Chapter 5

Chapter 3 and paper I discuss laser­assisted photoionization. Making use of the angular resolution of the VMIS, we investigated the interference between electrons released via different angular momentum channels from the 2p6 ground state of neon. Using a fitting algorithm, we characterized the amplitude and phase of the single­photon matrix elements of each angular momentum channel and thus the full ionization process. The difference in phase between the channels as well as the energy dependence of the extracted values is well reproduced by calculations based on angular­channel­resolved many­body perturbation theory. The presented method paves the way for further experiments, as its approach is universal and can be applied to other atomic systems. When it comes to molecules, the effect of the cc­transition has to be considered differently, as the electron in the continuum encounters a more complex potential. Nevertheless, angular­resolved RABBIT will provide novel insights into molecular dynamics on attosecond time scales.

During the analysis of the angle­resolved RABBIT experiments in neon we came across an interesting feature in the photoelectron spectrum shown in figure 5.1.

Within the peak corresponding to photoelectrons produced by the absorption of the 15thharmonic, a sub­structure can be identified. Mostly along the polarization axis, three sub­peaks are visible in the momentum map, as can be seen in figure 5.1 (a). As the 15thharmonic is the lowest order to overcome the ionization energy of neon, the 13th harmonic may excite the ground state electron to a resonance close to threshold, which is then ionized by absorption of two infrared photons.

This electron carries the same kinetic energy as that due to absorption of the 15th

0 px [arb. u.]

0 p y [arb. u.]

1 2 3

Kinetic energy [eV] 0

0.5 1 1.5 2

Intensity [arb. u.]

0 45 90 135 180

Emission angle [deg]

0 1 2

Delay [fs]

(a) (b)

(c)

SB 16

Figure 5.1: Sub-structures seen in the photoelectron spectrum for electrons produced by the absorption of the 15thharmonic. (a) shows the electron momentum map and (b) the angle-integrated photoelectron spectrum. In (c) the energy integrated range around 1 eV is shown as a function of angle and delay.

Figure 5.2: Ion kinetic energy distribution of protons resulting from the dissociation of adamantane after XUV induced ionization as a function of the delay.

harmonic, which means they interfere. The structure seen in the photoelectron spectrum can be caused by such an interference. Further analysis indicates that in the delay dependent angular distribution, shown in figure 5.1(c), oscillations at 45°

appear, out of phase with the oscillations at 90°. We are currently investigating these results further aiming to extract information allowing us to study resonant photoionization.

Lastly, the results of papers Iv and vI­vIII on the dissociation of organic molecules are introduced in chapter 4. Paper Iv discusses experimental results of the dia­

mondoid adamantane performed at the IXB, together with molecular dynamics calculations. The different fragmentation dynamics are identified using covariance analysis techniques. The dissociation of the doubly charged parent ion is followed by a Coulomb explosion process, leading predominantly to the C2H5and C8H11

fragmentation channels. Preliminary results are shown from an infrared­XUV pump­probe experiment, indicating time scales on the sub 100 fs scale. Looking ahead, we are currently working on the analysis of an XUV­XUV pump­probe ex­

periment performed at the FL26 at FLASH at DESY. The first preliminary results show time­dependent signals in many of the fragmentation channels. Addition­

ally, we see a shift of the momentum distribution in the protons, as shown in figure 5.2. The interpretation of the results is however still ongoing.

Papers vI and vII introduce results of time­resolved experiments in organic molecules performed at the FELs FLASH at DESY in Hamburg, Germany, and SACLA at the Spring8 facility in Sayo, Japan. Using infrared­XUV/x­ray pump­probe tech­

niques, the fragmentation landscape was studied. For the PAHs discussed in paper

vI and thiophene described in paper vII, the double and triple ionization of the parent molecule lead to a dissociation within hundreds of femtoseconds. Based on the results presented in paper vI a subsequent beamtime was granted and per­

formed in August 2021 with the aim to investigate the observed dynamics in further detail and extend the experiments to other PAHs.

On the way towards pump­probe experiments using intense XUV pulses on the attosecond time scale, this thesis provides a promising outlook for both HHG and FEL sources. However, technical advances are required in order to perform

”pure” experiments. For HHG, the attosecond resolution is realized, as shown in chapter 2.23, but most applications rely on techniques like RABBIT for which assumptions are required in order to extract information about the underlying physics. This works well for atomic systems, as in the case of neon presented in chapter 3, but is more complex for molecules and solids. For FELs on the other hand, the temporal resolution prevents the detection of ultrafast processes, like pre­dissociative charge migration. All in all however, HHG and FELs are on the way to delivering the required technical advances. The Extreme Light Infrastruc­

ture Attosecond Light Pulse Source (ELI­ALPS) facility in Szeged, Hungary and will take HHG on another level [112]. In particular the ”long” beamline with 55 m focusing and a 6 m gas cell will generate intese high­order harmonics on an un­

precedented scale. At FELs on the other hand, tunable isolated attosecond X­ray pulses were recently generated using at the Linac Coherent Light Source at the SLAC institute [27]. This gives a promising outlook towards experiments using XUV­XUV pump­probe techniques. Performing such experiments, in combina­

tion with advanced photoelectron and ­ion detection schemes, is the aim for these

”next­generation” ultrafast time­resolved experiments and will shine new light on the fundamental processes behind chemical and biological transformations.

In document Atomic and Molecular Dynamics Probed by Intense Extreme Ultraviolet Attosecond Pulses Peschel, Jasper (Page 104-108)