Investigating two-photon double ionization ofD2by XUV-pump–XUV-probe experiments (original) (raw)
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Physical Review A, 2010
Two-photon double ionization (TPDI) of D 2 is studied for 38-eV photons at the Free Electron Laser in Hamburg (FLASH). Based on model calculations, instantaneous and sequential absorption pathways are identified as separated peaks in the measured D + + D + fragment kinetic energy release (KER) spectra. The instantaneous process appears at high KER, corresponding to ionization at the molecule's equilibrium distance, in contrast to sequential ionization mainly leading to low-KER contributions. Measured fragment angular distributions are in good agreement with theory.
Electronic correlations in double ionization of atoms in pump-probe experiments
EPL (Europhysics Letters), 2010
The ionization dynamics of a two-electron atom in an attosecond XUV-infrared pump-probe experiment is simulated by solving the time-dependent two-electron Schrödinger equation. A dramatic change of the double ionization (DI) yield with variation of the pumpprobe delay is reported and the governing role of electron-electron correlations is shown. The results allow for a direct control of the DI yield and of the relative strength of double and single ionization.
Signatures of direct double ionization under xuv radiation
Physical Review A - Atomic, Molecular, and Optical Physics, 2005
In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under strong short wavelength radiation sources (45 eV), we present quantitative signatures of direct twophoton double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections PACS numbers: 32.80.Wr
Double ionization probed on the attosecond timescale
Nature Physics, 2014
Double ionization following the absorption of a single photon is one of the most fundamental processes requiring interaction between electrons 1-3. Information about this interaction is usually obtained by detecting emitted particles without access to real-time dynamics. Here, attosecond light pulses 4,5 , electron wave packet interferometry 6 and coincidence techniques 7 are combined to measure electron emission times in double ionization of xenon using single ionization as a clock, providing unique insight into the two-electron ejection mechanism. Access to many-particle dynamics in real time is of fundamental importance for understanding processes induced by electron correlation in atomic, molecular and more complex systems. The emergence of attosecond science (1 as = 10 −18 s) in the new millennium opened an exciting area of physics bringing the dynamics of electron wave functions into focus. The important goal of real-time visualization of the interplay between electrons and their role in molecular bonding now seems to be in reach. After a decade where attosecond light sources 4,5 were characterized and their potential demonstrated, the next phase will include the exploration of correlated electron dynamics in complex systems. A series of groundbreaking studies on single ionization (SI) in atoms using attosecond light pulses sheds light on the escaping electron and its interaction with the residual ion 6,8 , and the resulting coherent superposition of neutral bound states 9,10. Double ionization (DI) by absorption of a single photon is an inherently more challenging phenomenon, both experimentally and theoretically 1-3. The two-electron ejection can be understood only through interactions between electrons, and is usually discussed in terms of different mechanisms 11. In the knockout mechanism, the electron excited by interaction with the light field (the photoelectron) collides with another electron on its way out, resulting in two emitted electrons. In the shake-off mechanism, orbital relaxation following the creation of a hole ionizes a second electron. Electron correlations may also lead to indirect DI processes via highly excited states of the singly-charged ion 12. One-photon experimental investigations with the pair of electrons detected in coincidence can provide a fairly complete DI description without, however, following the dynamics of the electron correlation in real time. Multiphoton experimental investigations have been performed both on the femtosecond and attosecond timescales 13,14 , but DI in strong laser fields does not require electron correlation. In this work, we study DI of xenon in the near-threshold region using attosecond extreme ultraviolet (XUV) pulses for excitation
Two-color photoionization in xuv free-electron and visible laser fields
Physical Review A, 2006
Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg ͑FLASH at DESY͒ with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths.
Feasibility of coherent xuv spectroscopy on the 1S-2S transition in singly ionized helium
Physical Review A, 2009
The 1S-2S two-photon transition in singly ionized helium is a highly interesting candidate for precision tests of bound-state quantum electrodynamics ͑QED͒. With the recent advent of extreme ultraviolet frequency combs, highly coherent quasi-continuous-wave light sources at 61 nm have become available, and precision spectroscopy of this transition now comes into reach for the first time. We discuss quantitatively the feasibility of such an experiment by analyzing excitation and ionization rates, propose an experimental scheme, and explore the potential for QED tests.
Physical mechanisms encoded in photoionization yield from IR+XUV setups
The European Physical Journal D
We theoretically examine how and to which extent physical processes can be retrieved from two-color pump-probe experiments of atomic and molecular gases driven by an attosecond XUV pulse train and an infrared (IR) pulse. The He atom, the N2 molecule and Na clusters are investigated with time-dependent density functional theory. Results are interpreted on the basis of a simple model system. We consider observables most commonly used in experiments: ionization yield, photo-electron spectra, and angular distributions. We find that the basic time-dependent signatures are dominated by the interplay of IR laser and continuum electrons. System information, contained in the signal, will in general require careful disentangling from the effects of photon-electron dynamics.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2002
The mutual angular distributions of the two ejected electrons following direct photodouble ionization have been measured for D 2 at an excess energy (E) of 25 eV using linearly polarized light. These (γ , 2e) 'triple' differential cross sections (TDCSs) were obtained for asymmetric electron energy conditions with energy sharing ratios (R = E 2 /E 1 ) of R = 24, 11.5, 4 and 2.57. In all cases the 'reference' electron (energy = E 1 ) was oriented along the direction of the electric field vector (ε) and detected in coincidence with a second electron (energy = E 2 ) coplanar with ε and the photon beam direction (k γ ). For comparison, helium TDCSs were obtained for the same E and R values under nearly identical spectrometer conditions. These show very good agreement with the results of hyperspherical--matrix with semi-classical outgoing waves calculations, thus providing even more confidence in the D 2 TDCSs where there is as yet no accurate ab initio theory. The similarities and differences between the experimental results associated with the two targets are qualitatively discussed in terms of Feagin's model (Feagin J M 1998 J. Phys. B: At. Mol. Opt. Phys. 31 L729).
Characterization of the FLASH XUV-FEL pulses by two-color photoionization experiments
UVX 2008 - 9e Colloque sur les Sources Cohérentes et Incohérentes UV, VUV et X : Applications et Développements Récents, 2009
The current key performance indicators of the pump-probe facility at the Free Electron Laser in Hamburg (FLASH) are described. The temporal and spatial characteristics of both the extreme ultraviolet (XUV) Free Electron Laser and the temporally synchronized optical femtosecond Ti:Sapphire laser are determined by measuring two-color above threshold ionization in rare gases. Characteristic sidebands appear in the photoelectron spectrum under the combined action of both radiation fields providing a very sensitive probe for the spatial and temporal overlap of both pulses. The high spatial acceptance of a magnetic bottle electron spectrometer enabled us to obtain single-shot photoelectron spectra and to correct for the inherent time jitter between both light sources, which reduces the temporal resolution when recording spectra in average mode. As an illustration of the first application, experimental results on polarization control in the two-photon two-color ionization of atomic He are presented.
Nonsequential two-photon double ionization of helium
Physical Review A, 2008
We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process is fully described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime. PACS numbers: 32.80.Rm, 32.80.Fb, 42.50.Hz Correlated dynamical processes in nature poses unique challenges to experiments and theory. A prime example of this is the double ionization of helium by one-photon impact, which has been studied for more than 40 years. However, it is only during the last 15 years or so, that advances in theory, modeling and experiment have enabled scientists to gain a deeper insight into the role of electron correlations in this ionization process . The corresponding problem of two-photon double ionization of helium, in the photon energy interval between 39.4 and 54.4 eV, is an outstanding quantum mechanical problem that has been, and still is, subject to intense research worldwide, both theoretically and experimentally, employing state-of-the-art high-order harmonic [20-22] and free-electron (FEL) light sources . Despite all the interest and efforts that have been put into this research, major fundamental issues remain unresolved. What characterizes this particular three-body breakup process is that the electron correlation is a prerequisite for the process to occur, i.e., it depends upon the exchange of energy between the outgoing electrons, and as such it represents a clear departure from an independent-particle picture.
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The Journal of Chemical Physics, 2012
A time-dependent approach for the kinetic energy release (KER) spectrum is developed for a fragmentation of a diatomic molecule after an electronic decay process, e.g. Auger process. It allows one to simulate the time-resolved spectra and provides more insight into the molecular dynamics than the time-independent approach. Detailed analysis of the time-resolved emitted electron and KER spectra sheds light on the interrelation between wave packet dynamics and spectra.
Physical Review Letters
We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2p-3d boundbound transitions between the spin-orbit multiplets 3 P0,1,2 and 3 D1,2,3 of the transiently produced doubly charged Ne 2+ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4 ± 0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the AC Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron-laser radiation when the phase-locked pump and probe pulses precisely overlap in time.
Clocking Ultrafast Wave Packet Dynamics in Molecules through UV-Induced Symmetry Breaking
Physical Review Letters, 2012
We investigate the use of UV-pump-UV-probe schemes to trace the evolution of nuclear wave packets in excited molecular states by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the nuclear wave packet launched by the pump pulse in the neutral molecule is located. Hence, the variation of this asymmetry with the time delay between the pump and the probe pulses parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The performance of this method is illustrated for the H 2 molecule.
Physical Review A, 2012
Angular distributions of molecular above-threshold ionization (MATI) in bichromatic attosecond extreme ultraviolet (XUV) linear polarization laser pulses have been theoretically investigated. Multiphoton ionization in a prealigned molecular ion H 2 + produces clear MATI spectra which show a forward-backward asymmetry in angular and momentum distributions which is critically sensitive to the carrier envelope phase (CEP) φ, the time delay τ between the two laser pulses, and the photoelectron kinetic energies E e. The features of the asymmetry in MATI angular distributions are described well by multiphoton perturbative ionization models. Phase differences of continuum electron wave functions can be extracted from the CEP φ and time delay τ dependent ionization asymmetry ratio created by interfering multiphoton ionization pathways. At large internuclear distances MATI angular distributions exhibit more complex features due to laser-induced electron diffraction where continuum electron wavelengths are less than the internuclear distance.
X-ray multiphoton-induced Coulomb explosion images complex single molecules
Nature Physics
Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10−15 s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicti...
Physical Review A, 2012
We traced the femtosecond nuclear wave-packet dynamics in ionic states of oxygen and nitrogen diatomic molecules employing 38 eV XUV pump -XUV probe experiments at the Free Electron Laser in Hamburg (FLASH). The nuclear dynamics is monitored via the detection of coincident ionic fragments using a reaction microscope and a split-mirror setup to generate the pump and probe pulses. By comparing measured kinetic-energy-release spectra with classical and quantummechanical simulations, we identified electronic states of the molecular ions that are populated by ionization of the neutral molecule. For specific fragment charge states, this comparison allows us to assess the relevance of specific dissociation paths.
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Attosecond timescale analysis of the dynamics of two-photon double ionization of helium
New Journal of Physics, 2008
We consider the two-photon double ionization (DI) of helium and analyze electron dynamics on the attosecond timescale. We first re-examine the interaction of helium with an ultrashort XUV pulse and study how the electronic correlations affect the electron angular and energy distributions in the direct, sequential and transient regimes of frequency and time duration. We then consider pump-probe processes with the aim of extracting indirect information on the pump pulse. In addition, our calculations show clear evidence for the existence under certain conditions of direct two-color DI processes.
Using a new experimental method, physicists from the Max Planck Institute for Nuclear Physics in Heidelberg investigated the resonant two-photon ionization of helium with improved spectral resolution and angular resolution. [34]
Journal of Physics: Conference Series, 2010
We consider two-photon double ionization of helium by two xuv photons in the region around the sequential ionization threshold. We show that, on the attosecond timescale, the mechanism for double ionization is dominated by the absorption of one photon by each electron in the fundamental state He(1s 2 ). We examine the dynamics of two-photon double ionization of helium for an averaged photon energy ω =50 eV, with a pulse duration of two optical cycles. The double ionization rate, energy and angular distributions are calculated by solving the time-dependent Schrödinger equation. Results are discussed on the basis of a model.
Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser
New Journal of Physics, 2017
In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C 6 H 3 F 2 I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the timedependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.
Two-photon double ionization of the helium atom by ultrashort pulses
Journal of Physics B: Atomic, Molecular and Optical Physics, 2010
Two-photon double ionization of the helium atom was the subject of early experiments at FLASH and will be the subject of future benchmark measurements of the associated electron angular and energy distributions. As the photon energy of a single femtosecond pulse is raised from the threshold for two-photon double ionization at 39.5 eV to beyond the sequential ionization threshold at 54.4 eV, the electron ejection dynamics change from the highly correlated motion associated with nonsequential absorption to the much less correlated sequential ionization process. The signatures of both processes have been predicted in accurate \textit{ab initio} calculations of the joint angular and energy distributions of the electrons, and those predictions contain some surprises. The dominant terms that contribute to sequential ionization make their presence apparent several eV below that threshold. In two-color pump probe experiments with short pulses whose central frequencies require that the sequential ionization process necessarily dominates, a two-electron interference pattern emerges that depends on the pulse delay and the spin state of the atom.
Fully Differential Cross Sections for Photo-Double-Ionization of D2_
Physical Review Letters, 2004
We present the first report on the angular distributions of two electrons ejected from a fixed-in-space D 2 molecule by the absorption of a single photon. We focus on equal energy sharing of the two emitted electrons while the photon energy was 75.5 eV. We observed the theoretically predicted relaxation of one of the selection rules valid for He photo double ionization but removed for D 2 . For coplanar geometry of the two-electron escape we found a strong dependence of the electron angular distribution on the orientation of the molecular axis relative to the linear polarization axis of the light. This effect is reproduced qualitatively by a simple theoretical model in which a pair of photo ionization amplitudes is introduced for the light polarization parallel and perpendicular to the molecular axis. These amplitudes are calculated in the single-center approximation.
Rescattering Double Ionization of D2 and H2 by Intense Laser Pulses
Physical Review Letters, 2003
We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of D 2 (and H 2 ) molecules by short intense laser pulses. We find high-energy coincident D (and H ) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.
Physical Review A, 2012
We traced the femtosecond nuclear wave-packet dynamics in ionic states of oxygen and nitrogen diatomic molecules employing 38 eV XUV pump -XUV probe experiments at the Free Electron Laser in Hamburg (FLASH). The nuclear dynamics is monitored via the detection of coincident ionic fragments using a reaction microscope and a split-mirror setup to generate the pump and probe pulses. By comparing measured kinetic-energy-release spectra with classical and quantummechanical simulations, we identified electronic states of the molecular ions that are populated by ionization of the neutral molecule. For specific fragment charge states, this comparison allows us to assess the relevance of specific dissociation paths.
Physical Review Letters, 2009
We present a combined theoretical and experimental study of ultrafast wave-packet dynamics in the dissociative ionization of H 2 molecules as a result of irradiation with an extreme-ultraviolet (XUV) pulse followed by an infrared (IR) pulse. In experiments where the duration of both the XUV and IR pulses are shorter than the vibrational period of H 2 þ , dephasing and rephasing of the vibrational wave packet that is formed in H 2 þ upon ionization of the neutral molecule by the XUV pulse is observed. In experiments where the duration of the IR pulse exceeds the vibrational period of H 2 þ (15 fs), a pronounced dependence of the H þ kinetic energy distribution on XUV-IR delay is observed that can be explained in terms of the adiabatic propagation of the H 2 þ wave packet on field-dressed potential energy curves.
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