Time­resolved Fragmentation Dynamics studied with FELs

Pump­probe studies have also been performed at the FELs FLASH at DESY in Hamburg, Germany, and SACLA at the Spring8 facility in Sayo, Japan. Compared with HHG­based sources, FELs have the advantage of tunable wavelengths and higher amounts of photons per pulse. In particular, using photons in the x­ray regime enables probing of inner­shell electrons, rendering it possible to target the pump or probe only at specific atoms within the molecule.

Papers vI and vII present time­resoled results using an XUV/x­ray photon from an FEL and an infrared/UV photon from an optical laser. The studies were conducted on carbon­based molecules, namely three different polycyclic aromatic hydrocar­

bons (PAH) in paper vI and thiophene in paper vII. The authors contribution to these papers was mainly experimental, which is why this chapter, after a short introduction to FELs, only presents a summary of selected results.

4.2.1 Free­Electron Lasers

A Free­Electron Laser (FEL) is a source of photons emitted by electrons accel­

erated to relativistic velocities passing through an alternating magnetic field. The electrons are released into a vacuum and picked up by a linear accelerator, in which they reach kinetic energies up to several GeV. The relativistic electrons enter a set of opposing magnets, called undulators, with alternating magnetic fields, as illus­

trated in figure 4.9. The Lorentz force causes an oscillating motion of the electrons perpendicular to the magnetic field and their direction of motion. The alternating radial acceleration leads to the emission of synchrotron radiation in a cone directed towards the propagation direction of the electrons. So far, the emitted radiation

Electron accelerator

Undulator period

Photon beam Electron beam

N N N

N N N

S S S

S S S

Figure 4.9: Schematic illustration of the working principle of a FEL. The accelerated electron beam (black arrows) enters a set of undulators and is forced onto an oscillating motion. The radial accel-eration of the electrons produces synchrotron radiation (purple line). Microbunching of the electrons ensures constructive interference of the emitted field.

is created with a random phase. However, after a certain propagation distance through the undulators, the interaction between the generated field and electrons leads to a modulation of the electron density into microbunches, as shown in fig­

ure 4.9. As a consequence, the emitted radiation by the microbunches is in phase, increasing the emitted intensity. This process is called self­amplified spontaneous emission (SASE) [22, 105, 106] and is deployed at most XUV and x­ray FELs. The principle was first employed for photons in the XUV regime at the FEL FLASH facility in 2005 [107, 108]. Generally, FELs are available from the microwave [109]

to the x­ray regime [110], where the wavelength is set by the undulator period.

However, for high­energy photons in the x­ray regime the electron kinetic energy must be high enough, which is why linear accelerators of x­ray FELs are usually over several kilometers long.

4.2.2 Fragmentation Dynamics

Paper vII presents a study of the dissociation of the carbon­based molecule thio­

phene (C4H4S). Using 180 eV photons from the FEL SACLA, electrons were emit­

ted from the 2p orbital of the sulfur atom leading to a subsequent Auger process leaving the molecule in a dicationic state. Hence, the starting point was similar to the experiment described in the previous chapter, however with much higher excitation energies due to the x­ray induced ionization. An 800 nm optical laser monitored the evolution of the dynamics in a pump­probe setup. The resulting ions were detected by a momentum­imaging ion time­of­flight spectrometer with a Roentdek HEX120 detector [111]. Instead of detecting several ionization events per shot like in the previous experiment, only one event was identified, allowing an instant correlation of the fragmentation products. By doing so, it is not only possible to measure the yield of an isolated fragment, but also to instantaneously detect correlated ion pairs formed through the same fragmentation channel. This technique is called coincidence detection and generally requires a high repetition rate and/or long acquisition times.

Figure 4.10 shows the retrieved yield of selected fragmentation pairs as a function of the delay, where negative delays correspond to the infrared pulses arriving before the x­ray pulses. In panel 4.10(a), the yield of the strongest two­body break­up channel is shown, corresponding to C4SH²⁺₄ → CSH⁺+ C₃H⁺₃. A clear drop at

≈ 110 fs is seen with a slow rise at large delays. In 4.10(b), an example of an ion­

pair (C2H⁺₂,S⁺) from the three­body break­up channel C4SH²⁺₄ → C2H⁺_n + S⁺ + C₂H_4−n is presented. A remarkably different behavior can be seen: the yield of the ion pair increases drastically when the two pulses overlap and then quickly decreases. This behavior is more short­lived than the negative variation

-400 0 400 800

-400 0 400 800

-400 0 400 800

Delay [fs] Delay [fs] Delay [fs]

Ion pairs / 1000 events

3.5 4.0 4.5 5.0

3.5 4.0

3.0 3.5

2.5 25

30 35

(a) (b) (c)

Figure 4.10: Delay-dependent yields of different ion-pairs resulting from the dissociation of thiophene.

(a) shows the yield of (CSH⁺, C3H⁺₃) representing a two-body break-up channel. In (b) an example for an individual ion pair (C2H⁺₂,S⁺) resulting from a multibody break-up channel is shown, where (c) shows the yield of all ion pairs from multibody break-up channels. The dashed lines correspond to the yield of the corresponding ion pair upon ionization by only the x-ray pulse. Figure adapted from paper vII.

in the two­body break­up channel, which indicates that the two processes are not induced by the same dynamics. A similar enhancement is present in all ion pairs from multibody break­up channels, for which the combined yield is shown in 4.10(c).

Based on the results of self­consistent charge­density functional tight­binding sim­

ulations performed by our collaborators at the University of Turku, we can explain the experimental findings by a very fast opening of the ring structure of thiophene followed by the formation of a transient linear geometry of the parent dication.

The ring­opening is followed by the two­ and three­body break­up dissociation shown in figure 4.10. The lifetime of the transient linear geometry depends on the internal energy after the x­ray induced ionization and ranges from tens to hun­

dreds of femtoseconds. When the two pulses overlap in time, the effect of the infrared beam is primarily ionizing. Hence, the x­ray induced dicationic state is further transferred to a triply charged state. The primary consequence of this is the enhancement of the multibody break­up seen in figure 4.10.

Paper vI presents a similar study of the fragmentation dynamics of three PAHs, namely fluorene, phenanthrene and pyrene. An equally rich fragmentation land­

scape was detected for all molecules. Interestingly, a similar transient enhancement as in the previous experiment was seen in the time­resolved ion yield of selected fragments. In this study, the effect was also attributed to an enhanced dissociation due to the formation of an unstable triply charged state of the parent ion when the two pulses overlap in time. In addition, the paper determines the lifetime of the excited states of the single charged parent ions.

In general, the break­up into two or more fragments leads to a dynamical transfer of energy between the fragmentation products, which is why the momentum of the resulting ions is an interesting observable. In the next section an example of

Ion momentum [a.u.]

Ions / 106 events

0 50 100 150 200 250 300

0 100 200 300 400 500 600

4.5 eV 2.8 eV

transient off-transient difference

Figure 4.11: Momentum distribution of the S⁺ion for delay ranges outside (blue) and inside (red) the transient region. The red dots correspond to the difference between the two curves together with a fitted Gaussian function. Figure adapted from paper vII.

such a measurement is presented.

4.2.3 Time­resolved Ion Momentum Spectroscopy

The transient enhancement in the multibody fragmentation of thiophene, pre­

sented in the previous section, suggests an impact on the kinematics of the result­

ing ions. The additional charged fragments lead to an increased momentum of individual ions after dissociation. In paper vII, we present results of the ion mo­

mentum distribution of the S⁺ion, which are reproduced in figure 4.11. Shown are the momentum distributions within and outside the delay range of the transient enhancement as well as the difference between them. The first peak is attributed to O⁺₂ ions from a residual contamination in the chamber. Oxygen has the same mass as sulfur wherefore they cannot be distinguished from S⁺ions.

The main peak lies at 2.8 eV kinetic energy, while the difference curve shows a peak at 4.5 eV kinetic energy. Similarly to the results presented in section 4.1.3, such values can be explained by a Coulomb explosion following the dissociation. Paper vII puts forward results of a simple calculation, indicating that the formation of a triply charged transient linear geometry followed by a multibody break­up leads to S⁺ions of higher kinetic energies (∼ 4.2 eV) than the same process starting from a doubly charged state (∼ 2.2 eV). This supports the interpretation that in the transient delay region, which shows a shift towards higher kinetic energies, the en­

hancement of the multibody break­up is due to the more energetic fragmentation of the triply­charged system.

Chapter 5