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okay great uh thank you to the

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uh organizers for giving me a chance to

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give this talk and to you all for

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watching it uh my name is micah scheibel

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i'm from the georgia institute of

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technology and today i'll be talking

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about some recent measurements where we

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explore a mechanism called spin

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dichroism and its ability to form

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enantiomeric excesses in chiral amino

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acids

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so i'm sure i don't have to tell this

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crowd that chiral amino acids and and

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many other types of chiral molecules

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are uh found in in many different uh

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meteorites as well as in asteroids and

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comets uh and here i'm showing a few

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different amino acids that have been

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identified and in the parentheses is the

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largest measured enantiomeric excess of

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these chiral amino acids so these being

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the l form there's a 20 excess in the l

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over the corresponding d form

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interestingly enough when we look across

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a number of different sources for these

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chiral amino acids

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we see that very often they are

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enhanced towards the l enantiomer so

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here showing the

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measured

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excesses of l-isovalene in a number of

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different meteorite types all showing an

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excess towards the l-form and none

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really showing a statistically

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significant excess towards the d-form

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so the question that we're asking here

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is why do uh seemingly all of these

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these primitive meteorites display the

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same enantiomeric excesses or this the

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enantiomeric excesses of the same

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handedness

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so different forces in space which can

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drive enantioselectivity

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uh include several different abiotic

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mechanisms um the first being sort of

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just a chance or statistical fluctuation

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in the molecular populations which lead

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to an enhancement in one versus the

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other uh now if there's these uh

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excesses and meteorites were forming due

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to a chance mechanism we would expect

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both l and d forms

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uh alternatively amplification uh

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there's lots of different types of

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mechanisms that can drive amplification

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of a given

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chirality but all of these still require

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some initially small excess of one of

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the enantiomers versus the other so

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ultimately what we're searching for to

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explain why all of these meteorites are

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pushed in the same direction is

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something called a determinant force

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where where the excesses are at least

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initiated by some sort of physical force

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that is not dependent on the environment

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um so different determinate mechanisms

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which have been explored that uh

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induce enantiomeric excesses in organic

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molecules uh include first circularly

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polarized light uh where uh light from

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ionizing radiation such as uv light from

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distant stars can obtain a uh circular

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or actually a really unknown elliptical

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polarization as it passes through dust

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clouds on its way towards our solar

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system

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and those circularly polarized photons

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can induce an enantiomeric excess

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alternatively you can get preferential

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bonding or catalysis on chiral surfaces

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uh both of these have been explored and

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in general produce fairly small

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enantiomeric excesses

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a third mechanism which we are exploring

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in the current work is interactions of

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spin polarized electrons with chiral

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molecules uh so the question that we're

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asking is can spin polarized electrons

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create enantiomeric excesses within

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populations of chiral amino acids

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uh so first we'll just do a little bit

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about where we can expect these spin

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polarized electrons to come from um and

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what we're doing is we're

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looking at the ejection of these spin

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polarized electrons from a magnetized

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surface so you can imagine here uh our

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sort of

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schematic of an iron where all of the

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spins within the iron or at least the

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vast majority of them are pointing in

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the same direction

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and so when we have ionizing radiation

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come in it can excite one of those

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electrons which then scatters through

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the solid

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and then can escape the surface as a

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secondary electron

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so it turns out that because of the

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polarizing or the magnetization of that

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surface you have an unequal filling of

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the fermi energy levels

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within the solid and you get more or

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preferential scattering off of the prep

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slightly emptier levels so that leads to

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then a preferential polarization

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direction of the emitted secondary

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electrons from that surface

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um it turns out that a

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spin polarized electron in motion traces

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out a helical path um and that by

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reversing the substrate magnet

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uh magnetization direction we can also

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reverse the helicity of the ejected

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secondary electrons

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so the mechanism that we're exploring

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here spin dichroism consists in coupling

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between that electron helicity and the

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molecular chirality and exploring

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whether or not that coupling can lead to

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enantioselective damage and formation of

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enantiomeric excesses

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so here the idea is that we have some

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sort of ionizing radiation come in in

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these experiments we use x-rays they

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excite secondary electrons from the

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surface which then are ejected out and

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can interact with molecules absorbed

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um depending on the magnetization

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direction of that substrate you can

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either have plus or negative uh

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helicity spin polarized electrons

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and these may interact differently with

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the l or the i r car um

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enantiomers

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so the experimental setup that we

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designed was to first vapor deposit a

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molecule l-histidine uh and we used this

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one because it was well understood in in

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terms of its interactions with surfaces

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and and we knew that we could

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deposit it reliably on the surface

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and then we exposed those films of

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l-histidine to 695 ev soft x-rays uh

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that were generated by the advanced

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photon source synchrotron at argonne

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national lab

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those x-rays then interacted with our

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magnetized cobalt substrate to eject the

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spin polarized electrons

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could interact with and damage the vapor

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deposited l l-histidine

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we can then probe that damage using a

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technique called x-ray photoelectron

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spectroscopy

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and then

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determine how

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that difference in helicity

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affects the damaged cross-sections by

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reversing the magnetization of our

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substrate in situ

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what the xps measurements look like is

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shown here where i focused on the

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nitrogen uh component of our molecules

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nitrogen being uh

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very unique to the analyte molecule as

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all and also um on the chiral center so

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we see that the high resolution spectrum

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of nitrogen can be fit with five

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different peaks

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the first two uh being the largest uh

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are corresponding to the n1 and the n2

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components on the nitro uh the neutral

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histidine molecules while these smaller

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peaks out here the m3 and the n4 peaks

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correspond to nitrogen with on this

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wider ionic species

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um

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it turns out that these winter ionic

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species are important because

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depending on the thickness of our sample

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uh you you see more or less abundances

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of those of those winter ionic species

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um and so it turns out that for a thin

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film or a monolayer film sort of

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corresponding to this uh 0.3 nanometer

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thickness where the the n3 and the n4

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components of the switter ions are

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hardly there

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um you get more strongly interaction

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interactions with the molecules in the

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surface whereas with the thicker samples

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uh those those interactions aren't quite

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as strong

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so that the presence of multiple

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monolayers can weaken the molecular

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interactions between the the molecules

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and the surface

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potentially enhancing any measurement of

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this spindle dichroism in fact

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when we look at the

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abundance or the area of those peaks as

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a function of a radiation fluence or the

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cumulative electron fluence that these

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molecular films have been exposed to

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we see that they are well fit by

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exponential decays

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used to give us uh cross sections

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for the damage or what is the the damage

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cross-section for these molecules with

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the spin polarized electrons

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and so again it turns out that the

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thicker samples because there's more

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molecules there and they're less

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strongly interacting with the surface

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they give better and more precise

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exponential fits which ultimately give

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us better statistics and then we also

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see that it's pretty clear that the

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zwitter ionic species both in the thick

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and in the thinner samples those

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vitorianic species are damaged more

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readily than are the the neutral species

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so when we take measurements of these

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damaged cross sections uh at multiple

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different spots on the same sample so

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same deposition different spot uh we

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take about fifth 10 to 15 different

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spots in both of the magnetization

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conditions

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and then do a statistical comparison for

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those cross sections measured

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in both magnetization conditions and

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look for statistical significantly

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differences

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between those two populations

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and what we see that in fact the total

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cross-section and

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the cross-sections for this wider ionic

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species so strong evidence of spin

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dichroism so that the dependence that of

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the damage on that electron helicity

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for these l histidine molecules um if we

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assume that the the reverse is true for

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the d-histidine molecules which can be

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expected from previous measurements uh

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we get that if we had a racemic mixture

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of l-histidine on the surface

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we would produce actually a 17

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enantiomeric excess

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when only

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forty percent of the original molecular

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population remains uh so these are

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pretty strong effects of of this uh spin

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dichroism that we're showing here

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uh so in conclusion we've shown that uh

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elk hyrule lumentomeric excesses

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have been identified in many pretty much

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meteorites and uh although many

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mechanisms exist to explain these

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excesses

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uh ultimately the the cause of the

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symmetry breaking between y l versus d

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remains an open question

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um these experiments have shown that

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their strong and anti-selective

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preferences measured

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for spin polarized electrons ejected

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from a magnetized substrate

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in their interactions with deposited uh

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l-histidine amino acids uh it sort of

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remains to be seen how the spin

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polarized electron

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um

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molecule these spin polarized electrons

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can create these enantiomeric excesses

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are within different molecular

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populations um

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and and this is largely an unexplored

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mechanism uh within the field so with

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that i'd like to uh thank uh the the

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people who helped me out with this work

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uh tom and cermane georgia tech and

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richard at argonne as well as our

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funding from the doe and nasa post

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doctoral fellowship uh thank you all for