Synchronized pulses generated at 20 eV and 90 eV for attosecond pump–probe experiments

Fabris, Davide; Witting, Tobias; Okell, William; Walke, D. J.; Matía-Hernando, Paloma; Henkel, Jost; Barillot, T.; Lein, Manfred; Marangos, Jonathan P.; Tisch, J. W. G.

doi:10.1038/nphoton.2015.77

Cited by 57 publications

(48 citation statements)

References 32 publications

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“…Moreover, the generation of two XUV pulses with table-top sources is also possible, by using high-order high-harmonic generation (HHG) [24]. Great progress has been made in performing ultrafast measurements with HHG sources [25,26], despite the challenge of generating enough photon flux in the inefficient nonlinear conversion process.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray-pump–x-ray-probe spectroscopy

et al. 2016

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The capability of generating two intense, femtosecond x-ray pulses with a controlled time delay opens the possibility of performing time-resolved experiments for x-ray-induced phenomena. We have applied this capability to study the photoinduced dynamics in diatomic molecules. In molecules composed of low-Z elements, K-shell ionization creates a core-hole state in which the main decay mode is an Auger process involving two electrons in the valence shell. After Auger decay, the nuclear wave packets of the transient two-valence-hole states continue evolving on the femtosecond time scale, leading either to separated atomic ions or long-lived quasibound states. By using an x-ray pump and an x-ray probe pulse tuned above the K-shell ionization threshold of the nitrogen molecule, we are able to observe ion dissociation in progress by measuring the time-dependent kinetic energy releases of different breakup channels. We simulated the measurements on N 2 with a molecular dynamics model that accounts for K-shell ionization, Auger decay, and the time evolution of the nuclear wave packets. In addition to explaining the time-dependent feature in the measured kinetic energy release distributions from the dissociative states, the simulation also reveals the contributions of quasibound states.

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Section: Introductionmentioning

confidence: 99%

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray-pump–x-ray-probe spectroscopy

et al. 2016

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“…At present, most experiments that aimed at observing ultrafast dynamics have used a strong infrared field [3,15]; this may actually perturb and be driving the observed dynamics rather than allowing an observation of the dynamics intrinsic to the molecule (a complication that has motivated the development of the attosecond pump-attosecond probe experiments [41]). Several measurements have already been suggested to observe electron dynamics: time-resolved Auger spectra [42], photoelectron angular distributions [43], or x-ray absorption spectra [44,45]-the electronic transition probability being proportional to the time-dependent electronic density in each case.…”

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confidence: 99%

Electron dynamics following photoionization: Decoherence due to the nuclear-wave-packet width

et al. 2015

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The advent of attosecond techniques opens up the possibility to observe experimentally electron dynamics following ionization of molecules. Theoretical studies of pure electron dynamics at single fixed nuclear geometries in molecules have demonstrated oscillatory charge migration at a well-defined frequency but often neglecting the natural width of the nuclear wave packet. The effect on electron dynamics of the spatial delocalization of the nuclei is an outstanding question. Here, we show how the inherent distribution of nuclear geometries leads to dephasing. Using a simple analytical model, we demonstrate that the conditions for a long-lived electronic coherence are a narrow nuclear wave packet and almost parallel potential-energy surfaces of the states involved. We demonstrate with numerical simulations the decoherence of electron dynamics for two real molecular systems (paraxylene and polycyclic norbornadiene), which exhibit different decoherence time scales. To represent the quantum distribution of geometries of the nuclear wave packet, the Wigner distribution function is used. The electron dynamics decoherence result has significant implications for the interpretation of attosecond spectroscopy experiments since one no longer expects long-lived oscillations. The generation of attosecond pulses in the extreme ultraviolet range (using high harmonic generation in gases) [1,2] has opened up the possibility to probe dynamics in atoms, molecules, and solids with attosecond resolution [3-5]-the natural time scale of electronic motion. Attosecond techniques have since been developed and applied successfully to a range of problems, including the real-time observation of electronic relaxation in krypton atoms [6], the measurement of delays in photoemission of electrons in condensed-matter [7] and atomic [8] systems using the streaking technique, and the observation of electron dynamics in krypton atoms upon valence ionization using transient absorption spectroscopy [9].One key target of attosecond experiments remains the real-time observation and control of electron dynamics upon ionization in molecules [10][11][12][13][14][15][16][17]. The interference between electronic eigenstates, populated coherently, alternates between constructive and destructive and leads to oscillating motion of the electronic density with a period inversely proportional to the energy gap. This is "pure" electron dynamics (i.e., takes place even if the nuclei are fixed) and is often called charge migration in the literature [18] or hole migration if it is induced by electron correlation [19,20].A fascinating and outstanding question in the theoretical description of electron dynamics in molecules is the effect of the nuclei since most studies are carried out at a single fixed nuclear geometry (usually the equilibrium geometry of the neutral species) [21][22][23][24][25][26]. (Some studies include several conformers [22,27] but again with a single geometry per conformer.) These simulations predict long-lived oscillating motion in the electron...

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“…We can compute the number of complete oscillations of the average spin density before the amplitude is halved n 1 2 = t 1 2 /T; this is shown in Table I. Note that we have the optimum situation in PLN2 where the energy gap is small enough to allow 1.5 oscillations (n 1 2 ) before dephasing (t 1 2 ) occurs. In contrast for NBD the width is large and the period is small, and in PLN3 the period is large and the width is small, resulting in both cases in n 1 2 < 1.…”

Section: Resultsmentioning

confidence: 99%

“…1,2 Attosecond spectroscopy promises the prospect of real time observation of electronic motion, such as charge migration, 3 on its native time scale. 2,[4][5][6][7] In charge migration, several cationic states are populated coherently upon ionization of a neutral species, forming a non-stationary state where the hole oscillates between sites in the molecule (this is in contrast to charge transfer that is driven by nuclear motion).…”

Section: Introductionmentioning

confidence: 99%

“…Considering a single static nuclear geometry, the solution of the time-dependent electronic Schrödinger equation (in a.u.) for a two-state wavepacket reads Ψ(r,t; R) = c 0 e −i E 0 (R)t ψ 0 (r; R) + c 1 e −i E 1 (R)t ψ 1 (r; R), (1) where r and R are the electronic and nuclear coordinates, respectively. The hole oscillates with a frequency defined by the energy gap of the states involved,…”

Section: Introductionmentioning

confidence: 99%

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Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

Jenkins

Vacher

Twidale

et al. 2016

The Journal of Chemical Physics

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The observation of electronic motion remains a key target in the development of the field of attoscience. However, systems in which long-lived oscillatory charge migration may be observed must be selected carefully, particularly because it has been shown that nuclear spatial delocalization leads to a loss of coherent electron density oscillations. Here we demonstrate electron dynamics in norbornadiene and extended systems where the hole density migrates between two identical chromophores. By studying the effect of nuclear motion and delocalization in these example systems, we present the physical properties that must be considered in candidate molecules in which to observe electron dynamics. Furthermore, we also show a key contribution to nuclear delocalization arises from motion in the branching plane of the cation. For the systems studied, the dephasing time increases with system size while the energy gap between states, and therefore the frequency of the density oscillation, decreases with size (obeying a simple exponential dependence on the inter-chromophore distance). We present a system that balances these two effects and shows several complete oscillations in the spin density before dephasing occurs. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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Synchronized pulses generated at 20 eV and 90 eV for attosecond pump–probe experiments

Cited by 57 publications

References 32 publications

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray-pump–x-ray-probe spectroscopy

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray-pump–x-ray-probe spectroscopy

Electron dynamics following photoionization: Decoherence due to the nuclear-wave-packet width

Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

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