| 1. |
- Argenti, Luca, et al.
(författare)
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Attosecond photoelectron spectroscopy of helium doubly excited states
- 2023
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Ingår i: Physical Review Research. - 2643-1564. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- We describe a numerical method that simulates the interaction of the helium atom with sequences of femtosecond and attosecond light pulses. The method, which is based on the close-coupling expansion of the electronic configuration space in a B-spline bipolar spherical harmonic basis, can accurately reproduce the excitation and single ionization of the atom, within the electrostatic approximation. The time-dependent Schrödinger equation is integrated with a sequence of second-order split-exponential unitary propagators. The asymptotic channel-, energy-, and angularly resolved photoelectron distributions are computed by projecting the wave packet at the end of the simulation on the multichannel scattering states of the atom, which are separately computed within the same close-coupling basis. This method is applied to simulate the pump-probe ionization of helium in the vicinity of the 2s/2p excitation threshold of the He+ ion. This work confirms the qualitative conclusions of one of our earliest publications [Argenti and Lindroth, Phys. Rev. Lett. 105, 053002 (2010)], in which we demonstrated the control of the 2s/2p ionization branching ratio. Here we take those calculations to convergence and show how correlation brings the periodic modulation of the branching ratios in almost phase opposition. The residual total ionization probability to the 2s+2p channels is dominated by the beating between the sp+2,3 and the sp+2,4 doubly excited states, which is consistent with the modulation of the complementary signal in the 1s channel, measured in 2010 by Chang and coworkers [Gilbertson et al., Phys. Rev. Lett. 105, 263003 (2010)].
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| 2. |
- Argenti, Luca, et al.
(författare)
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Ionization branching ratio control with a resonance attosecond clock
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105, s. 053002-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the possibility to monitor the dynamics of autoionizing states in real-time and control the yields of different ionization channels in helium by simulating extreme ultraviolet (XUV) pump IR-probe experiments focused on the N=2 threshold. The XUV pulse creates a coherent superposition of doubly excited states which is found to decay by ejecting electrons in bursts. Prominent interference fringes in the photoelectron angular distribution of the 2s and 2p ionization channels are observed, along with significant out-of-phase quantum beats in the yields of the corresponding parent ions.
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| 3. |
- Argenti, Luca, et al.
(författare)
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Nondipole effects in helium photoionization
- 2010
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 43:23
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Tidskriftsartikel (refereegranskat)abstract
- An accurate calculation of the nondipole anisotropy parameter gamma in the photoionization of helium below the N = 2 threshold is presented. The calculated results are in fairly good agreement with the experimental results of Krassig et al (2002 Phys. Rev. Lett. 88 203002), but not as good as the accuracy of the calculation should have warranted. A careful examination of the possible causes for the observed discrepancies between theory and experiment seems to rule out any role either of the multipolar terms higher than the electric quadrupole, or of the singlet-triplet spin-orbit mixing. It is argued that such discrepancies might have an instrumental origin, due to the difficulty of measuring vanishingly small total cross sections sigma(tot) with the required accuracy. In such eventuality, it might be more appropriate to use a parameter other than gamma, such as for instance the drag current, to measure the nondipole anisotropy of the photoelectron angular distribution.
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| 4. |
- Argenti, Luca, et al.
(författare)
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Photoionization of helium by attosecond pulses : Extraction of spectra from correlated wave functions
- 2013
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 87:5, s. 053405-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wave packet resulting from the integration of the time-dependent Schrodinger equation in a finite-element discrete variable representation basis. These techniques are projection on products of hydrogenic bound and continuum states, projection onto multichannel scattering states computed in a B-spline close-coupling basis, and a technique based on exterior complex scaling implemented in the same basis used for the time propagation. These methods allow one to monitor the population of continuum states in wave packets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wave packets for two-photon double ionization.
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| 5. |
- Chew, Andrew, et al.
(författare)
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Attosecond transient absorption spectrum of argon at the L-2,L-3 edge
- 2018
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 97:3
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Tidskriftsartikel (refereegranskat)abstract
- Progress in high-harmonic generation has led to high-energy attosecond pulses with cutoff above the carbon 1s edge (283.8 eV). These pulses are essential to extend time-resolved spectroscopies to the water window in order to control electron dynamics in solvated organic species. Here we report a step towards this goal: the measurement, with subcycle time resolution, of the attosecond transient absorption spectrum of argon at the 2p(-1) L-2,L-3 edge (similar to 250 eV) in the presence of a short-wave infrared control pulse. The measurements, supported by theoretical simulations, demonstrate the concurrent role of Auger decay and tunnel ionization in the driven evolution of inner-valence holes of polyelectronic atoms.
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| 6. |
- Heuser, Sebastian, et al.
(författare)
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Angular dependence of photoemission time delay in helium
- 2016
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Ingår i: Physical Review A. - 2469-9926. ; 94:6
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Tidskriftsartikel (refereegranskat)abstract
- 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 1s(2) 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.
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| 7. |
- Lindroth, Eva, et al.
(författare)
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Atomic Resonance States and Their Role in Charge-Changing Processes
- 2012
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Ingår i: Advances in Quantum Chemistry, vol 63. - : Elsevier. - 9780123970091 ; , s. 247-308
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Bokkapitel (refereegranskat)abstract
- Resonant states play a governing role in many charge-changing processes in atoms; the photoionization of atoms in gas phase and the corresponding time-reversed process of dielectronic recombinations being the two most prominent and well-known examples. This is so thanks to their strongly localized character, which leads to large transition matrix elements and to the unusually long timescale at which their dynamics unfolds, if compared to free-electron wave packets. Many resonances correspond also to highly correlated electronic states. Hence, on the one hand, high-resolution extreme ultraviolet (XUV) and soft x-ray photoelectron spectroscopy, which give access to electronic resonances, provide unique insight into the effects of electronic correlation in the energy domain; on the other hand, new laser techniques able to produce light pulses of sub-femtosecond duration give a complementary, much needed, view of electronic correlation resolved in time. Here, we discuss resonances in atoms and atomic ions in all these respects. First, we survey how resonances can be accurately localized and characterized with complex scaling, standard scattering techniques and, for heavy ions, with relativistic many-body theory. The need for high precision is particularly stringent in the latter case since resonances located just above the ionization threshold, and thus whose parameters are most affected by numerical uncertainties, are often the most important for recombination. Second, we examine how the spectral properties of resonances translate in the time domain and how the course of the ionization process can be steered with external ultra-short driving laser pulses.
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| 8. |
- Lindroth, Eva, et al.
(författare)
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The Structure Behind it All
- 2009
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 194, s. 012001-
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Tidskriftsartikel (refereegranskat)abstract
- The talk discussed collisions where the structure of the systems involved plays a decisive role for the outcome of the event. For many types of charge changing processes the presence of resonant states can change the probability for a certain reaction by orders of magnitude. One example of this is electron-ion recombination where the resonant states are doubly or even multiply excited states lying above the ionization threshold of the recombined ion. The concept of a resonant state is discussed with the help of a simple model. The influence of such states is then illustrated through a few examples where some different calculational methods are compared with experiments. Finally, the possibility to also obtain accurate spectroscopical information from a collisional process is discussed
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| 9. |
- Marante, Carlos, et al.
(författare)
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Photoionization using the XCHEM approach : Total and partial cross sections of Ne and resonance parameters above the 2s(2)2p(5) threshold
- 2017
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 96:2
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Tidskriftsartikel (refereegranskat)abstract
- 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 2s(2)2p(5) and 2s(2)p(6) 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 beta 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 beta parameter are in good agreement with the experimental observations. We have found that the three lowest resonances preferentially decay into the 2p(-1) epsilon d continuum rather than into the 2p(-1) epsilon s one [Phys. Rev. A 89, 043415 (2014)], in agreement with previous expectations, and that in the vicinity of the resonances the partial 2p(-1) epsilon s cross section can be larger than the 2p(-1) epsilon d 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.
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| 10. |
- Mehmood, Saad, et al.
(författare)
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Coherence control in helium-ion ensembles
- 2021
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Ingår i: Physical Review Research. - 2643-1564. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- Attosecond pulses can ionize atoms in a coherent process. Since the emerging fragments are entangled, however, each preserves only a fraction of the initial coherence, thus limiting the chance of guiding the ion subsequent evolution. In this work, we use ab initio simulations of pump-probe ionization of helium above the 2s/2p threshold to demonstrate how this loss of coherence can be controlled. Thanks to the participation of 2ℓnℓ′ states, coherence between the ionic 2s and 2p states, which are degenerate in the nonrelativistic limit, results in a stationary, delay-dependent electric dipole. From the picosecond real-time beating of the dipole, caused by the fine-structure splitting of the n=2 manifold, it is possible to reconstruct all original ion coherences, including between antiparallel-spin states, which are a sensitive probe of relativistic effects in attosecond photoemission.
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