145 research outputs found
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 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 being ejected from the clusters. We
rationalize our data through a qualitative model of a finite-size non-thermal
plasma
Ultrafast relaxation of photoexcited superfluid He nanodroplets
The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, we study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free- electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental pho- toelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He ) within 1 ps. Subsequently, the bubble collapses and releases metastable He at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses
- …