric stretch is A1 what is bend you better look up. So what happens here is that since there is an energy match and there is a match of symmetry as well you have something called Fermi resonance in Fermi resonance energy can there is coupling between the modes and energy can get transferred between one mode and the other so it is not unusual to expect that if you excite OH stretch energy will go into OH band as well and then liberations have so many energy levels it is it may be quite expected that no matter which one you excite there will be energy transfer to liberations as well so finally when you make water molecule vibrate say excite the stretch that will that energy you give some energy for it to stretch right that energy will be dissipated by exciting bending motion and liberations as well and the times involved are always ultrafast because this V equal to 1 lives for about 200 femtosecond this V equal to 1 for OH band lives for about 170 femtosecond and we will see what happens when we look at the normal modes how they evolve okay. So just a reminder L2 liberation is rotational motion hindered by hydrogen bond so if this L2 band shifts to lower frequencies what does it indicate that means hydrogen bonds are weakened yeah so these are things that we will be looking for OH bending mode now OH bending mode generally we do not even pay attention to this in bulk water because each water molecule in any case is surrounded by so many water molecules but in a when you are going to make measurements in femtosecond time scale you need to take into account little more of a detail. And if you are going to work with water clusters where you have small number of water molecules which you can prepare artificially then also this becomes important this is what is actually known from this water cluster kind of experiment it is known that OH bending modes first of all depend the frequency of the OH bending modes it depends on number of intermolecular hydrogen bonds per molecule. So if it is equal to 4 then you have a higher frequency vibration if the number of intermolecular hydrogen bonds per molecule is less than 4 then you get a shift so the 4 seems to be a sort of a magic number here it does not matter whether you have well how many can you have per molecule that is another question the reason why 4 is a magic number how many hydrogen bonds can be there for water there can be 4 right you cannot have more than that so if it is not 4 for some reason then if some hydrogen bonds are broken then there is one kind of lower frequency vibration and if it is 4 if whatever number of hydrogen bonds could be formed is formed then you have a higher frequency vibration. So this is something that is known so the important pathway for energy transfer to surrounding hydrogen bonded network would be of course by coupling with intermolecular vibrational modes so the experiment that is done is pump a particular vibrational mode and probe all vibrational modes so this is an IR pump IR probe experiment okay pump by a femtosecond pulse in IR not visible or UV and probe in IR as well with that background we can come back to the earlier diagram and now I can tell you what these are this A B C these are the 3 spectra these are the spectra of 3 kinds of excitation frequencies used when you excite by C and I will tell you what the wavelength is then you essentially excite OH stretch when you excite this by using B you excite OH bend when you excite by A you essentially excite liberation of course it might have been better if you could excite here but there might have been a limitation of the experimental setup okay and these are the frequencies then 3150 centimeter and remember what I showed you earlier is a spectrum not a here this is actually a spectrum same scale and you will remember that a femtosecond pulse cannot be completely monochromatic there is always a spread of energies so what we are showing here is the modal frequency maximum of that spectrum IR spectrum so when you excite the OH stretching mode that has been done by using pulses which have maximum at 3150 centimeter inverse OH bending mode is excited using 1650 centimeter inverse maxima pulses and high frequency liberations are excited by 1350 centimeter inverse okay another reason for not exciting low frequency liberation is that I mean where will the energy go you can only have liberations if you pump at low frequency liberation right only when you excite something at higher energy can that energy go to lower energy modes if you pump a lower energy mode that energy cannot you cannot have joining up of photons in this case at least okay so remember this is a spectrum and this is a truncated spectrum actually as I think the probe had a little bit of limitation and in any case it makes no sense to probe the OH stretching mode the only thing that you can see there is a ground state bleach that is going to recover. So here the study is focused on mainly 1650 centimeter inverse what is that OH bending mode right and here you have all the liberations so what do you have before going into each of these quantitatively if you see when you excite the OH stretching mode remember OH stretching mode is how much 3150 centimeter inverse you get a bleach at 1700 centimeter inverse or so what does that mean why would there be a bleach when you pump the why would there be a bleach for the OH bending mode when you pump at OH stretch you pumping OH stretch and we see there is a ground state bleach in OH bend why would that happen and this is not nanoparticle nothing we will talk about nanoparticles later this is just liquid water what is the meaning of ground state bleach in this case population at v equal to 0 goes down population at some higher vibrational level goes up that is a hint total population more or less the same stretch yeah we pump by stretch then what happens I actually shown you the result already you have to connect with what I have shown you something that I have shown you the population of bend why remember Fermi resonance what happens in Fermi resonance there is direct energy transfer that means population of v equal to 2 yes population of v equal to 2 of OH bend takes place by direct energy transfer from the stretch remember yeah so a pump here right and that is v equal to 1 so there is energy transfer right so some population of v equal to 2 will be there so naturally there is depopulation of this OH bend energy transfer is taking place so it is an excited well vibrational excited state phenomenon that would cause that ground state bleach and here you see this transient absorption positive signal also coming for OH that is because of population of higher state okay so there is this Fermi resonance this is clear signature of that now what happens here there is ground state bleach of L2 as well and there is ground state bleach here little bit of growth here difficult to analyze this data but it happens that means since there is a bleach of L2 vibration that means L2 vibrations are also they also get excited that means energy gets transferred from stretch to bend and also to liberation so question is is it direct or is it 2 step now what happens when you excite OH bending more directly so here also you get a ground state bleach which is expected because you are exciting that particular vibration and you get this excited state absorption that is also expected once again you get a ground state bleach here and here you see it is a little different you see a buildup the buildup is stronger in high frequency vibrational mode you see that a little better which means when you make the molecule bend what is happening hydrogen bond network is affected and that is what will affect liberation as the that is what is taking place that is also sort of energy transfer and what happens when you excite high frequency liberations once again there is a ground state bleach here and once again there is this lower frequency transient absorption remember what we said about this higher frequency and lower frequency business for bending motion yeah yes if number of hydrogen bonds is less than 4 then you get this lower frequency absorption so that is why this might seem a little strange that I am exciting lower energy liberations and I see a ground state bleach in comparatively higher energy bend that is because by doing this exciting this lower energy liberations high frequency liberation what you do always is that you are moving like this hydrogen bonds will be affected they will be weakened that is why you get this phenomenon here so this is something very interesting because you might not at first glance expected right you might think that you excite lower energy what will happen this is what happens because we are not working with isolated molecules here it is a hydrogen bonded associated liquid and again you see there we see a rise of high frequency liberation again we see a ground state bleach here. Now qualitatively I think we understand what is going on quantitatively if you look at the traces this is what you see when you excite high frequency liberations directly the rise is actually instantaneous this little bit of what looks like a slope that is because of the instrument response function you are not using a delta pulse anyway when you excite OH bending mode do you see the rise yeah this one is instantaneous now compare this with this this is the instantaneous part and there is a rise why is there a rise look at this energy diagram once again excite the OH bending mode it is it leaves so 170 femtosecond and in this 170 femtosecond it can either come back to be equal to 0 or it can transfer energy to liberation the rise time that we get here is about 170 femtosecond which means that almost all the energy that you provide in OH bending goes into liberation mode that is why that higher V states of liberation actually grow it is no longer just instantaneous a little bit of instantaneous part will always be there because remember we are working with non monochromatic pulses so direct excitation is bound to be there and then you are exciting this high frequency liberations as well a little bit but that rise is very prominent and the rise is 170 femtosecond when you excite OH stretching mode then you see that instantaneous part decreases even more and just to the eye has in the growth become more prominent and has in the growth become longer yeah you can see it if you compare these here so here what happens is in fact there are 2 rise times now analysis of this kind of data is always problematic to fit a rise time itself is a non trivial task in order to fit kinetics to more than one rise time requires a lot of confidence as you know if you increase the number of parameters it is going to fit better anyway so your data quality has to be very good and you must be very confident about what you are doing you must make sure that you are just not making up some story that is not right so there are again model becomes important and 2 rise times here sort of okay because here you see 2 kinds of energy transfer one is from here to from stretch to bend and then from bend to here okay so again we are only providing you an overall view of this paper please read the paper and see how the data is analyzed so what they have done really is that they have fitted the entire thing to a kinetic scheme so it is like an intermediate right A to I to P when you do that you get a kinetic equation and that is what you have to fit your data to you cannot just fit to 2 rise or anything okay that is what they have done and they have extracted the 2 times they turn out to be 170 and 200 femtosecond so this is the thing when you pump high frequency liberations okay I do not have to go through each part of it but when you pump high frequency liberation this is what happens you get a broadband enhanced absorption rising towards small frequency which means again energy transfer is taking place and here you get this less than 100 femtosecond decay that is limited by the laser pulse and a very weak 0.9 picosecond component so that slow component is there it is not as if it is not there 0.9 picosecond means what 900 femtosecond okay you might not seem very slow but it is at least slower than less than 100 femtosecond so what this means this broadband enhanced absorption not easy to handle what it means is that when you excite the high frequency liberations then part of the energy goes right away and part of it goes a little slower which is may not be unexpected because there is always a period of this kind of liberation and motion 600 to 950 centimeter inverse there is a fast bleach and that is because of and because there is a shift of liberation L2 band to lower frequencies see what is happening here here something is rising is not it do you see the signal becomes more than 0 at the very end beyond 600 centimeter inverse so why is there a bleach here there is a bleach because due to high frequency liberations one thing that definitely happens is this hydrogen bond network is disrupted so like bending liberation also moves to lower frequencies if hydrogen bond is disrupted that is why you see that bleach and OH bending absorption there is a slight shift to lower frequency that we have already discussed why when you excite OH bend then in early time you get a bleach of v equal to 0 to v equal to 1 transition time constant is 170 femtosecond broad absorption at lower frequencies later for longer times you see there is a red shift of this absorption and liberation absorption gets reshaped why does that take place once again because of disruption of hydrogen bonded network okay and that is what is sort of discussed in this last paragraph and finally when you excite OH bend you see an instantaneous rise in the 1400 to 1610 centimeter inverse 1400 to 1610 and initial decay is 170 femtosecond as discussed already this is a v equal to 1 to 2 transition of the OH bending mode 1 to 2 longer times the signal is due to overall red shift of the liberation absorption okay and OH stage I think we have already discussed this for me resonance we do not have to go through it once again. So finally the story that we get is that of coupled vibration and this is I like this because it is a good example of how one can use pump probe technique to look at very intricate interactions that take place in a coupled system. So there they have looked at water because you know water is ubiquitous that is the liquid of life but one can think of experiments that one can design similar experiments on things that are more complicated of course IR experiment in presence of water if you want to look at motion of biomolecules for example this could be a good way to go the complicating factor there is that biomolecules are always in water water always has its very strong absorption because it is a solute anyways present in much more so unless you have absorption in some other region it may be difficult to follow but it may not be impossible. So finally this is the picture that has come out of the whole study you excite this I think this is taken from a movie that must be available in the somewhere on the website or in the associated contents of the paper but excite in 100 femtoseconds this is the situation hydrogen bond if I put it very qualitatively what happens is that within the first 100 femtoseconds the hydrogen bonding network is weakened and then longer times 3 picosecond or so hydrogen bonds actually get broken and that is what shows up in this red shift of bending vibration frequency and in shift of liberation frequency as we have discussed. So that is the story of how energy migrates from one mode to the other in associated systems like liquid water so I do not know whether anybody has done followed this up later on it might be interesting to look at forget biomolecules for the moment even water so water confined in a reverse micelle or water at the surface of a protein bound water what kind of energy migration do you have in case of bound water or inside a vesicle or maybe inside a cell for that matter but of course for that we want to do it inside a cell you have to work with an IR microscope not impossible it is a set we do not have it IR microscope coupled with femtosecond IR that is a problem but this paper is important because the thing is unfortunately in today's world a lot of research is driven by impact factor which is completely wrong and this paper is published in Jeff Eskeme which is the least impact factor in the Jeff Eskeme family it does not matter it is very good fundamental work and I think it is an indicator of lot of possibility that can open up in this kind of experiments for especially for water in confined environments and perhaps for other molecules as well okay so that is what I think is one of the most important pieces of work in this field in the last 10 15 years or so.