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| 2. |
- Alexandridi, C., et al.
(author)
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Attosecond photoionization dynamics in the vicinity of the Cooper minima in argon
- 2021
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In: Physical Review Research. - 2643-1564. ; 3:1
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Journal article (peer-reviewed)abstract
- Using a spectrally resolved electron interferometry technique, we measure photoionization time delays between the 3s and 3p subshells of argon over a large 34-eV energy range covering the Cooper minima in both subshells. The observed strong variations of the 3s−3p delay difference, including a sign change, are well reproduced by theoretical calculations using the two-photon two-color random-phase approximation with exchange. Strong shake-up channels lead to photoelectrons spectrally overlapping with those emitted from the 3s subshell. These channels need to be included in our analysis to reproduce the experimental data. Our measurements provide a benchmark for multielectronic theoretical models aiming at an accurate description of interchannel correlation.
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| 3. |
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| 4. |
- Arab, Khelifa, et al.
(author)
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Long Noncoding RNA TARID Directs Demethylation and Activation of the Tumor Suppressor TCF21 via GADD45A
- 2014
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In: Molecular Cell. - : Elsevier BV. - 1097-2765 .- 1097-4164. ; 55:4, s. 604-614
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Journal article (peer-reviewed)abstract
- DNA methylation is a dynamic and reversible process that governs gene expression during development and disease. Several examples of active DNA demethylation have been documented, involving genome-wide and gene-specific DNA demethylation. How demethylating enzymes are targeted to specific genomic loci remains largely unknown. We show that an antisense lncRNA, termed TARID (for TCF21 antisense RNA inducing demethylation), activates TCF21 expression by inducing promoter demethylation. TARID interacts with both the TCF21 promoter and GADD45A (growth arrest and DNA-damage-inducible, alpha), a regulator of DNA demethylation. GADD45A in turn recruits thymine-DNA glycosylase for base excision repair-mediated demethylation involving oxidation of 5-methylcytosine to 5-hydroxymethylcytosine in the TCF21 promoter by ten-eleven translocation methylcytosine dioxygenase proteins. The results reveal a function of lncRNAs, serving as a genomic address label for GADD45A-mediated demethylation of specific target genes.
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| 5. |
- Argenti, Luca, et al.
(author)
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Attosecond photoelectron spectroscopy of helium doubly excited states
- 2023
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In: Physical Review Research. - 2643-1564. ; 5:3
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Journal article (peer-reviewed)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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| 6. |
- Argenti, Luca, et al.
(author)
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Ionization branching ratio control with a resonance attosecond clock
- 2010
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105, s. 053002-
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Journal article (peer-reviewed)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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| 7. |
- Argenti, Luca, et al.
(author)
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Photoionization of helium by attosecond pulses : Extraction of spectra from correlated wave functions
- 2013
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In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 87:5, s. 053405-
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Journal article (peer-reviewed)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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| 8. |
- Baggesen, J.C., et al.
(author)
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Theory of attosecond absorption spectroscopy in krypton
- 2012
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In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 85, s. 013415-
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Journal article (peer-reviewed)abstract
- A theory for time-domain attosecond pump–attosecond probe photoabsorption spectroscopy is formulated and related to the atomic response. The theory is illustrated through a study of attosecond absorption spectroscopy in krypton. The atomic parameters entering the formulation such as energies and Auger widths, as well as wave functions and dipole coupling matrix elements, are determined by accurate many-body structure calculations. We create a hole in a valence shell by an attosecond pump, couple an inner-shell electron to the hole by an attosecond probe, and then monitor the formation of the hole in this manner. In a second example, a hole is created in an inner shell by the first pulse, and the second probe pulse couples an even more tightly bound state to that hole. The hole decays in this example by Auger electron emission, and the absorption spectroscopy follows the decay of the hole and the associated coherences in real time.
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| 9. |
- Bengtsson, Jakob, 1980-, et al.
(author)
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Modifying H− resonance asymmetries with short light pulses
- 2012
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In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 85:5, s. 053413-
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Journal article (peer-reviewed)abstract
- We present a method, based on time-dependent perturbation theory and complex rotation, to treat the interaction of a short light pulse with a correlated atomic system. The pulse is built from two short and weak pulses with Gaussian envelopes that are centred at two different frequencies.The method is applied to the negative hydrogen ion in the vicinity of a doubly excited resonanceand it is shown that the two light pulses can be used to alter the Fano profile of a resonance.
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| 10. |
- Bengtsson, Jakob, 1980-
(author)
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On the use of the uniform complex scaling-method for studying time-dependent systems
- 2012
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Doctoral thesis (other academic/artistic)abstract
- During the last few decades, laser technology has gone through a series of revolutionary improvements. With light pulses in the sub-femtosecond regime now experimentally available, an explicit time-dependent Hamilton operator is indispensable for a proper theoretical description of the interactions of atoms with such pulses. However, these theoretical studies are computationally very demanding, which motivates the search for new numerical methods and algorithms to approach time-dependent problems. This thesis contributes to this research field, with the main focus on the use of complex-scaled Hamilton operators. Thus, the formalism of complex scaling is studied in the context of its application to explicitly time-dependent atomic systems. Both non-relativistic and relativistic dynamics are investigated. The numerical advantages and the possibilities to extract physical quantities from complex-scaled wave functions are discussed. Of special interest is the ability to treat resonance states. These are multiply excited electronic states with sufficient energy to decay through Auger transitions to the surrounding continuum. With complex scaling, the Hamilton operator is non-Hermitian and such resonance states can be obtained as eigenstates. To analyze the non-bound part of the wave function requires essentially the construction of a second wave function; the left state vector. This additional wave function is, however, not easily constructed numerically in the complex scaling-method. To circumvent some of the numerical problems, we have proposed three different methods. These methods are based on Floquet theory, a propagation on a complex time-grid and time-dependent perturbation theory. By carefully investigating the numerical properties of the left state vector, we have thus studied the non-bound part of the system.
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