The Low Density Matter beamline (LDM) is devoted to experiments involving atomic, molecular and cluster beams in combination with the photons produced by the FERMI FEL sources. The LDM beamline features a modular end-station accommodating a broad range of detectors and target sources. The combined capabilities of FERMI (high brilliance, short pulse length, variable polarization, coherence), photon transport (variable-focusing optics) and LDM endstation open a wide range of research themes for rarefied matter under extreme irradiation conditions (multiple electronic excitation, multiple ionization, Coulomb explosion, non-linear optics, dichroism, and so on). A split-and-delay line, as well as a synchronized optical laser, allows studying the ultrafast dynamics of the target system upon electronic excitation, or irradiation with a tightly focused FEL pulse. The unique characteristics of FERMI have been heavily exploited in recent experiments performed at LDM.


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Interatomic or Intermolecular Coulombic Decay (ICD)

The chemical environment places a fundamental role on the electronically excited states of atoms, molecules and solids. If the excited atom or molecules is alone, it can decay only radiatively. While putting excited atom or molecule close to the neighborhood of other particles, a new more efficient decay mechanism may occur. Such an electronic decay was first predicted in a pioneering theoretical work by Cederbaum and co-workers and called Interatomic or Intermolecular Coulombic Decay (ICD) [1]. The electronically excited atom can transfer its energy in an extremely efficient way to a neighboring atom which then releases that energy by emission of one of its own outer shell electrons (see Fig.1). Since that time, various experimental and theoretical studies have shown that ICD is a rather common decay mode, widely encountered in loosely bound matter [2]. One of the most remarkable features of ICD is its extremely short lifetime, making it a highly efficient decay process for an excited atom embedded in an environment and, thus, a strong source of low-energy electrons. Therefore ICD is of current interest in attosecond and short wavelength free electron laser sciences [3-5]. In a broader context, ICD bridges the gap between fundamental research on the correlated motion of electrons and nuclei and more applied research, for example, on the use of low kinetic energy electrons in radiation chemistry.
Figure 1. The ICD process in Ne dimer: creation of a 2s hole in a neon dimer by photoionization; successive interatomic Coulombic decay: the 2s hole is filled by a 2p electron, the excess energy is transferred to the neighboring neon atom causing the ejection of one of its 2p electrons; the final state consists of two singly charged Ne+ ions, which undergo a Coulomb explosion [6].
[1] L.S. Cederbaum, J. Zobeley, F. Tarantelli, Phys. Rev. Lett. 79, 4778 (1997).
[2] U. Hergenhahn, J. Electron Spectrosc. Relat. Phenom. 184, 78 (2011).
[3] A. Dubrouil et al., J. Phys. B. 48, 204005 (2015).
[4] D. Iablonskyi e al., Phys. Rev. Lett.117, 276806 (2016).
[5] T. Takanashi et al, Phys. Rev. Lett. 118, 033202 (2017).
[6] T. Jahnke et al., Phys. Rev. Lett. 93, 163401 (2004).



Circular Dichroism of Atoms and Molecules

During the last years, studies of Circular Dichroism (CD), the different response of a sample to right- versus left-circularly polarized light, have attracted much attention due to the possibility to investigate the electronic and structural symmetry properties of matter. CD has a broad range of interest: from fundamental spin control to bio-chemistry (chiral molecules) and material science (e.g., magnetization) [1]. Dichroism in the strong field and multiphoton regime has been mainly investigated in the optical wavelength range. Free-electron lasers (FELs), with their unprecedented intensity over a large photon energy range from soft to hard X-ray extend such studies to shorter wavelengths [2]. In particular, experimental investigations of the CD in two-color near-infrared (NIR)–extreme-ultraviolet (XUV) multiphoton ionization, which was predicted theoretically [3], only started recently [4-6] with the operation of the seeded FEL FERMI.

Figure 2. a) Schematic representation of the two-color  multiphoton ionization process for He atom at XUV/NIR intensities 80mJ and 750mJ, respectively. Both XUV (48.4eV) and NIR (784 nm) pulses are left-hand circularly polarized. b) Typical VMI image cut through the 3D velocity and angular distribution obtained by Abel-inversion of the experimental image. c) Part of the photoelectron spectrum in the region of the He(1s) main line and the high-energy sidebands (SBs) measured at p/2 emission angle [4].
[1] G. A. Garcia, et al.,Nat. Commun. 4,  2132 (2013).
[2] A. K. Kazansky, et al.,Phys. Rev. Lett. 107,  253002 (2011).
[3] T. Ishikawa et al.,Nat. Photon. 6,  540 (2012).
[4] T. Mazza, et al., Nat. Commun. 5, 3648 (2014).
[5] T. Mazza, et al., J. Mod. Opt. 63, 367 (2016).
[6] M. Ilchen, et al., Phys. Rev. Lett. 118, 013002 (2017).


Coherent Control at FERMI

In a coherent control experiment, light pulses are used to guide the real-time evolution of a quantum system. This requires the coherence and the control of the pulses’ electric-field carrier waves. FERMI is the first fully coherent FEL, and possesses longitudinal coherence, which is not available at SASE FELs.  While many types of experiment performed with optical lasers have also been performed with SASE FELs, there have been none in the class of coherent control, because of the lack of longitudinal coherence. Only recently, the first demonstration of a coherent control experiment was performed [1]. By controlling the relative phase between two time-overlapped pulses with commensurate wavelengths, it was possible to guide the ionization pathways in neon (see Fig.3). Such a result demonstrates phase control at the attosecond level with FERMI, and opens the way for unique experiments in the EUV and soft X-ray regions, with complete control of the wavelength, polarization, phase and intensity. FERMI produces light with wavelengths down to 4 nm, providing access to core levels, and thus chemical specificity in coherent control experiments.
Figure 3. Scheme of the experiment: pulses of light (waves) emit electrons (green) from a neon atom (violet) [1].
[1] K.C. Prince et al, Nat. Photon. 10, 176 (2016).



Structure and Ultrafast Dynamics of Nanosystems

Short-wavelength FELs open new directions for exploring the structure and ultrafast dynamics of complex systems [1]. Main objectives are high-resolution structural and dynamical imaging of nanoparticles, macromolecules or viruses. Clusters and nanodroplets provide the perfect platform to develop a fundamental understanding how to extract various types of structures and complex dynamics, including the imaging of ultrafast electronic processes and transient states of matter in a single nanoparticle. Exposing single nanoparticles to the high photon fluxes required for single-shot diffractive imaging typically converts them into a plasma-like state, which is followed by various relaxation processes with signature behaviors such as absorption enhancement, bleaching, or suppression of electron emission. Especially interesting is the case of resonant excitation when numerous atoms within the same particle are excited simultaneously. It has been found that an efficient and extremely fast (fs-ps) interatomic Coulombic decay (ICD) [2], and collective auto-ionization [3] can drastically change the nanoplasma formation and dynamic.
Recently, diffraction patterns of single He nanodroplets were recorded at FERMI.  As size and shape differs from droplet to droplet (see Fig. 4), exploring the static and dynamic properties of individual droplets is experimentally challenging. Deformed droplets tilted out of the scattering plane produce features in the wide-angle diffraction pattern that break the point symmetry. When compared to a numerical model of non-superfluid rotating drops, experimental data show unexpectedly good agreement, considering that superfluidity and the formation of vortices in He nanodroplets with high angular momentum have been previously observed [4].
Figure 4. Wide-angle scattering images of He nanodroplets taken at the FERMI and their corresponding model shapes.

[1] T. Fennel, et al., Rev. Mod. Phys.82, 1793 (2010).
[2] A. C. LaForge et al. Sci. Reports 4, 3621 (2014).
[3] Y. Ovcharenko et al. Phys. Rev. Lett. 112, 073401 (2014).
[4] L. F. Gomez, Science 345, 906 (2014).


Control of Molecular Motion (Molecular Alignment)

Femtosecond pump–probe experiments have extensively been used to follow atomic and molecular motion in time [1]. Alignment at an FEL has been demonstrated previously using several approaches [2] but for very fast rotational dynamics the jitter of SASE (Self Amplified Spontaneous Emission) FELs can be problematic, although advanced data analysis has been demonstrated to recover the details in silico. In this respect, the advantage of using a jitter-free FEL as FERMI is immediately evident with the final goal of recording molecular movies with femtosecond, sub-Ångstrom resolution.
Recently the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at LDM/FERMI has been performed [3]. OCS molecules in a molecular beam were aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the S 2p edge, resulting in multiple ionization via Auger decay and subsequent Coulomb explosion of the molecules. The ionic fragments were collected using a time-of-flight mass spectrometer and the analysis of ion–ion covariance maps confirmed the correlation between fragments after Coulomb explosion (see Fig.5).
This result opens the way for a new class of experiments at LDM within the field of coherent control of molecular motion. Stronger alignment of molecules (than what was demonstrated for OCS) will be required to perform experiments using photoelectron holography techniques able to image the molecules from within, and allowing the recording of molecular movies on a femtosecond scale with picometer resolution.
Figure 5. Rotational revival alignment structure measured by monitoring the integrated signal for OC+ and S+ fragments while changing the time delay of the FEL pulse with respect to the alignment NIR pulse (left). The region of the OC+ and S+ channel in the partial covariance map at two different delays between the pump and probe pulses (right) [3].
[1] A.H. Zewail. J. Phys. Chem. A. 104,  5660 (2000).
[2] T. Kierspel et al., J. Phys. B 48, 204002 (2015).
[3] M. Di Fraia et al.,Phys. Chem. Chem. Phys. 19, 19733 (2017).

Molecular Photodynamics

Understanding the nature of molecular dynamics in detail, such as visualization of molecular bond breaking, charge-transfer processes, proton migration, Interatomic Coulombic decay (ICD), photoprotection, ring opening and isomerization phenomena induced by the interaction of photons with matter has become viable with the advent of ultrafast laser pulses [1].
Recently, experiments related to investigations of different ultrafast phenomena for various isolated organic molecules have been performed at FERMI by FELs-based time-resolved spectroscopy [2, 3] (for example, see Fig. 6).This class of spectroscopy is based on pump-probe techniques, where a pump pulse excites a sample and a probe pulse is used for probing the sample after an adjustable delay time.
Figure 6. A schematic overview of the relaxation mechanism of acetylacetone. The ground state S0 (darker blue), two singlet S2 (ππ*) (light blue) and S1 (nπ*) (orange), and two triplet T2 (nπ*) (light green) and T1 (ππ*) (green) states are shown. Excited state minima and minimum energy CIs (MECI) are indicated. Relative energies with respect to the electronic ground state minimum (S0min) are given. For details see [2].
[1]       N. Berrah. Phys. Chem. Chem. Phys. 19,19536 (2017).
[2]       R. J. Squibb et al. Nature Communications 9, 63 (2018).
[3]       S. Pathak et al. Nature Chemistry, 12, 795 (2020).
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