Results 11 - 20 of 133425
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[en] Highlights: • We examined the attainment of the Conical Intersection (CI) in Hipoxantine (Hx). • Charge transfer in the molecule is very important in the evolution of S0 and S1. • Aromaticity impairment and push pull systems in Hx are crucial in attaining its CI. • QTAIM offers valuable tools to study the photostability of nucleobases. We analyzed the evolution of the electron density across the S0 and S1 states potential energy curves of hypoxanthine (Hx) using the Quantum Theory of Atoms in Molecules (QTAIM). Examination of QTAIM energies and electronic populations indicates that charge transfer processes are important in the stabilization of the S1 state towards the Conical Intersection (CI) which confers to Hx its photostability. Our results point that the rise of energy of the S0 state approaching the CI is accompanied by a loss of aromaticity of hypoxanthine. Overall, the analyses presented herein give important insights on the photostability of nucleobases.
[en] We compute binding energies and root-mean-square radii for weakly bound systems of N=4 and 5 identical bosons. Ground and first excited states of an N-body system appear below the threshold for binding the system with N-1 particles. Their root-mean-square radii approach constants in the limit of weak binding. Their probability distributions are on average located in nonclassical regions of space which result in universal structures. Radii decrease with increasing particle number. The ground states for more than five particles are probably nonuniversal, whereas excited states may be universal.
[en] We discuss the discrepancy between the Cung et al. [Phys. Lett. B 68, 474 (1977)] calculation of the three-photon-annihilation contribution to the positronium ground-state energy, performed using the binding energy to regulate the infrared divergences, and the recent calculation of Adkins, Bui, and Zhu [Phys. Rev. A 37, 4071 (1988)], which used a photon mass to regulate these divergences. By using a simpler version of the binding-energy approach advocated by these authors, we confirm the value of the discrepancy they obtained. Furthermore, it is shown that the additional term needed in the binding-energy approach to produce agreement between the two methods is precisely the one required to make the binding-energy regularized amplitudes gauge invariant to all orders in the binding
[en] A perturbative expansion of the electron's Dirac Coulomb propagator around a nonrelativistic form is used to evaluate the one-loop p nonrecoil corrections to ground-state hyperfine splitting in p hydrogenic atoms. A contribution previously estimated as (α/π)(Zα)2 x (18.36 +- 5)E/sub F/ is found to be (α/π) (Zα)2 (15.10 +- 0.29)E/sub F/. Theory and experiment are compared for muonium hyperfine splitting and consequences for the fine-structure constant are discussed
[en] The geometries and relative energies of the nine lowest states of the ozone molecule have been determined in C/sub 2v/ symmetry from ab initio configuration interaction calculations in a [3s2p1d] contracted Gaussian basis. Calculations were carried out over a two-dimensional grid of points in C/sub 2v/ symmetry to locate the optimum geometrical parameters R and theta for each state. For the ground 1A1 state the calculated properties (with experimental values in parentheses) are as follows: R/sub e/=1.299 A (1.271 A), theta/sub e/=116.00 (116.80), ω1=1235 cm-1 (1110 cm-1) and ω2=707 cm-1 (705 cm-1). Of the excited states only the lowest 3B2 state is found to have an adiabatic excitation energy (0.92 eV) less than the dissociation energy (D/sub e/=1.13 eV) and hence to be a likely bound species. The 1B2 state responsible for the strong absorption in the Hartley band (4.7--5.8 eV) is stabilized by asymmetric distortions away from its equilibrium C2/sub v/ geometry (R/sub e/=1.405 A, theta/sub e/=1080); this finding suggests unequal bond lengths for this state or else purely dissociative behavior. The ring (21A1) state (R/sub e/=1.449 A, theta/sub e/=600) is found to lie 1.20 eV above the ground state, while the remaining five states have adiabatic excitation energies ranging from 1.4 to 3.6 eV. The implications for photodissociation of O3 are discussed
[en] Many important classes of surface reactions exhibit both high heats of reaction and large, positive activation energies. In addition, many surface reactions often occur in thermally isolated environments. As a result, significant autothermic effects are possible. In part I of this article, a generalized model of these effects is presented which describes the enhancement in reaction rate as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Reactant concentration and reaction order effects are also considered. Part II of this work presents the application of this model to numerous experimental plasma etching data
[en] We discuss theoretically the properties of an electromechanical oscillating system whose operation is based upon the cyclic conservative conversion between gravitational potential, kinetic and magnetic energies. The system consists of a superconducting coil subjected to a constant external force and to magnetic fields. The coil oscillates and has induced in it a rectified electrical current whose magnitude may reach hundreds of amperes. The design differs from that of most conventional superconductor machines since the motion is linear (and practically unnoticeable depending on frequency) rather than rotatory and it does not involve high speeds. Furthermore, there is no need for an external electrical power source to start up the system. We also show that the losses for such a system can be made extremely small for certain operational conditions, so that by reaching and keeping resonance the system's main application should be in the generation and storage of electromagnetic energy. (rapid communication)
[en] We investigate changes in superfluid weight in photo-excited states. This quantity is obtained by a microscopic calculation of the third-order nonlinear response function on the assumption that the pumping intensity is low. The resultant expression includes the energy dependence of self-energy explicitly. The importance of this dependence is shown by performing numerical calculations with the electron-boson interaction included. Moreover, it is found that the vertex correction makes a predominant contribution to the photoinduced change in superfluid weight. The similarity between the linear and photoinduced responses in its temperature dependence is discussed, and a quantitative estimation of the calculated results is made in comparison with that of the experiments. (author)
[en] The six doublet and the two quartet electronic states ("2Σ"+(2), "2Σ"-, "2Π(2), "2Δ, "4Σ"-, and "4Π) of the OH radical have been studied using the multi-reference configuration interaction (MRCI) method where the Davidson correction, core-valence interaction and relativistic effect are considered with large basis sets of aug-cc-pv5z, aug-cc-pcv5z, and cc-pv5z-DK, respectively. Potential energy curves (PECs) and dipole moment functions are also calculated for these states for internuclear distances ranging from 0.05 nm to 0.80 nm. All possible vibrational levels and rotational constants for the bound state X"2Π and A"2Σ"+ of OH are predicted by numerical solving the radial Schroedinger equation through the Level program, and spectroscopic parameters, which are in good agreements with experimental results, are obtained. Transition dipole moments between the ground state X"2Π and other excited states are also computed using MRCI, and the transition probability, lifetime, and Franck-Condon factors for the A"2Σ"+ - X"2Π transition are discussed and compared with existing experimental values.