Electronic structure in real time mapping valence electron rearrangements during chemical reactions

The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time resolved photoelectron spectroscopy with ultraviolet UV probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuum ultraviolet VUV region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time resolved VUV photoelectron spectroscopy. Finally, the importance of soft X ray free electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond time resolved soft X ray emission spectroscopy or resonant inelastic X ray scattering for mapping the electronic structure during chemical reactions is discusse

A high order harmonic generation apparatus for time and angle resolved photoelectron spectroscopy

We present a table top setup for time- and angle-resolved photoelectron spectroscopy to investigate band structure dynamics of correlated materials driven far from equilibrium by femtosecond laser pulse excitation. With the electron-phonon equilibration time being in the order of 1–2 ps it is necessary to achieve sub-picosecond time resolution. Few techniques provide both the necessary time and energy resolution to map non-equilibrium states of the band structure. Laser-driven high-order harmonic generation is such a technique. In our experiment, a grating monochromator delivers tunable photon energies up to 40 eV. A photon energy bandwidth of 150 meV and a pulse duration of 100 fs FWHM allow us to cover the k-space necessary to map valence bands at different k z and detect outer core states

Time resolved resonant inelastic X ray scattering A supreme tool to understand dynamics in solids and molecules

Dynamics in materials typically involve different degrees of freedom, like charge, lattice, orbital and spin in a complex interplay. Time resolved resonant inelastic X ray scattering RIXS as a highly selective tool can provide unique insight and follow the details of dynamical processes while resolving symmetries, chemical and charge states, momenta, spin configurations, etc. In this paper, we review examples where the intrinsic scattering duration time is used to study femtosecond phenomena. Free electron lasers access timescales starting in the sub ps range through pump probe methods and synchrotrons study the time scales longer than tens of ps. In these examples, time resolved resonant inelastic X ray scattering is applied to solids as well as molecular system

Magnetic dichroism in the 4f4f photoelectron spectra of free Eu atoms: Experimental proof of the atomic character of thin film Eu–Gd MCD

The 4f photoionization of Eu atoms oriented in the ground state was investigated by means of photoelectron spectroscopy with linearly polarized vacuum ultraviolet radiation. Strong magnetic dichroism could be observed for atoms oriented parallel/antiparallel to the VUV photon beam by optical pumping with circularly polarized laser radiation. The magnetic dichroism of the free atoms, well described within the general theory of photoelectron angular distribution, displays the same characteristic features as the CMD of thin Eu films on a ferromagnetic Gd substrate

Shot-to-Shot and Average Absolute Photon Flux Measurements of a Femtosecond Laser High-Order Harmonic Photon Source

The absolute flux of a femtosecond vacuum-ultraviolet (VUV) photon source based on the high-order harmonic generation of a femtosecondTi:sapphire laser and monochromatized with a grating monochromator is determined both on a shot-to-shot basis and averaged over seconds by acalibrated gas monitor detector. The average flux is compared with the average flux as determined with a calibrated GaAsP semiconductor photodiode. We found that the photodiode is a reliable and easy-to-use tool for estimating the order of magnitude of the average photon flux but that, due to saturation losses, it underestimates the average flux by up to −15%

Real Time Evolution of the Valence Electronic Structure in a Dissociating Molecule

Time resolved valence band photoelectron spectroscopy with a temporal resolution of 135 fs is used to map the entire occupied valence electronic structure of photoexcited gas phase Br 2 molecules during dissociation. The observed shifting and mixing of valence energy levels defines a transition period where the system appears to be intermediate between atoms and molecules. The surprisingly short bond breaking or dissociation time is determined by monitoring in real time how the photoelectron multiplet structure of the free atom arises from the valence states of the photoexcited molecul

Breakdown of the three-parameter model for the dichroism in the 4f4f photoelectron spectrum of laser-aligned Eu atoms

The dichroism in the 4f photoelectron spectra of laser-aligned Eu atoms has been determined with linearly polarized vacuum ultraviolet radiation. The experimental results disagree with the prediction of the three-parameter model based on the absolute magnitude and relative phase of the 4ftovarepsilon d,varepsilon g transition matrix elements. It is suggested that strong configuration interactions, confirmed in high-resolution photoelectron spectra, are the cause of the breakdown of the three-parameter model