Carbon and Nitrogen K-Edge NEXAFS Spectra of Indole, 2,3-Dihydro-7-azaindole, and 3-Formylindole.

The near-edge X-ray absorption fine structure (NEXAFS) spectra of indole, 2,3-dihydro-7-azaindole, and 3-formylindole in the gas phase have been measured at the carbon and nitrogen K-edges. The spectral features have been interpreted based on density functional theory (DFT) calculations within the transition potential (TP) scheme, which is accurate enough for a general description of the measured C 1s NEXAFS spectra as well as for the assignment of the most relevant features. For the nitrogen K-edge, the agreement between experimental data and theoretical spectra calculated with TP-DFT was not quite satisfactory. This discrepancy was mainly attributed to the many-body effects associated with the excitation of the core electron, which are better described using the time-dependent density functional theory (TDDFT) with the range-separated hybrid functional CAM-B3LYP. An assignment of the measured N 1s NEXAFS spectral features has been proposed together with a complete description of the observed resonances. Intense transitions from core levels to unoccupied antibonding π* states as well as several transitions with mixed-valence/Rydberg or pure Rydberg character have been observed in the C and N K-edge spectra of all investigated indoles.

Experimental and Theoretical Photoemission Study of Indole and Its Derivatives in the Gas Phase.

The valence and core-level photoelectron spectra of gaseous indole, 2,3-dihydro-7-azaindole, and 3-formylindole have been investigated using VUV and soft X-ray radiation supported by both an ab initio electron propagator and density functional theory calculations. Three methods were used to calculate the outer valence band photoemission spectra: outer valence Green function, partial third order, and renormalized partial third order. While all gave an acceptable description of the valence spectra, the last method yielded very accurate agreement, especially for indole and 3-formylindole. The carbon, nitrogen, and oxygen 1s core-level spectra of these heterocycles were measured and assigned. The double ionization appearance potential for indole has been determined to be 21.8 ± 0.2 eV by C 1s and N 1s Auger photoelectron spectroscopy. Theoretical analysis identifies the doubly ionized states as a band consisting of two overlapping singlet states and one triplet state with dominant configurations corresponding to holes in the two uppermost molecular orbitals. One of the singlet states and the triplet state can be described as consisting largely of a single configuration, but other doubly ionized states are heavily mixed by configuration interactions. This work provides full assignment of the relative binding energies of the core level features and an analysis of the electronic structure of substituted indoles in comparison with the parent indole.

Ultrafast charge carrier dynamics in quantum confined 2D perovskite.

We studied the charge carrier dynamics in 2D perovskite NBT2PbI4 by ultrafast optical pump-THz probe spectroscopy. We observed a few ps long relaxation dynamics that can be ascribed to the band to band carrier recombination, in the absence of any contribution from many-body and trap assisted processes. The transient conductivity spectra show that the polaron dynamics is strongly modulated by the presence of a rich exciton population. The polarization field resulting from the exciton formation acts as the source of a restoring force that localizes polarons. This is revealed by the presence of a negative imaginary conductivity. Our results show that the dynamics of excitons in 2D perovskites at room temperature can be detected by monitoring their effect on the conductivity of the photoinduced polaronic carrier.

Impulsive laser-induced alignment of OCS molecules at FERMI.

We demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at the Low Density Matter Beamline (LDM) at the free-electron laser FERMI. OCS molecules in a molecular beam were impulsively aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the sulphur 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. The analysis of the CO+ and S+ channels allowed us to extract the rotational dynamics, which is in agreement with our theoretical description as well as with previous experiments. This result opens the way for a new class of experiments at LDM within the field of coherent control of molecules with the possibilities that a precisely synchronized optical-pump XUV-probe laser setup like FERMI can offer.

Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields.

High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.

The Low Density Matter (LDM) beamline at FERMI: optical layout and first commissioning.

The Low Density Matter (LDM) beamline has been built as part of the FERMI free-electron laser (FEL) facility to serve the atomic, molecular and cluster physics community. After the commissioning phase, it received the first external users at the end of 2012. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline.

Charge transfer and penning ionization of dopants in or on helium nanodroplets exposed to EUV radiation.

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work, we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n = 2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants.

Fast stabilization of a high-energy ultrafast OPA with adaptive lenses.

The use of fast closed-loop adaptive optics has improved the performance of optical systems since its first application. Here we demonstrate the amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier devoted to Attosecond Science exploiting two high speed adaptive optical systems for the correction of static and dynamic instabilities. The exploitation of multi actuator adaptive lenses allowed for a minimal impact on the optical setup.

Laser induced strong-field ionization gas jet tomography.

We introduce a novel in-situ strong field ionization tomography approach for characterizing the spatial density distribution of gas jets. We show that for typical intensities in high harmonic generation experiments, the strong field ionization mechanism used in our approach provides an improvement in the resolution close to factor of 2 (resolving about 8 times smaller voxel volume), when compared to linear/single-photon imaging modalities. We find, that while the depth of scan in linear tomography is limited by resolution loss due to the divergence of the driving laser beam, in the proposed approach the depth of focus is localized due to the inherent physical nature of strong-field interaction and discuss implications of these findings. We explore key aspects of the proposed method and compare it with commonly used single- and multi-photon imaging mechanisms. The proposed method will be particularly useful for strong field and attosecond science experiments.

sp hybridization in free carbon nanoparticles--presence and stability observed by near edge X-ray absorption fine structure spectroscopy.

The presence and stability of sp hybridized atoms in free carbon nanoparticles was investigated by NEXAFS spectroscopy. The experiments show that a predominant fraction of carbon atoms is found in linear sp-chains and that conversion into sp(2) structures proceeds already at low temperature and in the gas phase.

Effect of axial torsion on sp carbon atomic wires.

Ab initio calculations within density-functional theory combined with experimental Raman spectra on cluster-beam deposited pure-carbon films provide a consistent picture of sp-carbon chains stabilized by sp;{3} or sp;{2} terminations, the latter being sensitive to torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level and to switch on and off the on-chain pi-electron magnetism.