字幕列表 影片播放 列印英文字幕 We are at the [diamond] [light] source just outside Oxford in England [we're] here for the next week or so [to] do an experiment and we are standing in the synchrotron right now You can see it all around [I] Think that [one] is a particle accelerator Where electrons are circulated around put in a roughly circular or a pretty well circular trajectory? Very very very close to the speed of light. We are not so much interested on like Particle physicists We're not so much interested in the particles of themselves what we are interested in for our experiments is the fact that when you take a charged particle like an electron and Circulate it very very close to the speed [of] light or accelerate it very close to speed of light What it does is it gives off? What's called synchrotron radiation. So this is radiation that goes all the way from infrared up to very very hard very very [high-energy] X-Rays and we as condensed Matter physicists solid-state physicist nano scientists Whatever you want to call us what we are most interested in is harnessing that light Shining it onto a sample and learning about what the atoms and molecules And that sample are doing we walk around is there's a sort of hissing noise And the hissing noise comes from nitrogen gas now because of this very bright light that we have it's very very focused But it can get quite hot so we are standing right above. What is called Beamline eyes zero name What's a beam line? Well? You have the electrons circulating around. How do you get the light out well? What happens is you have tubes funnels through which the light travels and also as you can see Us going off into the distance you might be able to see this in terms of just coming off tangentially to the to the ring We're on the surfaces and interfaces line just here just down here and we will be collecting that light Coming from those electrons and shining it onto our sample down in this experimental hatch down here So to get beam time you have to like anything in science you have to write a grant proposal you have to get the time you have to get the funding for the work and We wrote that proposal quite some time ago almost a year ago now So we were hugely excited when we got the time and they were hugely excited to be here to actually be able to exploit this Synchrotron radiation So what we're looking at here is obviously we need to have optics to get this light down But this isn't optics these are X-Ray optics And we need to be able to get those x-rays onto our sample to focus them down the synchrotron is about 10 billion times Brighter than the sun So this is an immense engineering challenge with very very clever people doing this work To ensure that we can get that those very bright x-Rays and get them on to our sample so that [the] Tube you can see Coming down here is actually the tube that is carrying the beam [to] [our] to our chamber, and it's also a high vacuum So it's a pressure comparable to that you get on the surface of the moon the electrons are not [allowed] to the electrons are basically Circulating away over there what [we're] what is coming down This Tube is the light generated by those electrons, and that's going to come into or similarly ultra high vacuum chamber We're going to have those x-rays and they're going to impinge on our sample And then we're going to find out wonderful things about our particular molecule We're in the experimental hochberg This is where we actually do the experiments and the light actually comes in through this Tube here And then we're going to chase out through the tumors ETc some more optics some very tips of Diagnosis, and then we're coming all along all along Into our experimental chamber here. It's an ultra high vacuum Chamber [Stainless-steel] Pressure in here very very low again comparable to the surface of the moon and our sample sits in there So the beam is coming into our chamber, and it's going to the beam is going to hit our sample And it's going to eject the electrons out and we want to measure the energies at which those electrons come out It's actually something called the photoelectric effect which it wasn't for a relativity that Einstein won. His nobel prize It was actually for the photoelectric effect something called photon emission that's what we're doing here one part just one part of what we're doing here photons in, electrons out and Every material has [its] own signature in terms of its photo emission spectrum, so this is our electron energy analyzer this wonderful hemispherical analyzer and it's literally called a hemispherical analyzer and The way we measured the energies [of] the electrons is that we have a couple of spheres in here, and we put a voltage on those spheres and then what we do is we [clear] off the centripetal Force on the electrons against that that voltage so we play off the centripetal force against the electrostatic force and only those electrons that I've got the right and Kinetic energy the right speed are actually going to make it through and then we vary the voltages and we can collect a spectrum that way Precisely, that's exactly what it does. It's an energy pencil. So it it allows us to detect how many electrons We've got of a certain energy all these different arms you can see It's also allowing us to move samples around in vacuum without Breaking the vacuum and so we can use these arms to move the sample around What they're doing over here? What Sam's doing at the moment is sorting out? The the important aspects that this experiment [let] me tell you about the experiment We're going to do so this is the [molecule] were interested, and it's a absolutely fascinating molecule It's a [C60] it's [a] what's called a Buckminsterfullerene We've got a single water molecule, and this is made. We don't make this we're not half clever enough We are physicist we are not clever enough to do this. This is made by our colleagues in Southampton They do something called molecular surgery they open up a hole in the cage they drop a water molecule in and then they seal up the hole just with wet chemistry it's Phenomenal the water molecule is trapped within the kids. It's in kids its incarcerated within within this molecule Why is that interesting [well] first of all you've got a single water molecule not interacting with any other water molecules [this] is not something you find very often in nature the C60 is about a nanometer across once you confine Molecules once you confine atoms and once you confine particles to small spaces That's when you start to see very interesting quantum effects What we're really interested [in] is can we exploit those effects when we take this molecule and put it down onto a surface And the first question to ask is when you put it down onto a surface and the cage bonds to the surface does the molecule Inside does the water molecule feel the effect of the surface Or is this just like a faraday cage does it completely screen it out So that [the] water molecule doesn't see the surface there are a number of different things We do both up the first is to exploit this photoelectric effect this photo emission effect photons come in We excite electrons out [of] the oxygen and we can look at the spectrum [of] those Electrons to work out, what's happening with the water molecule? Where do we get those photons? Well we get them from the [beamline] why do we need the beamline and why is a synchrotron? Why don't we just do this with an X-Ray source back in Nottingham? Why don't we do [it] in a lab the great thing about a synchrotron? Is that you can tune the photon energy? across a very wide range in this case from hundred Electron volts of the killer Electron volts that's a Remarkable tool because we can tune in on [particular] parts of the spectrum and work out what's going on the challenge of this experiment? it's a really fun experiment to do is we take a silver surface which everybody is busily preparing at the moment and We're going to put our molecules down onto that surface the first challenge is how do we get the molecules down into the surface? well what we do fortunately would [C60] we can take it as a powder, and that's how we guess we get about a milligram of these molecules at the black powder we put it into an oven it's literally an oven and We [bolt] [it] [onto] the chamber We heat it up and the molecules very nicely Sublime they go directly from the solid phase to the gas phase and they stream off as a beam of molecules They hit the surface and they stick and if we do that in just the right way It's [taking] us a bit of time to work out just What that [right] way is what if we do it in just the right [way]? Then you can get a nice ordered film an ordered mono layer a single layer of molecules on the surface Then we take a beam of photons They come in and we look at the electrons of a given out by the water we also interested actually in the kids itself So we also look at the electrons coming out from the carbon That's the first stage of the experiment, but [actually] the technique. [we'll] use in here in parallel And a real advantage of this particular beam line because it can do this technique. It's something called X-Ray standing with analysis and that's That is a fascinating technique and here's where my lack of a crystal comes into play what we do is. We have a crystal There's [a] surface of a crystal. We have planes of atoms What we have here is our beam coming in now if we tune our photon energy of [our] beam To something called the Bragg Condition right what that means is if we get the wavelength of our beam? So it's the right is the same size as the spacing of the crystals well half of the crystal planes What will happen is that the beam will get diffracted? So we have a beam coming in here, and we have a beam diffracted which comes back out and when those two beams interfere What happens is we get to get something which is called a standing wave it's pretty well exactly what happens on a guitar string you get a standing wave [set] [up] [on] a guitar string in terms of the interference of [travelling] waves on a guitar string and Now the great [thing] is if we have our molecule at the surface We have this standing wave field which has got a periodicity It's a weird so it's got a periodicity the molecule bears in this Wave that's the best way of thinking about it. You've got the molecule Which is sitting in this wave field and if you change the energy of the incoming beam just a little bit what you can do is you can tune the Maxima and minima you can shift them back and forth [and] By shifting them back and forth you shift them back and Forth through the oxygen And then what happens is if you look at the electrons coming out from the oxygen you will see a characteristic profile depending on where the Water molecule is sitting with respect to the surface planes the reason we're doing all [of] this is to find out where in the kids The molecule is sitting and moreover We're not even restricted to those planes what we can do is you can rotate our sample and we can triangulate? the position after we do the same thing again and Triangulate the position of a single molecule within the cage we have done some preliminary work [were] fairly confident It's going to be sitting fairly close to the key to the center of the cage which is Relatively surprising because when these molecules when the fullerene molecules go down onto the surface, what happens is was charged from the silver surface Electrons go into the molecule bonds are quite strong bond and yet the water just sits in the middle of the kids [well] I don't care what seemed seemingly does it seems to be completely screened from its environment Which in terms of actually being able to exploit this effect to be able to look at those? That water molecule explode in a device for example is really [quite] interesting Good luck Tim [laughing] they are going to turn on the beam we will be fried we will absolutely be fried You're looking into the chamber absolutely so you're looking into the preparation one of the preparation chambers this thing coming down with the bellows is a Manipulator that allows us to move the sample up and down. You've got all these various What are called feedthroughs? Which allow you to feed electrical signals in and out from [the] sample for example to heat it for example or to measure the temperature? Of the sample the windows are obviously obviously this is a wonderful invention called a leak valve It allows you to leak in very very small [quantities] of gas And that's what we need to do to prepare our sample. What we do is We bombarded with argon ions business is happening here in [terms] of the measurement is happening and over here in these chambers over here That's where the beam yeah in terms of where the beam hits the sample and here is going to our analyzer This large cylindrical Tube. This is [our] analyzer Thank you for 10 weeks [I] think I guess yeah by 10 weeks since we did the beam time so we've done some analysis And when I say we I'm using the royal way, I must admit. I have not been heavily involved with the analysis. It's been driven
B1 中級 物理實驗的解剖--60個符號 (Anatomy of a Physics Experiment - Sixty Symbols) 4 0 林宜悉 發佈於 2021 年 01 月 14 日 更多分享 分享 收藏 回報 影片單字