A Week in the Lab: Just-So Stories
At the end of the last installement, I promised "just-so sotries" to go with the three graphs I posted (one, two, three). So, what's going on here?
The second graph is the most complicated of the three, but it's probably the easiest place to start. This is a plot of the initial amplitude of my curve fits as a function of pressure. That amplitude is basically a measure of how much stuff hits the detector when I don't put any voltage on the screening wires to steer charged particles away. There are two important features of this graph: at high pressure, if you increase the pressure, you increase the signal, and at low pressure, if you increase the pressure you decrease the signal. This suggests that there are two different effects at work here.
The high pressure result is easy to explain: as you increase the pressure, you increase the number of atoms passing through the discharge region. More atoms passing through means more atoms excited to various states, and more atoms ionized, thus, more free electrons to hit the detector.
The lower pressure result is a little trickier, but the key to figuring that part out is that the pressure measured is the pressure in the whole chamber, not just the discharge region. Higher pressure means more stuff between the discharge region and the detector, which means there are more chances for an electron to hit something, get deflected, and not make it to the detector. As you increase the pressure, you decrease the chances that a free electron created in the discharge region will get detected, and thus you decrease the signal.
When you put these two competing (and very non-linear) effects together, you get the "U" shape seen in the graph of amplitude vs. pressure. At high pressure, the sheer number of particles created is large enough that substantial numbers get through, even though there's a lot of gas in the way. At lower pressure, the rapid increase in the distance an electron can move without being deflected makes up for the fact that fewer electrons are being created. The total number is lower, but a larger fraction of them survive to be detected, so the net signal is higher.
The survival effect can also be used to explain the third graph, of the characteristic stopping voltage vs. pressure. Here we see a smooth decrease in stopping voltage as we increase the pressure. That sort of makes sense in terms of electrons suffering collisions as they come across the chamber. At higher pressure, the electrons hit more things as they pass through the chamber, and they end up losing a bunch of energy. The lower-energy electrons are more easily steered away from the detector, so the stopping voltage is lower. At lower pressure, the electrons cruise right across without hitting anything, so it takes a more substantial screening potential to ward them off.
That leaves only the graph of final current vs. pressure to explain. This is a little trickier, but my current theory is this: In addition to producing a lot of electrons, the discharge operation also produces metastable atoms and positive ions. metastables or positive ions striking the detector plate will also cause a current to flow, but you can argue that the current ought to have the opposite sign from that caused by the electrons. Metastables and positive ions are also a little more fragile than the electrons are-- they're bigger, move more slowly, and can be de-excited or neutralized before making it to the detector.
So, the idea is this: The discharge spits out enough stuff that there's always something hitting the detector. At high pressure, this signal is mostly electrons, because they're the only thing that survives across the chamber. As you decrease the pressure, you get more positive ions and metastable atoms surviving the trip, which pulls the final current down, and eventually causes it to change sign. At low pressure and high (negative) screening potential, the signal on the detector should be dominated by positive ions and metastable atoms.
That's the theory, here. The problem I have now is that I don't know how to sort those two out from one another. All I really care about is the metastable atoms, and if I could get rid of the positive ions, that would make my life easier. At the moment, though, I don't have a way of getting rid of both positive ions and electrons, so I'm sort of stuck.
Or, I could be telling the wrong just-so stories about these graphs, and what's going on here could be something completely different. There are a couple of things that hint that this "I'm an idiot" theory might be the right one, but I'll have to do some more tests before I know for sure. Of course, then the question becomes "How much time do I want to spend trying to understand the plasma discharge source, when that's not really the point of this experiment?"
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