Intense ultrashort X-ray pulses produced by modern free-electron lasers (FELs) allow one to probe biological systems, inorganic materials and molecular reaction dynamics with nanoscale spatial and femtoscale temporal resolution. These experiments require the knowledge, and possibly the control, of the spectro-temporal content of individual pulses. FELs relying on seeding have the potential to produce spatially and temporally fully coherent pulses. Here we propose and implement an interferometric method, which allows us to carry out the first complete single-shot spectro-temporal characterization of the pulses, generated by an FEL in the extreme ultraviolet spectral range. Moreover, we provide the first direct evidence of the temporal coherence of a seeded FEL working in the extreme ultraviolet spectral range and show the way to control the light generation process to produce Fourier limited pulses. Experiments are carried out at the FERMI FEL in Trieste.
COBISS.SI-ID: 4070139
We present a detailed theoretical study of two-photon above threshold excitation to the lowest dipole forbidden 1Se and 1De autoionizing states of helium with free-electron laser pulses. We consider the case where the formation of these transient states is detected by monitoring the yield of atoms in the metastable 1s2s states, which are reached in the radiative cascade decays of the autoionizing states. We describe the two-photon near-resonant driving with an effective Hamiltonian operator within the density matrix formalism, and we report the relevant parameters of the model, such as the field-induced shifts, the effective Rabi frequencies, and the cascade branching ratios of both the autoionizing states and the 1sns states, to which the autoionizing states decay. We compare the calculated metastable atom yield to the vacuum ultraviolet florescence yield and to the electron yield. We discuss the effects of the intensity of the source, admixture of higher harmonics in the incident beam, polarization of the incident light, and the pulse chirp on the metastable atom yield.
COBISS.SI-ID: 28657447
We present an experimental and theoretical study of resonant inelastic x-ray scattering (RIXS) in the carbon disulphide CS2 molecule near the sulfur K-absorption edge. We observe a strong evolution of the RIXS spectral profile with the excitation energy tuned below the lowest unoccupied molecular orbital (LUMO) absorption resonance. The reason for this is twofold. Reducing the photon energy in the vicinity of the LUMO absorption resonance leads to a relative suppression of the LUMO contribution with respect to the emission signal from the higher unoccupied molecular orbitals, which results in the modulation of the total RIXS profile. At even larger negative photon-energy detuning from the resonance, the excitation energy dependence of the RIXS profile is dominated by the onset of electron dynamics triggered by a coherent excitation of multiple electronic states. Furthermore, our study demonstrates that in the hard x-ray regime, local ionzation of the S 1s core hole occurs in CS2 during the RIXS process because of the orientational dephasing of interference between the waves scattering on the two sulfur atoms. Core-hole localization leads to violation of the symmetry selection rules for the electron transitions observed in the spectra.
COBISS.SI-ID: 28800807
We investigate bidirectional femtosecond charge transfer dynamics using the core−hole clock implementation of resonant photoemission spectroscopy from 4,4′-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core−hole creation in all systems, opening an additional decay channel for the core−hole, involving electron donation from substrate to the molecule. Furthermore, using the core−hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.
COBISS.SI-ID: 2895460
To broaden our analytical capabilities with molecular imaging in addition to the existing elemental imaging with micro-PIXE, a linear Time-Of-Flight mass spectrometer for MeV Secondary Ion Mass Spectrometry (MeV-SIMS) was constructed and added to the existing nuclear microprobe at the J.Stefan Institute. We measured absolute molecular yields and damage cross-section of reference materials, without significant alteration of the fragile biological samples during the duration of measurements in the mapping mode. We explored the analytical capability of the MeV-SIMS technique for chemical mapping of the plant tissue of medicinal cannabis leaves. A series of hand-cut plant tissue slices were prepared by standard shock-freezing and freeze-drying protocol and deposited on the Si wafer. We show the measured MeV-SIMS spectra showing a series of peaks in the mass area of cannabinoids, as well as their corresponding maps. The indicated molecular distributions at masses of 345.5 u and 359.4 u may be attributed to the protonated THCA and THCA-C4 acids, and show enhancement in the areas with opened trichome morphology.
COBISS.SI-ID: 29064487