Probing Electron Correlation on the Attosecond Timescale

• This thesis describes how photoemission stimulated by an attosecond pulse train (APT) can be used to extract information on electron correlation in simple quantum systems such as atoms. The emission of the electron by an APT induces a reorganization of the electrons remaining in the ion core. This reorganization causes a change in the trajectory of the emitted electron. An infrared (IR) field is used to probe the delay induced by the ion's potential and the electron's reorganization via an interferometric technique. This thesis focuses on how the delay can be measured in various atomic systems and on how physical information about the electron correlations may be extracted. The first chapter of the thesis presents a brief overview of the attosecond techniques which have been used. It describes how the APTs are generated via a non-linear process called High Order Harmonic Generation (HHG), and how these pulses are characterized temporally using the so-called RABITT technique (reconstruction of attosecond beating by interference in a two-photon transitions). Finally, the different parts of the experimental set-up are described: the laser system, the APT generation chamber and the different detectors. The second part focuses on a theoretical description of the photo-ionization process. The delays measured in the RABITT technique are derived and interpreted using perturbation theory. The influence of electron correlation on the delay is then investigated in the case of a Fano resonance and in double photoionization. The third chapter describes experimental results obtained in various atomic systems. A comparison is made between the photoemission delays from the outer valence shells of argon, neon, and helium; between the inner and outer valence shells of argon; between the on-resonance and off-resonance delays for argon levels interacting in a Fano resonance; and between the delay induced by single and double photoionization in xenon. The experimental results are compared with calculations using several different atomic codes.

• 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.

Subject headings

NATURVETENSKAP  -- Fysik -- Atom- och molekylfysik och optik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Atom and Molecular Physics and Optics (hsv//eng)

Keyword

Fysicumarkivet A:2014:Guénot

Publication and Content Type

dok (subject category)

vet (subject category)