nm0692: right well yesterday i was er [0.5] talking about the [2.2] idea of a 
track [0.3] in [0.2] radiation chemistry [0.4] where [0.3] as the particle 
moves along [0.6] it's losing energy we know it loses energy at a great rate we 
know the energy of the particle [0.4] we know the range of the particle [0.8] 
what happens to this energy how does it get transmitted to the medium [0.5] and 
[0.2] this [0.2] gave an indication [0.3] that the principal [0.2] acts of 
energy deposition [0.4] are in the form of ionization [0.4] and also [0.2] o-, 
of excitation as well [0.2] that that's [0.2] an overall picture [0.6] now [0.
4] that being the case you might say well [0.2] all right you've got ions in 
excited states [0.3] or so you say [0.5] er [0.4] what happens immediately 
after [0.4] this [0.4] event has occurred [0.6] and [0.2] i suppose [0.4] the 
first thing that happens [0.5] is that the electrons [0.7] which are formed [0.
6] in the ionization act [0.4] these are very small [0.4] and they're very 
mobile [1.4] and they're certainly very small compared with the [0.2] cations 
that are formed 'cause those are going to be molecular size [0.4] so the 
electrons tend to diffuse away very rapidly [0.5] simply because they 
are so small [0.6] and er [1.4] they have a a very high mobility [0.9] what 
about the er [1.0] the cations that are formed [0.4] well if you just consider 
a general i put R-H-plus but it could be anything [0.4] but suppose it was i 
don't know hexane or something [0.4] then [0.2] the [0.7] cation radical [0.4] 
which is the thing you get by taking the electron out of the molecule [1.0] er 
[0.5] will [1.4] react with electrons that are [0.2] are nearby the ones that 
haven't got away so to speak [0.4] and you get a certain amount of ion 
recombination [0.7] and when this electron returns to that cation [0.5] it's 
very likely [0.4] to form it in an excited state [0.2] so you can get 
additional excited states [0.2] from [0.3] from ion recombination [1.4] but 
also what can happen [0.9] is that the R-H-plus the cation radical [0.7] er is 
a very powerful proton donor [0.7] and it will give a proton to almost anything 
in sight [0.7] er and so [0.3] this will tend to happen [0.4] and perhaps the 
best known example of this is if you imagine [0.3] R-H-plus is [0.5] H-two-O-
plus [0.6] the water cation [0.4] then [0.2] that will give away a proton to a 
nearby water molecule [0.4] 
to give H-three-O-plus [1.0] and you're left with O-H behind [2.2] so you do 
get these er [1.6] proton transfers occurring [1.3] how many ions do we get [1.
6] how do you measure the efficiency of a radiation chemistry [1.0] process [0.
6] well in photochemistry it was very easy because you had the idea of a photon 
[1.0] and the photon [0.7] entered a molecule [1.0] and so much no s-, a 
certain percentage of the molecule was converted [0.7] to a product [0.4] and 
you had the idea of a quantum yield [0.4] the idea that you could destroy [0.5] 
such a percentage of molecules [0.7] you would form such a percentage of 
product molecules [0.4] and you had a quantum yield for this and this was 
always less than one [0.5] unless you had a chain reaction [0.4] so it's a very 
very clear idea [0.6] but with a gamma ray quantum i the point i was trying to 
make yesterday it's completely different [0.4] because the gamma ray quantum [0.
4] has got enormous amounts of energy [0.7] and [0.8] or the alpha particle has 
[0.4] and you don't have the notion [0.3] of just one molecule be-, be-, being 
converted by one 
quantum [0.6] you've got the n-, the idea of along the track [0.2] hundreds of 
thousands of molecules being converted [0.6] so how do we cope with this idea 
[0.5] well we cope with it by saying [1.3] how many molecules [0.2] how or how 
many ions do we get [0.7] for a certain amount of energy deposited [0.7] and 
there is a definition of a thing called a G value [0.8] and the G value [0.6] 
which is a measure of the product yield or indeed the react-, the [0.2] 
destruction of the reactant 'cause er that would be then the negative G value 
but [0.5] they normally normally you look at products to measure positive G 
values [1.0] they are the number of molecules [0.2] formed [0.3] or destroyed 
[0.5] for every hundred electron volts [2.7] and so you if you think about it a 
hundred electron volts [1.5] ionization of a typical organic is about ten 
eleven or twelve let's so say ten for the sake of argument [0.6] then [0.5] you 
would say well if we got ten [0.6] prod-, if we got ten ions [0.5] that would 
be [0.2] a hundred per cent efficient [1.2] but in fact you don't get ten ions 
[0.3] you get typically [0.4] two or three ions [0.6] maybe 
slightly more than three sometimes but not much more than three [0.7] and so 
the ionization efficiency [0.8] is not enormously high [1.4] but of course [1.
4] you also get the excited states as well [0.7] and so if you're at the 
excited state yield which is also often about two or three [0.5] but well for 
some systems [0.9] you're using about fifty per cent of the of the ou-, energy 
[1.0] chemically productively [0.4] what happens to the rest [0.5] well the 
rest [0.3] you get ion recombination the ions don't escape at all they they si-,
they're formed and they recombine instantly [0.2] don't measure them [0.3] 
'cause they're not around to be measured any more [1.8] under those conditions 
[1.7] you simply end up by getting excited states [0.4] and some of these 
simply end up by heating up the medium [0.3] so you you you actually simply do 
a lot of vibration excitation of the medium [0.3] so the medium will become 
warm [0.3] as you put radiation through it [1.1] so [0.3] the energy is 
partitioned between ionization events [0.4] excitation events [0.3] and merely 
thermal warming [0.8] and this [0.2] G value [0.6] as i've [0.2] put down 
there is the analogue of the quantum yield [0.2] it's not as precise in a way 
[0.4] you can't picture it as as nicely [0.7] but you've got to start from 
somewhere [0.4] and you say well if i've got a million electron volts [0.3] how 
m-, what do i get for each hundred electron volts that i deposit in the medium 
[0.5] and so that is your [0.2] fundamental parameter the G value [0.3] and 
whenever you're discussing radiation chemistry of anything [0.4] you begin [0.
3] by saying well what is what is its G value [0.3] is it er [1.0] one or [1.0] 
three or five or [0.4] and if it's a lot more [0.3] than [0.6] than ten or if 
it becomes hundreds again it's a you got a chain reaction [0.4] running away [1.
3] now [1.6] let's think about some of the er [1.9] experiments [0.2] to get 
some idea of what these G values are [0.4] the very simplest experiment you can 
do [0.3] is using an ionization chamber and i'll draw one of these it's a very 
simple idea [0.7] if you imagine you've got a glass vessel [0.8] with two 
electrodes in [1.1] you got a you apply a voltage [0.4] that's voltage there [1.
1] and you have an ammeter for measuring current [0.4] there [2.5] now [0.6] 
normally of 
course [1.6] if you have that [0.2] globe filled with [0.5] argon [0.3] or 
helium [0.7] or [0.5] hexane vapour or something propane [1.0] if you apply a 
voltage [1.0] that's not enormous [1.2] then there's no current [0.8] because 
the gases are insulating [0.6] i know if you take it to a very very high 
voltage you should get diametric breakdown [0.3] we're not thinking about that 
at all [0.4] we're thinking of quite low voltages [0.3] get no current [0.5] 
but if you bring up [0.9] a [0.4] other a radioisotope [0.4] to the outside [0.
4] there [0.3] close to the glass [0.4] vessel so if we have isotope [3.0] this 
is giving out [0.7] radiation [1.0] like so [0.8] or you could fire a very very 
fast electron beam [0.7] into the globe [1.6] if you're still applying the 
voltage [1.5] you don't get a current [0.6] I [0.4] becomes greater than nought 
[0.6] with radiation [4.1] I [0.5] becomes greater than zero [1.1] without 
radiation [2.2] I [0.3] equals zero [0.7] so clearly the radiation is producing 
ions in the gas [1.1] and you know they're there [0.8] because the whole thing 
is carrying a current [0.5] once you apply a voltage [1.1] positive ions are 
moving to one electrode [0.6] negative ions are moving 
to another [0.7] so immediately we know [0.6] that [0.3] the application [0.8] 
of ionizing radiation to a gas [0.6] produces ions [0.7] and you might say well 
[0.8] er [1.5] what sort of [0.4] yields do we get [0.7] from gases [0.7] and 
maybe from liquids [0.4] well from gases [0.6] the [0.2] ion yield is [0.2] 
sort of [0.5] reasonable it's [0.2] say [0.2] one [0.6] to three [0.3] depends 
on the gas [0.5] for liquids [1.6] interestingly if you if you fill that globe 
with [0.2] liquid hexane or liquid benzene [0.8] G liquid it's nought-point-one 
[1.6] now why should that be [0.7] well the answer again if we go back to 
photochemistry [3.7] a lot of [1.0] photochemical acts [0.6] give you [0.7] two 
species formed very close to each other [2.2] if they [0.8] can get away from 
each other [0.4] you get products [1.0] but if they [0.8] are forced [0.2] by 
being surrounded by solvent [1.0] back onto each other [0.4] they recombine [0.
7] and so what's happening in the liquid [0.3] is if you get [0.9] a positive 
charge and a neg-, negative charge formed [0.4] and these are surrounded [2.7] 
by solvent molecules [3.4] like so [0.4] and this of course is a positive 
solvent molecule and that's a [0.3] either a negative solvent molecule or an 
electron [0.5] 
the thing is trapped [0.6] and you've got basically what's called a cage effect 
the thing is trapped [1.3] in a solvent so i'll say solvent cage effect [0.7] 
that's very important in photochemistry [1.7] it's very important in radiation 
chemistry as well [0.7] so [0.3] in liquids [0.4] you've got the solvent cage 
effect operating [0.4] in a gas phase you haven't once the ions are formed zoom 
[0.5] they they go charging off towards the electrodes [0.2] so there's quite a 
difference there [0.7] so [0.7] that was the f-, the first indication [0.3] 
that you were getting ionization [0.8] in the gases [0.2] to quite high yields 
[0.4] but in the liquids [0.9] to very low yields [0.2] and for a long time [0.
2] that experiment [0.9] convinced everybody [0.7] that ionization [0.7] was 
not important in liquids [1.6] not at all [0.7] because [0.6] the conductivity 
experiments indicated the yields are very low [1.0] and that that sort of held 
[0.4] sway for a long time [0.7] but then there were some [0.3] very clever 
experiments [0.4] that were done [0.4] by a chap [0.4] called Hamill [0.7] and 
i'll [0.8] as i've got these on a [0.5] a slide i'll put the slide on [2.7] and 
it goes as follows [6.4] Hamill worked [1.5] 
with [1.0] solutions [0.6] they were quite dilute say a millimolar [0.8] of 
things like naphthalene and biphenyl [0.3] and he worked with lots of other 
ones as well but they're just examples [0.6] in [0.2] what are called glass 
glassy forming solvents or [0.6] er [0.2] glass forming solvents [1.4] 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 [1.4] they give 
a perfect glass they don't crystallize [1.3] they've got an irregular structure 
[0.4] you've got the methyl group here [0.4] tetrahydrofuran [0.5] crystallizes 
when you freeze it [0.2] in liquid nitrogen [0.5] methyltetrahydrofuran is less 
symmetrical [0.4] and it forms a glass [1.6] pentane [0.2] will crystallize if 
you freeze it [0.3] you put a methyl group on get three-methylpentane [1.4] it 
gives a perfect glass [0.6] and if you've got a perfect glass [0.7] it means 
you can freeze the thing in a cell a caught cell [0.6] and then run an optical 
spectrum [0.7] at liquid nitrogen temperature [0.9] in an ordinary 
spectrophotometer [1.8] and [0.3] what did 
Hamill do [0.4] well [0.5] he rotated these solutions in these glassy forming 
solvents and then ran U-V spectra [0.8] and for example [2.2] if you er [0.6] 
take [1.2] naphthalene [1.4] C-ten-H-eight [0.8] this is co-, naphthalene's 
colourless [0.8] dissolve it up in T-H-F or three-methyl-pentane [0.4] if you 
gamma irradiate it [1.2] the thing goes deep green [0.3] 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 [0.3] not easy to do 'cause you'd need [0.4] 
er [0.8] er a gamma source to do it [0.2] and we we don't have one here [0.8] 
er [1.6] to get deep green colour [0.6] this is this has got a it's quite a 
structured spectrum it's not just a broad band [0.4] and it's exactly the same 
spectrum as this thing [0.7] in other words if you take a sodium film [0.3] a 
film of so-, if you purify sodium [0.7] in fact you heat it up and evaporate it 
[0.4] and then could allow the sodium vapour to get to get condense on glass [0.
3] you get [0.6] a [0.2] mirror [0.3] formed [0.2] a silver mirror [0.4] of 
sodium [0.8] if you take that silver mirror of sodium which is very very pure 
[0.9] and you react it [0.5] 
with [1.1] a dilute solution of naphthalene [0.2] in an ether [0.7] you end up 
by getting a deep green solution [0.6] and this is sodium-plus [0.3] C-ten-H-et 
[0.2] H-eight- [0.3] minus [0.2] with a free radical symbol [0.3] er it's a 
free radical [0.5] and it's it's strongly paramagnetic [0.3] and it shows an E-
S-R spectrum quite a complex one [0.4] but it can be perfectly analysed in 
terms of er [0.4] in terms of naphthalene [0.5] so [2.5] you've got this formed 
[0.3] now we know [0.3] the extinction coefficient [0.3] of this [0.7] thing [0.
2] because we can [0.5] measure [0.7] how much sodium has been consumed or [0.
5] we can indeed er [1.8] er [1.0] we can quantitate it reasonably well [1.0] 
and so if we know the extinction of the [0.4] of this we can work out the yield 
[0.5] of these [1.1] cat-, [0.2] anions [0.3] which are formed in the glass [0.
8] and if you work it out [0.6] it comes out to be about two [0.5] for 
hydrocarbons [0.3] and three [0.3] for alcohols and ethers [0.6] and this 
retracts from all people's thinking because [0.3] what it meant was [0.3] that 
if you [0.2] operate at low temperature [1.9] where everything was frozen down 
[1.6] and you gamma irradiate it [1.1] the electrons [0.8] formed [0.5] by 
attacking [0.6] the 
solvent [0.5] migrated through the medium [0.4] and ended up [0.8] on the 
naphthalene [1.1] now this is quite this is a very important idea here [0.2] 
which i which i i i want to stress so i'll i'll th-, i'll go on about it a bit 
[1.1] when you do photochemical excitation [0.6] you match [0.3] the quantum [0.
9] of the light [1.7] to the energy level [0.2] of the solute [1.2] okay [0.4] 
you have a solvent and a solute [0.6] the solvent normally doesn't absorb any 
light [0.3] you're looking at the solute [0.3] and that [0.4] has spectra-, 
spectral levels [0.5] that correspond to the irradiation wavelength in other 
words you're [0.3] pumping a particular state within the solute molecule [0.8] 
that's photochemistry [0.6] in radiation chemistry it's completely different [1.
0] as the [1.3] gamma ray traverses a medium or the alpha particle whatever it 
is [1.0] it [0.3] interacts with the solvent [0.4] and it ionizes the solvent 
[1.6] you might say what happens to the solute [0.3] does it ionize that as 
well [0.7] well [0.3] it's purely statistical [0.7] it will [0.5] excite 
electrons in whatever it's passing by [1.9] and of course statistically [1.4] 
if you take something like 
methanol or pentane [0.6] then take a litre of that and work out how many m-, 
how many moles there are in liquid pentane [1.0] it's about ten molar [0.4] 
liquid pentane [0.2] is about ten moles per litre [0.8] the naphthalene is ten-
to-the-minus-three [0.4] so it's a a ten-thousandfold excess of the solvent [0.
7] and so ninety-nine- [0.7] point-nine-nine per cent [0.3] of all the energy 
[1.1] goes into the solvent [0.3] and nought-point-nought-one per cent goes 
into solute [0.4] so [0.2] all of the [0.5] reactions shown by the solute [1.7] 
reflect what's happened to the solvent [1.6] and with this solute [0.2] it's 
picking up the electrons that have been formed in the solvent [0.6] they've 
moved through this medium at low temperature [0.5] and they've al-, alighted [0.
3] and been trapped [0.7] [cough] [0.2] on the solute to give this green colour 
[1.3] and [0.8] this process of having a very small amount of something there 
[0.8] that captures [1.0] the negative charge in the system [0.9] so you can 
then look at it optically [1.0] that's called a scavenger [0.4] okay [0.4] so 
it's playing the role [0.5] of a scavenger [1.4] i'll i've actually writing on 
the glass at 
the moment so i'll [0.5] get back onto the [2.8] so the naphthalene [0.4] is [0.
2] C-ten-H-eight [1.0] acts [1.0] as [0.5] a scavenger [2.5] and the [0.7] this 
was a [0.8] the key [0.2] to [1.3] getting a much better understanding what was 
going on [0.4] so we now know that in the solid organic matrices [0.4] you know 
frozen down [0.5] we had high ion yields [1.5] and that result [0.2] is 
complete opposite [0.4] to the one we got [0.5] using the conductivity cell [0.
7] and in the conductivity cell it's almost what's happening properly there [0.
6] is that er [0.4] you're simp-, is-, you're simply [0.7] not you haven't got 
a very efficient means of g-, of [0.2] gathering the charge [0.5] here [0.5] 
you've got these solute molecules spread right through the system and they c-, 
they captured electrons before they can [0.4] find positive ions to [0.7] 
collide with [0.5] or even destroy each other which er is i-, you know [0.2] 
can also happen [0.7] and there is such a reaction which i'll [0.3] perhaps 
i'll talk about later [1.0] well that was a position in [0.6] frozen matrices 
[0.4] and a question then arose [0.3] well [0.3] oh that's fine for frozen 
matrices [0.8] what about liquids [0.7] well [1.1] the answer 
to that came when people developed a technique called pulse radiolysis [1.3] we 
talked about flash photolysis before that's where you take [0.5] a flash lamp 
[0.8] or a laser that's pulsed [0.4] and you administer [0.2] a flash of light 
or a pulse of light from the laser [0.4] to the sample [0.5] and you look [1.9] 
spectroscopically on a very short time base nanoseconds or even shorter than 
nanoseconds as to what's going on [0.9] in pulse radiolysis exactly the same 
idea [0.4] what you've got this time [0.5] instead of radiolysing with a 
continuous beam [0.2] which is what you normally do with a cobalt source or a 
[0.3] linear accelerator [0.9] with a linear accelerator you can [0.2] arrange 
the electronics [0.6] to chop the beam [1.1] and you can chop it [0.4] right 
down [1.2] into nanosecond pulses [0.7] and so [0.9] just as in flash 
photolysis [0.2] you had a nanosecond flash of light [0.6] in pulse radiolysis 
[0.7] you've got [0.6] a nanosecond [0.2] burst of electrons [0.5] you can 
pulse an electron beam [0.6] fairly easily [0.5] and get [0.3] a pulse of 
electrons [0.4] so it's exactly the same type of idea [0.9] and with that of 
course you can 
then [0.7] attack liquids with these electron pulses [0.2] and look at [0.8] 
ions that are formed [0.4] and what you have to do [0.4] is to put in [0.6] 
again [1.0] a scavenger [1.0] most [0.2] aromatics [0.2] er sorry most 
aliphatics [0.5] and ethers if you imagine that you were to ionize an ether [0.
9] you get an ether cation or ether radical and electron [1.3] those those 
aliphatic radicals absorb right down in the U-V they're very hard to see [0.3] 
so if you want to know what's going on what you again do [0.3] is to put in [0.
6] another scavenger [0.6] er maybe naphthalene again or biphenyl but this time 
[0.4] you're putting it at room temperature [0.5] instead of of a very low 
temperature [0.9] so it's the same experiment in a way that Hamill did [0.2] 
except that it's in the liquid phase [1.0] to begin with [0.3] so you're 
looking at the mobility of the of the ions which you don't get in the [0.4] 
frozen matrix [0.2] and also [1.0] er [0.2] you're [0.5] using fast [0.2] 
spectroscopic recording you're recording optical spectra [0.9] on the 
nanosecond timescale [0.5] and if you do that [0.6] then you find [0.4] that [0.
3] you get er [1.6] ion yields [0.5] er [0.2] in water 
about two-point-three [0.2] this is using nanosecond [0.6] N-S [0.8] in 
methanol about one-point-three [0.6] and [0.2] in hexane about point-five [0.2] 
so the ion yields are much higher [1.2] than the pure conductivity experiment 
indicated [0.4] and this is using [0.2] the scavengers [0.5] but this time [1.
1] in [0.3] a l-, in the liquid phase [0.2] so again [0.4] we've got direct 
physical evidence [0.3] for [0.4] quite [0.2] large yields of ions [0.3] even 
in hydrocarbons [0.3] from [1.1] the r-, the radiolysis experiment [1.9] 
there's another thing that you can do [0.7] and which was done [2.1] 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 [0.7] and the question raised 
in people's minds [0.5] okay this is what you see at ten-to-the-minus-nine [0.
4] but what if we could actually shorten the pulse further [0.6] would we see 
earlier events after all in the track model [0.2] you had the pictures of these 
spurs [0.2] with positive and negative ions [0.4] and the question was [0.4] 
would it be conceivable [0.5] to see [0.4] these er scavengers [0.6] capturing 
electrons or maybe capturing positive charges as 
well [1.3] on a very very short timescale [2.4] giving ion yields higher than 
what you see at one nanosecond because at one nanosecond [0.6] some of your 
naphthalene minuses [0.5] might have combined [0.2] with solvent cations [0.3] 
or naphthalene pluses [0.4] also formed [0.3] and so people then began [0.3] to 
to develop [0.5] shorter timescales [0.2] and they developed [0.3] picosecond 
pulse radiolysis [0.4] and that's ten-to-the-minus-twelve of a second [0.9] so 
here we go [0.5] picosecond pulse radiolysis ten-to-the-minus-twelve of a 
second [0.4] basically it get gives higher ion yields [0.5] and so if you 
actually [0.3] measure the ion yield [0.3] as a function of the [0.3] pulse 
length that you're applying [0.3] you find that ten-to-the-minus-twelve of a 
second [0.4] everything [0.4] gives ion yields of three [0.4] at ten-to-the-
minus-nine it's dropped down to about sort of two or one-point-five [0.5] 
microsecond i-, it's fairly stable [0.4] and then gradually of course [0.3] 
with even longer pulses it gets very low [0.4] and you get to the figure [0.2] 
that you record [0.3] in a conductivity experiment [0.5] so [0.6] you actually 
find the ion yield 
depends on time [0.5] and it depends on that [0.4] because [0.3] you form these 
things inhomogenously [0.6] el-, in the spurs [0.8] they're formed [0.6] they 
begin to migrate away [1.0] on a very short timescale they they alight on the 
solute and give you say a solute minus that er [0.2] you can see at the very 
short timescales [0.6] but [0.7] not long afterwards some of these have already 
recombined [0.5] with positive ions in the medium [0.3] to give lower yields [1.
3] well [0.7] this [0.5] was the picture with ions [0.7] what about excited 
states [1.0] well there are various ways of forming the excited states [0.3] 
and various ways of trying to see them [0.5] let's let's so let's look now [0.
4] at [0.5] excited states [0.6] measurement of [1.0] excitation yields in 
radiolysis [1.4] what we think happens [0.3] and what i've done to start with 
[0.3] is simply take a very simple system [0.6] and i've taken cyclohexane [0.
8] now you might wonder [0.3] why cyclohexane [0.6] and the answer is [0.4] 
that [0.2] if you're looking at free radical chemistry [0.6] cyclohexane's 
quite a good thing to work with because [0.3] if you look at cyclohexane [1.7] 
you've got hy-, the-, two 
pairs of hydrogens all the way around [1.6] it doesn't matter which of those 
bonds you break you always end up [1.0] with the same radical C-six-H-eleven [1.
4] if you add hexane [0.7] you can take a hydrogen off [0.5] the methyl group 
at the end [0.2] or the next C-H-two in [0.2] or the next C-H-two after that so 
immediately you got three different radicals [0.4] and the chemistry becomes 
much more complicated [0.4] so cyclohexane is very much a preferred solvent to 
work with [0.4] because you can get it very very pure [0.9] and it gives just 
one radical [0.5] there's only one C-six-H-eleven [0.8] so whichever that one w-
, whichever those bonds you break you always have the same radical [0.7] now if 
you [0.4] take cyclohexane and irradiate it and i've just put [0.2] this is the 
symbol by the way that sort of er [0.7] lightning looking symbol [0.2] er with 
the little gam-, gamma above [0.4] that's the er [1.3] symbol for [0.4] high 
energy radiation ionizing radiation [0.4] er i put gamma [0.2] i could have put 
alpha if it was alpha radiation [0.5] and you get [0.7] three things happening 
you get the cation [1.1] and again 
that's a [0.2] a radical cation [0.2] initially so i'll perhaps i'll put a dot 
there just to remind you [0.6] the electron [0.8] and the C-six-H-twelve-star 
[0.7] now we can measure this by scavenging [0.2] at low temperatures [0.3] we 
can measure that as well by scavenging at low temperatures [0.6] what if you 
put anthracene in the system [0.8] what [0.4] happens if anthracene is there [1.
2] right [0.8] well i if i've got anthracene A [0.7] what happens is this [0.5] 
the electron goes onto the anthracene to give anthracene-minus [2.0] 
the hole [0.9] present in the C-six-H-twelve-plus-dot [0.4] goes onto the 
anthracene [0.3] to give anthracene-plus [1.3] it's much [0.2] easier to ionize 
anthracene than it is to anthrace-, to ionize cyclohexane [0.3] so 
thermodynamically [0.4] the positive charge will migrate [0.3] from this 
molecule [0.2] onto the anthracene [0.3] to give anthracene-plus [0.7] any 
excited states [0.8] will be of very high energy 'cause they're going to be 
sigma sigma star states you remember there are no pi-electrons there [0.4] but 
there are excited states formed [0.7] and they give [0.4] the excitation to the 
anthracene [0.4] to give singlet anthracene-plus [0.6] and triplet anthracene 
[0.9] meanwhile [0.6] er what can also happen in the system [0.2] is the 
anthracene-plus [0.7] and the anthracene-minus [0.3] if you don't track them [0.
5] at a low temperature but if you have them at room temperature [0.8] these 
will find each other [1.1] and annihilate each other in other words the plus 
will destroy the minus to give excited states [0.6] so you'll get yet more 
singlet anthracene [0.2] and triplet 
anthracene [0.8] the singlet anthracene [0.3] will emit [0.5] fluorescence [0.
2] so you can see fluorescence [0.3] coming out of the system [0.4] and you can 
measure it [2.1] it is the case [1.5] that if you take [1.6] a solvent such as 
cyclohexane [0.3] or benzene or toluene [0.3] or [0.2] some other solvents [0.
6] and put into the solvents [0.8] a molecule that's a good fluorescing agent 
[1.0] and you expose it to radiation [0.8] it will emit [0.4] light it emits 
fluorescence [0.4] so you're not putting any light in [0.6] you're only putting 
ionizing radiation into this system [0.7] and you're getting fluorescence out 
[0.6] and this is the basis of many detectors [0.4] and this is a a a a a very 
slight digression [0.4] but [0.7] very many of the most important experiments 
that have been done in [0.5] cos-, in radiochemistry [0.3] but also 
astrophysics have relied on this [0.6] because [1.5] if you [0.4] think about 
things like cosmic rays that are constantly pulsing through the earth [0.6] 
they're pulsing through you as well [1.5] are there ways of detecting them [0.
4] well if you take [0.4] er [0.2] either a plastic block [0.3] loaded with 
anthracene [0.5] or maybe a solution 
of toluene [0.3] with some anthracene in it [0.6] and you take it down to the 
bottom of a coal shaft [1.2] you can actually see it fluorescing [0.4] and it's 
fluorescing [0.3] because of cosmic rays [0.6] passing right through the earth 
[0.2] and passing through that [1.2] sample [0.5] and f-, giving you light [0.
4] so we are all the time [0.4] under constant irradiation [0.4] from [0.4] 
outside the solar system [0.7] and you can detect that [0.4] using these [0.3] 
systems [1.4] and these th-, these systems [0.3] er [0.3] they have a special 
name in in that technology [0.5] they're called liquid scintillators [0.5] so 
if i write if i put in brackets under here liquid scintillators [9.1] they're a 
a very good way of measuring [0.3] ionizing radiation [1.2] 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 [0.4] and they are used again for measuring radiation [0.6] if 
there's been a leakage or something you go you can either go round with a 
gigacounter [0.2] which is one possibility [0.4] or you can 
go round with a solid state scintillator [0.2] and you measure [0.6] the er [2.
3] the scintillation spectrum [0.6] okay [0.3] well if you de-, er [0.2] that's 
that was a bit of an aside but i thought i'd mention it in case er [0.4] you 
sort of read about it sometime and wondered how it all linked up with this [1.
0] the [0.3] G value that's a hundred electron volt yield of singlet anthracene 
is about two [0.9] and the yield of triplet anthracene [0.4] measured by pulse 
radiolysis this time [0.8] is also about two [0.3] so you get quite high 
excited state yields [0.4] from [0.2] radiolysis [0.4] of something like [0.7] 
cyclohexane [0.5] hexane [0.6] er dioxan [0.2] benzene and toluene [0.2] and 
all those sorts of solvents [0.8] high yields from organic solvents 
particularly [1.4] if there are no [0.4] er [0.3] 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 [2.8] that's using [0.
8] spectroscopic methods [1.7] this detection of excited states [0.6] we've [0.
9] looked at the fluorescence emission [0.6] from the anthracene say or 
whatever else you were 
using [0.8] you can look at the triplet states 'cause they won't emit at room 
temperature 'cause you know about that anyway from photochemistry [0.3] but you 
can see them in p-, er by you doing triplet triplet abdorp-, triplet triplet 
absorption [0.5] as in pulse radiolysis [0.5] what about chemical effects [0.4] 
well you can also induce [1.1] cis-trans isomerization [0.8] i mentioned in t-, 
[0.2] er w-, in the photochemistry lectures that er [0.4] one of the things 
that er you can do [0.2] is to make molecules [0.6] isomerize by shining light 
on them [0.7] but you can also do it with radiation [0.5] and if you take [0.2] 
liquid benzene [0.9] and add [0.4] a small amount of [0.3] cis-butene [0.4] 
which is what that is [0.8] and gamma irradiate it [1.2] it turns into a 
mixture of cis and trans-butene [0.5] so you can bring about isomerization [1.
0] what how does this happen [0.4] well [0.5] i-, [0.6] from the previous 
overhead you ought to have a good idea [0.6] but i'll i'll write it all i'll 
put it down again [0.3] to indicate [1.1] got [0.5] this is benzene [0.9] 
benzene [0.4] undergoes radiolysis [0.9] and this is a perfectly general 
equation you see you've got it for 
cyclohexane you've now got it for benzene [0.6] it applies to every molecule 
there is [0.4] so [0.8] you know if you were given [0.2] say a question [0.4] 
in which you were given some molecule you hadn't come across before [0.4] and 
asked what happens if you radiolyse this [1.0] the first thing to write down is 
M-plus [0.5] minus an M-star where the molecule is M [0.5] then you've got 
everything that can happen [1.1] in the spur [0.2] i-, along the track [1.5] 
well this is benzene-plus minus benzene excited state benzene exci-, excited 
state is singlet and triplet [0.5] you get them both formed [0.5] and what 
happens in particular [0.7] is that triplet benzene [0.8] will react [0.3] with 
this [0.5] and it will transfer its energy [0.3] so we've got energy transfer 
[0.4] to give excited [0.7] cis-butene [0.5] plus ground state benzene so i 
could maybe [0.3] just write in ground state benzene just to remind you that 
the [0.7] you've got energy transfer so i if i put E-N [0.9] energy transfer [0.
4] there [0.9] and this excited state falls back to the ground state [0.5] and 
it falls back into a mixture of cis and trans [0.8] and of course [0.8] er [0.
9] 
this has another go then this can go round the circuit again and get converted 
into trans [0.5] the isomerization yield [1.0] is two-point-three [0.5] you 
convert two-point-three molecules of cis-butene [0.3] for every [0.3] hundred 
electron volts [0.5] of [0.7] energy that falls on there [0.5] and if that [0.
7] if it's a case there's a fifty-fifty chance of falling this way or that way 
[0.5] it means [0.4] the excited state yield [0.6] the triplet state yield [0.
4] of the cis-butene is in fact twice that it's four-point-six [0.5] 'cause 
half of them [0.4] go this way [0.4] and half of them go that way [0.7] all 
right [0.9] so [0.3] that [0.3] is [0.6] an indication [0.2] of how you can use 
a scavenger [0.8] and this time it's the cis-butene [0.3] to register [0.4] 
what's going on in the benzene [0.4] you rely on the scavenger [0.4] picking up 
[0.5] the intermediates [1.5] of course if you [0.6] were as a person were 
undergoing [0.4] gamma radiolysis [1.7] in the water in your body you would get 
positi-, you'd get [0.4] ion-, ionization [1.8] and you get the intermediates 
formed from that [0.2] and then your D-N-A in your proteins acts as the 
scavengers [0.3] so [0.2] this picture i've 
given you [0.3] of scavengers [0.2] small amounts of s-, materials [0.4] in a 
bulk solvent system [0.5] is relevant to radiobiology [1.2] because in 
radiobiology [0.4] eighty per cent of you is water eighty per cent [0.3] and so 
if you [0.4] get [0.3] radiation effects in water [0.3] these will [0.2] 
transmit themselves [0.5] to the building blocks [0.3] of your cells [0.2] okay 
but [0.3] that's for a later lecture but i mention it now [0.5] this picture [0.
2] carries all the way through this notion of scavenging [1.1] now [0.2] i've 
talked about the [1.1] capturing the n-, the [0.3] electrons [0.8] i've talked 
about capturing the excited states [0.7] the last thing i want to talk about is 
capturing the positive ions [0.3] how do we know [0.5] well well we there must 
be positive ions there 'cause we see the negative ions [0.4] and you can't have 
negative ions without positive ions but it would be nice to see them as well [0.
5] and what you can do [0.5] is this [1.8] you can take [1.5] a [1.2] a 
molecule [1.9] which we know from electrochemistry [1.4] is very easily 
oxidized [0.8] to give [2.1] a highly coloured [0.3] species [0.8] and a nice 
example [0.6] is [1.2] triphenylamine [0.3] nitrogen 
with three phenol groups on [0.7] this is a pale [0.4] pale yellow material [0.
5] crystalline [0.5] if you dissolve it in [0.5] cyclohexane [0.4] and submit 
it to pulse radiolysis [0.5] you get a brilliant colour [0.8] just for a short 
time [0.5] but you can measure that [0.3] and it's known [0.5] from 
electrochemical oxidation [0.2] of triphenylamine [0.7] so [0.2] er i-, er it's 
a known species [0.3] the extinction coefficient's known as well [0.5] and 
what's happening [0.4] is you're getting this this er [0.5] you're getting C-
six-H-twelve-plus [0.4] with an odd little [0.2] little radical dot [0.8] 
formed that's your solvent cation radical [0.8] this is your [0.3] amine [0.9] 
this gives up an electron far more readily than this does [0.2] so the electron 
[0.3] passes from here [0.4] onto there to neutralize it and give you back the 
solvent [0.6] this becomes [0.8] the cation [0.8] and you can measure these 
cations [0.6] er [0.7] by pulse radiolysis that's what er P-R stands for [0.5] 
at room temperature [0.4] or you can do it at liquid nitrogen temperature as 
well [0.3] and you get the same coloration there it's a blue-green colour [0.9] 
so this is called a positive ion scavenger [0.4] and 
it's been chosen [0.3] because it's got a very low ionization energy [0.3] and 
it's marvellous [0.3] at er picking up positive ions in that sort of way [1.2] 
okay [0.4] so those are [0.9] all [1.4] experiments [0.7] either at low 
temperature [0.7] in some cases [0.5] matrix isolation as i've called it before 
[0.5] or room temperature [0.8] to measure [0.6] cations [0.4] here [0.5] 
excited states there [0.2] and electrons on the previous overhead [0.6] so that 
is how [0.3] we actually get at [0.5] the [0.2] the yields o-, of materials [2.
1] now [0.5] er [2.5] i'm going to skip [0.2] the scavenger equation 'cause i'm 
going to come back to that later on [1.3] what i'd like to do now [0.2] is to 
say a little bit about water [1.1] water is probably the most important system 
looked at [0.5] mainly because as i said [0.4] we are [0.6] we consist largely 
of water [0.8] so there's huge interest [0.3] there must must be over a 
thousand papers on the radiation chemistry of water and probably half a dozen 
books as well [1.0] er [0.3] what happens [0.2] with water [1.2] well [0.2] in 
a way it's quite interesting [0.5] if you take [0.4] a vessel [1.1] and you fi-,
you fill it almost to the top [1.4] put a stopper in [2.6] er [0.3] 
take this vessel of pure water completely full [2.5] and if you gamma irradiate 
it [1.8] for days [1.9] and then you do a a big analysis of it [1.2] what 
happens [0.4] and the answer is [1.3] almost nothing [1.1] water appears [0.2] 
to survive [1.3] radiolysis [0.6] well that's a very strange sort of thing with 
hydrocarbons [0.6] you get all sorts of products you get dimers you get 
hydrogen you get goodness knows what [0.4] with methanol you get ethylene 
glycol [0.6] plus hydrogen [0.5] but water [0.6] is extraordinary [0.5] it's a-,
it appears to be amazingly radiation resistant [1.1] and so [0.2] for quite a 
while [0.8] certainly [0.2] in in the earlier parts in the earlier y-, y-, 
years of this century [0.5] people thought water was very boring [0.5] because 
it didn't seem to do anything [0.7] er [0.9] but then [0.7] people got used to 
got used to this this idea of using scavengers and they began to look at water 
[0.6] with [0.2] scavengers [1.2] so they they took water [0.7] and added 
scavengers [0.5] so the first point to note is water on its own [0.4] appears 
[0.5] not to do anything [0.9] but if you put in scavengers [0.4] and the two 
i've chosen 
are iron-two and cerium-four [0.6] iron-two is a reducing agent [0.7] cerium-
four is an oxidizing agent [1.2] what do we think we get in [1.8] well let us 
suppose [0.4] and course i i know i know the answer to this but let us suppose 
[0.2] that water does indeed undergo radiolysis [0.5] to give an oxidizing 
species [0.5] and a reducing species [0.5] but let's also suppose [0.3] that 
they are extremely good [0.5] at going back again to water [0.9] right [1.6] 
let's [0.4] look at it in more detail now what [0.2] what [0.5] from the 
experience with cyclohexane and benzene what mi-, might we expect to see [1.0] 
well water [0.4] we might have thought would ionize [0.3] so we write H-two-O-
plus and E-minus [0.7] and H-two-O-star [0.8] that would be a very good 
starting point [0.4] what happens to those things [0.8] well the first thing 
that happens [0.9] er [0.8] and i want to concentrate on this side of the 
picture [0.4] so perhaps i'll [0.5] cover that up for the moment [1.1] H-two-O-
plus is known in mass spectrometry you can you can ionize water in a mass 
spectrometer [0.5] and what's known is a very very fast reaction [0.3] between 
H-two-O-plus [0.2] and 
water [0.3] to give H-three-O-plus and O-H radical [1.4] so [0.2] we might have 
thought well [0.2] perhaps we're getting some O-H radical [2.5] what about the 
electrons [0.6] well [0.8] er [0.7] an electron [1.3] the kind of electron that 
goes along a wire in a torch or in a T-V set [0.5] or to this overhead 
projector [0.5] if you put an electron in water [1.2] it becomes solvated [2.7] 
i don't know what you know about [0.2] solutions of sodium in ammonia or 
potassium in ammonia but if you take ammonia [0.5] and you dissolve potassium 
or sodium [0.4] have you done that have you done an experiment in the lab [1.1] 
with [0.2] sodium in ammonia have you ever had liquid ammonia as a reagent [1.
9] maybe not [0.8] well if you take ammonia [0.8] it's a liquid [0.6] it er 
boils at minus-thirty-three but it's got quite a high [1.0] latent heat of 
vaporization so it s-, hangs around for quite a while before it all evaporates 
[0.3] it's around for really quite a long time [0.6] if you put sodium in 
ammonia [0.2] it dissolves and gives you a deep blue solution [1.1] and these 
deep blue solutions are p-, are p-, highly magnetic [0.7] and basically you've 
got N-A-plus [1.0] 
and free electrons [0.4] in the ammonia [0.9] and those free electrons are 
solvated by the ammonia molecule which has got an electron [0.2] with say six 
ammonias round it [0.4] well here [0.3] you got an electron with six waters 
round it that's called the aquated electron or the hydrated electron [0.6] okay 
[0.5] and you can s-, you can see that that is there [0.2] in pulse radiolysis 
[0.6] if you just pulse radiolyse pure water [0.5] you get [0.2] a band at 
seven-hundred [0.5] and this thing [0.5] of course [0.4] i mentioned before [0.
5] in the photochemistry section of the course [0.2] if you flash photolyse [0.
8] iodide ion [0.8] or ferrocyanide ion [0.3] again you get this broad band [0.
4] peaking at seven-hundred [0.2] that's right out in the red so it's a blue 
colour [1.2] you see this [0.3] and you do indeed get electrons [0.5] the O-H 
radicals [0.7] well [0.7] i mentioned cerium-four and iron-three [1.4] you take 
water [0.4] and radiolyse it with iron-two there [1.0] the iron-two becomes 
iron-three [1.4] if you take water with cerium-four there [0.8] the cerium-four 
becomes reduced to cerium-three [1.5] if we put these two 
scavengers in [1.6] we get oxidation [0.5] of this [0.2] and reduction of that 
[0.6] that was the first hint [0.3] that there were oxidizing species in water 
[0.7] these are very classical type experiments using those things [0.8] e-, 
but in the pulse radiolysis [0.5] we can do this [0.3] and we can see the O-H 
radicals [0.5] here [0.3] which are formed in that step there [0.5] they attack 
various things [0.3] they attack [0.5] benzene [0.2] s-, [0.2] add on to 
benzene to give this thing [0.5] and that has got a known optical absorption 
spectrum this is you can s-, you can make this thing photochemically [0.6] so 
we we know the o-, O-H is adding to benzene [0.4] to give this [0.5] but we 
also know that O-H if we have a different experiment [0.6] it attacks [0.2] 
various things [0.4] and if we have [0.2] sodium thiocyanate [0.8] er [0.4] N-A-
[0.2] S-C-N- [0.5] minus it's a scavenger [1.0] then [0.4] we get electron 
transfer reaction electron transfer [0.4] we get S-C-N-dot in other words the O-
H pulls the electron off there [0.3] to give O-H-minus hydroxide [0.4] which 
then reproteinates to water [0.7] meanwhile this has formed S-C-N [0.4] and 
this actually loves [0.2] to add to an S-C-N-minus 
to give this thing [0.5] and that is very highly coloured [0.3] and you can see 
it in pulse radiolysis [0.9] so in the pulse radiolysis experiment [0.2] 
without the scavenger [0.2] you get this [0.7] with the scavenger [0.3] you see 
this [0.5] or you see that [0.8] and so [0.5] although water [0.4] appears [0.
6] not to give any [0.2] reaction to radiation [0.4] what's happening is you 
are getting [0.4] yields of oxidizing species and reducing species [0.5] the 
oxidizing species [1.2] is in fact O-H [0.6] and the reducing species [0.4] is 
in fact E-minus [0.5] and so thi-, this is basically [0.3] the guts of water 
radiolysis [0.4] i put H-two-O-star there and excited state of water [0.6] are 
they very important [0.7] well [0.5] surprisingly [0.2] perhaps [0.5] they're 
not [0.6] er you might have thought there might be a reasonable yield [0.7] but 
in fact [2.3] when you [0.5] radiolyse water [1.5] you [0.7] form these two in 
the spur [0.9] and there's a big difference between water and hexane [0.5] in 
hexane [0.2] if you've got two charges a few angstroms away [1.6] they have an 
enormous coulombic attraction [0.7] because the cou-, the attraction between 
two charges is given by Coulomb's law [0.3] 
Q-one-Q-two [0.3] over epsilon-R [0.3] squared [1.8] and [1.3] Q-, Q-, Q-one 
and Q-two are both one [1.0] it's the epsilon that counts the dielectric 
constant [0.8] the dielectric constant or electric permittivity [0.5] of hexane 
is about two [1.0] has any-, does anyone have any idea what it is for water [0.
4] the dielectric constant of water or electric permittivity of water [1.0] any 
[0.3] any feel for that [1.1] well it water's a highly dielectric medium [0.3] 
and it the figure's about eighty [0.8] so the [0.2] force between the positive 
and neg-, negative charges [1.0] in hexane [0.3] is enormous [0.3] because [0.
4] on the bottom line you've got epsilon being two [0.3] but in water [0.3] 
epsilon's eighty [0.4] so the coulombic attraction is forty times weaker [0.3] 
so there's a much better chance they'll get away [0.5] and not recombine [0.5] 
to give excited states [0.6] and indeed [0.5] the excited states of water [0.6] 
er [0.4] are purely dissociative if you excite water [0.8] er with a very deep 
U-V quantum [1.8] you get excited water [0.3] but it breaks into [0.3] hydrogen 
atoms and O-H radicals [0.2] and so [0.2] you do not have stable excited states 
of water [0.3] if they are 
formed at all [0.2] they dissociate [0.2] to O-H [0.2] and hydrogen [0.3] and 
immediately the hydrogen atom [0.5] will form an electron [0.3] because if you 
think about it [4.1] the solvated electron [0.7] will with react with a proton 
[0.5] to give you [2.0] a hydrogen atom [0.9] so you can convert electrons to 
hydrogen atoms that's the H atom [2.2] that's the proton [1.1] and that's the 
solvated electron [2.6] and so [0.3] these are the three very very simple 
species [0.3] but you always knew [0.5] that the hydrogen atom [0.2] was a 
proton plus an electron [0.3] so it's not surprising [0.4] that in water [0.9] 
you actually get the hydrogen atom dissociating into those two bits [0.6] or 
they can recombine to give that [0.2] and you've actually got [0.3] there is a 
P-K-A [0.4] the hydrogen atom [0.2] it's a lit-, it's a little tiny acid all in 
its own right [0.8] well on that amazing thought [0.5] i'll [0.2] i'll bring 
things to an end today [0.5] so we've talked about [1.2] radiolysis [0.3] using 
scavengers of organic systems [0.3] and i've finished up by talking a bit about 
water [0.5] the big difference is the organic systems [0.2] have excited states 
that's a very important part of their radiolysis [0.8] water [0.5] is entirely 
[0.4] ionization [0.3] to give [0.2] O-H-dot [0.4] and E-minus those are the 
two principal species [0.3] and they dominate radiation biology [0.9] okay