Auger Electron Spectroscopy, or AES, is a way to study the surface physics and atomic structure of a material. It is also the type of physics experiment that you would only think of if you happened to be a stupendous badass. (Her name was Lise Meitner and she was among the great geek chicks of all time. Unfortunately, cute German geek chicks couldn't buy love at the store in 1923, so the effect is named after Pierre Auger instead, who came up with it 3 years later.)

AES is like learning about dentistry by examining the aftermath of a high-velocity hockey puck's encounter with a mouth. When electrons are smashed into the surface of a solid, two things are emitted: more electrons and X-rays. This suggests two courses of action when you see electrons smashing into solids: protecting your gonads and measuring the kinetic energy of the electrons that are getting spat out.

An electron that just bullseyed an atom can knock out a core electron, which creates an unstable ionization. Atoms don't like for their core electrons to be holes, so an outer orbital electron drops down to fill the gap. Ah, electrons dropping to lower states -- sounds tranquil, like the clicking of little balls together on wire pendulums, or a pastoral waterfall. Actually, the appropriate mental image, adjusted for scale, would be of a grand piano smashing into the ground. Energy is released, and it has to go somewhere. This is where the X-rays come from, but in other cases, an electron can get spat out.

What good does that do? Who cares? What the hell are we talking about? Each electron in an atom belongs to an orbital with a well defined and discrete energy level, unless it doesn't, but we're ignoring the exceptions right now. By energy level, I am referring to the binding energy as being relative to a vacuum -- meaning that it is the energy it takes to rip the electron completely away from the atom. Because these energy levels are discrete, the kinetic energy of the ejected electron is equal to the binding energy of the core electron minus the total of the binding energy of the ejected electron and the electron that drops to fill the core hole. Got that?

The ultimate goal is to smash tons of electrons into the surface in question while detecting the kinetic energy of electrons that are emitted, creating a graph where the x-axis is energy and the y-axis is number of electrons counted. Spikes on the graph will correspond to energy level differences between orbitals, and can be matched up with AES graphs of the elements to identify their features.

There are problems. But when man is confronted with problems in his electron-smashing scheme, he doesn't give up that easily, no. Like a small child trying to unwrap candy, he goes and solves them.

You are smashing electrons into the surface of an object. What problems would you encounter? Well, first of all, air is going to get in the way. So, we remove it by conducting AES in ultra high vacuum on the order of 10^-10 torr, which involves things like sapphire valves, bell jars, manifold pumps, all necessary when reducing atmospheric pressure to a ten trillionth of its sea-level value. What else? Well, you aren't going to see any electrons coming from any deeper than 5 or 10 nanometers deep into the surface of the object. But this is not really a problem. We can use AES to study surface impurities or surface plasmons or other two dimensional quantum phenomena.

But when it comes to the actual detection, a whole slew of issues arise. Electrons bounce, which means you can pick up electrons that are the same energy as the ones you're firing. Electrons also can lose energy in various ways that are not important here, meaning that an electron that started out with a given energy can end up with a smaller one, confusing your chart. Instead of a nice, clear series of spikes, a typical AES count chart has positive slope leading up to a huge cliff where the elastic peak is, which is the energy of the backscattered electrons. Above this no electrons should really exist. Below this there are a few nooks and crannies, but certainly not a clear picture. But once differentiated, it is much easier to see the relevant features and compare them with prediction and earlier experiment.

Some advice for the novice AES system user: monitor the emission current. This is a measure of how many electrons you are accellerating toward the test surface. You want this to be low, on the order of a microAmp, because otherwise you run the danger of creating an argon lamp that costs as much as a house. This is bad. Also, you want to maintain the test surface at a positive voltage relative to the outside of the bell jar, so that the electrons won't get distracted like a husband wandering through a sports bar on his way to pick up some milk and bread. Not too much, though, because you do want electrons to make it to the detector. A few volts ought to do it.

Armed with this knowledge, along with a Ph. D. in applied solid state physics, you are now prepared to purchase and operate your very own AES system.

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