Attosecond spectroscopy promises a real-time probe of electronic dynamics, such as those involved in nonsequential two-photon double ionization (NSDI) and charge migration driven by electron correlation and electronic relaxation. A XUV-pump-XUV-probe setup is considered a general method for implementing attosecond spectroscopy, similar to the pump-probe scheme that has been successfully applied in femtosecond spectroscopy. However, major technical issues need to be solved in order to carry out such experiments in attosecond time-domain. In this talk I will discuss our effort in developing such techniques through two different routes: 1) intense high harmonic generation and 2) attosecond angular streaking. In the first approach, we achieved ion-electron coincidence detection of two-photon double ionization of an atomic system using intense XUV pulses produced through high harmonic generation. In the second approach, employing a novel attosecond angular streaking technique, we probed electron dynamics taking place in the first 500 attoseconds after pumping. These techniques will ultimately bring time-resolved spectroscopy to the attosecond domain.

r0pt Figure Modelling strong-field induced radiation near the ionization threshold requires careful treatment of excited state dynamics. Solution of the time dependent Schrodinger equation (TDSE) provides accurate data in this regime, but the computational time needed prohibits direct calculation of the macroscopic response (e.g., from a gas jet) due to the range of intensities to be considered. We apply a method of interpolation of (precalculated) TDSE results as a function of laser intensity at a given wavelength to simulate the macroscopic propagation of the high harmonic signals using the discrete dipole approximation. This allows investigation of the angular dependence of harmonic and off-harmonic radiation near and below the ionization threshold (see Fig. 1). The method can be extended beyond the hydrogen atom through the Single Active Electron (SAE) approximation or Time Dependent Density Functional Theory (TDDFT). Results for the helium atom (SAE) and N₂⁺ molecule (TDDFT) will be presented. This work was supported by AFOSR MURI (Grant No. FA9550-16-1-0121) and DOE-BES (Award No. DE-SC0001771)

In this work, we perform femtosecond M_2,3-edge XANES on low-spin iron complexes using a high harmonic generation (HHG) light source. Low-spin iron complexes hold promise as affordable alternatives to ruthenium-based photosensitizers. Conventional iron complexes with polypyridyl ligands are unsuitable because the excitation energy is dissipated in 200 fs by ultrafast population of the low-lying quintet metastable state. This formally ∆S=2 process is conjectured to proceed through a metal-centered triplet intermediate state, but spectroscopic evidence of such an intermediate has remained elusive. Here we use femtosecond M_2,3-edge XANES to elucidate the role of metal-centered intermediate states. The transient response of Fe(phen)₃(SCN)₂ following excitation in the MLCT band clearly shows the presence of a metal-centered ³T₁ state. The transient response also shows oscillations consistent with the evolution of a vibrational wave-packet on the quintet surface. We also recorded the M_2,3-edge transient response of Fe[(CF₃)₂bpca]₂, a complex with a weaker ligand field. A comparison of the photophysical behaviors of Fe(phen)₃(SCN)₂ and Fe[(CF₃)₂bpca]₂ sheds light on the influence of crystal field strength on the relaxation mechanism of the MLCT state. Our findings add a hitherto missing piece to the existing picture of photophysics of iron complexes.

Tungsten hexacarbonyl and iron pentacarbonyl are two of the common precursors used for Focused Electron Beam Induced Deposition (FEBID). FEBIP is one of the new techniques explored by the nanotechnology industry to build sub-10nm structures. Velocity Map Imaging (VMI) technique has been used in the study of the Fe(CO)5 and W(CO)6 compounds to refining Malli L. Gulzari et al. [3], Rosenberg G. et al. [4] the electron beam induced deposition process by giving the essential details on this molecules, such as cross sections, kinetic energies, angular distributions. Neustetter M. et al. [5], Thon R. et al. [6] A molecular analysis of surfaces and deposition of Me2Au(acac) has been performed using electron stimulated desorption (ESD) and temperature programmed desorption (TPD) revealing the presence of C2H2, AuII, CO, O+. A plain Copper substrate has been used for deposition of the compound at a pressure of  10-8 mbar for 2 min, equivalent to 3.4L. This work is part of the ELENA Horizon 2020 Training Network, www.elena-eu.org was developed as a European Initiative to support the emerging nanotechnology market and as a response to the increasing demand in this field.

Circularly polarized laser pulse can induce electronic ring currents within a single molecule, that are expected to persist for as long as nanoseconds. Photoinduced ring currents in molecules have not yet been observed experimentally, and most theoretical studies have considered rather weak laser intensity - below 10¹² W/cm², which limits the strength of the induced current. r0.47 We study the properties of the ring current that can be induced in benzene, over the range of laser pulse frequencies and intensities. Simulations within Time-Dependent Density Functional Theory (TDDFT) are compared to results obtained within Tight Binding method. In the regime of the excitation of the persistent ring current one can observe influence of the resonance enhanced ionization (REMPI) on the current. Our results indicate that circularly-polarized femtosecond laser pulse can be used to control ionization and efficient ionization leads to a persistent hole current. Consequently for higher intensities of the laser pulse the leading contribution from the hole current induces the change of the direction of the total ring current (see Figure 1). Furthermore we present how the mechanism can be used to switch between the two regimes of co- and counter propagating ring currents during the laser pulse and discuss properties of ring currents in other molecules.

Double ionization has been well studied in atoms and diatomic molecules. However, polyatomic molecules, with more complicated electronic structure, have not been studied as extensively. We use few cycle intense ultrafast laser pulses and coincidence velocity map imaging to investigate strong field double ionization in molecules such as CH₂IBr and 1,3-Cyclohexadiene. By using a time stamping camera to make vector momentum measurements of electrons and ions, we are able to distinguish between multiple double ionization channels. Different double ionization channels which result in different fragment ion pairs show different electron correlation patterns, indicating that the double ionization dynamics are influenced by the orbitals from which the electrons are removed.

In this work we perform ultrafast transient M_2,3-edge XANES on divalent cobalt-manganese(N,N,N-tri(2-(2-pyridylamino)ethyl)amine)Cl, a heterobimetallic system with directly-interacting dual metal centers held within a non-innocent ligand scaffold. The strong metal-metal interaction facilitated by the ‘double-decker’ type ligand scaffold leads to a highly multiconfigurational electronic structure with relaxation pathways unavailable in monometallic analogues. With the ultrashort broadband probe pulse used in transient high-harmonic generation spectroscopy, we are able to perform M_2,3-edge XANES on both metal edges simultaneously with high specificity for each metal center and ligand environment. By combining transient XANES with transient UV-visible spectroscopy we have compiled a full picture of the electronic relaxation dynamics of this complex molecule. Photoinduced MM’CT is followed by distinct electronic dynamics at both metal edges and within the ligand scaffold, suggesting complex interplay of the Co, Mn, and ligand redox centers. Increased understanding of the relation of function to metal-specific photodynamics will help lay essential groundwork for the development of multimetallic catalysts with efficiencies comparable to those found in nature.

r0pt Figure When a helium atom is excited into a superposition of a 1s state and a np+ state, the electron wave packet produces a ring current with attosecond scale charge density dynamics. The dynamics in the field free wave packet manifest in a time dependent relative phase between the ground and excited state. In this talk, we will discuss an interference scheme that allows for time resolved imaging of the relative phase of the bound states to be extracted from a photoelectron spectrum. The presented results are obtained from ab initio simulations of the time dependent Schrödinger equation in the single active electron and dipole approximations. Effects of pulse length, polarization and other parameters of the imaging pulse on the obtained data will be discussed. This work was supported by DOE-BES (Award No. DE-SC0001771) and NSF JILA Physics Frontier Center (Grant No. PHY 1734006)