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.