Interatomic or Intermolecular Coulombic Decay (ICD)

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].

 

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].

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.
 

[1] K.C. Prince et al, Nat. Photon. 10, 176 (2016).

 

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].
 

[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).

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.
 

[3] M. Di Fraia et al.,Phys. Chem. Chem. Phys. 19, 19733 (2017).