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D2 molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured D+ ion fragment momenta and angular distributions that originate from two different four-step dissociative ionization pathways after four-photon absorption (one XUV + three NIR). We show that, even for very low dissociation kinetic energy release ≤ 240 meV, specific electronic excitation pathways can be identified and isolated in the final ion momentum distributions. With the aid of ab initio electronic structure and time-dependent Schrödinger equation calculations, angular momentum, energy, and parity conservation are used to identify the excited neutral molecular states and molecular orientations relative to the polarization vectors in these different photoexcitation and dissociation sequences of the neutral D2 molecule and its D2+ cation. In one sequential photodissociation pathway, molecules aligned along either of the two light polarization vectors are excluded, while another pathway selects molecules aligned parallel to the light propagation direction. The evolution of the nuclear wave packet on the intermediate B1ςu+ electronic state of the neutral D2 molecule is also probed in real time.

D$_2$ molecules, excited by linearly cross-polarized femtosecond extremeultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highlystructured D$^+$ ion fragment momenta and angular distributions that originatefrom two different 4-step dissociative ionization pathways after four photonabsorption (1 XUV + 3 NIR). We show that, even for very low dissociationkinetic energy release $\le$~240~meV, specific electronic excitation pathwayscan be identified and isolated in the final ion momentum distributions. Withthe aid of {\it ab initio} electronic structure and time-dependentSchr\"odinger equation calculations, angular momentum, energy, and parityconservation are used to identify the excited neutral molecular states andmolecular orientations relative to the polarization vectors in these differentphotoexcitation and dissociation sequences of the neutral D$_2$ molecule andits D$_2^+$ cation. In one sequential photodissociation pathway, moleculesaligned along either of the two light polarization vectors are excluded, whileanother pathway selects molecules aligned parallel to the light propagationdirection. The evolution of the nuclear wave packet on the intermediate \Bstateelectronic state of the neutral D$_2$ molecule is also probed in real time.

New experimental results are presented on the dissociation dynamics of small neutral molecules in time-resolved photoexcitation-photoionization experiments by ultrashort vacuum- and extreme-ultraviolet laser pulses and charged particle momentum imaging.

We have performed ultrafast three-dimensional ion momentum imaging spectroscopy on the dissociative single ionization of oxygen molecules using attosecond pulse trains with a broad energy spectrum of 5-30 eV. High-resolution momentum imaging allows clear identification of vibrational structures corresponding to the predissociation of highly excited cationic states. By adding a pump infrared field that is synchronized with and polarized orthogonally to the XUV pulse train, and an additional probe IR field, we demonstrate how the yield of O+ ions can be steered between different dissociation channels by coherently controlling the coupling between multiple O2+∗ electronic states on an attosecond time scale. Time-dependent calculations in a single active electron approximation allow a qualitative analysis of ion yields for two orientations of the molecular axis.

Understanding electron-electron correlations in matter ranging from atoms to solids represents a grand challenge for both experiment and theory. These correlations occur on attosecond timescales and have only recently become experimentally accessible. In the case of highly excited systems, the task of understanding and probing correlated interactions is even greater. In this work, we combine state-of-the-art light sources and advanced detection techniques with ab initio calculations to unravel the role of electron-electron correlation in D2 photoionization by mapping the dissociation of a highly excited D2+ molecule. Correlations between the two electrons dictate the pathways along which the molecule dissociates and lead to a superposition of excited ionic states. Using 3D Coulomb explosion imaging and electron-ion coincidence techniques, we assess the relative contribution of competing parent ion states to the dissociation process for different orientations of the molecule with respect to the laser polarization, which is consistent with a shake-up ionization process. As a step toward observing coherent superposition experimentally, we map the relevant nuclear potentials using Coulomb explosion imaging and show theoretically that such an experiment could confirm this coherence via two-path interference.

We used ultrashort femtosecond vacuum ultraviolet (VUV) and infrared (IR) pulses in a pump-probe scheme to map the dynamics and nonequilibrium dissociation channels of excited neutral H2 molecules. A nuclear wave packet is created in the BΣu+1 state of the neutral H2 molecule by absorption of the ninth harmonic of the driving infrared laser field. Due to the large stretching amplitude of the molecule excited in the BΣu+1 electronic state, the effective H2+ ionization potential changes significantly as the nuclear wave packet vibrates in the bound, highly electronically and vibrationally excited B potential-energy curve. We probed such dynamics by ionizing the excited neutral molecule using time-delayed VUV-or-IR radiation. We identified the nonequilibrium dissociation channels by utilizing three-dimensional momentum imaging of the ion fragments. We found that different dissociation channels can be controlled, to some extent, by changing the IR laser intensity and by choosing the wavelength of the probe laser light. Furthermore, we concluded that even in a benchmark molecular system such as H2∗, the interpretation of the nonequilibrium multiphoton and multicolor ionization processes is still a challenging task, requiring intricate theoretical analysis.

We present a new experimental setup for measuring ultrafast nuclear and electron dynamics of molecules after photo-excitation and ionization. We combine a high flux femtosecond vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) source with an internally cold molecular beam and a 3D momentum imaging particle spectrometer to measure electrons and ions in coincidence. We describe a variety of tools developed to perform pump-probe studies in the VUV-XUV spectrum and to modify and characterize the photon beam. First benchmark experiments are presented to demonstrate the capabilities of the system.

Table-top sources of extreme ultraviolet (XUV) light based on high-harmonic generation (HHG) provide novel insight into the fundamental properties of molecules, nanomaterials, or correlated solids and are of interest for advanced applications. Extending HHG to high repetition rates is important for experiments, yet efficient XUV conversion of correspondingly weak driving pulses is challenging. Here, we demonstrate an efficient source of femtosecond XUV pulses at 50-kHz repetition rate using ultraviolet second-harmonic pulses focused tightly into Kr. As a result, a photon flux of about 10^14 s^-1 is generated at 22.3 eV, corresponding to 2?10^-4 conversion efficiency which surpasses our similar, direct-driven harmonics by two orders-of-magnitude. This enhancement exceeds the expected dipole scaling, evidencing improved phase-matching for ultraviolet-driven HHG under tight focusing as corroborated by simulations. Spectral isolation ofa single harmonic renders this efficient 50-kHz XUV source a highly valuable tool for ultrafast photoemission, nanoscale imaging and other experiments.

D_{2} molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured D^{+} ion fragment momenta and angular distributions that originate from two different four-step dissociative ionization pathways after four-photon absorption (one XUV + three NIR). We show that, even for very low dissociation kinetic energy release ≤ 240 meV, specific electronic excitation pathways can be identified and isolated in the final ion momentum distributions. With the aid of ab initio electronic structure and time-dependent Schrödinger equation calculations, angular momentum, energy, and parity conservation are used to identify the excited neutral molecular states and molecular orientations relative to the polarization vectors in these different photoexcitation and dissociation sequences of the neutral D_{2} molecule and its D_{2}^{+} cation. In one sequential photodissociation pathway, molecules aligned along either of the two light polarization vectors are excluded, while another pathway selects molecules aligned parallel to the light propagation direction. The evolution of the nuclear wave packet on the intermediate B^{1}Σ_{u}^{+} electronic state of the neutral D_{2} molecule is also probed in real time.