4,623 research outputs found
Quantum optimal control of photoelectron spectra and angular distributions
Photoelectron spectra and photoelectron angular distributions obtained in
photoionization reveal important information on e.g. charge transfer or hole
coherence in the parent ion. Here we show that optimal control of the
underlying quantum dynamics can be used to enhance desired features in the
photoelectron spectra and angular distributions. To this end, we combine
Krotov's method for optimal control theory with the time-dependent
configuration interaction singles formalism and a splitting approach to
calculate photoelectron spectra and angular distributions. The optimization
target can account for specific desired properties in the photoelectron angular
distribution alone, in the photoelectron spectrum, or in both. We demonstrate
the method for hydrogen and then apply it to argon under strong XUV radiation,
maximizing the difference of emission into the upper and lower hemispheres, in
order to realize directed electron emission in the XUV regime
Universality of the collapse transition of sticky polymers
The universality of the swelling of the radius of gyration of a homopolymer
relative to its value in the state, independent of polymer-solvent
chemistry, in the crossover regime between and athermal solvent
conditions, is well known. Here we study, by Brownian dynamics, a polymer model
where a subset of monomers is labelled as "stickers". The mutual interaction of
the stickers is more attractive than those of the other ("backbone") monomers,
and has the additional important characteristic of "functionality" ,
i.e., the maximum number of stickers that can locally bind to a given sticker.
A saturated bond formed in this manner remains bound until it breaks due to
thermal fluctuations, a requirement which can be viewed as an additional
Boolean degree of freedom that describes the bonding. This, in turn, makes the
question of the order of the collapse transition a non-trivial one.
Nevertheless, for the parameters that we have studied (in particular,
), we find a standard second-order collapse, using a
renormalised solvent quality parameter that takes into account the increased
average attraction due to the presence of stickers. We examine the swelling of
the radius of gyration of such a sticky polymer relative to its value in the
altered state, using a novel potential to model the various excluded
volume interactions that occur between the monomers on the chain. We find that
the swelling of such sticky polymers is identical to the universal swelling of
homopolymers in the thermal crossover regime. Additionally, for our model, the
Kuhn segment length under conditions is found to be the same for
chains with and without stickers.Comment: 13 pages, 10 figures, supplementary material (see ancillary
directory), to appear in Soft Matte
Self-Organized Dynamical Equilibrium in the Corrosion of Random Solids
Self-organized criticality is characterized by power law correlations in the
non-equilibrium steady state of externally driven systems. A dynamical system
proposed here self-organizes itself to a critical state with no characteristic
size at ``dynamical equilibrium''. The system is a random solid in contact with
an aqueous solution and the dynamics is the chemical reaction of corrosion or
dissolution of the solid in the solution. The initial difference in chemical
potential at the solid-liquid interface provides the driving force. During time
evolution, the system undergoes two transitions, roughening and
anti-percolation. Finally, the system evolves to a dynamical equilibrium state
characterized by constant chemical potential and average cluster size. The
cluster size distribution exhibits power law at the final equilibrium state.Comment: 11 pages, 5 figure
Prospects for discovering supersymmetric long-lived particles with MoEDAL
We present a study on the possibility of searching for long-lived
supersymmetric partners with the MoEDAL experiment at the LHC. MoEDAL is
sensitive to highly ionising objects such as magnetic monopoles or massive
(meta)stable electrically charged particles. We focus on prospects of directly
detecting long-lived sleptons in a phenomenologically realistic model which
involves an intermediate neutral long-lived particle in the decay chain. This
scenario is not yet excluded by the current data from ATLAS or CMS, and is
compatible with astrophysical constraints. Using Monte Carlo simulation, we
compare the sensitivities of MoEDAL versus ATLAS in scenarios where MoEDAL
could provide discovery reach complementary to ATLAS and CMS, thanks to looser
selection criteria combined with the virtual absence of background. It is also
interesting to point out that, in such scenarios, in which charged staus are
the main long-lived candidates, the relevant mass range for MoEDAL is
compatible with a potential role of Supersymmetry in providing an explanation
for the anomalous events observed by the ANITA detector.Comment: 12 pages, 6 figures; preliminary results presented in
arXiv:1903.11022; matches published version in EPJ
Suppression of hole decoherence in ultrafast photoionization
In simple one-photon ionization, decoherence occurs due to entanglement between ion and photoelectron. Therefore, the preparation of coherent superpositions of electronic eigenstates of the hole in the photoion is extremely difficult. We demonstrate for the xenon atom that the degree of electronic coherence of the photoion in attosecond photoionization can be enhanced if the influence of many-body interactions is properly controlled. A mechanism at low photon energies involving multiphoton ionization is found, suppressing the loss of coherence through ionization into the same photoelectron partial waves. The degree of coherence found between the 4 d0 and 5 s hole states is, on the one hand, limited by Auger decay of the 4 d0 hole. On the other hand, increasing the population ratio such that a significant portion of the state is in a true superposition of both states renders the maximization of the degree of coherence difficult
Gas-Liquid Nucleation in Two Dimensional System
We study the nucleation of the liquid phase from a supersaturated vapor in
two dimensions (2D). Using different Monte Carlo simulation methods, we
calculate the free energy barrier for nucleation, the line tension and also
investigate the size and shape of the critical nucleus. The study is carried
out at an intermediate level of supersaturation(away from the spinodal limit).
In 2D, a large cut-off in the truncation of the Lennard-Jones (LJ) potential is
required to obtain converged results, whereas low cut-off (say, is
generally sufficient in three dimensional studies, where is the LJ
diameter) leads to a substantial error in the values of line tension,
nucleation barrier and characteristics of the critical cluster. It is found
that in 2D, the classical nucleation theory (CNT) fails to provide a reliable
estimate of the free energy barrier. It underestimates the barrier by as much
as 70% at the saturation-ratio S=1.1 (defined as S=P/PC, where PC is the
coexistence pressure at reduced temperature ). Interestingly,
CNT has been found to overestimate the nucleation free energy barrier in three
dimensional (3D)systems near the triple point. In fact, the agreement with CNT
is worse in 2D than in 3D. Moreover, the existing theoretical estimate of the
line tension overestimates the value significantly.Comment: 24 pages, 8 figure
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