Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Ionization of a two-electron atom in a strong electromagnetic field

A one-dimensional model of a helium atom in an intense field of a femtosecond electromagnetic pulse has been constructed using the Hartree technique. “Exact” calculations have been compared to the approximations of “frozen” and “passive” electrons. A nonmonotonic dependence of the single-electron ionization probability on the radiation intensity has been detected. Minima in the ionization probability are due to multiphoton resonances between different atomic states due to the dynamic Stark effect. We suggest that the ionization suppression is due to the interference stabilization in this case. Zh. éksp. Teor. Fiz. 112, 470–482 (August 1997)     

The role of photoelectronic processes in the formation of a fluorescent plume by 248-nm laser irradiation of single crystal NaNO3

 Visible fluorescent “plumes” are readily produced when nominally transparent ionic materials are exposed to pulsed UV laser irradiation. Over a wide range of laser fluences where plumes are observed, however, the photon and electron densities are inadequate to support multiphoton ionization and inverse bremsstrahlung, which are often used to explain plasma production and excitation of atomic spectral lines. We present evidence that the great majority of charged particles (electrons and positive ions) comprising the plume at the onset of formation in defect-laden NaNO₃ are emitted directly from the surface. A model is described wherein the required electron energy to excite and eventually ionize neutral atoms is provided by electrostatic interactions in the expanding plume. The time evolution of the “overlap” between the expanding charge cloud and thermally emitted neutrals accounts for the time evolution of the atomic line emissions after the laser pulse. Received: 15 August 1996/Accepted: 16 August 1996     

Generation of surface features in films deposited by matrix-assisted pulsed laser evaporation: the effects of the stress confinement and droplet landing velocity

Coarse-grained molecular dynamics simulations are applied to investigate the origins of the surface features observed in films deposited by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The formation of transient balloon-like structures with a polymer-rich surface layer enclosing matrix vapor, observed in earlier simulations of slow heating of polymer-matrix droplets, has been explored in this work at higher rates of thermal energy deposition. Tensile stresses generated in the regime of partial stress confinement are found to induce an internal boiling in the overheated droplets and associated generation of “molecular balloons” at thermal energy densities at which no homogeneous boiling takes place without the assistance of tensile stresses. Simulations of the dynamic processes occurring upon the collision of a polymer-matrix droplet with a substrate provide the molecular-level pictures of the droplet impact phenomenon and reveal the connections between the droplet landing velocity and the shapes of the polymer features observed in scanning electron microscopy images of films deposited in MAPLE experiments. The distinct types of surface features observed in MAPLE experiments, namely, wrinkled “deflated balloons,” localized arrangements of interconnected polymer filaments, and elongated “nanofibers,” are shown to emerge from different scenarios of droplet landing and/or disintegration observed in the simulations.     

Dipoles and band alignment for benzene/Au(111) and C<Subscript>60</Subscript>/Au(111) interfaces

A local orbital DFT-approach combined with a “scissor”-operator is used to obtain the Charge Neutrality Level and the screening parameter in the benzene/Au(111) and C₆₀/Au(111) interfaces. The “pillow” dipole and interface Fermi level are also calculated. The total dipole induced across the interface is compared with the experimental evidences: while the agreement for C₆₀/Au(111) is excellent, for benzene/Au(111), some discrepancies appear that are discussed in the light of other models.     

Anticipative control of switched queueing systems

The relevant dynamics of a queueing process can be anticipated by taking future arrivals into account. If the transport from one queue to another is associated with transportation delays, as it is typical for traffic or productions networks, future arrivals to a queue are known over some time horizon and, thus, can be used for an anticipative control of the corresponding flows. A queue is controlled by switching its outflow between “on” and “off” similar to green and red traffic lights, where switching to “on” requires a non-zero setup time. Due to the presence of both continuous and discrete state variables, the queueing process is described as a hybrid dynamical system. From this formulation, we derive one observable of fundamental importance: the green time required to clear the queue. This quantity allows to detect switching time points for serving platoons without delay, i.e., in a “green wave” manner. Moreover, we quantify the cost of delaying the start of a service period or its termination in terms of additional waiting time. Our findings may serve as a basis for strategic control decisions.     

Observation of dark dot splitting pattern in terbium gallium garnet

We have observed a new polarisation pattern in terbium gallium garnet which is both laser excited and mechanically stressed. The pattern can be characterized by a splitting from a “one dot” structure to a “two dots” structure. We have found that the new pattern is very sensitive to the orientation and the strength of the external forces applied to the sample. Thanks to this sensitivity and to the image simplicity, this new effect may be used in a vectorial force sensor and actuator. Received: 25 March 1999 / Accepted: 20 April 1999 / Published online: 25 August 1999     

Mechanisms for similarity based cooperation

Cooperation based on similarity has been discussed since Richard Dawkins introduced the term “green beard” effect. In these models, individuals cooperate based on an aribtrary signal (or tag) such as the famous green beard. Here, two different models for such tag based cooperation are analysed. As neutral drift is important in both models, a finite population framework is applied. The first model, which we term “cooperative tags” considers a situation in which groups of cooperators are formed by some joint signal. Defectors adopting the signal and exploiting the group can lead to a breakdown of cooperation. In this case, conditions are derived under which the average abundance of the more cooperative strategy exceeds 50%. The second model considers a situation in which individuals start defecting towards others that are not similar to them. This situation is termed “defective tags”. It is shown that in this case, individuals using tags to cooperate exclusively with their own kind dominate over unconditional cooperators.     

The carrier “antibinding” in quantum dots: a charge separation effect

We show that the carrier “antibinding” observed recently in semiconductor quantum dots, i.e., the fact that the ground state energy of two electron-hole pairs goes above twice the ground-state energy of one pair, can entirely be assigned to a charge separation effect, whatever its origin. In the absence of external electric field, this charge separation comes from different “spreading-out” of the electron and hole wavefunctions linked to the finite height of the barriers. When the dot size shrinks, the two-pair energy always stays below when the barriers are infinite. On the opposite, because barriers are less efficient for small dots, the energy of two-pairs in a dot with finite barriers, ends by behaving like the one in bulk, i.e., by going above twice the one-pair energy when the pairs get too close. For a full understanding of this “antibinding” effect, we have also reconsidered the case of one pair plus one carrier. We find that, while the carriers just have to spread out of the dot differently for the “antibinding” of two-pairs to appear, this “antibinding” for one pair plus one carrier only appears if this carrier is the one which spreads out the less. In addition a remarkable sum rule exists between the “binding energies” of two pairs and of one pair plus one carrier.     

Description of electrons correlation in1,3S-states of negative ion of positronium

The energy of ground^1,3S-states of negative positronium ion in the hyperspherical coordinates method has been calculated in Born-Oppenheimer and adiabatic approximations accounting the contribution of angle and radial correlation. Quasi-crossing points for autodetaching adiabatic potentials were revealed, which forms a structure in nonadiabatic potential. The received energy values have been compared with results of calculations performed using other methods. It was shown that in order to receive energy values with accuracy of 10⁻⁴ a.u. it is necessary to include more than 70 elements into the basis for determination of adiabatic potentials or to use Padé approximation on the basis dimension. Presented at the 11th Colloquium “Quantum Groups and Integrable Systems”, Prague, 20–22 June 2002. The work is supported by the Slovak Agency for Science, Grant 2/7157/20.     

Discrete photodetection of quantum jumps on the V configuration of atomic levels

A theory of the discrete photodetection of quantum jumps on the V configuration of atomic levels has been developed. A three-level source atom is placed in a cavity excited by a resonance fluorescence field. The cavity is tuned to exact resonance with an atomic transition. The cavity mode state is tested by a flux of unexcited (at the entrance) probe atoms passing through the cavity. The energy states of the outgoing probe atoms are detected by ionization chambers, which are assumed ideal. This a posteriori statistical information is indirectly related to the numerical characteristics of a measured quantum system consisting of the source atom and cavity mode. The “tuning” conditions for a discrete photodetector, i.e., the rules for choosing the parameters and durations of the interactions of the cavity mode with the probe and source atoms, intensities of the pump and probe fields that are necessary for observing quantum jumps from the “bright” state to the “dark” one and vice versa, have been determined. A two-state model that describes the dynamics of a quantum jump has been analyzed. The formulas have been obtained for the observable characteristics of quantum jumps: the mean residence time of the quantum system in quasistationary states (durations of the bright and dark periods), probabilities of quantum jumps, mean excitation levels of the quantized cavity mode, etc.     

Medium effects in the production and π0γ decay of ω-mesons in pA collisions in the GeV region

The ω resonance production and its π⁰γ decay in pA reactions close to threshold is considered within the Intranuclear Cascade (INC) model. The π⁰γ invariant-mass distribution shows two components which correspond to the ω decay “inside” and “outside” the nucleus, respectively. The “inside” component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for the ω-meson and its decaying pion. The relative contribution of the “inside” component is analyzed in detail for different kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between the π⁰ and γ momenta allows to separate events related to the “inside”ω decay from different sources of background when uncorrelated π⁰'s and γ's are produced. Received: 2 April 2001 / Accepted: 5 June 2001     

Third order response to a multifrequency photon field in semiconductors

This paper contains a detailed calculation of the photoinduced current density at third order in the coupling between a semiconductor and a multifrequency photon field, starting from its standard textbook expression which reads in terms of a triple commutator. Due to a major intrinsic problem linked to this triple commutator, such a derivation has been made possible quite recently only, thanks to the tools developed in the composite-boson many-body theory we have recently constructed. The photoinduced current density is shown to ultimately read in a compact form, in terms of the “Pauli scatterings” and “Coulomb scatterings” for exciton-exciton interactions introduced in this theory. Representation of this third order response in Shiva diagrams, which visualize interactions between excitons, is also given to better grasp the physics of the various contributions.     

The Origins of the Field Concept in Physics

The term, “field,” made its first appearance in physics as a technical term in the mid-nineteenth century. But the notion of what later came to be called a field had been a long time in gestation. Early discussions of magnetism and of the cause of the ocean tides had long ago suggested the idea of a “zone of influence” surrounding certain bodies. Johannes Kepler's mathematical rendering of the orbital motion of Mars encouraged him to formulate what he called “a true theory of gravity” involving the notion of attraction. Isaac Newton went on to construct an eminently effective dynamics, with attraction as its primary example of force. Was his a field theory? Historians of science disagree. Much depends on whether a theory consistent with the notion of action at a distance ought qualify as a “field” theory. Roger Boscovich and Immanuel Kant later took the Newtonian concept of attraction in new directions. It was left to Michael Faraday to propose the “physical existence” of lines of force and to James Clerk Maxwell to add as criterion the presence of energy as the ontological basis for a full-blown “field theory” of electromagnetic phenomena.     

Accelerated simulation of heat transfer in magnetic fluid hyperthermia

We have created a fast algorithm for calculating the temperature profile in a living tissue treated by magnetic fluid hyperthermia. Our algorithm solves an equation by the finite difference method. This equation includes the “heat sink” and “K-effective” effects. The algorithm need not an initial solution and it has the peculiarity that it makes a recursive division of the network, reducing thus the calculation period 5–9 times.     

The exciton many-body theory extended to any kind of composite bosons

We have recently constructed a many-body theory for composite excitons, in which the possible carrier exchanges between N excitons can be treated exactly through a set of dimensionless “Pauli scatterings” between two excitons. Many-body effects with free excitons turn out to be rather simple because these excitons are the exact one-pair eigenstates of the semiconductor Hamiltonian, in the absence of localized traps. They consequently form a complete orthogonal basis for one-pair states. As essentially all quantum particles known as bosons are composite bosons, it is highly desirable to extend this free exciton many-body theory to other kinds of “cobosons” — a contraction for composite bosons — the physically relevant ones being possibly not the exact one-pair eigenstates of the system Hamiltonian. The purpose of this paper is to derive the “Pauli scatterings” and the “interaction scatterings” of these cobosons in terms of their wave functions and the interactions which exist between the fermions from which they are constructed. It is also explained how to calculate many-body effects in such a very general composite boson system.     

Electronic structure and local interactions on a S(100) 2×1 surface with submonolayer Ba overlayers

The electronic structure and ionization energy for the system Ba/Si(100)2×1 have been studied as functions of the submonolayer coverage. It is found that there is an energy gap in the surface states spectrum and that the Ba/Si(100)2×1 interface is semiconducting up to 1.5 monolayers of Ba. Two surface bands induced by Ba adsorption have been detected. The evolution of the spectrum with increasing degree of Ba coverage points to the existence of two nonequivalent “adsorption sites,” which differ in binding energy by 0.11 eV. The development of the Ba-induced bands is found to terminate at a coverage corresponding to the minimum ionization energy and close to one monolayer. The adsorption bond is shown to have a primarily covalent character. Zh. éksp. Teor. Fiz. 114, 2145–2152 (December 1998)     

Elastic Green's function of icosahedral quasicrystals

The elastic theory of quasicrystals considers, in addition to the “normal” displacement field, three “phason” degrees of freedom. We present an approximative solution for the elastic Green's function of icosahedral quasicrystals, assuming that the coupling between the phonons and phasons is small. Received: 18 December 1997 / Accepted: 6 March 1998