Nonlocal Resonance Research Articles

Calculation of rate constants for dissociative attachment of low-energy electrons to hydrogen halides HCl, HBr, and HI and their deuterated analogs

Calculations of rate constants for the process of dissociative attachment of low-energy electrons to hydrogen halides HCl, HBr, and HI and for the reverse process of associative detachment based on the nonlocal resonance model are reported. The calculated data are of importance for the modeling of plasma processes, environmental chemistry, etc. The calculated dissociative attachment rate constants are found to be in good agreement with existing experimental data. It is shown that at low temperatures the rate constants are very sensitive to small changes of the parameters of the nonlocal resonance model used for the calculation of the rate constants and represent a severe test of the theory. The isotopic effect and its dependence on the temperature is also discussed. The calculations of rate constants for the reverse process of associative detachment are also reported and discussed.

Effects of interchannel coupling in associative detachment: electron spectra for H+Cl- and H+Br- collisions.

We present experimental and theoretical energy spectra of the electrons detached in collisions of slow Cl- and Br- ions with atomic hydrogen. Nonlocal resonance theory predicts two kinds of features in the spectra: steplike structures associated with rovibrational onsets and steep rises associated with interchannel coupling, the latter being absent in a calculation using the simpler local-complex potential theory. Our experimental spectra confirm the presence of both types of structures and thus the necessity of including interchannel coupling to properly describe the product-state distribution.

Resonances and Threshold Phenomena in Low-Energy Electron Collisions with Hydrogen Halides: New Experimental and Theoretical Results

Low-energy electron collisions with HCl and HBr and the deuterated compounds have been investigated by experimental and theoretical methods. New experimental results have been obtained on relative differential cross-sections for vibrational excitation and dissociative electron attachment. Measurements with high energy resolution for rotationally cooled molecules have revealed, in addition to shape resonance, threshold peaks and Wigner cusps, the existence of surprisingly sharp oscillatory structures in the elastic and v = 0 → 1 vibrational excitation cross-sections in a narrow range below the dissociative attachment threshold. The theoretical analysis is based on an improved nonlocal resonance model which has been constructed on the basis of ab initio fixed-nuclei scattering phase shifts for HCl and HBr and accurate ab initio calculations of the bound part of the HCl− and HBr− potential-energy functions. The high degree of agreement which has been obtained between experiment and theory for all channels indicates that the mechanisms responsible for the rich threshold structures in the collision cross-sections are completely understood.

Dissociative attachment of low-energy electrons to vibrationally excited hydrogen molecules

Dissociative electron attachment to hot hydrogen molecules is studied in the framework of nonlocal resonance model. The method based on the use of the Bateman approximation, well known in nuclear physics, is adapted for solving the Lippmann-Schwinger integral equation of the nonlocal resonance model and applied to the calculation of cross sections of inelastic resonant electron-molecule collisions. The proposed method is compared with the Schwinger-Lanczos algorithm used extensively for the treatment of these processes. It is shown that the Bateman approximation is very useful and efficient for treating the non-separable nonlocal potentials appearing in the integral kernels of the nonlocal resonance models. The calculated cross sections for the dissociative attachment of electrons to vibrationally excited hydrogen molecules are of importance for astrophysics.

On the types of instability of spatially restricted systems

A classification of instabilities in spatially restricted systems is presented, which generalizes a classification considered in book [1]. It is shown that, if a system has no active boundaries and the waves are not amplified in an infinite homogeneous medium, which corresponds to the absence of solutions of the dispersion equation with the negative imaginary part of the wave vector at the real frequency, then only nonamplified instabilities with a nonlocal resonance can be developed. The development of nonamplified instability is considered in a spatially restricted system through which a flux propagates, when along with natural waves the excitation of the waves of fluxes playing a key role in the development of the instability is taken into account.

Inelastic low-energy electron collisions with the HBr and DBr molecules: Experiment and theory

Low-energy electron collisions with the HBr and DBr molecules are addressed from the experimental and theoretical points of view. Relative differential cross sections for the excitation of vibrational levels of HBr and DBr up to $v=6$ have been measured as a function of the incident electron energy in the range 0--4 eV. In addition to the shape resonance near 2 eV collision energy, intense and narrow threshold peaks are found for the excitation of the $v=1$ level of HBr and the $v=1$ and $v=2$ levels of DBr. Measurements with high resolution for rotationally cooled molecules have revealed the existence of sharp oscillatory structures in the elastic and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections in a narrow range below the dissociative-attachment threshold. The dissociative-attachment cross section has been measured with high resolution of the incident electrons in the range 0.2--1.4 eV. The theoretical analysis is based on an improved nonlocal resonance model, which has been constructed on the basis of existing fixed-nuclei electron-HBr scattering phase shifts and accurate ab initio calculations of the bound part of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function. This purely ab initio-based model is used to calculate integral electron-scattering and dissociative-attachment cross sections for HBr and DBr. The theoretical cross sections agree very well with the experimental data. The observed threshold peaks and Wigner cusp structures in the vibrational excitation functions are correctly reproduced. The sharp structures in the $v=\stackrel{\ensuremath{\rightarrow}}{0}0$ and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections below the dissociative-attachment threshold, consisting of a superposition of boomerang-type oscillations and quasibound levels of the outer well of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function, are quantitatively described by the theory. The high degree of agreement between experiment and theory indicates that the essentials of low-energy electron-HBr collision dynamics are completely understood.

Dissociative electron attachment rates for H2 and its isotopes

Dissociative electron attachment to rovibrationally excited H2, HD, HT, D2, DT, and T2 molecules in their ground electronic states via the ∑u+2 shape resonance is studied within the framework of the nonlocal resonance theory. Rate constants for selected processes are presented in the electron energy range up to 5 eV. Maximum rate of 2.8×10−8 cm3 s−1 is found for attachment to all these molecules.

Associative detachment in low-energy collisionsbetween hydrogen atoms and atomic halogen anions

Using an improved nonlocal resonance theory, total crosssections, rovibrational final state distributions and electronspectra for low-energy (<2 eV) associative detachment (AD)collisions between hydrogen atoms and negative halogen ionsX- (X = F, Cl, Br) as well as deuterium atoms and F- ions have been calculated. The model involves realisticlong-range interactions for the H + X- systems andtakes into account the dependence of the dipole-modifiedthreshold exponent on internuclear distance. Good agreement isfound between experimental data for the total AD cross sectionsand the theoretical results. In the calculated electron energyspectra nonlocal effects are manifest through step structurewhich is (almost) independent of collision energy; thisstructure should therefore be observable in experiments withrather modest collision energy resolution. For illustration,electron spectra are presented which correspond to realisticexperimental conditions with regard to the energy widths of theelectron spectrometer and the collision energy distribution. Therovibrational final-state distributions in the HX(v, J)molecules formed are reported for a thermal kinetic energy distributionat T ≈ 300 K; for HF, DF and HCl the rotational-summedvibrational populations pv are compared with previoustheoretical and experimental results.

Low-energye−H2scattering: Separation of dissociative attachment and dissociation channels

Electron impact dissociation and dissociative attachment (DA) via the ${}^{2}{\ensuremath{\Sigma}}_{u}^{+}$ shape resonance in $e\ensuremath{-}{\mathrm{H}}_{2}$ collisions is treated by the quasiclassical version of the nonlocal resonance theory. Optical theorem is applied to analyze all the energetically accessible channels. Collisionally induced dissociation and dissociative attachment cross sections for vibrationally or rotationally excited ${\mathrm{H}}_{2}$ molecule are presented in the electron energy range up to 5 eV. It is shown that the DA cross sections we published recently [Phys. Rev. A 61, 052705 (2000)] in fact represent the sum of the cross sections for the DA and dissociation processes in the energy range above the dissociation threshold.

Dissociative electron attachment to vibrationally and rotationally excitedH2and HF molecules

The vibrational and rotational state dependence of dissociative attachment (DA) in low-energy $e\ensuremath{-}{\mathrm{H}}_{2}$ and e-HF collisions is studied within the framework of nonlocal resonance theory. The dynamics of nuclear motion in the nonlocal complex potential is treated by the quasiclassical approach. The resonance energy, width function, and level-shift function are taken from our previous calculations based on the projection operator formalism. Results for all vibrational states up to the dissociative attachment thresholds and selected rotational states are presented. Our cross sections for ${\mathrm{H}}_{2}$ exhibit a plateau structure in the energy range above the dissociation threshold, and this structure gets more pronounced with higher $v$ or J. The dissociative attachment cross section for vibrationally and rotationally hot HF $(T=1150$ K) agrees well with available experimental data. For ${\mathrm{H}}_{2}$ in thermal equilibrium at 1400 K, the agreement between experiment and theory is less satisfactory.

Associative detachment, dissociative attachment, and vibrational excitation of HCl by low-energy electrons

An improved nonlocal resonance model for electron-HCl collisions is constructed. The model takes account of the dependence of the dipole-modified threshold exponent on the internuclear distance. The long-range part of the ${\mathrm{HCl}}^{\ensuremath{-}}$ potential-energy function has been determined from the most accurate ab initio calculations which are available. Only the s-wave component of the electronic continuum wave function is taken into account. Cross sections for vibrational excitation, dissociative attachment, and associative detachment have been calculated. For all three collision processes, the cross sections calculated with this model are in better agreement with experiment than previous calculations. The calculations reproduce very well even fine details of the threshold structures in the vibrational excitation cross sections of HCl.

Capture and loss of valence electrons during low energy H + and H − scattering from LaB 6(100), Cs/Si(100), graphite and LiCl

The mechanism of electron attachment to low-energy (100 eV) protons has been investigated during scattering from graphite, Si(100), LaB 6(100), and LiCl. The primary H + ion survives neutralization when scattered from highly ionized target species, such as Cs +, La 3+, Li +, and Cl −, existing on the topmost surface layer. In terms of H − ion formation at the metal and semiconductor surfaces, multiple scattering from the solid is dominant relative to single surface scattering because the former has a much larger cross-section than the latter, indicating that H − is caused by the non-local resonant tunneling process. At the LiCl surface, on the other hand, a close atomic encounter with individual target ions is found to be important for H − ion formation. This is because the non-local resonance process is prohibited due to the existence of the large band gap and the charge state of scattered hydrogen is determined by the transient chemisorption state very close to the surface.

Low-energy reactive ion scattering as a probe of surface femtochemical reaction: H+ and H− formation on ionic compound surfaces

Capture and loss of valence electrons during low-energy (50–500 eV) proton scattering from some alkali–halide surfaces such as LiCl, NaCl, and KF have been investigated in comparison with those from the TiO2(110) and Cs-adsorbed Si(100) surfaces. The primary H+ ion survives neutralization when scattered from the highly ionized target species existing on the surface. For H− ion formation, a close atomic encounter with individual target ions is found to be important; the H− ion is formed more efficiently on the cationic site than on the anionic site despite the fact that the valence electron is spacially localized on the latter. This is because the charge state of scattered hydrogen is determined during a transient chemisorption state and amphoteric hydrogen tends to be coordinated negatively (positively) on the cationic site (the anionic site). The final charge state of scattered hydrogen is fixed at a certain bond-breaking distance (∼5.0 a.u.) from the surface where the well-defined atomic orbital of hydrogen evolves. The competing nonlocal resonance tunneling is suppressed at the ionic-compound surfaces due to the existence of a large band gap, so that hydrogen is scattered without losing the memory of such a transient chemisorption state.

Generalization of the nonlocal resonance model for low-energy electron collisions with hydrogen halides: the variable threshold exponent

The nonlocal resonance model developed previously for the description of low-energy electron collisions with hydrogen halides is generalized to include the dependence of the dipole-modified threshold exponent on the internuclear distance. An efficient computational scheme has been developed to deal with the resulting nonseparability of the nonlocal complex potential for the nuclear motion within the Schwinger-Lanczos approach. The results reveal that the R-dependence of the threshold exponent has a significant effect on the threshold peaks in the vibrational excitation cross sections of HCl. The shape and intensity of the calculated threshold peak in the 0→1 vibrational excitation channel, in particular, are in much better agreement with experimental data than previous results. For the electron-HBr and electron-HI collision systems the effects of the R-dependence of the threshold exponent are not significant.

Nuclear dynamics of the H collision complex beyond the local approximation: associative detachment and dissociative attachment to rotationally and vibrationally excited molecules

An improved nonlocal resonance model for the description of the nuclear dynamics of the collision complex is developed. The model is based on the ab initio electron- scattering data of Berman et al at short internuclear distances, the ab initio data of Senekowitsch et al for the bound state of at intermediate distances, and exhibits the correct polarization interaction at large internuclear distances. Cross sections for associative detachment (AD) and dissociative attachment (DA) to rotationally and vibrationally excited molecules were calculated for a wide range of energies and angular momenta with full inclusion of nonlocal effects. The calculated AD rate constant at 300 K is , which is significantly larger than the experimental value reported by Schmeltekopf et al. A series of narrow low-energy orbiting resonances was found in the AD cross section. Our results compare well with previous calculations of DA cross sections within the nonlocal resonance model. The DA cross section for vibrationally and rotationally hot has been calculated and compared with the experimental data of Allan and Wong. It is demonstrated that the local-complex-potential approximation is an excellent approximation for the associative detachment process, whereas it fails severely for the DA process in .

Electron attachment and vibrational excitation in hydrogen iodide: calculations based on the nonlocal resonance model

The nonlocal resonance model developed earlier for the description of the collision of low-energy electrons with HCl and HBr has been adapted to the electron-HI collision system. The parameters of the model have been determined by fitting experimental high-resolution data for the attachment cross section in HI in the energy range 0 - 170 meV. Moreover, ab initio electronic-structure data for the electronic ground-state potential-energy function of HI- have been taken into account. Within the resulting model, cross sections for vibrational excitation and dissociative attachment processes in HI and DI have been calculated over a reasonably wide energy range. No high-resolution experimental data are available for comparison.

Calculation of dissociative attachment of electrons to diatomic molecules by the Schwinger–Lanczos approach

Numerical studies of resonant scattering of electrons by diatomic molecules with full account of the nonlocal level shift and resonance width operators are carried out with emphasis on the various approximations of the nonlocal potentials. The Schwinger–Lanczos approach proposed recently by Meyer, Horáček and Cederbaum [Phys. Rev. A 43, 3587 (1991)] is applied and its performance is investigated. The efficiency of the method is further improved by introducing a new local complex potential. Very accurate values of the dissociative attachment cross sections for a d-wave resonance model are reported.

Calculation of cross sections for vibrational excitation and dissociative attachment in electron collisions with HBr and DBr.

The nonlocal resonance model developed earlier for the description of low-energy inelastic and reactive electron-HCl collisions has been adapted to the electron-HBr collision system. The parameters of the model have been determined by fitting the eigenphase sum in the fixed-nuclei approximation to the data of an {ital ab} {ital initio} {ital R}-matrix calculation of Morgan, Burke, and collaborators. The Schwinger-Lanczos method has been employed to solve the nuclear scattering problem with a nonlocal, complex, and energy-dependent effective potential. Fully converged cross sections have been obtained on a dense grid of energies for many vibrational excitation, deexcitation, and dissociative channels in both HBr and DBr. The computed cross sections are generally in good agreement with experiment as far as data are available. {copyright} {ital 1996 The American Physical Society.}

Semi-analytical modelling of non-local ion-cyclotron resonance heating in toroidal geometry

A semi-analytical approach is proposed for computing the non-local response of a toroidal plasma to an electromagnetic perturbation. Although the perturbed distribution function as well as the absorbed power are also computed, the focus is on the associated quasi-linear diffusion operator. Different decorrelation models allow one to recapture the well-known ideal collisionless plasma result where the wave–particle energy exchange is exactly at resonance, as well as the more realistic plasma response where the interaction region is widened. The choice of the independent variables also allows one to carry out all computations except the integration of the guiding-centre motion analytically.

Application of the Schwinger-Lanczos method to the calculation of dissociative attachment of electrons to diatomic molecules

The Schwinger-Lanczos method proposed recently by Meyer et al. (1991) is applied to the calculation of dissociative attachment cross section of electrons to diatomic molecules within a nonlocal resonance model. As a specific example the dissociative attachment cross section for e-+HCl is calculated.