On the intertubes today I’m seeing a lot of references to “Electron filmed for the first time” (digg, msnbc, Live Science.) For a decent explanation that doesn’t involve radically distorting quantum theory, I recommend this Physical Review Focus article (and, of course, nothing compares to the original PRL…although it must be said, as always, that four pages is not enough, damnit!) Note that, if I understand correctly, the movie “filmed” above is a movie in “momentum space” and, of course, we’re not really talking about the observation of a single electron, but of the momentum distribution of the coherent electron wave packets. But what really confuses me is what the actual “movie” is supposed to represent.
Okay so what is this paper about? Now I’m not a chemist, so my picture may well be off, but I think I get the jist of what is going on. Roughly what we are talking about is taking an atom, ionizing it, and then pushing the coherent electron beam created during the ionization up against the resulting ion. In previous experiments, it seems that what groups have done is to use a strong laser beam to ionize electrons and then use the same laser beam to push the electron wave packet up against the atom. Here the authors decouple the process of creating the coherent electron wave packet and accelerating it. As I understand it, an attosecond XUV pulse is used to ionize the atom and create the electron wave packet and then a IR laser is used then used to guide this electron wave packet. Whereas in previous experiments, since the same laser was used to both ionize and then move the electrons, the process of accelerating the electrons back was quite imprecise (the electron wave packets were created across a range of different conditions.) By decoupling the ionization and the acceleration the authors of this paper have been able to create less violent and well controlled electron scattering events.
So my sketch picture of what one of these experiments looks like is as follows. Attosecond pulse comes along. Knocks electrons out of the atom. IR laser is around, so exactly when the attosecond pulse creates the electron wave packet determines exactly what electromagnetic field the electrons will see. Electrons then move in this field, and scatter off of the now ionized atom. The whole thing is done in an electric field such that the resulting electrons will then drift toward a detector. The electron wave packets drift and disperse as they fly toward a detector. Repeated versions of this experiment give a good “picture” in momentum space of the electron wave packet intensity. By varying the IR laser pulse phase, you can change the momentum distribution of the electron wavepacket which is created by the attosecond pulse. Thus, similar to a stroboscope, where you flash light at a frequency which appears to freeze a periodic motion, by varying when the attosecond pulse creates the electron wave packet, you basically create the same initial electron wave packet. Repeated application of this stroboscopic measurement at different IR laser phases then can be used to put together a movie.
Okay so here is where I get confused. It’s not exactly clear to me what I should be tacking from this movie. Disregarding the fact that it is a movie in momentum space of the electron wave packet, this really isn’t a movie of the dynamics of an electron being scattered off the ion, is it? Really it is a snapshot of the electron wave packet being created at different delays from the attosecond pulse, and hence of different ways in which the IR laser and ion scattering influences the subsequent electron dynamics. Am I missing something here? Any chemist or physicist care to clear things up for this thoroughly confused computer scientist?