XUV/X-ray light and fast ions for ultrafast chemistry

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Abstract

The deposition of large amounts of energy in a molecule by XUV/X-ray photon absorption or fast-ion collision, triggers a set of complex ultrafast electronic and nuclear dynamics that allow a deep understanding and control of chemical reactivity. This themed issue showcases the research performed in the understanding, monitoring and control of these processes.

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This issue collects contributions from groups working in this field worldwide, but many of the articles have been written in collaboration between European research groups that have participated in the last 4 years – from June 2013 until June 2017 – in the ‘XUV/X-ray light and fast ions for ultrafast chemistry’ (XLIC) COST Action (http://www.xlic.eu). This European project gathered together researchers from 25 European countries which included experimental groups working with different techniques, mainly collision with ions, synchrotron radiation and ultrashort laser pulses, and theoretical groups with expertise in different methodologies such as quantum chemistry calculations and molecular dynamics simulations in both the ground and excited states, including the treatment of the electronic continuum, laser-molecule interaction and strong laser fields.

The study of excited molecules under high energetic conditions has opened the window to understanding a large variety of fundamental chemical processes. One representative example is the use of X-ray Free Electron Lasers (FELs) to excite the 1s core electrons of C in C₆₀. Under high-fluence conditions each C atom absorbs multiple photons and new complex electronic dynamics as secondary ionization and recombination events, besides photoionization and Auger relaxation, are observed.¹ The use of high energy photons also allows extreme sensitivity which reveals the coupling between electronic and nuclear degrees of freedom. For instance, by using X-ray synchrotron radiation it is possible to have site selectivity in valence-shell photoionization and, as shown in the case of HCl,² to obtain information on the atomic composition of the molecular orbitals due to the rotational dependence of the photoionization profiles.

International collaborations are especially important in this field as the combination of different techniques allows us to access information that cannot be obtained by a single one. A recent example is provided by the study of thymidine,³ where the use of both vacuum ultraviolet (VUV) synchrotron radiation and collisions with O⁶⁺ ions in separate experiments, supported by theoretical calculations, allowed the determination of the distribution of energy deposited in the ionized molecule as a result of the collision.

One of the most important experimental developments in recent years has come from techniques that generate and control ultrashort laser pulses. At the end of the 80s the pioneering work of Ahmed Zewail allowed imaging of the course of a chemical reaction in the femtosecond time scale.⁴ In the last decade, with the advent of FEL and High Harmonic Generation (HHG) techniques, chemistry is entering the attosecond time domain,^5,6 in which it is possible to image electronic motion and to obtain control over the electronic movement that drives chemical reactions. The monitoring and control of ultrafast dissociation reactions of a large variety of systems has been achieved in recent years by several groups, for example Fe(CO)₅ complexes,⁷ proton detachment from acetylene,⁸ polycyclic aromatic hydrocarbons (PAHs),⁹ chiral molecules¹⁰ or biomolecules such as phenylalanine.¹¹

To understand the interaction and decay mechanisms following ultrashort energy deposition in molecules, it is also necessary to develop new theoretical methods, in many instances beyond the Born–Oppenheimer approximation, to describe ultrafast electron dynamics and the connection between electronic and nuclear degrees of freedom.¹² It is also important to note that electron dynamics in highly excited states often result in interferences between different electronic excitation channels, fully understanding these processes is a hot research area in which theory and experiment are still required to explore these phenomena in atoms.^13–16

The present themed issue collects a series of articles showing recent advances in all of these topics. In her Perspective article (DOI: 10.1039/c7cp01996c), Norah Berrah introduces how the use of FELs and the generation of very short pulses of few fs duration can be applied to study molecular dynamics following the ionization of inner-shell electrons.

Collisions with highly charged ions (HCI) is one of the experimental techniques used to generate multiply charged molecules that can evolve to lead to the formation of highly reactive intermediates or stable doubly-charged species in the gas phase. The production of these reactive compounds is illustrated in the study of the collision of γ-aminobutyric acid with Ar⁹⁺ (DOI: 10.1039/c7cp00903h), or the collision of C⁴⁺ with hydrated clusters of 5-Br-uracil (DOI: 10.1039/c7cp02233f). An interesting extension of this technique to very large systems is given by Milosavljević et al. (DOI: 10.1039/c7cp02075a), in the study of the collision of Xe²⁵⁺ with the cytochrome c protein with initial charge states ranging from −9 to −17. Charge transfer is not the only mechanism relevant in these collisions, direct knockout processes can strongly contribute to the total fragmentation cross sections, as shown by Zettergren et al. (DOI: 10.1039/c7cp01583f) in their study of the collision of charged porphyrins with He and Ne atoms, leading to the formation of highly reactive species. For instance, when the molecules are embedded in a cluster, as in 1,2 butadiene (DOI: 10.1039/c7cp02090b), knockout mechanisms explain the formation of new chemical bonds. Theoretical treatments necessary to understand the collisional processes and predict charge transfer are given in two articles for the case of tetrahydrofuran (DOI: 10.1039/c7cp02100c) and HCN polymers (DOI: 10.1039/c7cp00826k).

The study of collisions has direct applications in astrochemistry, where the modelling of the abundance of complex molecules requires information on both the stability of the different species and the reactive cross-sections and branching ratios for the most relevant reactions. In the article from Cernuto et al. (DOI: 10.1039/c7cp00827a) these quantities are obtained for the reaction of He ions with dimethyl ether. Fullerenes, from small to giant ones, are a huge family of compounds of great relevance in astrochemistry. The prediction of the stabilities of all possible isomers is a challenge for theory, but as proposed in DOI: 10.1039/c7cp01598d a simple model, based on structural motifs, can allow the prediction and understanding of the stability of these fullerenes.

Synchrotron radiation is a second technique often used to generate highly excited molecules. In the article of Sorensen et al. (DOI: 10.1039/c7cp01667k) electrons from the inner valence region of cyclopropane are extracted in double photoionization experiments. The dissociation channels are analysed as a function of the photon energy, showing selective population of different dication states, which act as gateway states for different dissociation channels. Synchrotron radiation can also be used to map the potential energy surface and the vibrational structure of the ground state, the analysis of the resonant inelastic X-ray scattering (RIXS) spectra of H₂O, D₂O and HDO (DOI: 10.1039/c7cp01215b), obtained using the Swiss Light Source, allows us to not only understand the details and dynamics of the X-ray molecular scattering process, but also to obtain information about the vibrational structure and potential energy surfaces of the ground state not obtainable by other techniques. Femtosecond pulses and photoelectron photoion coincidence momentum spectroscopy are complementary techniques that also help us to understand the electronic and nuclear photodynamics following multiphoton absorptions, as shown for the case of NO₂ (DOI: 10.1039/C7CP02057K).

The use of ultrashort laser pulses generated by FEL or HHG light sources are the third techniques used in several articles of this issue to generate highly excited molecules. HHG is used to get insight into the mechanisms, which immediately proceed from the action of ionizing radiation on the DNA building blocks, thymine and thymidine (DOI: 10.1039/c7cp02803b), or ultrafast proton migration in benzene (DOI: 10.1039/c7cp02255g). Ultrafast Coulomb explosion induced by an X-ray FEL pulse is explored for the case of diiodomethane (DOI: 10.1039/c7cp01669g) and new experiments done at FERMI (DOI: 10.1039/c7cp01812f) demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) using 200 fs pulses from a near-infrared laser, allowing the possibility to perform future experiments where alignment of small molecules is required.

The correct interpretation of experiments in which molecules are highly excited or ionized and ultrafast dynamics take place, requires new theoretical methods to obtain an accurate description of electron dynamics in highly excited states. Several articles in this issue deal with the development and application of these new methodologies. The article of Suraud et al. (DOI: 10.1039/c7cp00995j) deals with one of the main challenges in the field that is the analysis of attosecond dynamics induced by irradiation of a train of attosecond XUV pulses in the presence of an infrared (IR) field and applies it to systems such as C₃ or Na_n clusters. Emmanouilidou et al. obtain molecular Auger rates and single-photon ionization cross sections of molecular nitrogen interacting with FEL radiation (DOI: 10.1039/c7cp02345f). Dundas et al. study the electron dynamics of acetylene ionization using a linearly-polarized VUV pulse (DOI: 10.1039/c7cp01661a). Martin et al. (DOI: 10.1039/c7cp01856h) study more complex systems, such as glycine, and explain the ultrafast charge migration and charge fluctuations inside the molecule resulting from the coherent superposition of ionic states produced by the broadband attosecond pulse. Gräfe et al. (DOI: 10.1039/c7cp01832k) show that the analysis of asymmetries in the photoelectron momentum distribution can be used as a tool to monitor intrinsic and field-driven electron dynamics and transient population transfer, allowing the differentiation of adiabatic and non-adiabatic dynamics.

This issue also collects new theoretical developments in the description of other experiments such as nonresonant inelastic electron and X-ray scattering cross sections (DOI: 10.1039/c7cp02054f), time-resolved X-ray scattering (DOI: 10.1039/c7cp01831b), new processes such as hot electron emission following the absorption of a single photon (DOI: 10.1039/c7cp01705g) or light-induced nonadiabatic phenomena (DOI: 10.1039/c7cp02164j).

Accurate calculations in the excited state are given to describe the dynamics of the rotational predissociation process of LiH electronic excited states (DOI: 10.1039/c7cp02097j), multi-channel photoionization of NO₂ in the vicinity of a conical intersection (DOI: 10.1039/c7cp01643c), the correlated dynamics of electrons and nuclei of the HCN molecule induced by an intense UV femtosecond optical pulse (DOI: 10.1039/C7CP02048A), the resonant dynamic Stark shift based on a wave-packet propagation technique (DOI: 10.1039/C7CP02146A), the photodynamics of 2-thiouracil (DOI: 10.1039/c7cp02258a), the photophysics of a copper phenanthroline (DOI: 10.1039/c7cp00436b) or the energy flow in peptides (DOI: 10.1039/c7cp01768e) and technical, but important aspects to make these calculations affordable, such as how to accelerate electronic structure calculations by using graphical processing units (GPUs) (DOI: 10.1039/c7cp01473b).

In summary this is a fast developing research field in which international collaborations are crucial to promote the exchange of expertise of teams working with different techniques and to reinforce theory-experiment collaborations. In this respect, COST Actions provide a unique framework to promote these exchanges, as have been shown in the XLIC Action. Finally, we want to thank all contributors to this issue, which we think gives a quite complete overview of the multidisciplinary research done in this emerging field.

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