Probing Electron Correlation on the Attosecond Timescale

Sammanfattning: Popular Abstract in English Imagine you are sunbathing on a sandy beach, while reading the latest Dan Brown book. Once the book is finished, you look at the waves and notice that in one place, they decelerate and become large. Excited by your reading, you imagine that beneath those larger waves there is a ship containing a treasure in it (most likely sunk by the Illuminati during World War II). But, since you are rather tired from your reading, and you do not want to go into the water, you first try to figure out if your idea makes sense. So you start to estimate the size of the ship by looking at where the waves start to grow, and at how much larger they become. Finally, in a last attempt, you try to solve the relevant hydrodynamic equations, but fall asleep. Since the advent of synchrotron light sources, scientists have tried to do something similar, but replacing the ship by an atom or molecule, the waves by an electron wave packet, and the treasure by a published article. One difference though, is that the electron (wave) does not come from far away but is created by the atom itself, when it absorbs a photon. This is the photoelectric effect. Another difference is that we cannot "see" the electrons, when they are directly on top of the atom, but only far away, once they have reached the detector. Luckily, scientists have three tools which are helpful when it comes to extracting information about the atom. First, the frequency of the light (the separation between the wave crests) can be changed more or less easily. Second, the angular distribution (i.e. the direction of the waves.) may also be controlled. The third and last tool has emerged with the recent development of attosecond science, and is the temporal investigation of electron wave packets. Unlike on the beach where the waves may be timed with a watch on your wrist, there is no clock fast enough to see an electron moving. Instead, the phase of the wave is measured (i.e. how the wave's peak is offset with respect to another, unperturbed wave). One way to measure the phase is to perform interferometry. This adds the wave we are interested in to another, unperturbed, wave. The addition of these two waves results in a new wave, whose size depends on the peak positions of the two waves (i.e. their phase). For example, if the peaks are at the same place, then the resulting wave will be twice as big. If the peaks of one wave correspond to the troughs of the other wave, then when adding the two waves they will cancel out. So, by measuring the size of this new compound wave we can work out the original phase.