Verne L. Jacobs

The Influence of Autoionization Accompanied by Excitation on Dielectronic Recombination and Ionization Equilibrium

In the process of dielectronic recombination, the doubly excited state formed by radiationless capture may autoionize preferentially into an excited state of the recombining ion. This additional autoionization process has not been discussed in previous treatments of dielectronic recombination. The dielectronic recombination rates for certain nonhydrogenic Fe ions, although still larger than the direct radiative recombination rates, are found to be substantially reduced by the inclusion of the additional autoionization rate in the branching ratio for the stabilizing radiative transition. Consequently, the temperatures of maximum equilibrium abundance are significantly lower than those predicted by recent calculations. Finally, the radiative energy loss rate coefficients are calculated for radiation processes involving electron Fe-ion collisions in high-temperature plasmas. Electron impact excitation of resonance line radiation is the dominant radiative cooling mechanism in steady-state plasmas at temperatures where ions with bound electrons are abundant. However, it is found that the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.

Dielectronic recombination rates, ionization equilibrium, and radiative energy-loss rates for neon, magnesium, and sulfur ions in low-density plasmas. [in solar corona

Results of detailed and systematic calculations are presented for the total dielectronic recombination rate coefficients for the ions of Ne, Mg, and S in a low-density predominantly hydrogen plasma. The new recombination rates are used to calculate solar corona ionization-equilibrium distributions of the ions. The most important effect of dielectronic recombination for ions in corona equilibrium is found to be a shift in the maximum-abundance temperatures toward higher temperatures, which are in some cases reduced from those predicted on the basis of the simple Burgess formula.

Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon

We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time.

High-Resolution Measurement of the Kα Spectrum of Fe XXV-XVIII: New Spectral Diagnostics of Nonequilibrium Astrophysical Plasmas

We present laboratory measurements of high-resolution spectra of iron Kα emission under transient ionization conditions similar to those that are believed to exist in stellar flares and young supernova remnants. Taking advantage of our high spectral resolution (λ/Δλ ≥ 2000), we identify a number of transitions that can serve as diagnostics of ionizing plasmas. By varying the excitation energy in the experiments, we constrain the effects of the electron distribution on these diagnostic lines. Using our measured line ratios, we deduce values for the ionization time, η = Net, in the plasma, which agree with the actual values to ~20% accuracy. This result gives us confidence to our ability to derive similar constraints on astrophysical plasmas from remote X-ray spectroscopic observations.

The influence of autoionization accompanied by excitation on the dielectronic recombination and the ionization equilibrium of silicon ions

Abstract : The dielectronic recombination rate coefficients have been calculated for the various ionization stages of silicon. Account has been taken of all stabilizing radiative transitions nd all autoionization processes which involve a single-electron electric-dipole transition of the recombining ion core. For certain ions the dielectronic recombination rates, although still larger than the direct radiative recombination rates, are found to be substantially reduced when account is taken of the effects of a previously neglected autoionization process in which the excited recombining ion core undergoes a Delta n = o transition to a lower excited state. The Temperatures at which these ions have their maximum abundance in corona equilibrium are significantly reduced when use is made of the new dielectronic recombination rates. Calculations are also presented for the total rates of radiative energy loss from isothermal steady-state plasmas due to the line and continum emission of silicon ions. (Author)

A general framework for numerical simulation of improvised explosive device (IED)-detection scenarios using density functional theory (DFT) and terahertz (THz) spectra.

We have developed a general framework for numerical simulation of various types of scenarios that can occur for the detection of improvised explosive devices (IEDs) through the use of excitation using incident electromagnetic waves. A central component model of this framework is an S-matrix representation of a multilayered composite material system. Each layer of the system is characterized by an average thickness and an effective electric permittivity function. The outputs of this component are the reflectivity and the transmissivity as functions of frequency and angle of the incident electromagnetic wave. The input of the component is a parameterized analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated using density functional theory (DFT) and parameter adjustment according to any additional information that may be available, e.g., experimentally measured spectra or theory-based assumptions concerning spectral features. A prototype simulation is described that considers response characteristics for THz excitation of the high explosive β-HMX. This prototype simulation includes a description of a procedure for calculating response spectra using DFT as input to the S-matrix model. For this purpose, the DFT software NRLMOL was adopted.

Dielectric Response of High Explosives at THz Frequencies Calculated Using Density Functional Theory

We present in this study calculations of the ground-state resonance structures associated with the high explosives β-HMX, PETN, RDX, TNT1, and TNT2 using density functional theory (DFT). Our objective is the construction of parameterized dielectric-response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric-response functions provide the basis for analyses pertaining to the dielectric properties of explosives. In particular, these dielectric-response functions provide quantitative initial estimates of spectral-response features for subsequent adjustment with knowledge of additional information, such as laboratory measurements and other types of theory-based calculations. With respect to qualitative analyses, these spectra provide for the molecular-level interpretation of response structure. The DFT software GAUSSIAN was used for the calculations of the ground-state resonance structures presented here.

Photorecombination rates of hydrogenic and nonhydrogenic states

The authors present analytical approximations for photoionization cross sections from hydrogenic and nonhydrogenic nl-subshells. Their analytical representations satisfy the requirement that the S[sub 2] oscillator strength sum rule must not be violated. Using the analytical approximations for the cross sections, the photorecombination rates are given as functions of temperature in terms of hypergeometric functions.

Ground state resonance structure calculated by density functional theory for estimating the dielectric response of the high explosive PETN

We present calculations of ground state resonance structure associated with the high explosive PETN using density functional theory (DFT), which is for the construction of parameterized dielectric response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric functions provide for different of types of analyses concerning the dielectric response of explosives. In particular, these dielectric response functions provide quantitative initial estimates of spectral response features for subsequent adjustment with respect to additional information such as laboratory measurements and other types of theory based calculations. With respect to qualitative analysis, these spectra provide for the molecular level interpretation of response structure. The DFT software NRLMOL was used for the calculations of ground state resonance structure presented here.

K‐Alpha Emission Spectra From Non‐Equilibrium Ionizing Plasmas

Kα X‐ray emission spectra from highly charged Fe ions have been theoretically predicted using a detailed and systematic spectral model. Account has been taken of the fundamental atomic radiative‐emission processes associated with inner‐shell electron collisional excitation and ionization, as well as dielectronic recombination. Particular emphasis has been directed at extreme non‐equilibrium or transient‐ionization conditions, which can occur in astrophysical and tokamak plasmas. Good agreement has been found in comparisons with spectral observations on the EBIT‐II electron beam ion trap at the Lawrence Livermore National Laboratory. We have identified spectral features that can serve as diagnostics of the electron density, the line‐formation mechanism, and the charge‐state distribution.