Search

We investigate how attosecond XUV pump/IR probe schemes can be used to exert control on the ionization dynamics of the hydrogen molecule. The aim is to play with all available experimental parameters in the problem, namely the XUV pump–IR probe delay, the energy and emission direction of the produced photo-ions, as well as combinations of them, to uncover control strategies that can lead to preferential electron ejection directions. We do so by accurately solving the time-dependent Schrodinger equation, with inclusion of both electronic and nuclear motions, as well as the coupling between them. We show that both the IR pulse and the nuclear motion can be used to break the molecular inversion symmetry, thus leading to asymmetric molecular-frame photoelectron angular distributions. The preferential electron emission direction can thus be tuned by varying the pump–probe delay, by choosing specific ranges of proton kinetic energies, or both. We expect that similar control strategies could be used in more complex molecules containing light nuclei.

Capturing electronic dynamics in real time has been the ultimate goal of attosecond science since its beginning. While for atomic targets the existing measurement techniques have been thoroughly validated, in molecules there are open questions due to the inevitable copresence of moving nuclei, which are not always mere spectators of the phototriggered electron dynamics. Previous work has shown that not only can nuclear motion affect the way electrons move in a molecule, but it can also lead to contradictory interpretations depending on the chosen experimental approach. In this Letter we investigate how nuclear motion affects and eventually distorts the electronic dynamics measured by using two of the most popular attosecond techniques, reconstruction of attosecond beating by interference of two-photon transitions and attosecond streaking. Both methods are employed, in combination with ab initio theoretical calculations, to retrieve photoionization delays in the dissociative ionization of H2, H2→H?, in the region of the Q1 series of autoionizing states, where nuclear motion plays a prominent role. We find that the experimental reconstruction of attosecond beating by interference of two-photon transitions results are very sensitive to bond softening around the Q1 threshold (27.8 eV), even at relatively low infrared (IR) intensity (I0∼1.4×1011 W/cm2), due to the long duration of the probe pulse that is inherent to this technique. Streaking, on the other hand, seems to be a better choice to isolate attosecond electron dynamics, since shorter pulses can be used, thus reducing the role of bond softening. This conclusion is supported by very good agreement between our streaking measurements and the results of accurate theoretical calculations. Additionally, the streaking technique offers the necessary energy resolution to accurately retrieve the fast-oscillating phase of the photoionization matrix elements, an essential requirement for extending this technique to even more complicated molecular targets., Physical Review Letters, 128 (6), ISSN:0031-9007, ISSN:1079-7114

${\mathrm{D}}_{2}$ molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured ${\mathrm{D}}^{+}$ 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 $\ensuremath{\le}$ 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 Schr\"odinger 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 ${\mathrm{D}}_{2}$ molecule and its ${\mathrm{D}}_{2}^{+}$ 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 $B{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ electronic state of the neutral ${\mathrm{D}}_{2}$ molecule is also probed in real time.

Author(s): Bello, RY; Lucchese, RR; Rescigno, TN; McCurdy, CW | Abstract: We demonstrate a theoretical treatment of dissociative single ionization of the LiH molecule using two-color UV-UV pulse sequences that makes use of a highly correlated description of both the ionization continuum and target molecular ion and neutral states to which it is coupled. The present results emphasize how the details of the ionization process at various internuclear distances combine to form a lens through which such experiments image the dynamics of intermediate electronic states populated by the pump pulse. While ionization yields (dissociative and nondissociative) provide information about the amplitudes and phases that build up the molecular wave packet in the neutral states, molecular frame photoelectron angular distributions exhibit the changing character of those states, i.e., from ionic to covalent. In addition, the time-dependent mean kinetic energy of the wave packet on neutral states is clearly mapped onto the kinetic energy release of the atomic fragments produced by the probe ionization pulse.

We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of nonresonant one-color two-photon single-valence ionization of neutral ${\mathrm{N}}_{2}$ molecules. Using 9.3-eV photons produced via high-order harmonic generation and a three-dimensional momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of ${\mathrm{N}}_{2}$ populates the $X$ $^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}, A$ $^{2}\mathrm{\ensuremath{\Pi}}_{u}$, and $B$ $^{2}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ ionic states of ${\mathrm{N}}_{2}{}^{+}$, where the photoelectron angular distributions associated with the $X$ $^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$ and $A$ $^{2}\mathrm{\ensuremath{\Pi}}_{u}$ states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.

We investigate the two-photon double ionization of atomic magnesium induced by ultrashort pulses. Though the initial and final state symmetries are comparable to the same process in helium, in stark contrast the range of photon energies for which nonsequential ionization is the only open pathway is narrow (less than 1 eV) in magnesium. Thus several sequential ionization pathways feature heavily in these processes. Nonetheless, it is found that for pulse durations between 0.25 and 2.0 fs, the joint angular dependence of the ejected electrons can depend sensitively on pulse length, varying between the strictly back-to-back ejection characteristic of nonsequential ionization to other distributions. The significance of excited-state correlating configurations in representing the initial state of magnesium is discussed in the light of their consequences for the resulting angular distributions at photon energies where sequential ionization can access intermediate states that lie nearby in energy, particularly for longer pulses.

Author(s): Bello, RY; Yip, FL; Rescigno, TN; Lucchese, RR; McCurdy, CW | Abstract: By decomposing the initial-state wave function into its unique natural orbital expansion, as defined in the 1950s by Lowdin and used in modern studies of entanglement, we analyze the role of electron correlation in the initial state of an atom or molecule in determining the angular distribution of one-photon double ionization. Final-state correlation of the two ejected electrons is treated completely in numerically accurate calculations as the initial states of He, H-, and H2 are built up from correlating configurations in strict order of decreasing natural orbital occupations. In the two-electron atoms it is found that the initial-state correlation plays a sometimes modest but generally measurable role. In striking contrast, for H2 a large number of correlating configurations in the ground state is often necessary to produce angular distributions even approximately resembling the correct ones. One-photon double photoionization of oriented H2 is found to be particularly sensitive to left-right correlation along the bond.

The interaction of an extreme-ultraviolet attosecond pulse with a molecular system suddenly removes electrons, which can lead to significant changes in the chemical bonding and hence to rearrangements of the residual molecular cation. The timescales of the electronic and nuclear dynamics are usually very different, thus supporting separate treatment. However, when light nuclei are involved, as in most organic and biological molecules containing atomic hydrogen, the correlation between electronic and nuclear motion cannot be ignored. Using an advanced attosecond pump–probe spectroscopic method, we show that the coupling between electronic and nuclear motion in H2 leaves a clear trace in the phase of the entangled electron–nuclear wave packet. This requires us to re-evaluate the physical meaning of the measured phase, which depends on the energy distribution between electrons and nuclei. The conclusions are supported by ab initio calculations that explicitly account for the coupling between electronic and nuclear dynamics.

Author(s): Martin, L; Bello, RY; Hogle, CW; Palacios, A; Tong, XM; Sanz-Vicario, JL; Jahnke, T; Schoffler, M; Dorner, R; Weber, T; Martin, F; Kapteyn, HC; Murnane, MM; Ranitovic, P | Abstract: 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.

Recent research interests have been raised in uncovering and controlling ultrafast dynamics in excited neutral molecules. In this work we generalize the Monte Carlo wave packet (MCWP) approach to XUV-pump--IR-probe schemes to simulate the process of dissociative double ionization of ${\mathrm{H}}_{2}$ where singly excited states in ${\mathrm{H}}_{2}$ are involved. The XUV pulse is chosen to resonantly excite the initial ground state of ${\mathrm{H}}_{2}$ to the lowest excited electronic state of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+}$ symmetry in ${\mathrm{H}}_{2}$ within the Franck-Condon region. The delayed intense IR pulse couples the excited states of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+}$ symmetry with the nearby excited states of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$ symmetry. It also induces the first ionization from ${\mathrm{H}}_{2}$ to ${{\mathrm{H}}_{2}}^{+}$ and the second ionization from ${{\mathrm{H}}_{2}}^{+}$ to ${\mathrm{H}}^{+}+{\text{H}}^{+}$. To reduce the computational costs in the MCWP approach, a sampling method is proposed to determine in time the dominant ionization events from ${\mathrm{H}}_{2}$ to ${{\mathrm{H}}_{2}}^{+}$. By conducting a trajectory analysis, which is a unique possibility within the MCWP approach, the origins of the characteristic features in the nuclear kinetic energy release spectra are identified for delays ranging from 0 to 140 fs and the nuclear dynamics in the singly excited states in ${\mathrm{H}}_{2}$ is mapped out.

In this study we investigate the dissociative photoionization of molecular hydrogen H2, addressing the influence of autoionizing states and nuclear motion on the photoelectron dynamics. Experimental results are compared with ab initio calculations.

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid. Fecha de lectura: 17-10-2017.

The ultrafast decay of highly excited electronic states is resolved with a molecular clock technique, using the vibrational motion associated to the ionic bound states as a time-reference. We demonstrate the validity of the method in the context of autoionization of the hydrogen molecule, where nearly exact full dimensional ab-initio calculations are available. The vibrationally resolved photoionization spectrum provides a time-energy mapping of the autoionization process into the bound states that is used to fully reconstruct the decay in time. A resolution of a fraction of the vibrational period is achieved. Since no assumptions are made on the underlying coupled electron-nuclear dynamics, the reconstruction procedure can be applied to describe the general problem of the decay of highly excited states in other molecular targets, We acknowledge the financial support from the FP7 Marie Curie ITN CORINF, the European Research Council under the ERC grant 290853 XCHEM, the European COST Action CM1204 XLIC and the MINECO Project No. FIS201342002-R. AP acknowledges Ramón y Cajal Programme from MINECO (Spain). LM acknowledges support from DFG priority programme 1840 QUTIF. Theoretical calculations were obtained at the Mare Nostrum BSC and CCC-UAM computer centers

Nature Communications 9(1), 2259 (2018). doi:10.1038/s41467-018-04740-5, The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H2 two-electron wave function in which electron–electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources., Published by Nature Publishing Group UK, [London]

An ultrashort XUV laser pulse ionizes the D2 molecule creating an electronic and nuclear wave packet, with the dominant contributions from the 2sσg and 2pπu ionic states. A delayed interaction with a 780 nm IR field ejects the second electron, leading to the Coulomb explosion of the molecule, whose nuclear fragments, recorded in coincidence, map the dynamics associated to those two ionic excited states. By varying the orientation of the light polarization, one can control the molecular dynamics by modifying the ratio between the ionic states. Experimental and ab initio theoretical data are jointly reported.