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At least 37 records · Page 2Linked to original sources
arXiv2008

How do drops evaporate?

We consider evaporation of pure liquid drops on a thermally conductive substrate. Two evaporative models are considered: one that concentrates on the liquid phase in determining evaporative flux, and the other one that centers on the gas/vapor phase. A single governing equation for the evolution of drop thickness, including both models, is developed. Experiments are used to estimate relevant parameters. We show how the derived governing equation can be used to predict which evaporation model is appropriate under considered experimental conditions.

physics.flu-dyn↗
arXiv2008

A singular perturbation approach to the steady-state 1D Poisson-Nernst-Planck modeling

The reduced 1D Poisson-Nernst-Planck (PNP) model of artificial nanopores in the presence of a permanent charge on the channel wall is studied. More specifically, we consider the limit where the channel length exceed much the Debye screening length and channel's charge is sufficiently small. Ion transport is described by the nonequillibrium steady-state solution of the PNP system within a singular perturbation treatment. The quantities, 1/lambda -- the ratio of the Debye length to a characteristic length scale and epsilon -- the scaled intrinsic charge density, serve as the singular and the regular perturbation parameters, respectively. The role of the boundary conditions is discussed. A comparison between numerics and the analytical results of the singular perturbation theory is presented.

physics.chem-ph↗
arXiv2008

Dye-Sensitized Nanostructured TiO2 Film Based Photoconductor

Grooves were etched in a conductive layer of a conductive, transparent glass, and a nanoporous TiO2 film was deposited on both the conductive and nonconductive area. The width of the grooves was 100 $μ$m and 150 $μ$m. A transparent TiO2 film was dye-sensitized, covered with an electrolyte, and sandwiched with a cover glass. The conductivity of the dye-sensitized TiO2 film permeated with electrolyte was studied in the dark and under illumination, and was observed to be dependent on light intensity, wavelength and applied voltage. This study shows that dye-sensitized nanoporous films can be used as a wavelength dependent photoconductor.

physics.chem-ph↗
arXiv2009

Two-Level Chromophore and Irreversibility

This paper ("Two-Level Chromophore and Irreversibility", which can be found at arXiv:0804.0086) has been withdrawn by the author due to too many errors and misleading statements. This paper has been superseded by Section 2.11 and Chapter 7 (and other sections) of my Ph.D. dissertation ("Pressure Broadening and Pressure Shift of Diatomic Iodine at 675 nm"), which can be found at arXiv:0910.5053.

physics.chem-ph↗
arXiv2008

Atomic Structures of all the Twenty Essential Amino Acids and a Tripeptide, with Bond Lengths as Sums of Atomic Covalent Radii

Recently, the bond lengths of the molecular components of nucleic acids and of caffeine and related molecules were shown to be sums of the appropriate covalent radii of the adjacent atoms. Thus, each atom was shown to have its specific contribution to the bond length. This enabled establishing their atomic structures for the first time. In this work, the known bond lengths for amino acids and the peptide bond are similarly shown to be sums of the atomic covalent radii. Based on this result, the atomic structures of all the twenty essential amino acids and a tripeptide have been presented.

physics.gen-ph↗
arXiv2008

A Liquid Film Motor

It is well known that electro-hydrodynamical effects in freely suspended liquid films can flow the liquid. Here we report a purely electrically driven rotation in water and some other liquid suspended films with full control on the velocity and the chirality of the rotating vortices. The device, which is called ``film motor'', consists of a quasi two-dimensional electrolysis cell in an external in-plane electric field, crossing the mean electrolysis current density. If either the external field or the electrolysis voltage exceeds some threshold (while the other one is not zero), the liquid film begins to rotate. The device works perfectly with both DC and AC fields.

cond-mat.soft↗
arXiv2008

Ionic Kratzer bond theory and vibrational levels for achiral covalent bond HH

A dihydrogen Hamiltonian reduces to the Sommerfeld-Kratzer-potential, adapted for field quantization according to old-quantum theory. Constants omega_e, k_e and r_e needed for the H_2 vibrational system derive solely from hydrogen mass m_H. For H_2, a first principles ionic Kratzer oscillator returns the covalent bond energy within 0,08 % and all levels within 0,02 %, 30 times better than the Dunham oscillator and as accurate as early ab initio QM.

physics.gen-ph↗
arXiv2008

How the surrounding water changes the electronic and magnetic properties of DNA

Strong influence of water molecules on the transport and magnetic properties of DNA, observed in this study, opens up real opportunities for application of DNA in molecular electronics. Interaction of the nucleobases with water molecules leads to breaking of some of the $π$ bonds and appearance of unbound pi electrons. These unbound electrons contribute significantly to the charge transfer at room temperature by up to 1000 times, but at low temperature the efficiency of charge transfer is determined by the spin interaction of the two unbound electrons located on the intrastrand nucleobases. The charge exchange between the nucleobases is allowed only when the spins of unbound electrons are antiparallel. Therefore, the conductance of the DNA molecule can be controlled by a magnetic field. That effect has potential applications for developing a nanoscale spintronic device based on the DNA molecule, where efficiency of spin interaction will be determined by the DNA sequence.

physics.chem-ph↗
arXiv2013

The dipole-quadrupole theory of surface-enhanced Raman scattering

The review is devoted to explanation of SERS in terms of the dipole and quadrupole light-molecule interactions arising in surface fields strongly varying in space in the region of the strongly irregular surface roughness. The main SERS characteristics, the theory of electromagnetic fields near some model kinds of rough surfaces and some other systems, the theory of SERS Raman tensor for arbitrary and symmetrical molecules, selection rules and analysis of the SER spectra, some anomalies in the SER spectra of symmetrical molecules for some specific conditions, electrodynamic forbiddance of the quadrupole scattering mechanism for the methane molecule and molecules with cubic symmetry groups are considered. The huge enhancement and blinking of the SERS signal arising in the phenomenon of Single Molecule detection by the SERS method are explained. The above theory is compared with some another SERS mechanisms, and the phenomena accompanying SERS are accounted for. It is demonstrated that the theory is in a good agreement with the experiment and explains quite a number of characteristics related to the SERS phenomenon.

physics.chem-ph↗
arXiv2009

Analysis of a stochastic chemical system close to a SNIPER bifurcation of its mean-field model

A framework for the analysis of stochastic models of chemical systems for which the deterministic mean-field description is undergoing a saddle-node infinite period (SNIPER) bifurcation is presented. Such a bifurcation occurs for example in the modelling of cell-cycle regulation. It is shown that the stochastic system possesses oscillatory solutions even for parameter values for which the mean-field model does not oscillate. The dependence of the mean period of these oscillations on the parameters of the model (kinetic rate constants) and the size of the system (number of molecules present) is studied. Our approach is based on the chemical Fokker-Planck equation. To get some insights into advantages and disadvantages of the method, a simple one-dimensional chemical switch is first analyzed, before the chemical SNIPER problem is studied in detail. First, results obtained by solving the Fokker-Planck equation numerically are presented. Then an asymptotic analysis of the Fokker-Planck equation is used to derive explicit formulae for the period of oscillation as a function of the rate constants and as a function of the system size.

physics.chem-ph↗
arXiv2008

Using strong electromagnetic fields to control x-ray processes

Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular physics has begun. Not only is is possible to control outer-shell electron dynamics with intense ultrafast optical lasers, but now control of inner-shell processes has become possible by combining intense infrared/optical lasers with tunable sources of x-ray radiation. This marriage of strong-field laser and x-ray physics has led to the discovery of methods to control reversibly resonant x-ray absorption in atoms and molecules on ultrafast timescales. Using a strong optical dressing field, resonant x-ray absorption in atoms can be markedly suppressed, yielding an example of electromagnetically induced transparency for x rays. Resonant x-ray absorption can also be controlled in molecules using strong non-resonant, polarized laser fields to align the framework of a molecule, and therefore its unoccupied molecular orbitals to which resonant absorption occurs. At higher laser intensities, ultrafast field ionization produces an irreversible change in x-ray absorption. Finally, the advent of x-ray free electron lasers enables first exploration of non-linear x-ray processes.

physics.atom-ph↗
arXiv2009

The "weighted ensemble" path sampling method is statistically exact for a broad class of stochastic processes and binning procedures

The "weighted ensemble" method, introduced by Huber and Kim, [G. A. Huber and S. Kim, Biophys. J. 70, 97 (1996)], is one of a handful of rigorous approaches to path sampling of rare events. Expanding earlier discussions, we show that the technique is statistically exact for a wide class of Markovian and non-Markovian dynamics. The derivation is based on standard path-integral (path probability) ideas, but recasts the weighted-ensemble approach as simple "resampling" in path space. Similar reasoning indicates that arbitrary nonstatic binning procedures, which merely guide the resampling process, are also valid. Numerical examples confirm the claims, including the use of bins which can adaptively find the target state in a simple model.

physics.comp-ph↗
arXiv2008

Maria Goeppert Mayer's Theoretical Work on Rare-Earth and Transuranic Elements

After the discovery of element 93 neptunium by Edwin McMillan and Philip H. Abelson in 1941, Maria Goeppert Mayer applied the Thomas-Fermi model to calculate the electronic configuration of heavy elements and predicted the occurrence of a second rare-earth series in the vicinity of elements 91 or 92 extending to the transuranic elements. Mayer was motivated by Enrico Fermi, who was at the time contemplating military uses of nuclear energy. Historical development of nuclear science research leading to Mayer's publication is outlined. Mayer's method is introduced with the aid of a computer, which enables students to visualize her description of eigenfunctions, particularly the sudden change of spatial distribution and eigenenergy at the beginning of the rare-earth series. The impact of Mayer's work on the periodic table is discussed.

physics.chem-ph↗
arXiv2009

Dipole emission and coherent transport in random media II. Density of states vs. index of refraction

This is the second of a series of papers devoted to develop a microscopical approach to the dipole emission process and its relation to coherent transport in random media. In this Letter, we deduce a relation between the transverse decay rate of an emitter in a virtual cavity and the complex refraction index of the host medium. We argue on the possibility of a criterion for inhibition/enhancement of spontaneous emission in function of the transition frequency and the correlation length of the host scatterers. In addition, we study the radiative/non-radiative nature of the net power emission through a microscopical analysis of the scattering events involved. This study reveals essential discrepancies with previous interpretations.

physics.optics↗
arXiv2009

Reactive Collisions of Ultracold Molecules Confined in quasi-2D Geometry

We present a formalism for rigorous calculations of cross sections for inelastic and reactive collisions of ultracold atoms and molecules confined by laser fields in quasi-2D geometry. Our results show that the elastic-to-inelastic ratios of collision cross sections are enhanced in the presence of a laser confinement and that the threshold energy dependence of the collision cross sections can be tuned by varying the confinement strength and external magnetic fields. The enhancement of the elastic-to-inelastic ratios is inversely proportional to $\sqrt{ε/\hbar ω_0}$, where $ε$ is the kinetic energy and $ω_0$ is the oscillation frequency of the trapped particles in the confinement potential.

physics.atom-ph↗
arXiv2008

Dependence of response functions and orbital functionals on occupation numbers

Explicitly orbital-dependent approximations to the exchange-correlation energy functional of density functional theory typically not only depend on the single-particle Kohn-Sham orbitals but also on their occupation numbers in the ground state Slater determinant. The variational calculation of the corresponding exchange-correlation potentials with the Optimized Effective Potential (OEP) method therefore also requires a variation of the occupation numbers with respect to a variation in the effective single-particle potential, which is usually not taken into account. Here it is shown under which circumstances this procedure is justified.

cond-mat.mtrl-sci↗