On the initiation of chemical reactions by electronic excitations in molecular solids

In this article we briefly review the results of theoretical simulations for the initiation of chemistry processes in high-explosive crystals from a solid-state-physics viewpoint. We analyze the possibility of initiation of chemical reactions from excited electronic states. In other words, we look for conditions that facilitate electronic excitations in the crystal. Specifically, we describe modifications to the electronic structure of RDX (cyclotrimethylene trinitramine) induced by lattice defects and by a shock wave traversing the solid. Our approach is based on ab initio Hartree–Fock band-structure calculations with electronic correlation corrections. An excitonic mechanism and a hole model, suggested earlier, are discussed in connection with the most recent experimental and theoretical advances in ultrafast optical techniques. We also consider here possible new avenues in the development of detonation theory. Received: 3 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-301/744-4451, E-mail: KuklaMM@ih.navy.mil     

Magnetic excitations in solids

This article presents a review of low to moderate frequency magnetic excitations, termed magnons or spin waves, in magnetically ordered materials. The emphasis is on intuitive behavior rather than analytical theory. Topics include spin waves, magnetostatic modes, dipole-exchange modes, surface anisotropy, dispersion properties, nonlinear effects, and relaxation. These phenomena are illustrated with experimental examples based on magnon light scattering results as well as conventional microwave techniques.     

On the theory of electronic correlations in solids

A formalism is presented for the computation of long range as well as short range correlations in the electronic ground state of a solid. It is based on a Local Approach to the correlation problem which was successfully tested before for small molecules. The method connects two approaches to the correlation problem. One is that of the RPA which is often applied in solid state physics and describes the long range aspects of the correlated electronic motions. The other is that of quantum chemistry which tries to calculate to high accuracy the short range part of the correlation problem. The method is variational in nature. The approximations which have been made exclude the application of the present formalism to strongly correlated systems such as valence fluctuating systems.     

On the phase transitions for the electronic excitations in semiconductors

A model Hamiltonian for a system of interacting electrons, holes and Wannier excitons is derived. This system of electronic excitations is assumed to be in a quasi-equilibrium state. With the aid of Bogolubov's variational principal the thermodynamic potential is calculated. Using the most general mean-field Hamiltonian as a trial Hamiltonian, a set of coupled integral equations is obtained for the self-energies. These equations are solved numerically for equal effective masses of the electrons and holes. Below a critical temperature ofk _B T _c0.65E _ex ^b whereE _ex ^b is the exciton binding energy, we find a first order phase transition from an exciton rich phase into a degenerate electron-hole phase. The mechanical and thermal stability of both phases is proven. Below a critical temperaturek _B T _c0.11E _ex ^b the exciton system becomes degenerate (Bose-Einstein condensation). A complete phase diagram of these three phases is given.This is a project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft     

Universality of anomalous excitations in amorphous solids

A general proof for the universal existence of anomalous, non-Debye harmonic excitations in disordered solids is proposed. Starting from an ideal amorphous solid with fixed mean positions of all atoms and harmonic interactions, it was demonstrated that any increase of disorder leads stringently to an increase of the spectral density for low (and also very high) frequencies. That means, the density of states shows additional contributions to the low-frequency part of the standard Debye density, even though the wavelength of corresponding phonon like excitations is of an order of magnitude on which the amorphous solid is still homogeneous and the sound waves are not strongly effected by the disorder.     

Femtosecond mid-infrared spectroscopy of low-energy excitations in solids

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 m has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-T_C superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. PACS 78.67.De; 73.21.Fg; 07.57.Hm; 42.65.Re     

Localized electronic excitations in nickel oxide

Valence-electron ionization and optical excitation energies of NiO have been calculated by the SCF?Xα cluster method and transition-state procedure. Ni 3d-like spin orbitals are localized above the O 2p band, leading to well defined peaks in the photoemission spectra. Ligand-to-metal charge-transfer excitations are responsible for strong optical absorption between 4 and 7eV.     

Relaxation of electronic excitations in strontium titanate

The transient absorption spectra and kinetics were studied for undoped, lead doped and high purity SrTiO 3 single crystals. The pulsed electron beam induced transient absorption is studied in all crystals. The strong absorption at 0.8 v eV was observed only in high purity SrTiO 3 . This absorption is suggested to arise from intrinsic electron polaron. The bound electron polarons are likely responsible for absorption band at 1.4 v eV. The main luminescence band under excitation pulse is observed at 2.75 v eV. The luminescence decay is faster than that of transient absorption.     

Multiplication of electronic excitations in AgCl crystals

The kinetics of pulse conduction in silver chloride single crystals is investigated in the temperature range 12–300 K upon picosecond excitation with x-ray bremsstrahlung. It is found that the experimentally observed concentration of conduction electrons is nearly twice as high as the concentration of electron-hole pairs generated by an exciting pulse. This effect is associated with the chain multiplication of band charge carriers.     

Femtosecond mechanisms of electronic excitations in crystalline materials

The lattice dynamics of crystals is investigated in the course of high-power electronic excitation. It is revealed that, at W _e < W _eo , atoms and ions are displaced from their regular sites for 100–300 fs. Subsequent relaxation of the crystal lattice in response to a strong local electric field induced by the collisional displacement of ions occurs for 10–50 ns in an oscillatory manner with a period of 0.5–1.5 ps.     

Ionic displacement caused by electronic excitations

Electronic excitations are often accompanied by displacement of the ions from their ground-state equilibrium positions. This leads to line broadening of optical spectra, Stokes shifts, conformational changes, and photoinduced reactions. Here we discuss approaches to these features within ab-initio methods, in particular within many-body perturbation theory. A number of various systems, including molecules, point defects, polymers, and surfaces, are discussed to illustrate issues like localization and self-trapping that are relevant for a detailed understanding of the interrelation between excited states and geometrical structure. To cite this article: M. Rohlfing, C. R. Physique 10 (2009).     

Collective electronic excitations in clusters in the vicinity of metal surfaces

We study the collective excitations of metal clusters approaching a metal surface. Using a simple model for the frequency-dependent dielectric constant ε(ω) and the multiple scattering method, we numerically investigate the shift in the plasmon resonance due to the coupling of the collective modes of the sphere with those of its mirror image. Results of the model calculation are verified by means of ab initio theory. As a prototype system we study Na₉ ⁺ cluster on the Cu(100) surface. The representation of the solid surface by a cluster of several, typically 54 substrate atoms, is used in combination with a high level configuration interaction (CI) calculation. PACS 31.15.Dv; 36.40.Gk; 73.20.-f