Dynamics of hollow atom formation in intense x-ray pulses probed by partial covariance mapping

The Role of Momentum Partitioning in Covariance Ion Imaging Analysis

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data.

Transient responses of double core-holes generation in all-attosecond pump-probe spectroscopy

Double core-holes (DCHs) show remarkable and sensitive effects for understanding electron correlations and coherence. With advanced modulation of x-ray free-electron laser (XFEL) facility, we propose the forthcoming all-attosecond XFEL pump-probe spectroscopy can decipher the hidden photon-initiated dynamics of DCHs. The benchmark case of neon is investigated, and norm-nonconserving Monte-Carlo wavefunction method simulates non-Hermitian dynamics among vast states, which shows superiority in efficiency and reliability. In our scheme, population transfer to DCHs is sequentially irradiated by pump and probe laser. By varying time delay, Stark shifts and quantum path interference of resonant lines sensitively emerge at specific interval of two pulses. These ubiquitous multi-channel effects are also observed in phase-fluctuating pulses, derived from extra phases of impulsive Raman processes by pump laser. Non-perturbation absorption/emission verifies the uniquely interchangeable role of two pules in higher intensity. Our results reveal sensitive and robust responses on pulse parameters, which show potential capacity for XFEL attosecond pulse diagnosis and further attosecond-timescale chemical analysis.

Multiparticle Cumulant Mapping for Coulomb Explosion Imaging

We extend covariance velocity map ion imaging to four particles, establishing cumulant mapping and allowing for measurements that provide insights usually associated with coincidence detection, but at much higher count rates. Without correction, a fourfold covariance analysis is contaminated by the pairwise correlations of uncorrelated events, but we have addressed this with the calculation of a full cumulant, which subtracts pairwise correlations. We demonstrate the approach on the four-body breakup of formaldehyde following strong field multiple ionization in few-cycle laser pulses. We compare Coulomb explosion imaging for two different pulse durations (30 and 6 fs), highlighting the dynamics that can take place on ultrafast timescales. These results have important implications for Coulomb explosion imaging as a tool for studying ultrafast structural changes in molecules, a capability that is especially desirable for high-count-rate x-ray free-electron laser experiments.

Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis

We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot^-1). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.

Cumulant mapping as the basis of multi-dimensional spectrometry

Cumulant mapping employs a statistical reconstruction of the whole by sampling its parts. The theory developed in this work formalises and extends ad hoc methods of 'multi-fold' or 'multi-dimensional' covariance mapping. Explicit formulae have been derived for the expected values of up to the 6th cumulant and the variance has been calculated for up to the 4th cumulant. A method of extending these formulae to higher cumulants has been described. The formulae take into account reduced fragment detection efficiency and a background from uncorrelated events. Number of samples needed for suppressing the statistical noise to a required level can be estimated using Matlab code included in Supplemental Material. The theory can be used to assess the experimental feasibility of studying molecular fragmentations induced by femtosecond or X-ray free-electron lasers. It is also relevant for extending the conventional mass spectrometry of biomolecules to multiple dimensions.

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d_3/2 and 4d_5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Multi-reference approach to the computation of double core-hole spectra

Double Core-Hole (DCH) states of small molecules are assessed with the restricted active space self-consistent field and multi-state restricted active space perturbation theory of second order approximations. To ensure an unbiased description of the relaxation and correlation effects on the DCH states, the neutral ground-state and DCH wave functions are optimized separately, whereas the spectral intensities are computed with a biorthonormalized set of molecular orbitals within the state-interaction approximation. Accurate shake-up satellite binding energies and intensities of double-core-ionized states (K^-2) are obtained for H₂O, N₂, CO, and C₂H_2n (n = 1-3). The results are analyzed in detail and show excellent agreement with recent theoretical and experimental data. The K^-2 shake-up spectra of H₂O and C₂H_2n molecules are here completely characterized for the first time.

Transient resonant Auger-Meitner spectra of photoexcited thymine

We present the first investigation of excited state dynamics by resonant Auger-Meitner spectroscopy (also known as resonant Auger spectroscopy) using the nucleobase thymine as an example. Thymine is photoexcited in the UV and probed with X-ray photon energies at and below the oxygen K-edge. After initial photoexcitation to a ππ* excited state, thymine is known to undergo internal conversion to an nπ* excited state with a strong resonance at the oxygen K-edge, red-shifted from the ground state π* resonances of thymine (see our previous study Wolf, et al., Nat. Commun., 2017, 8, 29). We resolve and compare the Auger-Meitner electron spectra associated both with the excited state and ground state resonances, and distinguish participator and spectator decay contributions. Furthermore, we observe simultaneously with the decay of the nπ* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the nπ* state and the ground state resonances. We assign these contributions to population transfer from the nπ* state to a ππ* triplet state via intersystem crossing on the picosecond timescale based on simulations of the X-ray absorption spectra in the vibrationally hot triplet state. Moreover, we identify signatures from the initially excited ππ* singlet state which we have not observed in our previous study.

Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with time-resolved recoil-frame covariance imaging

The photodissociation dynamics of strong-field ionized methyl iodide (CH3I) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources.

Three-dimensional covariance-map imaging of molecular structure and dynamics on the ultrafast timescale

Ultrafast laser pump-probe methods allow chemical reactions to be followed in real time, and have provided unprecedented insight into fundamental aspects of chemical reactivity. While evolution of the electronic structure of the system under study is evident from changes in the observed spectral signatures, information on rearrangement of the nuclear framework is generally obtained indirectly. Disentangling contributions to the signal arising from competing photochemical pathways can also be challenging. Here we introduce the new technique of three-dimensional covariance-map Coulomb explosion imaging, which has the potential to provide complete three-dimensional information on molecular structure and dynamics as they evolve in real time during a gas-phase chemical reaction. We present first proof-of-concept data from recent measurements on CF₃I. Our approach allows the contributions from competing fragmentation pathways to be isolated and characterised unambiguously, and is a promising route to enabling the recording of 'molecular movies' for a wide variety of gas-phase chemical processes.

Multielectron-Ion Coincidence Spectroscopy of Xe in Extreme Ultraviolet Laser Fields: Nonlinear Multiple Ionization via Double Core-Hole States

Ultrafast multiphoton ionization of Xe in strong extreme ultraviolet free-electron laser (FEL) fields (91 eV, 30 fs, 1.6×10^{12}  W/cm^{2}) has been investigated by multielectron-ion coincidence spectroscopy. The electron spectra recorded in coincidence with Xe^{4+} show characteristic features associated with two-photon absorption to the 4d^{-2} double core-hole (DCH) states and subsequent Auger decay. It is found that the pathway via the DCH states, which has eluded clear identification in previous studies, makes a large contribution to the multiple ionization, despite the long FEL pulse duration compared with the lifetime of the 4d core-hole states.

Hard x-ray spectroscopy and dynamics of isolated atoms and molecules: a review

We present here a review of the most significant recent achievements in the field of HAXPES (hard x-ray photoelectron spectroscopy) on isolated atoms and molecules, and related spectroscopies. The possibility of conducting hard x-ray photoexcitation and photoionization experiments under state-of-the art conditions in terms of photon and electron kinetic energy resolution has become available only in the last few years. HAXPES has then produced structural and dynamical information at the level of detail already reached in the VUV and soft-x-ray ranges. The much improved experimental conditions have allowed extending to the hard x-ray range some methods well established in soft x-ray spectroscopies. Investigations of electron and nuclear dynamics in the femtosecond (fs, 10^-15 s) and even attosecond (as, 10^-18 s) regime have become feasible. Complex relaxation phenomena following deep-core ionization can now be enlightened in great detail. Other phenomena like e.g. recoil-induced effects are much more important in fast photoelectron emission, which can be induced by hard x-rays. Furthermore, a new kind of ionic states with double core holes can be observed by x-ray single-photon absorption. Future perspectives are also discussed.

Photochemical pathways in nucleobases measured with an X-ray FEL

The conversion of light energy into other molecular energetic degrees of freedom is often dominated by ultrafast, non-adiabatic processes. Femtosecond spectroscopy with optical pulses has helped in shaping our understanding of crucial processes in molecular energy-conversion. The advent of new, ultrashort and bright X-ray free electron laser sources opens the possibility to use X-ray-typical element and site sensitivity for ultrafast molecular research. We present two types of spectroscopy, ultrafast Auger and ultrafast X-ray absorption spectroscopy, and discuss their sensitivity to molecular processes. While Auger spectroscopy is able to monitor bond distance changes in the vicinity of an X-ray created core hole, near-edge absorption spectroscopy can deliver high-fidelity information on non-adiabatic transitions involving lone-pair orbitals. We demonstrate these features on the example of the UV-excited nucleobase thymine, investigated at the oxygen K-edge. We find a C-O bond elongation in the Auger data in addition to ππ*/ nπ* non-adiabatic transition in X-ray near-edge absorption. We compare the results from both methods and draw a conclusive scenario of non-adiabatic molecular relaxation after UV excitation. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

The LAMP instrument at the Linac Coherent Light Source free-electron laser

The Laser Applications in Materials Processing (LAMP) instrument is a new end-station for soft X-ray imaging, high-field physics, and ultrafast X-ray science experiments that is available to users at the Linac Coherent Light Source (LCLS) free-electron laser. While the instrument resides in the Atomic, Molecular and Optical science hutch, its components can be used at any LCLS beamline. The end-station has a modular design that provides high flexibility in order to meet user-defined experimental requirements and specifications. The ultra-high-vacuum environment supports different sample delivery systems, including pulsed and continuous atomic, molecular, and cluster jets; liquid and aerosols jets; and effusive metal vapor beams. It also houses movable, large-format, high-speed pnCCD X-ray detectors for detecting scattered and fluorescent photons. Multiple charged-particle spectrometer options are compatible with the LAMP chamber, including a double-sided spectrometer for simultaneous and even coincident measurements of electrons, ions, and photons produced by the interaction of the high-intensity X-ray beam with the various samples. Here we describe the design and capabilities of the spectrometers along with some general aspects of the LAMP chamber and show some results from the initial instrument commissioning.

Short-wavelength free-electron laser sources and science: a review

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

Cationic double K-hole pre-edge states of CS2 and SF6

Recent advances in X-ray instrumentation have made it possible to measure the spectra of an essentially unexplored class of electronic states associated with double inner-shell vacancies. Using the technique of single electron spectroscopy, spectra of states in CS₂ and SF₆ with a double hole in the K-shell and one electron exited to a normally unoccupied orbital have been obtained. The spectra are interpreted with the aid of a high-level theoretical model giving excellent agreement with the experiment. The results shed new light on the important distinction between direct and conjugate shake-up in a molecular context. In particular, systematic similarities and differences between pre-edge states near single core holes investigated in X-ray absorption spectra and the corresponding states near double core holes studied here are brought out.

Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method's impressive electronic structure sensitivity for excited-state investigations.Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast ππ*/nπ* electronic relaxation of hetero-organic molecules.

Correlated Electron Dynamics at Surfaces Investigated via He^{2+} Ion Neutralization

The neutralization of a single He^{2+} ion near a Ir surface leads to the emission of an electron pair. Via coincidence spectroscopy we give evidence that a sizable amount of these electron pairs originate from a correlated single step neutralization of the ion involving a total of four electrons from the metal. These correlated electron pairs cannot be explained in the common picture of two consecutive and independent neutralization steps. We infer a characteristic time scale for the correlated electron dynamics in the metal of 40-400  as.

Study of double core hole excitations in molecules by X-ray double-quantum-coherence signals: a multi-configuration simulation

The multi-configurational self-consistent field method is employed to simulate the two-dimensional all-X-ray double-quantum-coherence (XDQC) spectroscopy, a four-wave mixing signal that provides direct signatures of double core hole (DCH) states. The valence electronic structure is probed by capturing the correlation between the single (SCH) and double core hole states. The state-averaged restricted-active-space self-consistent field (SA-RASSCF) approach is used which can treat the valence, SCH, and DCH states at the same theoretical level, and applies to all types of DCHs (located on one or two atoms, K-edge or L-edge), with both accuracy and efficiency. Orbital relaxation introduced by the core hole(s) and the static electron correlation is properly accounted for. The XDQC process can take place via different intermediate DCH state channels by tuning the pulse frequencies. We simulate the XDQC signals for the three isomers of aminophenol at 8 pulse frequency configurations, covering all DCH pathways involving the N1s and O1s core hole (N1sN1s, O1sO1s and N1sO1s), which reveal different patterns of valence excitations.

CFEL-ASG Software Suite (CASS): usage for free-electron laser experiments with biological focus

CASS [Foucar et al. (2012). Comput. Phys. Commun.183, 2207-2213] is a well established software suite for experiments performed at any sort of light source. It is based on a modular design and can easily be adapted for use at free-electron laser (FEL) experiments that have a biological focus. This article will list all the additional functionality and enhancements of CASS for use with FEL experiments that have been introduced since the first publication. The article will also highlight some advanced experiments with biological aspects that have been performed.

Charge and Nuclear Dynamics Induced by Deep Inner-Shell Multiphoton Ionization of CH3I Molecules by Intense X-ray Free-Electron Laser Pulses

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

Theoretical tracking of resonance-enhanced multiple ionization pathways in X-ray free-electron laser pulses

We present an extended Monte Carlo rate equation approach to examine the inner-shell ionization dynamics of atoms in an intense x-ray free-electron laser (XFEL) pulse. In addition to photoionization, Auger decay, and fluorescence processes, we include bound-to-bound transitions in the rate equation calculations. Using an efficient computational scheme, we account for "hidden resonances" unveiled during the course of an XFEL pulse. For Ar, the number of possible electron configurations is increased ten-billion-fold over that required under nonresonant conditions. We investigated the complex ionization dynamics of Ar atoms exposed to an 480-eV XFEL pulse, where production of ions above charge state 10+ is not allowed via direct one-photon ionization. We found that resonance-enhanced x-ray multiple ionization pathways play a dominant role in producing these nominally inaccessible charge states. Our calculated results agree with the measured Ar ion yield and pulse-duration dependence. We also predict the surprising ion yields reported earlier for Kr and Xe. The Monte Carlo rate equation method enables theoretical exploration of the complex dynamics of resonant high-intensity x-ray processes.

Multiple ionization and fragmentation dynamics of molecular iodine studied in IR-XUV pump-probe experiments

The ionization and fragmentation dynamics of iodine molecules (I(2)) are traced using very intense (∼10(14) W cm(-2)) ultra-short (∼60 fs) light pulses with 87 eV photons of the Free-electron LASer at Hamburg (FLASH) in combination with a synchronized femtosecond optical laser. Within a pump-probe scheme the IR pulse initiates a molecular fragmentation and then, after an adjustable time delay, the system is exposed to an intense FEL pulse. This way we follow the creation of highly-charged molecular fragments as a function of time, and probe the dynamics of multi-photon absorption during the transition from a molecule to individual atoms.

Emerging photon technologies for probing ultrafast molecular dynamics

The understanding of physical and chemical changes at an atomic spatial scale and on the time scale of atomic motion is essential for a broad range of scientific fields. A new class of femtosecond, intense, short wavelength lasers, the free electron lasers, has opened up new opportunities to investigate dynamics in many areas of science. For chemical dynamics to advance however, a rigorous, quantitative understanding of dynamical effects due to intense X-ray exposure is also required. We illustrate this point by reporting here an experimental and theoretical investigation of the interaction of C(60) molecules with intense X-ray pulses, in the multiphoton regime. We also describe the potential of new available instrumentation and explore their potential impact in physical, chemical and biological sciences when they are coupled with emerging photon technologies.

Femtosecond X-ray-induced explosion of C60 at extreme intensity

Understanding molecular femtosecond dynamics under intense X-ray exposure is critical to progress in biomolecular imaging and matter under extreme conditions. Imaging viruses and proteins at an atomic spatial scale and on the time scale of atomic motion requires rigorous, quantitative understanding of dynamical effects of intense X-ray exposure. Here we present an experimental and theoretical study of C60 molecules interacting with intense X-ray pulses from a free-electron laser, revealing the influence of processes not previously reported. Our work illustrates the successful use of classical mechanics to describe all moving particles in C60, an approach that scales well to larger systems, for example, biomolecules. Comparisons of the model with experimental data on C60 ion fragmentation show excellent agreement under a variety of laser conditions. The results indicate that this modelling is applicable for X-ray interactions with any extended system, even at higher X-ray dose rates expected with future light sources.

Analysis of a measurement scheme for ultrafast hole dynamics by few femtosecond resolution X-ray pump–probe Auger spectroscopy

Ultrafast hole dynamics created in molecular systems as a result of sudden ionisation is the focus of much attention in the field of attosecond science. Using the molecule glycine we show through ab initio simulations that the dynamics of a hole, arising from ionisation in the inner valence region, evolves with a timescale appropriate to be measured using X-ray pulses from the current generation of SASE free electron lasers. The examined pump–probe scheme uses X-rays with photon energy below the K edge of carbon (275–280 eV) that will ionise from the inner valence region. A second probe X-ray at the same energy can excite an electron from the core to fill the vacancy in the inner-valence region. The dynamics of the inner valence hole can be tracked by measuring the Auger electrons produced by the subsequent refilling of the core hole as a function of pump–probe delay. We consider the feasibility of the experiment and include numerical simulation to support this analysis. We discuss the potential for all X-ray pump-X-ray probe Auger spectroscopy measurements for tracking hole migration.