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As a first step towards meeting the recent demand for new computational tools capable of reproducing molecular-ionization continua in a wide energy range, we introduce a hybrid Gaussian-B-spline basis (GABS) that combines short-range Gaussian functions, compatible with standard quantum-chemistry computational codes, with B splines, a basis appropriate to represent electronic continua. We illustrate the performance of the GABS hybrid basis for the hydrogen atom by solving both the time-independent and the time-dependent Schrödinger equation for a few representative cases. The results are in excellent agreement with those obtained with a purely B-spline basis, with analytical results, when available, and with recent above-threshold ionization spectra from the literature. In the latter case, we report fully differential photoelectron distributions which offer further insight into the process of above-threshold ionization at different wavelengths ; Work supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 290853, European COST Action No. CM1204 XLIC, and MICINN Project No. FIS2010-15127

This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in The Journal of Physical Chemistry Letters, copyright © 2018 American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.jpclett.7b03220 ; Direct measurement of autoionization lifetimes by using time-resolved experimental techniques is a promising approach when energy-resolved spectroscopic methods do not work. Attosecond time-resolved experiments have recently provided the first quantitative determination of autoionization lifetimes of the lowest members of the well-known Hopfield series of resonances in N2. In this work, we have used the recently developed XCHEM approach to study photoionization of the N2molecule in the vicinity of these resonances. The XCHEM approach allows us to describe electron correlation in the molecular electronic continuum at a level similar to that provided by multireference configuration interaction methods in bound state calculations, a necessary condition to accurately describe autoionization, shakeup, and interchannel couplings occurring in this range of photon energies. Our results show that electron correlation leading to interchannel mixing is the main factor that determines the magnitude and shape of the N2photoionization cross sections, as well as the lifetimes of the Hopfield resonances. At variance with recent speculations, nonadiabatic effects do not seem to play a significant role. These conclusions are supported by the very good agreement between the calculated cross sections and those determined in synchrotron radiation and attosecond experiments ; This work has been supported by the ERC advanced grant 290853 - XCHEM - within the seventh framework programme of the European Union, the ERC proof-of-concept grant 780284 - Imaging-XChem - within the Horizon2020 Framework Programme, and the MINECO projects FIS2013-42002-R and FIS2016-77889-R (AEI/FEDER, UE). We also acknowledge computer time from CCC-UAM and Marenostrum Supercomputer Centers. L.A. acknowledges support from the TAMOP NSF Grant No. 1607588, as well as UCF funding

We present an in-depth theoretical study of N2 photoionization in the region between the second (2Πu) and third (2Σu+) ionization thresholds. In this region, the electronic continuum includes the Hopfield series of autoionizing states, corresponding to excitations to nsσd, ndσd, and ndπg molecular orbitals. Calculations have been performed by using the xchem code, which makes use of a Gaussian and B-spline hybrid basis in the framework of a close-coupling approach. We provide total and partial photoionization cross sections for all open channels, energy positions, and widths for the five lowest resonances of each series and, when resonances are well isolated from each other, Fano and Starace parameters. We also discuss how the coupling between the two series of overlapping resonances, nsσd and ndσd, affects their energies and autoionization widths. These results show the potential of the xchem method to describe resonant photoionization in molecules ; This work has been supported by the ERC Advanced Grant No. 290853 – XCHEM – within the Seventh Framework Program of the European Union, the ERC Proof-of-Concept Grant No. 780284 – Imaging-XChem – within the Horizon 2020 Framework Programme, and MINECO Project No. FIS2016-77889-R (AEI/FEDER, UE). L.A. acknowledges support from the TAMOP NSF through Grant No. 1607588, as well as UCF funding

Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum (ATAS) of helium dressed by a few-cycle visible pulse [C. Ott, Nature (London) 516, 374 (2014)] provide evidence of the inversion of Fano profiles. With the support of accurate ab initio calculations that reproduce the results of the latter experiment, here we investigate the new physics that arise from ATAS when the laser intensity is increased. In particular, we show that (i) previously unnoticed signatures of the dark 2p2 1S doubly excited state are observed in the experimental spectrum, (ii) inversion of Fano profiles is predicted to be periodic in the laser intensity, and (iii) the ac Stark shift of the higher terms in the sp2,n+ autoionizing series exceeds the ponderomotive energy, which is the result of a genuine two-electron contribution to the polarization of the excited atom ; We acknowledge computer time from the CCC-UAM and Marenostrum Supercomputer Centers and financial support from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 290853 XCHEM, the MINECO project FIS2013-42002-R, the ERAChemistry Project PIM2010EEC-00751, the European COST Action XLIC CM1204, the Marie Curie ITN CORINF, and the CAM project NANOFRONTMAG. C.O. and T.P. acknowledge financial support by the Max-Planck Research Group Program of the Max-Planck Gesellschaft (MPG) and the European Research Council (ERC, Grant No. X-MuSiC-616783)

The XCHEM approach interfaces well established quantum chemistry packages with scattering numerical methods in order to describe single-ionization processes in atoms and molecules. This should allow one to describe electron correlation in the continuum at the same level of accuracy as quantum chemistry methods do for bound states. Here we have applied this method to study multichannel photoionization of Ne in the vicinity of the autoionizing states lying between the 2s22p5 and 2s2p6 ionization thresholds. The calculated total photoionization cross sections are in very good agreement with the absolute measurement of Samson et al. [J. Electron Spectrosc. Relat. Phenom. 123, 265 (2002)], and with independent benchmark calculations performed at the same level of theory. From these cross sections, we have extracted resonance positions, total autoionization widths, Fano profile parameters, and correlation parameters for the lowest three autoionizing states. The values of these parameters are in good agreement with those reported in earlier theoretical and experimental work. We have also evaluated β asymmetry parameter and partial photoionization cross sections and, from the latter, partial autoionization widths and Starace parameters for the same resonances, not yet available in the literature. Resonant features in the calculated β parameter are in good agreement with the experimental observations. We have found that the three lowest resonances preferentially decay into the 2p-1ϵd continuum rather than into the 2p-1ϵs one [Phys. Rev. A 89, 043415 (2014)], in agreement with previous expectations, and that in the vicinity of the resonances the partial 2p-1ϵs cross section can be larger than the 2p-1ϵd one, in contrast with the accepted idea that the latter should amply dominate in the whole energy range. These results show the potential of the XCHEM approach to describe highly correlated process in the ionization continuum of many-electron systems, in particular molecules, for which the XCHEM code has been specifically designed ; We acknowledge computer time from the CCC-UAM and Marenostrum Supercomputer Centers, and financial support from the European Research Council under the European Union's Seventh Framework Programme (No. FP7/2007-2013)/ERC Grant Agreement No. 290853 XCHEM, the MINECO Projects No. FIS2013-42002-R and No. FIS2016-77889-R, and the European COST Action XLIC CM1204 and STSM CM1204-26542. L.A. acknowledges support from the TAMOP NSF Grant No. 1607588, as well as UCF fundings. E.L. and T.K. acknowledge support from the Swedish Research Council, Grant No. 2016-03789

We have determined spectral phases of Ne autoionizing states from extreme ultraviolet and midinfrared attosecond interferometric measurements and ab initio full-electron time-dependent theoretical calculations in an energy interval where several of these states are coherently populated. The retrieved phases exhibit a complex behavior as a function of photon energy, which is the consequence of the interference between paths involving various resonances. In spite of this complexity, we show that phases for individual resonances can still be obtained from experiment by using an extension of the Fano model of atomic resonances. As simultaneous excitation of several resonances is a common scenario in many-electron systems, the present work paves the way to reconstruct electron wave packets coherently generated by attosecond pulses in systems larger than helium ; Work supported by the ERC proof-of-concept Grant No. 780284-Imaging-XChem within the seventh framework program of the European Union, the MINECO Project No. FIS2013-42002-R, the EU-H2020- LASERLABEUROPE-654148, the ANR Projects No. ANR-15-CE30-0001-CIMBAAD, No. ANR-11- EQPX0005-ATTOLAB, and No. ANR-10-LABX-0039- PALM, the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award no. DEGF02-04ER15614, and the NSF Grant No. PHY-1607588. Calculations were performed at CCC-UAM and Marenostrum Supercomputer Center. F. M. acknowledges support from the "Severo Ochoa" Programme for Centres of Excellence in R&D (MINECO, Grant No. SEV-2016- 0686) and the "María de Maeztu" Programme for Units of Excellence in R&D (Grant No. MDM-2014-0377)

Time delays of electrons emitted from an isotropic initial state with the absorption of a single photon and leaving behind an isotropic ion are angle independent. Using an interferometric method involving XUV attosecond pulse trains and an IR-probe field in combination with a detection scheme, which allows for full three-dimensional momentum resolution, we show that measured time delays between electrons liberated from the 1s2 spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the common linear polarization axis of the ionizing XUV light and the IR-probing field. Such time delay anisotropy, for which we measure values as large as 60 as, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general and significant effect that must be taken into account in all measurements of time delays involving photoionization processes ; S.H, C.C, L.G., and U.K. acknowledge support by the ERC advanced Grant No. ERC-2012-ADG_20120216 within the seventh framework program of the European Union and by the NCCR MUST, funded by the Swiss National Science Foundation. M.L. acknowledges support from the ETH Zurich Postdoctoral Fellowship Program. A.J.G., L.A., and F.M. acknowledge the support from the European Research Council under the ERC Grant No. 290853 XCHEM, from the European COST Action No. CM1204 XLIC, the MINECO Project No. FIS2013-42002-R, the ERA-Chemistry Project No. PIM2010EEC- 00751, and the European Grant No. MC-ITN CORINF. Calculations were performed at the Centro de Computacion Científica of the Universidad Autónoma de Madrid (CC-UAM) and the Barcelona Supercomputing Center (BSC). I.I. and A.S.K. acknowledge support of the Australian Research Council (Grant No. DP120101805) and the use of the National Computational Infrastructure Facility. J.M.D. acknowledges support from the Swedish Research Grants No. 2013-344 and No. 2014-3724. E.L. acknowledges support from the Swedish Research Council, Grant No. 2012-3668. Moreover, this research was supported in part by the Kavli Institute for Theoretical Physics (National Science Foundation under Grant No. NSF PHY11-25915) and by NORDITA, the Nordic Institute for Theoretical Physics