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Papers by David Micha

The geometrical structures of M+(Ar)n ions, with n=1–14, have been studied by the minimization of... more The geometrical structures of M+(Ar)n ions, with n=1–14, have been studied by the minimization of a many-body potential surface with a simulated annealing procedure. The minimization method is justified for finite systems through the use of an information theory approach. It is carried out for eight potential-energy surfaces constructed with two- and three-body terms parametrized from experimental data and abinitio

The Journal of Physical Chemistry C, 2012

ABSTRACT Recent work on light absorption by model surfaces of Si has shown that Ag adsorbates inc... more ABSTRACT Recent work on light absorption by model surfaces of Si has shown that Ag adsorbates increase the intensity of photoinduced electronic transitions at lower photon energies. Furthermore, another set of recent results for Si quantum dots (QDs) has shown that P and Al dopants shift the light absorbance toward lower photon energies. In this report, the optical absorbance of Si QDs with P and Al dopants and either one or three Ag adsorbed atoms has been calculated with TD-DFT using the PW91/PW91 density functionals to compare with our previous results. In general, the presence of Ag adsorbates shows both a decrease in the HOMO-LUMO gap and a drastic increase in the absorbance below 4 eV. The addition of dopants leads to a combined effect where the energy gap is further decreased to values below 2 eV. The molecular orbitals for the initial and final states involved in transitions with large oscillator strengths were also calculated, which qualitatively show the excited electrons moving toward the Ag during excitation. This study indicates that stronger absorption in the visible, near-UV, and near-IR parts of the spectrum can be achieved with a combination of Ag adsorbate clusters and doping.

The Journal of Chemical Physics, Dec 28, 2007

An alkali atom-noble gas cluster system is considered as a model for solvation effects in optical... more An alkali atom-noble gas cluster system is considered as a model for solvation effects in optical spectra, within a quantum-classical description based on the density operator of a many-atom system and its partial Wigner transform. This leads to an eikonal-time-dependent molecular orbital treatment suitable for a time-dependent description of the coupling of light emission and atom dynamics in terms of the time-dependent electric dipole of the whole system. As an application, we consider an optically excited lithium atom as the dopant in a helium cluster at 0.5K. We describe the motions of the excited Li atom interacting with a cluster of He atoms and calculate the time-dependent electric dipole of the Li -He99 system during the dynamics. The electronic Hamiltonian is taken as a sum of three-body Li-He diatomic potentials including electronic polarization and repulsion, with l-dependent atomic pseudopotentials for Li and He, while we use a modified pair potential for He-He. The calculations involve the coupling of 12 quantum states with 300 classical degrees of freedom. We present results for the dynamics and spectra of a Li atom interacting with a model cluster surface of He atoms and also interacting with a droplet of He. We have found that the Li atom is attracted or repulsed from the He surface, depending on the orientation of its 2p orbitals. The spectra and dynamics of Li inside and at the surface of a cluster are found to be strongly dependent on its electronic states, its velocity direction, and whether light is present during emission or not.

The Journal of Chemical Physics, 2016

A new general computational procedure is presented to obtain photoconductivities starting from at... more A new general computational procedure is presented to obtain photoconductivities starting from atomic structures, combining ab initio electronic energy band states with populations from density matrix theory, and implemented for a specific set of materials based on Si crystalline slabs and their nanostructured surfaces without and with adsorbed Ag clusters. The procedure accounts for charge mobility in semiconductors in photoexcited states, and specifically electron and hole photomobilities at Si(111) surfaces with and without adsorbed Ag clusters using ab initio energy bands and orbitals generated from a generalized gradient functional, however with excited energy levels modified to provide correct bandgaps. Photoexcited state populations for each band and carrier type were generated using steady state solution of a reduced density matrix which includes dissipative medium effects. The present calculations provide photoexcited electronic populations and photoinduced mobilities resulting from applied electric fields and obtained from the change of driven electron energies with their electronic momentum. Extensive results for Si slabs with 8 layers, without and with adsorbed Ag clusters, show that the metal adsorbates lead to substantial increases in the photomobility and photoconductivity of electrons and holes.

Electronic excitation and charge transfer in atom-atom collisions are described within a first pr... more Electronic excitation and charge transfer in atom-atom collisions are described within a first principles dynamics based on the eikonal description of atomic motions and the time-dependent Hartree-Fock approximation for the many-electron system, in an Eik/TDHF procedure. Diatomic systems involving protons, alkali and rare gas atoms are studied with hamiltonians including atomic core pseudopotentials, to ascertain their accuracy and efficiency in problems dealing with time-dependent many-electron phenomena. Calculations are based on the use of cartesian gaussian expansion functions including electron translation factors.[1] The equations for coupled electronic and nuclear motions are solved with a relax-and-drive numerical procedure useful in the description of coupled fast (electronic) and slow (nuclear) motions, and also applicable to problems involving light emission and collisions at metal surfaces.[2] 1. K. Runge and D. A. Micha, Phys. Rev. A 53, 1388 (1996). 2. D. A. Micha, J. Phys. Chem. 103, 7562 (1999).

The Journal of Chemical Physics, Feb 1, 2011

A mixed quantum-classical formulation is developed for a quantum subsystem in strong interaction ... more A mixed quantum-classical formulation is developed for a quantum subsystem in strong interaction with an N-particle environment, to be treated as classical in the framework of a hydrodynamic representation. Starting from the quantum Liouville equation for the N-particle distribution and the corresponding reduced single-particle distribution, exact quantum hydrodynamic equations are obtained for the momentum moments of the single-particle distribution coupled to a discretized quantum subsystem. The quantum-classical limit is subsequently taken and the resulting hierarchy of equations is further approximated by various closure schemes. These include, in particular, (i) a Grad-Hermite-type closure, (ii) a Gaussian closure at the level of a quantum-classical local Maxwellian distribution, and (iii) a dynamical density functional theory approximation by which the hydrodynamic pressure term is replaced by a free energy functional derivative. The latter limit yields a mixed quantum-classical formulation which has previously been introduced by I. Burghardt and B. Bagchi, Chem. Phys. 134, 343 (2006).

Chem Phys Lett, 1974

A computational scheme is reported which uses a reference potential approximation to provide an e... more A computational scheme is reported which uses a reference potential approximation to provide an efficient numerical solution of the multi-channel Volterra integral equation. Applications to rotational inelastic scattering and quasi-bound states of the He-N 2 system are presented, together with a discussion of the effects of varying the number of closed channels.

Physical Review a, Aug 1, 1994

We describe the coupling of electronic and nuclear motions in slow atomic collisions using a comb... more We describe the coupling of electronic and nuclear motions in slow atomic collisions using a combination of the eikonal and time-dependent Hartree-Fock (TDHF) approximations. Starting with an eikonal representation of the total wave function, a wave function is constructed from classical trajectories in a way suitable for describing atomic collisions with velocities down to a fraction of an atomic unit. The TDHF formulation is developed in terms of its density operator. The differential equations coupling the density operator to the nuclear motions have been solved with a procedure developed to account for the coupling of fast (electronic) and slow (nuclear) degrees of freedom. This is based on a local-interaction picture and on a temporal linearization of the equations, allowing for the integration of the electronic density over large time intervals. Density-matrix equations are derived in a basis of traveling atomic orbitals, and numerical results are presented for H++H and He2++H. Good agreement is found with experimental results for H++H, comparing integral electron transfer cross sections from 2 to 2000 eV. In addition, an analysis of the time dependence of atomic orbital populations provides insight on electronic rearrangement during collisions and shows that even very small contributions from the driving forces of the nuclei on the electrons have a cumulative effect on the density operator that can substantially change final populations.

Chem Phys Lett, 2003

A recent density matrix treatment of femtosecond photodesorption has been extended to include fre... more A recent density matrix treatment of femtosecond photodesorption has been extended to include frequency chirping effects. It describes the dissipative dynamics of desorption and substrate electronic excitations, with an optical Bloch equation and rate equations for the substrate temperature and electron density, including the electron-phonon coupling. Results on CO/Cu(0 0 1) are presented for yield vs. pulse fluence, showing agreement with experimental values, and for chirping effects. Variations of the chirped frequency by ±5% give large changes in the yields. The calculations suggest that chirping can be used to control desorption yields, and to investigate the strength of electron-phonon coupling in metals.

Chem Phys, 1976

Stripping reactions of K with Br2, BrI, and I2 at hyperthermal energies have been theoretically s... more Stripping reactions of K with Br2, BrI, and I2 at hyperthermal energies have been theoretically studied with particular emphasis on the electron transfer and direct rearrangement stages of the reactions. Electron transfer was investigated with a stochastic model based on Landau-Zener transition probabilities for atom-diatomic collisions. Results show that vibrational distributions of the negative diatomic halogen ions immediately after electron transfer are determined by a vertical transition mechanism. A comparison was then made between experimental and theoretical velocity distributions of products K+I2 and Br2, with a model based on diatomic momentum distributions. This provides evidence that a fraction of the vibrationally excited Br-2 and I-2 ions relax as they react with K+. The theoretical results are consistent with available experimental results. This is also the case for K+BrI, in connection with calculated velocity and angular distributions of products, and branching ratios.

The Journal of Chemical Physics

The resonance energies of long-lived states in elastic scattering of atoms by solid surfaces are ... more The resonance energies of long-lived states in elastic scattering of atoms by solid surfaces are related to the trajectories of poles of the scattering matrix in the planes of the complex components of the reciprocal vector G. Resonance trajectories, similar to Regge-pole trajectories, are discussed for scattering of He by LiF(001) at fixed angle and varying wavelength. This approach gives insight into the ordering of resonances. A construction is described in the plane of the G vector components to identify possible resonance energies and to discuss their high-energy behavior.

We present results for Li and Na collisions with He and Ne, on the state-to-state integral cross ... more We present results for Li and Na collisions with He and Ne, on the state-to-state integral cross section for electronic excitation in the keV range, using a first principles treatment of the quantum dynamics [1-3]. The same procedure has also been used to study the spin-orbit recoupling dynamics of Na*-He in the hyperthermal energy range. The time-evolution of electronic excitation

Advances in Quantum Chemistry, 2004

Starting with the Liouville–von Neumann equation for the statistical density operator, a partial ... more Starting with the Liouville–von Neumann equation for the statistical density operator, a partial Wigner transform is introduced to derive equations coupling quantum degrees of freedom with quasiclassical ones, as frequently encountered in molecular systems undergoing electronic transitions. Further consideration of the limit of short wavelengths in phase space leads to the introduction of effective potentials, and a treatment suitable for

Physical Review A, 1989

Vibrational transitions in collisions of two polyatomic systems are described in terms of a Hamil... more Vibrational transitions in collisions of two polyatomic systems are described in terms of a Hamiltonian bilinear in momentum and position operators, for several degrees of freedom. The relative motion is assumed to be classical and leads to time-dependent coefficients in the Hamiltonian. ...