Mutual neutralization in collisions of H+ with Cl.

The cross section and final state distribution for mutual neutralization in collisions of H+ with Cl- have been calculated using an ab initio quantum mechanical approach. It is based on potential energy curves and nonadiabatic coupling elements for the six lowest 1Σ+ states of HCl computed with the multireference configuration interaction method. The reaction is found to be driven by nonadiabatic interactions occurring at relatively small internuclear distances (R < 6 a0). Effects on the mutual neutralization cross section with respect to the asymptotic form of the potential energy curves, inclusion of closed channels, as well as isotopic substitution are investigated. The effect of spin-orbit interaction is investigated using a semiempirical model and found to be small. A simple two-state Landau-Zener calculation fails to predict the cross section

Photodetachment cross sections of the C2nH- (n=1-3) hydrocarbon-chain anions

We report theoretical results of the low-energy photodetachment cross sections of the C2H-, C4H-, and C6H- hydrocarbon-chain anions. The complex Kohn variational technique is used to calculate molecular-frame transition dipole moments from the anion ground state to a photoelectron in the continuum of the neutral radical. We employ the Franck-Condon approximation and include interchannel electronic coupling to determine the low-energy photodetachment cross sections and asymmetry parameters. We discuss the behavior of the cross section, especially near thresholds, and describe the role of electronic resonances and excited channels. The theoretical results reproduce the main characteristics of recent measurements of absolute photodetachment cross sections

A Genetic Search in Frequency Space for Stabilizing Atoms by High-Intensity Laser Fields

The goal of this paper is to explore the power of stochastic search methods, in particular genetic algorithms, to solve a challenging problem in experimental physics. The problem is to find an optimum frequency to stabilize atoms by high-intensity laser fields. The standard approach to search for optimal laser parameters has been by trial and error. This is the first known application of a genetic algorithm technique to model atomic stabilization. Genetic algorithms worked well for this problem as a way to automate the search in a time efficient manner. A parallel platform is used to perform the genetic search efficiently. Locating the best frequency to achieve a suppression of ionization, which is predicted to occur at high intensities, can help design a laboratory experiment and tune to that frequency in order to identify a stabilization effect. The genetic algorithms did successfully identify this optimum frequency. It is indeed possible to extend the number of unknown tunable laser parameters, beyond searching merely over frequency space. For instance, optimal pulse shape and pulse duration can also be included. While conducting such a search in multi-dimensional parameter space, parallel genetic algorithms can offer an advantage to the tedious trial and error procedures

Theory of dissociative recombination of highly-symmetric polyatomic ions

A general first-principles theory of dissociative recombination is developed for highly-symmetric molecular ions and applied to H$_3$O$^{+}$ and CH$_3^+$, which play an important role in astrophysical, combustion, and laboratory plasma environments. The theoretical cross-sections obtained for the dissociative recombination of the two ions are in good agreement with existing experimental data from storage ring experiments

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals

A first-principle theoretical approach to study the process of radiative electron attachment is developed and applied to the negative molecular ions CN$^-$, C$_4$H$^-$, and C$_2$H$^-$. Among these anions, the first two have already been observed in the interstellar space. Cross sections and rate coefficients for formation of these ions by radiative electron attachment to the corresponding neutral radicals are calculated. For completeness of the theoretical approach, two pathways for the process have been considered: (i) A direct pathway, in which the electron in collision with the molecule spontaneously emits a photon and forms a negative ion in one of the lowest vibrational levels, and (ii) an indirect, or two-step pathway, in which the electron is initially captured through non-Born-Oppenheimer coupling into a vibrationally resonant excited state of the anion, which then stabilizes by radiative decay. We develop a general model to describe the second pathway and show that its contribution to the formation of cosmic anions is small in comparison to the direct mechanism. The obtained rate coefficients at 30~K are $7\times 10^{-16}$cm$^3$/s for CN$^-$, $7\times 10^{-17}$cm$^3$/s for C$_2$H$^-$, and $2\times 10^{-16}$cm$^3$/s for C$_4$H$^-$. These rates weakly depend on temperature between 10K and 100 K. The validity of our calculations is verified by comparing the present theoretical results with data from recent photodetachment experiments

Classical Model for Electronically Non-Adiabatic Collision Processes: Resonance Effects in Electronic-Vibrational Energy Transfer

A recently developed classical model for electronically nonadiabatic collision processes is applied to electronic-vibrational energy transfer in a collinear atom~diatom system, A + BC(v=1) + A*+ BC(v=0), which closely resembles Br-H{sub 2}. This classical model, which treats electronic as well as heavy particle (i.e., translation, rotation, and vibration) degrees of freedom by classical mechanics, is found to describe the resonance features in this process reasonably well. The usefulness of the approach is that it allows one to extend standard Monte Carlo classical trajectory methodology to include electronically non-adiabatic processes in a dynamically consistent way

Successful Aging and the Epidemiology of HIV

By 2015, it is estimated that nearly half of those living with HIV in the US will be 50 years of age and older. This dramatic change in the demographics of this clinical population represents unique challenges for patients, health care providers, and society-at-large. Fortunately, because of highly active antiretroviral therapy (HAART) and healthy lifestyle choices, it is now possible for many infected with HIV to age successfully with this disease; however, this depends upon one’s definition of successful aging. It is proposed that successful aging is composed of eight factors: length of life, biological health, cognitive efficiency, mental health, social competence, productivity, personal control, and life satisfaction. Unfortunately, HIV and medication side effects can compromise these factors, thus diminishing one’s capacity to age successfully with this disease. This article explores how HIV, medication side effects from HAART, and lifestyle choices can compromise the factors necessary to age successfully. Implications for practice and research are posited

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment.

Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10-8 cm3 s-1 were found for the reaction of C+ with Br- and I-, and a rate constant of 4.2 ± 1.1 × 10-9 cm3 s-1 was measured for the reaction with Cl-. The C+ + Cl- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10-10 cm3 s-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C+ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant

Elastic electron scattering from formic acid

Following our earlier study on the dynamics of low energy electron attachment to formic acid, we report the results of elastic low-energy electron collisions with formic acid. Momentum transfer and angular differential cross sections were obtained by performing fixed-nuclei calculations employing the complex Kohn variational method. We make a brief description of the technique used to account for the polar nature of this polyatomic target and compare our results with available experimental data

Classical model for laser‐induced nonadiabatic collision processes

By synthesizing earlier work of Orel and Miller and of Meyer, McCurdy, and Miller, a model for describing laser-induced electronically non-adiabatic collision processes is constructed which treats all degrees of freedom -- heavy particle (i.e,, translation, rotation, and vibration), electronic, and photon by classical mechanics. This then makes it relatively easy to carry out calculations to simulate such processes within a dynatnically consistent framework. Application is made to the test case H + LiF {yields} Li + HF reaction considered by Light and Altenberger-Siczek. The most interesting feature revealed by these classical calculations is maxima in the reaction probability as a function of initial translational energy at energies below the laser-free threshold. It is seen that this structure can be understood as a Franck-Condon-like effect