nm0692: right well yesterday i was er talking about the idea of a track in radiation chemistry where as the particle moves along it's losing energy we know it loses energy at a great rate we know the energy of the particle we know the range of the particle what happens to this energy how does it get transmitted to the medium and this gave an indication that the principal acts of energy deposition are in the form of ionization and also o-, of excitation as well that that's an overall picture now that being the case you might say well all right you've got ions in excited states or so you say er what happens immediately after this event has occurred and i suppose the first thing that happens is that the electrons which are formed in the ionization act these are very small and they're very mobile and they're certainly very small compared with the cations that are formed 'cause those are going to be molecular size so the electrons tend to diffuse away very rapidly simply because they are so small and er they have a a very high mobility what about the er the cations that are formed well if you just consider a general i put R-H-plus but it could be anything but suppose it was i don't know hexane or something then the cation radical which is the thing you get by taking the electron out of the molecule er will react with electrons that are are nearby the ones that haven't got away so to speak and you get a certain amount of ion recombination and when this electron returns to that cation it's very likely to form it in an excited state so you can get additional excited states from from ion recombination but also what can happen is that the R-H-plus the cation radical er is a very powerful proton donor and it will give a proton to almost anything in sight er and so this will tend to happen and perhaps the best known example of this is if you imagine R-H-plus is H-two-O-plus the water cation then that will give away a proton to a nearby water molecule to give H-three-O-plus and you're left with O-H behind so you do get these er proton transfers occurring how many ions do we get how do you measure the efficiency of a radiation chemistry process well in photochemistry it was very easy because you had the idea of a photon and the photon entered a molecule and so much no s-, a certain percentage of the molecule was converted to a product and you had the idea of a quantum yield the idea that you could destroy such a percentage of molecules you would form such a percentage of product molecules and you had a quantum yield for this and this was always less than one unless you had a chain reaction so it's a very very clear idea but with a gamma ray quantum i the point i was trying to make yesterday it's completely different because the gamma ray quantum has got enormous amounts of energy and or the alpha particle has and you don't have the notion of just one molecule be-, be-, being converted by one quantum you've got the n-, the idea of along the track hundreds of thousands of molecules being converted so how do we cope with this idea well we cope with it by saying how many molecules how or how many ions do we get for a certain amount of energy deposited and there is a definition of a thing called a G value and the G value which is a measure of the product yield or indeed the react-, the destruction of the reactant 'cause er that would be then the negative G value but they normally normally you look at products to measure positive G values they are the number of molecules formed or destroyed for every hundred electron volts and so you if you think about it a hundred electron volts ionization of a typical organic is about ten eleven or twelve let's so say ten for the sake of argument then you would say well if we got ten prod-, if we got ten ions that would be a hundred per cent efficient but in fact you don't get ten ions you get typically two or three ions maybe slightly more than three sometimes but not much more than three and so the ionization efficiency is not enormously high but of course you also get the excited states as well and so if you're at the excited state yield which is also often about two or three but well for some systems you're using about fifty per cent of the of the ou-, energy chemically productively what happens to the rest well the rest you get ion recombination the ions don't escape at all they they si-, they're formed and they recombine instantly don't measure them 'cause they're not around to be measured any more under those conditions you simply end up by getting excited states and some of these simply end up by heating up the medium so you you you actually simply do a lot of vibration excitation of the medium so the medium will become warm as you put radiation through it so the energy is partitioned between ionization events excitation events and merely thermal warming and this G value as i've put down there is the analogue of the quantum yield it's not as precise in a way you can't picture it as as nicely but you've got to start from somewhere and you say well if i've got a million electron volts how m-, what do i get for each hundred electron volts that i deposit in the medium and so that is your fundamental parameter the G value and whenever you're discussing radiation chemistry of anything you begin by saying well what is what is its G value is it er one or three or five or and if it's a lot more than than ten or if it becomes hundreds again it's a you got a chain reaction running away now let's think about some of the er experiments to get some idea of what these G values are the very simplest experiment you can do is using an ionization chamber and i'll draw one of these it's a very simple idea if you imagine you've got a glass vessel with two electrodes in you got a you apply a voltage that's voltage there and you have an ammeter for measuring current there now normally of course if you have that globe filled with argon or helium or hexane vapour or something propane if you apply a voltage that's not enormous then there's no current because the gases are insulating i know if you take it to a very very high voltage you should get diametric breakdown we're not thinking about that at all we're thinking of quite low voltages get no current but if you bring up a other a radioisotope to the outside there close to the glass vessel so if we have isotope this is giving out radiation like so or you could fire a very very fast electron beam into the globe if you're still applying the voltage you don't get a current I becomes greater than nought with radiation I becomes greater than zero without radiation I equals zero so clearly the radiation is producing ions in the gas and you know they're there because the whole thing is carrying a current once you apply a voltage positive ions are moving to one electrode negative ions are moving to another so immediately we know that the application of ionizing radiation to a gas produces ions and you might say well er what sort of yields do we get from gases and maybe from liquids well from gases the ion yield is sort of reasonable it's say one to three depends on the gas for liquids interestingly if you if you fill that globe with liquid hexane or liquid benzene G liquid it's nought-point-one now why should that be well the answer again if we go back to photochemistry a lot of photochemical acts give you two species formed very close to each other if they can get away from each other you get products but if they are forced by being surrounded by solvent back onto each other they recombine and so what's happening in the liquid is if you get a positive charge and a neg-, negative charge formed and these are surrounded by solvent molecules like so and this of course is a positive solvent molecule and that's a either a negative solvent molecule or an electron the thing is trapped and you've got basically what's called a cage effect the thing is trapped in a solvent so i'll say solvent cage effect that's very important in photochemistry it's very important in radiation chemistry as well so in liquids you've got the solvent cage effect operating in a gas phase you haven't once the ions are formed zoom they they go charging off towards the electrodes so there's quite a difference there so that was the f-, the first indication that you were getting ionization in the gases to quite high yields but in the liquids to very low yields and for a long time that experiment convinced everybody that ionization was not important in liquids not at all because the conductivity experiments indicated the yields are very low and that that sort of held sway for a long time but then there were some very clever experiments that were done by a chap called Hamill and i'll as i've got these on a a slide i'll put the slide on and it goes as follows Hamill worked with solutions they were quite dilute say a millimolar of things like naphthalene and biphenyl and he worked with lots of other ones as well but they're just examples in what are called glass glassy forming solvents or er glass forming solvents there were quite a few of these around i mentioned them in the photochemistry section of the course these are solvents that when you freeze them down they give a perfect glass they don't crystallize they've got an irregular structure you've got the methyl group here tetrahydrofuran crystallizes when you freeze it in liquid nitrogen methyltetrahydrofuran is less symmetrical and it forms a glass pentane will crystallize if you freeze it you put a methyl group on get three-methylpentane it gives a perfect glass and if you've got a perfect glass it means you can freeze the thing in a cell a caught cell and then run an optical spectrum at liquid nitrogen temperature in an ordinary spectrophotometer and what did Hamill do well he rotated these solutions in these glassy forming solvents and then ran U-V spectra and for example if you er take naphthalene C-ten-H-eight this is co-, naphthalene's colourless dissolve it up in T-H-F or three-methyl- pentane if you gamma irradiate it the thing goes deep green it s-, it goes a deep green colour it's quite a spectacular experiment to do it's a nice demonstration but of course it's not easy to do 'cause you'd need er er a gamma source to do it and we we don't have one here er to get deep green colour this is this has got a it's quite a structured spectrum it's not just a broad band and it's exactly the same spectrum as this thing in other words if you take a sodium film a film of so-, if you purify sodium in fact you heat it up and evaporate it and then could allow the sodium vapour to get to get condense on glass you get a mirror formed a silver mirror of sodium if you take that silver mirror of sodium which is very very pure and you react it with a dilute solution of naphthalene in an ether you end up by getting a deep green solution and this is sodium-plus C-ten-H-et H-eight- minus with a free radical symbol er it's a free radical and it's it's strongly paramagnetic and it shows an E-S-R spectrum quite a complex one but it can be perfectly analysed in terms of er in terms of naphthalene so you've got this formed now we know the extinction coefficient of this thing because we can measure how much sodium has been consumed or we can indeed er er we can quantitate it reasonably well and so if we know the extinction of the of this we can work out the yield of these cat-, anions which are formed in the glass and if you work it out it comes out to be about two for hydrocarbons and three for alcohols and ethers and this retracts from all people's thinking because what it meant was that if you operate at low temperature where everything was frozen down and you gamma irradiate it the electrons formed by attacking the solvent migrated through the medium and ended up on the naphthalene now this is quite this is a very important idea here which i which i i i want to stress so i'll i'll th-, i'll go on about it a bit when you do photochemical excitation you match the quantum of the light to the energy level of the solute okay you have a solvent and a solute the solvent normally doesn't absorb any light you're looking at the solute and that has spectra-, spectral levels that correspond to the irradiation wavelength in other words you're pumping a particular state within the solute molecule that's photochemistry in radiation chemistry it's completely different as the gamma ray traverses a medium or the alpha particle whatever it is it interacts with the solvent and it ionizes the solvent you might say what happens to the solute does it ionize that as well well it's purely statistical it will excite electrons in whatever it's passing by and of course statistically if you take something like methanol or pentane then take a litre of that and work out how many m-, how many moles there are in liquid pentane it's about ten molar liquid pentane is about ten moles per litre the naphthalene is ten-to-the-minus-three so it's a a ten-thousandfold excess of the solvent and so ninety-nine- point-nine-nine per cent of all the energy goes into the solvent and nought-point-nought-one per cent goes into solute so all of the reactions shown by the solute reflect what's happened to the solvent and with this solute it's picking up the electrons that have been formed in the solvent they've moved through this medium at low temperature and they've al-, alighted and been trapped [cough] on the solute to give this green colour and this process of having a very small amount of something there that captures the negative charge in the system so you can then look at it optically that's called a scavenger okay so it's playing the role of a scavenger i'll i've actually writing on the glass at the moment so i'll get back onto the so the naphthalene is C-ten-H-eight acts as a scavenger and the this was a the key to getting a much better understanding what was going on so we now know that in the solid organic matrices you know frozen down we had high ion yields and that result is complete opposite to the one we got using the conductivity cell and in the conductivity cell it's almost what's happening properly there is that er you're simp-, is-, you're simply not you haven't got a very efficient means of g-, of gathering the charge here you've got these solute molecules spread right through the system and they c-, they captured electrons before they can find positive ions to collide with or even destroy each other which er is i-, you know can also happen and there is such a reaction which i'll perhaps i'll talk about later well that was a position in frozen matrices and a question then arose well oh that's fine for frozen matrices what about liquids well the answer to that came when people developed a technique called pulse radiolysis we talked about flash photolysis before that's where you take a flash lamp or a laser that's pulsed and you administer a flash of light or a pulse of light from the laser to the sample and you look spectroscopically on a very short time base nanoseconds or even shorter than nanoseconds as to what's going on in pulse radiolysis exactly the same idea what you've got this time instead of radiolysing with a continuous beam which is what you normally do with a cobalt source or a linear accelerator with a linear accelerator you can arrange the electronics to chop the beam and you can chop it right down into nanosecond pulses and so just as in flash photolysis you had a nanosecond flash of light in pulse radiolysis you've got a nanosecond burst of electrons you can pulse an electron beam fairly easily and get a pulse of electrons so it's exactly the same type of idea and with that of course you can then attack liquids with these electron pulses and look at ions that are formed and what you have to do is to put in again a scavenger most aromatics er sorry most aliphatics and ethers if you imagine that you were to ionize an ether you get an ether cation or ether radical and electron those those aliphatic radicals absorb right down in the U-V they're very hard to see so if you want to know what's going on what you again do is to put in another scavenger er maybe naphthalene again or biphenyl but this time you're putting it at room temperature instead of of a very low temperature so it's the same experiment in a way that Hamill did except that it's in the liquid phase to begin with so you're looking at the mobility of the of the ions which you don't get in the frozen matrix and also er you're using fast spectroscopic recording you're recording optical spectra on the nanosecond timescale and if you do that then you find that you get er ion yields er in water about two-point-three this is using nanosecond N-S in methanol about one-point- three and in hexane about point-five so the ion yields are much higher than the pure conductivity experiment indicated and this is using the scavengers but this time in a l-, in the liquid phase so again we've got direct physical evidence for quite large yields of ions even in hydrocarbons from the r-, the radiolysis experiment there's another thing that you can do and which was done all of these experiments were done on a nat-, a nanosecond timescale it's ten- to-the-minus-nine of a second okay ten-to-the-minus-nine and the question raised in people's minds okay this is what you see at ten-to-the-minus-nine but what if we could actually shorten the pulse further would we see earlier events after all in the track model you had the pictures of these spurs with positive and negative ions and the question was would it be conceivable to see these er scavengers capturing electrons or maybe capturing positive charges as well on a very very short timescale giving ion yields higher than what you see at one nanosecond because at one nanosecond some of your naphthalene minuses might have combined with solvent cations or naphthalene pluses also formed and so people then began to to develop shorter timescales and they developed picosecond pulse radiolysis and that's ten-to-the-minus-twelve of a second so here we go picosecond pulse radiolysis ten-to-the-minus-twelve of a second basically it get gives higher ion yields and so if you actually measure the ion yield as a function of the pulse length that you're applying you find that ten- to-the-minus-twelve of a second everything gives ion yields of three at ten-to- the-minus-nine it's dropped down to about sort of two or one-point-five microsecond i-, it's fairly stable and then gradually of course with even longer pulses it gets very low and you get to the figure that you record in a conductivity experiment so you actually find the ion yield depends on time and it depends on that because you form these things inhomogenously el-, in the spurs they're formed they begin to migrate away on a very short timescale they they alight on the solute and give you say a solute minus that er you can see at the very short timescales but not long afterwards some of these have already recombined with positive ions in the medium to give lower yields well this was the picture with ions what about excited states well there are various ways of forming the excited states and various ways of trying to see them let's let's so let's look now at excited states measurement of excitation yields in radiolysis what we think happens and what i've done to start with is simply take a very simple system and i've taken cyclohexane now you might wonder why cyclohexane and the answer is that if you're looking at free radical chemistry cyclohexane's quite a good thing to work with because if you look at cyclohexane you've got hy-, the-, two pairs of hydrogens all the way around it doesn't matter which of those bonds you break you always end up with the same radical C-six-H-eleven if you add hexane you can take a hydrogen off the methyl group at the end or the next C-H- two in or the next C-H-two after that so immediately you got three different radicals and the chemistry becomes much more complicated so cyclohexane is very much a preferred solvent to work with because you can get it very very pure and it gives just one radical there's only one C-six-H-eleven so whichever that one w-, whichever those bonds you break you always have the same radical now if you take cyclohexane and irradiate it and i've just put this is the symbol by the way that sort of er lightning looking symbol er with the little gam-, gamma above that's the er symbol for high energy radiation ionizing radiation er i put gamma i could have put alpha if it was alpha radiation and you get three things happening you get the cation and again that's a a radical cation initially so i'll perhaps i'll put a dot there just to remind you the electron and the C-six-H-twelve-star now we can measure this by scavenging at low temperatures we can measure that as well by scavenging at low temperatures what if you put anthracene in the system what happens if anthracene is there right well i if i've got anthracene A what happens is this the electron goes onto the anthracene to give anthracene-minus the hole present in the C-six-H-twelve-plus-dot goes onto the anthracene to give anthracene-plus it's much easier to ionize anthracene than it is to anthrace-, to ionize cyclohexane so thermodynamically the positive charge will migrate from this molecule onto the anthracene to give anthracene-plus any excited states will be of very high energy 'cause they're going to be sigma sigma star states you remember there are no pi-electrons there but there are excited states formed and they give the excitation to the anthracene to give singlet anthracene-plus and triplet anthracene meanwhile er what can also happen in the system is the anthracene-plus and the anthracene-minus if you don't track them at a low temperature but if you have them at room temperature these will find each other and annihilate each other in other words the plus will destroy the minus to give excited states so you'll get yet more singlet anthracene and triplet anthracene the singlet anthracene will emit fluorescence so you can see fluorescence coming out of the system and you can measure it it is the case that if you take a solvent such as cyclohexane or benzene or toluene or some other solvents and put into the solvents a molecule that's a good fluorescing agent and you expose it to radiation it will emit light it emits fluorescence so you're not putting any light in you're only putting ionizing radiation into this system and you're getting fluorescence out and this is the basis of many detectors and this is a a a a a very slight digression but very many of the most important experiments that have been done in cos-, in radiochemistry but also astrophysics have relied on this because if you think about things like cosmic rays that are constantly pulsing through the earth they're pulsing through you as well are there ways of detecting them well if you take er either a plastic block loaded with anthracene or maybe a solution of toluene with some anthracene in it and you take it down to the bottom of a coal shaft you can actually see it fluorescing and it's fluorescing because of cosmic rays passing right through the earth and passing through that sample and f-, giving you light so we are all the time under constant irradiation from outside the solar system and you can detect that using these systems and these th-, these systems er they have a special name in in that technology they're called liquid scintillators so if i write if i put in brackets under here liquid scintillators they're a a very good way of measuring ionizing radiation you can use liquid ones and also you could there are s-, there are crystal ones as well there are there's some some very nice crystalline scintillators that you can make solid state ones and they are used again for measuring radiation if there's been a leakage or something you go you can either go round with a gigacounter which is one possibility or you can go round with a solid state scintillator and you measure the er the scintillation spectrum okay well if you de-, er that's that was a bit of an aside but i thought i'd mention it in case er you sort of read about it sometime and wondered how it all linked up with this the G value that's a hundred electron volt yield of singlet anthracene is about two and the yield of triplet anthracene measured by pulse radiolysis this time is also about two so you get quite high excited state yields from radiolysis of something like cyclohexane hexane er dioxan benzene and toluene and all those sorts of solvents high yields from organic solvents particularly if there are no er organic radicals there sorry if there are n-, sorry if there are no hydroxy groups there they they that disturbs things but i i'll talk about that later as well that's using spectroscopic methods this detection of excited states we've looked at the fluorescence emission from the anthracene say or whatever else you were using you can look at the triplet states 'cause they won't emit at room temperature 'cause you know about that anyway from photochemistry but you can see them in p-, er by you doing triplet triplet abdorp-, triplet triplet absorption as in pulse radiolysis what about chemical effects well you can also induce cis-trans isomerization i mentioned in t-, er w-, in the photochemistry lectures that er one of the things that er you can do is to make molecules isomerize by shining light on them but you can also do it with radiation and if you take liquid benzene and add a small amount of cis-butene which is what that is and gamma irradiate it it turns into a mixture of cis and trans-butene so you can bring about isomerization what how does this happen well i-, from the previous overhead you ought to have a good idea but i'll i'll write it all i'll put it down again to indicate got this is benzene benzene undergoes radiolysis and this is a perfectly general equation you see you've got it for cyclohexane you've now got it for benzene it applies to every molecule there is so you know if you were given say a question in which you were given some molecule you hadn't come across before and asked what happens if you radiolyse this the first thing to write down is M-plus minus an M-star where the molecule is M then you've got everything that can happen in the spur i-, along the track well this is benzene-plus minus benzene excited state benzene exci-, excited state is singlet and triplet you get them both formed and what happens in particular is that triplet benzene will react with this and it will transfer its energy so we've got energy transfer to give excited cis-butene plus ground state benzene so i could maybe just write in ground state benzene just to remind you that the you've got energy transfer so i if i put E-N energy transfer there and this excited state falls back to the ground state and it falls back into a mixture of cis and trans and of course er this has another go then this can go round the circuit again and get converted into trans the isomerization yield is two-point-three you convert two-point- three molecules of cis-butene for every hundred electron volts of energy that falls on there and if that if it's a case there's a fifty-fifty chance of falling this way or that way it means the excited state yield the triplet state yield of the cis-butene is in fact twice that it's four-point-six 'cause half of them go this way and half of them go that way all right so that is an indication of how you can use a scavenger and this time it's the cis-butene to register what's going on in the benzene you rely on the scavenger picking up the intermediates of course if you were as a person were undergoing gamma radiolysis in the water in your body you would get positi-, you'd get ion-, ionization and you get the intermediates formed from that and then your D-N-A in your proteins acts as the scavengers so this picture i've given you of scavengers small amounts of s-, materials in a bulk solvent system is relevant to radiobiology because in radiobiology eighty per cent of you is water eighty per cent and so if you get radiation effects in water these will transmit themselves to the building blocks of your cells okay but that's for a later lecture but i mention it now this picture carries all the way through this notion of scavenging now i've talked about the capturing the n-, the electrons i've talked about capturing the excited states the last thing i want to talk about is capturing the positive ions how do we know well well we there must be positive ions there 'cause we see the negative ions and you can't have negative ions without positive ions but it would be nice to see them as well and what you can do is this you can take a a molecule which we know from electrochemistry is very easily oxidized to give a highly coloured species and a nice example is triphenylamine nitrogen with three phenol groups on this is a pale pale yellow material crystalline if you dissolve it in cyclohexane and submit it to pulse radiolysis you get a brilliant colour just for a short time but you can measure that and it's known from electrochemical oxidation of triphenylamine so er i-, er it's a known species the extinction coefficient's known as well and what's happening is you're getting this this er you're getting C-six-H-twelve-plus with an odd little little radical dot formed that's your solvent cation radical this is your amine this gives up an electron far more readily than this does so the electron passes from here onto there to neutralize it and give you back the solvent this becomes the cation and you can measure these cations er by pulse radiolysis that's what er P-R stands for at room temperature or you can do it at liquid nitrogen temperature as well and you get the same coloration there it's a blue-green colour so this is called a positive ion scavenger and it's been chosen because it's got a very low ionization energy and it's marvellous at er picking up positive ions in that sort of way okay so those are all experiments either at low temperature in some cases matrix isolation as i've called it before or room temperature to measure cations here excited states there and electrons on the previous overhead so that is how we actually get at the the yields o-, of materials now er i'm going to skip the scavenger equation 'cause i'm going to come back to that later on what i'd like to do now is to say a little bit about water water is probably the most important system looked at mainly because as i said we are we consist largely of water so there's huge interest there must must be over a thousand papers on the radiation chemistry of water and probably half a dozen books as well er what happens with water well in a way it's quite interesting if you take a vessel and you fi-, you fill it almost to the top put a stopper in er take this vessel of pure water completely full and if you gamma irradiate it for days and then you do a a big analysis of it what happens and the answer is almost nothing water appears to survive radiolysis well that's a very strange sort of thing with hydrocarbons you get all sorts of products you get dimers you get hydrogen you get goodness knows what with methanol you get ethylene glycol plus hydrogen but water is extraordinary it's a-, it appears to be amazingly radiation resistant and so for quite a while certainly in in the earlier parts in the earlier y-, y-, years of this century people thought water was very boring because it didn't seem to do anything er but then people got used to got used to this this idea of using scavengers and they began to look at water with scavengers so they they took water and added scavengers so the first point to note is water on its own appears not to do anything but if you put in scavengers and the two i've chosen are iron-two and cerium-four iron-two is a reducing agent cerium-four is an oxidizing agent what do we think we get in well let us suppose and course i i know i know the answer to this but let us suppose that water does indeed undergo radiolysis to give an oxidizing species and a reducing species but let's also suppose that they are extremely good at going back again to water right let's look at it in more detail now what what from the experience with cyclohexane and benzene what mi-, might we expect to see well water we might have thought would ionize so we write H-two-O-plus and E-minus and H-two-O-star that would be a very good starting point what happens to those things well the first thing that happens er and i want to concentrate on this side of the picture so perhaps i'll cover that up for the moment H-two-O-plus is known in mass spectrometry you can you can ionize water in a mass spectrometer and what's known is a very very fast reaction between H-two-O-plus and water to give H-three-O-plus and O-H radical so we might have thought well perhaps we're getting some O-H radical what about the electrons well er an electron the kind of electron that goes along a wire in a torch or in a T-V set or to this overhead projector if you put an electron in water it becomes solvated i don't know what you know about solutions of sodium in ammonia or potassium in ammonia but if you take ammonia and you dissolve potassium or sodium have you done that have you done an experiment in the lab with sodium in ammonia have you ever had liquid ammonia as a reagent maybe not well if you take ammonia it's a liquid it er boils at minus-thirty-three but it's got quite a high latent heat of vaporization so it s-, hangs around for quite a while before it all evaporates it's around for really quite a long time if you put sodium in ammonia it dissolves and gives you a deep blue solution and these deep blue solutions are p-, are p-, highly magnetic and basically you've got N- A-plus and free electrons in the ammonia and those free electrons are solvated by the ammonia molecule which has got an electron with say six ammonias round it well here you got an electron with six waters round it that's called the aquated electron or the hydrated electron okay and you can s-, you can see that that is there in pulse radiolysis if you just pulse radiolyse pure water you get a band at seven-hundred and this thing of course i mentioned before in the photochemistry section of the course if you flash photolyse iodide ion or ferrocyanide ion again you get this broad band peaking at seven-hundred that's right out in the red so it's a blue colour you see this and you do indeed get electrons the O-H radicals well i mentioned cerium-four and iron-three you take water and radiolyse it with iron-two there the iron-two becomes iron-three if you take water with cerium-four there the cerium-four becomes reduced to cerium- three if we put these two scavengers in we get oxidation of this and reduction of that that was the first hint that there were oxidizing species in water these are very classical type experiments using those things e-, but in the pulse radiolysis we can do this and we can see the O-H radicals here which are formed in that step there they attack various things they attack benzene s-, add on to benzene to give this thing and that has got a known optical absorption spectrum this is you can s-, you can make this thing photochemically so we we know the o-, O-H is adding to benzene to give this but we also know that O-H if we have a different experiment it attacks various things and if we have sodium thiocyanate er N-A- S-C-N- minus it's a scavenger then we get electron transfer reaction electron transfer we get S-C-N-dot in other words the O-H pulls the electron off there to give O-H-minus hydroxide which then reproteinates to water meanwhile this has formed S-C-N and this actually loves to add to an S-C-N-minus to give this thing and that is very highly coloured and you can see it in pulse radiolysis so in the pulse radiolysis experiment without the scavenger you get this with the scavenger you see this or you see that and so although water appears not to give any reaction to radiation what's happening is you are getting yields of oxidizing species and reducing species the oxidizing species is in fact O-H and the reducing species is in fact E-minus and so thi-, this is basically the guts of water radiolysis i put H-two-O-star there and excited state of water are they very important well surprisingly perhaps they're not er you might have thought there might be a reasonable yield but in fact when you radiolyse water you form these two in the spur and there's a big difference between water and hexane in hexane if you've got two charges a few angstroms away they have an enormous coulombic attraction because the cou-, the attraction between two charges is given by Coulomb's law Q-one-Q-two over epsilon-R squared and Q-, Q-, Q-one and Q-two are both one it's the epsilon that counts the dielectric constant the dielectric constant or electric permittivity of hexane is about two has any-, does anyone have any idea what it is for water the dielectric constant of water or electric permittivity of water any any feel for that well it water's a highly dielectric medium and it the figure's about eighty so the force between the positive and neg-, negative charges in hexane is enormous because on the bottom line you've got epsilon being two but in water epsilon's eighty so the coulombic attraction is forty times weaker so there's a much better chance they'll get away and not recombine to give excited states and indeed the excited states of water er are purely dissociative if you excite water er with a very deep U-V quantum you get excited water but it breaks into hydrogen atoms and O-H radicals and so you do not have stable excited states of water if they are formed at all they dissociate to O-H and hydrogen and immediately the hydrogen atom will form an electron because if you think about it the solvated electron will with react with a proton to give you a hydrogen atom so you can convert electrons to hydrogen atoms that's the H atom that's the proton and that's the solvated electron and so these are the three very very simple species but you always knew that the hydrogen atom was a proton plus an electron so it's not surprising that in water you actually get the hydrogen atom dissociating into those two bits or they can recombine to give that and you've actually got there is a P-K-A the hydrogen atom it's a lit-, it's a little tiny acid all in its own right well on that amazing thought i'll i'll bring things to an end today so we've talked about radiolysis using scavengers of organic systems and i've finished up by talking a bit about water the big difference is the organic systems have excited states that's a very important part of their radiolysis water is entirely ionization to give O-H-dot and E-minus those are the two principal species and they dominate radiation biology okay