Adsorption of cytosine and aza derivatives of cytidine on Au single crystal surfaces

The adsorption of cytosine on the Au(111) and Au(110) surfaces has been studied using both aqueous deposition and evaporation in vacuum to prepare the samples. Soft X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the electronic structure and orientation of the adsorbates. In addition, three derivatives of cytosine, 6-azacytosine, 6-azacytidine and 5- azacytidine, were studied. Monolayer films of the latter three samples were adsorbed on Au(111) from aqueous solution, and the nature of bonding was determined. Spectra have been interpreted in the light of published calculations of free cytosine molecules and new ab initio calculations of the other compounds. Surface core level shifts of Au 4f imply that all of these compounds are chemisorbed. Cytosine adsorbs as a single tautomer, but in two chemical states with different surface-molecule bonding. For deposition in vacuum, a flat-lying molecular state bonded through the N(3) atom of the pyrimidine ring dominates, but a second state is also present. For deposition from solution, the second state dominates, with the molecular plane no longer parallel to the surface. This state also bonds through the N(3) atom, but in addition interacts with the surface via the amino group. Two tautomers of 6-azacytosine were observed, and they and 6-azacytidine adsorb with similar geometries, chemically bonding via the azacytosine ring. The ribose ring does not appear to perturb the adsorption of azacytidine compared with azacytosine. The azacytosine ring is nearly but not perfectly parallel to the surface, like 5-azacytidine, which adsorbs as an imino tautomer. ...Comment: 40 pages, 3 tables and 8 figure

A review of recent progress in understanding the spontelectric state of matter

The spontelectric state of matter is exemplified by the presence of static, spontaneous electric fields extending throughout thin films of dipolar solids. The spontelectric state was discovered using a low energy electron beam technique, using the ASTRID storage ring at Aarhus University. Following a resume of the characteristics and of a model for the spontelectric effect, a description is given of the counter-intuitive behaviour of fields in films of methyl formate as a function of deposition temperature, T. It is found that films for T ≤ 77.5 K show the expected decrease in the field with increasing T but, for T ≥ 77.5 K, an increase in the field for higher T is revealed. Analysis of these results illustrates the non-linear and non-local characteristics of the spontelectric state. Recently it has been shown that Reflection-Absorption Infrared Spectroscopy (RAIRS) provides a new and independent technique for the detection of the spontelectric effect, through the observation of vibrational Stark shifts in spectra of films. Stark shifts for nitrous oxide are demonstrated to be in harmony with electric fields measured using the electron beam technique. The method is then applied to carbon monoxide, showing that this material displays the spontelectric effect between deposition temperatures of 20 K and 26 K

Observation and Control of Laser-Enabled Auger Decay

Single photon laser enabled Auger decay (spLEAD) has been redicted theoretically [Phys. Rev. Lett. 111, 083004 (2013)] and here we report its first experimental observation in neon. Using coherent, bichromatic free-electron laser pulses, we have detected the process and coherently controlled the angular distribution of the emitted electrons by varying the phase difference between the two laser fields. Since spLEAD is highly sensitive to electron correlation, this is a promising method for probing both correlation and ultrafast hole migration in more complex systems.Comment: 5 pages, 3 figure

Collective Autoionization in Multiply-Excited Systems: A novel ionization process observed in Helium Nanodroplets

Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with subsequent decay paths which have been scarcely investigated to date. Here, we show using helium nanodroplets as an example that these systems can decay by a new type of process, named collective autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances much greater than that of direct single-photon ionization. This novel collective ionization process is expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed to high intensity radiation fields

Real-time dynamics of the formation of hydrated electrons upon irradiation of water clusters with extreme ultraviolet light

Free electrons in a polar liquid can form a bound state via interaction with the molecular environment. This so-called hydrated electron state in water is of fundamental importance e.g.~in cellular biology or radiation chemistry. Hydrated electrons are highly reactive radicals that can either directly interact with DNA or enzymes, or form highly excited hydrogen (H∗) after being captured by protons. Here, we investigate the formation of the hydrated electron in real-time employing XUV femtosecond pulses from a free electron laser, in this way observing the initial steps of the hydration process. Using time-resolved photoelectron spectroscopy we find formation timescales in the low picosecond range and resolve the prominent dynamics of forming excited hydrogen states

Evolution and ion kinetics of a XUV-induced nanoplasma in ammonia clusters

High-intensity extreme ultraviolet (XUV) pulses from a free-electron laser can be used to create a nanoplasma in clusters. In Ref. [Michiels et al. PCCP, 2020; 22: 7828-7834] we investigated the formation of excited states in an XUV-induced nanoplasma in ammonia clusters. In the present article we expand our previous study with a detailed analysis of the nanoplasma evolution and ion kinetics. We use a time-delayed UV laser as probe to ionize excited states of H and H$_2^+$ in the XUV-induced plasma. Employing covariance mapping techniques, we show that the correlated emission of protons plays an important role in the plasma dynamics. The time-dependent kinetic energy of the ions created by the probe laser is measured, revealing the charge neutralization of the cluster happens on a sub-picosecond timescale. Furthermore, we observe ro-vibrationally excited molecular hydrogen ions H$_2^{+*}$ being ejected from the clusters. We rationalize our data through a qualitative model of a finite-size non-thermal plasma

Tracking the ultraviolet-induced photochemistry of thiophenone during and after ultrafast ring opening

Photoinduced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic structure and molecular dynamics calculations of the excited- and ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state are shown to be driven by ballistic S–C bond extension and to be complete within 350 fs. Theory and experiment also enable visualization of the rich ground-state dynamics that involve the formation of, and interconversion between, ring-opened isomers and the cyclic structure, as well as fragmentation over much longer timescales