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Charge transfer processes between weakly bound entities play an important role in various chemical and biological environments. In this combined experimental and theoretical work, we investigate the nature of charge-transfer processes in homogeneous atomic and heterogeneous atomic-molecular clusters. Our results reveal fundamentally different processes to be at play in pure argon clusters compared to mixed argon-nitrogen systems: We demonstrate that the former species decay via photon-mediated charge transfer while a nonradiative direct process is found dominant in the atomic-molecular cases. Our results are of general interest for studies on charge redistribution in more complex and biologically relevant samples where molecules are involved.

Physical review / X 10(4), 041056 (2020). doi:10.1103/PhysRevX.10.041056, The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry., Published by APS, College Park, Md.

Synopsis Results from the first soft X-ray user experiment at the European XFEL on nonlinear photon-matter interaction will be presented. Angle-resolved electron time-of-flight spectroscopy employed at the AQS (Atom-ic-like Quantum Systems) endstation of the SQS (Small Quantum Systems) instrument reveals insight into the character of resonances in highly transient, core ionized neon ions, i.e. Ne+ 1s12s22p6 → Ne+* 1s02s22p6np, together with their respective relaxation dynamics. Enabled by the unique properties of the European XFEL, novel perspectives on efficient nonlinear spectroscopy will be discussed.

We combine a quantum dynamical propagator that explicitly accounts for quantum mechanical time ordering with optimal control theory. After analyzing its performance with a simple model, we apply it to a superconducting circuit under so-called Pythagorean control. Breakdown of the rotating-wave approximation is the main source of the very strong time-dependence in this example. While the propagator that accounts for the time ordering in an iterative fashion proves its numerical efficiency for the dynamics of the superconducting circuit, its performance when combined with optimal control turns out to be rather sensitive to the strength of the time-dependence. We discuss the kind of quantum gate operations that the superconducting circuit can implement including their performance bounds in terms of fidelity and speed., 16 pages, 11 figures

A new model is presented for a reflector producing a shaped narrow beam when illuminated by a point feed. The deviation from an idealised (Dunbar) model which has zero beam width in the geometrical optics approximation is regulated by a single arbitrary function tau , introduced to enhance the physical optics performance. In terms of tau , the phase distribution on a plane aperture is readily computed. Reflector profiles are then calculated by integration along the characteristics of a first order non-linear partial differential equation, leading to explicit formulae involving only ordinary quadratures. Initial data arises naturally on the central-section curve. It is shown that for continuous reflectors a well-known design due to Brunner (1971), frequently used in surveillance radar, is mathematically inconsistent.

The reflexion of outward travelling cylindrical gravitational waves by a thin cylindrical shell of matter is discussed. The shell is regarded as a surface distribution, and it is shown that if it is perfectly rigid, total reflexion occurs. Quasi-periodic incident waves are found to lead to a completely periodic standing wave state between the reflector and a central wave-source, the field outside the system remaining static. In the treatment of standing waves the components of the stress-energy tensor in the shell are determined and the construction for a model source is given.

If two empty regions of space-time are entirely separated by matter, little information of the nature of one region can be gained by knowledge of the other. Examples of a hollow Schwarzschild sphere and a hollow static torus are considered. In the first of these, it is shown that the size of the flat interior is not at all limited by the smallness of the Schwarzschild mass constant, the outer spherical boundary, or the condition that the sphere be constructed only of matter of positive density with positive stress-energy scalar.

A device making use of the linearity of one of the field equations for Rosen’s cylindrically symmetric metric is employed to demonstrate a correspondence between some outgoing toroidal gravitational waves and imploding-exploding cylindrical waves. An exact form of axially symmetric metric for a smooth toroidal pulse in the region ｜ z ｜ < t (in coordinates which approximate to cylindrical polars when the field is weak) is constructed, and extensions to the region ｜ z ｜ < t are considered. The solution remains valid throughout the reflexion of the wave as it meets itself on the symmetry axis, and displays this process clearly. As a particular example, a solution of Weber & Wheeler for a non-singular cylindrical pulse is converted into one for a time-symmetric toroidal pulse, and the distant field at the moment of time-symmetry is briefly examined.

Known cylindrical wave solutions of the free-space gravitational field equations are superimposed on a static field and extended to the interior of a generating cylinder of matter. It is shown that the waves propagate energy. The interior and exterior field of a static cylinder of approximately constant density is obtained.

A construction is given for an exact solution of Einstein’s empty space-time field equations representing an axially sjonmetric pulse wave with the properties that (i) the wave front is a sphere, outside of which all space-time is flat; (ii) a finite, expanding line singularity forms the axial diameter of this sphere. Because of (i), the solution would need to be generalized before a physical source could be introduced. The presence of radiation is confirmed by the fact that at a great distance from the centre of the wave the Riemann tensor is asymptotically of Petrov’s canonical type II (with vanishing scalar invariants).

Further properties of a vacuum metric obtained in an earlier paper, representing a spherical-fronted gravitational wave (which, however, possesses cylindrical symmetry) are derived. This metric is constructed by introducing ‘bending terms’ into Rosen’s metric for cylindrical waves. The way the transition occurs from properties characteristic of cylindrical waves to those of spherical waves is examined. The asymptotic behaviour of the optical parameters and the null tetrad components of the Weyl (Riemann) tensor along an outgoing null geodesic is determined. An interpretation of the axial singularity is given by comparison with an analogous feature in classical electromagnetic wave propagation.

Various definitions of uniform acceleration are possible in relativity theory. Two of these are considered with particular reference to their physical realizations.

It is well known that there is no static axisymmetric two-body solution of Einstein's gravitational field equations, if it is assumed that the bodies are separated in a certain definite sense. In this paper it is shown, by the construction of a complete physically sensible model, that static two-body solutions do exist for systems in which one body is hollow and contains the other. The stability of the particular system described is briefly discussed.

The source of a Kerr space-time is generally assumed to be reducible to a disc, although the singularity in the solution is in the form of a ring. This is because the maximally-extended exterior manifold is multiply-connected, being formed by two sheets joined through the ring. However, it is possible, in principle, to construct 'minimal' sources (reducible to the singularity) for space-times with such topology. This is illustrated in the Kerr case with zero mass constant, and some possibly general features are indicated.

IMPORTANT information in astronomy may be gained in the not-too-distant future by the observation of gravitational waves. But the nonlinearity of the gravitational field equations in general relativity makes it difficult to estimate effects which might arise through collisions between waves, even when the nature of the impinging waves is known. In general, direct superposition of solutions for individual waves will lead to violation of the vacuum equations. One thus has to determine, in a particular situation, whether there exists a valid vacuum solution, non-singular in the wave region, which represents the gravitational field before, during and after the collision, or whether instead the eventual formation of physical singularities is predicted. For example, Penrose1 has shown that in the collision of (exact) weak “sandwich” plane waves2, each will warp the other until singularities occur in the wave-front.