Nonlocal Resonance Model Research Articles

Autodetachment of Diatomic Carbon Anions from Long-Lived High-Rotation Quartet States.

We show that strong molecular rotation drastically modifies the autodetachment of C_{2}^{-} ions in the lowest quartet electronic state a^{4}Σ_{u}^{+}. In the strong-rotation regime, levels of this state only decay by a process termed "rotationally assisted" autodetachment, whose theoretical description is worked out based on the nonlocal resonance model. For autodetachment linked with the exchange of six rotational quanta, the results reproduce a prominent, hitherto unexplained electron emission signal with a mean decay time near 3ms, observed on stored C_{2}^{-} ions from a hot ion source.

Resonant collisions of electrons with O2 via the lowest-lying 2Πg state of O2−

We report the results of an ab initio study of resonant vibrational excitation of molecular oxygen by electron impact at low energies where the lowest-lying ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{g}$ resonant state of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ dominantly contributes to the cross sections. The contribution of this resonance to the dissociative electron attachment cross section is also determined and the origin of its unusual oscillations is discussed. Calculations were performed within the nonlocal resonance model describing the nuclear dynamics of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ after electron capture into the resonant state. The model was constructed using potential-energy curves obtained with standard quantum-chemical methods and eigenphase sums from the fixed-nuclei $R$-matrix calculations of electron scattering off ${\mathrm{O}}_{2}$. The effect of the spin-orbit interaction is taken into account when determining the potential-energy curves of the molecular negative ion. The vibrational excitation cross sections are compared with other available theoretical and experimental results.

Electron-impact vibrational excitation of isocyanic acid HNCO

In a combined experimental and theoretical study, we probe the vibrational excitation of isocyanic acid induced by electron impact in the energy range up to 5 eV. Experimentally, we report differential elastic and vibrationally inelastic cross sections at the scattering angle of ${135}^{\ensuremath{\circ}}$. Theoretically, we characterize the involved resonant states using a regularized analytical continuation method. We also apply a nonlocal resonance model to calculate cross sections for vibrational mode that involves the N-H stretching motion. The model reproduces all the features observed in the experiment: efficient excitation at threshold, sharp cusps in the excitation functions, and the formation of an ${A}^{\ensuremath{'}}$ shape resonance. There is a second $({A}^{\ensuremath{'}\ensuremath{'}})$ resonance visible in the spectra; however, it excites only selected vibrational modes. The origins of this selectivity are discussed.

Lowest autodetachment state of the water anion

The potential energy surface of the ground state of the water anion H2O− is carefully mapped using multireference CI calculations for a large range of molecular geometries. Particular attention is paid to a consistent description of both the O−+H2 and OH−+H asymptotes and to a relative position of the anion energy to the ground state energy of the neutral molecule. The autodetachment region, where the anion state crosses to the electronic continuum is identified. The local minimum in the direction of the O− + H2 channel previously reported by Werner et al. [J. Chem. Phys. 87, 2913 (1987)] is found to be slighly off the linear geometry and is separated by a saddle from the autodetachment region. The autodetachment region is directly accessible from the OH−+H asymptote. For the molecular geometries in the autodetachment region and in its vicinity we also performed fixed-nuclei electron-molecule scattering calculations using the R-matrix method. Tuning of consistency of a description of the correlation energy in both the multireference CI and R-matrix calculations is discussed. Two models of the correlation energy within the R-matrix method that are consistent with the quantum chemistry calculations are found. Both models yield scattering quantities in a close agreement. The results of this work will allow a consistent formulation of the nonlocal resonance model of the water anion in a future publication.

Metastable states of diatomic hydrogen anions

Transient states of the negative hydrogen molecule H−2 have drawn attention as intermediate reaction complexes in important molecular reactions like associative detachment (AD: H− + H → H−2→ H−2 + e−) and dissociative attachment (DA: e− + H2 → H−2 → H + H−). Recently it has been shown that metastable states of hydrogen molecular anions exist that defy both auto detachment and spontaneous dissociation for several microseconds (in the case of H−2) and even milliseconds (in the case of D−2). Here we present Coulomb explosion measurements for H−2 and D−2 that provide detailed information on the shape of the molecular wave function. We compare these experimental results with calculated wave functions obtained using a nonlocal resonance model. Our measurements confirm the predicted stabilization of H−2 and D−2 in states of high angular momentum.

Electron scattering in HCl: An improved nonlocal resonance model

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is fitted to ab initio electron-scattering eigenphase sums calculated using the Schwinger multichannel method, while the long-range part is based on the ab initio potential-energy curve of the bound anion HCl${}^{\ensuremath{-}}$. This model significantly improves the agreement of nonlocal resonance calculations with recent absolute experimental data on dissociative electron attachment cross sections for HCl and DCl. It also partly resolves an inconsistency in the temperature effect in dissociative electron attachment to HCl present in the literature. Finally, the present model reproduces all qualitative structures observed previously in elastic scattering and vibrational-excitation cross sections.

Improved nonlocal resonance model for electron – HCl collisions

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is based on ab-initio electron scattering eigenphase sums calculated using the Schwinger Multichannel method and the long-range part on the potential curve of the bound HCl− anion. We have calculated cross sections for vibrational excitation and dissociative electron attachment. For all collision processes where quantitative experimental data are available, the new model agrees with experiments better than the previous model.

Absolute cross sections for dissociative electron attachment to HCl, HBr, and their deuterated analogs

We have measured absolute dissociative electron attachment cross sections for $\mathrm{H}X$ and $\mathrm{D}X$ ($X=\mathrm{Cl}$ and Br) using a trochoidal electron spectrometer equipped with a total ion collection collision chamber, operated both in a field-free passive ion collection mode, and an active ion collection mode, with an extracting electric field. Our results for ${\mathrm{Br}}^{\ensuremath{-}}$ from HBr are in an excellent quantitative agreement with recent predictions of the nonlocal resonance model, but the cross sections for ${\mathrm{Cl}}^{\ensuremath{-}}$ from HCl and DCl are, respectively, 2.2 times and 2.8 times smaller than the theoretical predictions. Cross sections for the production of ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ in the $6--10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ range are much smaller than those for ${X}^{\ensuremath{-}}$ and the isotope effect is much weaker, consistent with the assignment to core-excited Feshbach resonances. There is one exception, however, the $9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ band in HCl has a large isotope effect.

Long-lived metastable anions of hydrogen halides

We search for narrow resonances in cross-sections for electron collisions with HCl, DCl, HBr and DBr molecules, calculated with the nonlocal resonance model. Narrow resonances corresponding to long-lived metastable states of anionic molecules are indeed found in both elastic and vibrational excitation cross-sections. The largest lifetime τ = 0.11 ms is predicted for HBr − with rotational quantum number J = 20. For HCl we find maximum τ = 0.6 ns for J = 22 and τ = 0.6 μs for J = 33 in DCl. A surprising isotope effect is found for DBr, where the largest lifetime is τ = 1.5μs, i.e., much smaller than for HBr.

Correlation effects in R-matrix calculations of electron-F2 elastic scattering cross sections

Correlation effects are studied in electron scattering off the fluorine molecule. Fixed-nuclei approximation R-matrix calculations of the elastic collision cross sections are presented for a set of internuclear distances at three levels of correlation. The aim of this work is to study the role of electronic correlation on the properties of the 2Sigmau resonance. The Feshbach-Fano R-matrix method of resonance-background separation is used to study the effect of inclusion of various levels of correlation on the energy and width of the 2Sigmau resonance. Data required for construction of the nonlocal resonance model (construction of a discrete state and its coupling to the continuum) which allows the calculation of inelastic processes such as dissociative electron attachment and vibrational excitation [W. Domcke, Phys. Rep. 208, 97 (1991)] including the correlation are presented.

Giant structures in low-energy electron–deuterium-iodide elastic scattering cross section

Calculations of the resonance part of the cross section of elastic electron scattering with $\mathrm{DI}$ molecule at energies below the dissociative attachment threshold based on the use of the nonlocal resonance model of Hor\'a\ifmmode \check{c}\else \v{c}\fi{}ek, Nakamura and Domcke [Z. Phys. D 42, 181 (1997)] reveal a series of sharp peaks of giant amplitudes of several thousands of ${\mathrm{\AA{}}}^{2}$. These structures are interpreted as demonstrations of outer well resonances in the negative molecular ion state potential and should be observable experimentally provided the energy resolution of the beam is in the range of few meV.

Dissociative electron attachment to HBr: A temperature effect

The effects of rovibrational temperature on dissociative electron attachment to hydrogen bromide has been investigated from the experimental and theoretical point of view. Theoretical calculations based on the nonlocal resonance model predict a strong temperature effect on the Br{sup -} fragment ion yield due to population of higher vibrational and rotational states. A crossed beam experimental setup consisting of a temperature controlled effusive molecular beam and a trochoidal electron monochromator has been used to confirm this prediction. The high degree of agreement between experiment and theory indicates the validity of the theoretical model and its underlying physical picture.

Long-lived anionic states ofH2, HD,D2, andT2

A detailed investigation of the long-lived states of the hydrogen molecule anion has been performed within the ab initio--based nonlocal resonance model. The resonances in the $X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Sigma}_{u}^{+}$ electronic state of $\mathrm{H}_{2}{}^{\ensuremath{-}}$ are stabilized against autodetachment by molecular rotation and can be interpreted as orbiting states of $\mathrm{H}+{\mathrm{H}}^{\ensuremath{-}}$ with high angular momenta $(J=22\ensuremath{-}27)$. The lifetimes and energies of all long-lived states have been calculated for the hydrogen molecular anion and its isotopic analogs. Recent experimental values of Heber et al. [Phys. Rev. A 73, 060501(R) (2006)] for lifetimes of $\mathrm{H}_{2}{}^{\ensuremath{-}}$, $\mathrm{H}{\mathrm{D}}^{\ensuremath{-}}$, and $\mathrm{D}_{2}{}^{\ensuremath{-}}$ can be explained within the model. For $\mathrm{T}_{2}{}^{\ensuremath{-}}$, we predict a large number of states with lifetimes up to $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{s}$.

Dissociative electron attachment and vibrational excitation of the chlorine molecule

This paper is aimed at the theoretical investigation of the inelastic processes taking place in resonant collisions of low-energy electrons with the chlorine molecule. Dissociative electron attachment and vibrational excitation of ${\mathrm{Cl}}_{2}$ by electron impact is investigated in the energy range $0--1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, where the $^{2}\ensuremath{\Sigma}_{u}^{+}$ resonance plays the central role. The calculations were carried out within the framework of the nonlocal resonance model. This approach makes it possible to calculate the integrated cross sections of the above-mentioned processes for a variety of initial and final rovibrational states of the target molecule. The present model is constructed on the basis of ab initio fixed-nuclei $R$-matrix calculations using the so-called Feshbach-Fano $R$-matrix method. The Schwinger-Lanczos algorithm was utilized to solve the Lippmann-Schwinger equation describing the motion of the nuclei.

Dissociative electron attachment and vibrational excitation ofH2by low-energy electrons: Calculations based on an improved nonlocal resonance model. II. Vibrational excitation

We treat vibrational excitation of hydrogen by low-energy electrons using an improved nonlocal resonance model. The model is based on accurate ab initio data for the $^{2}\ensuremath{\Sigma}_{u}^{+}$ shape resonance and takes full account of the nonlocality of the effective potential for nuclear motion. Integral vibrational excitation cross sections were calculated for numerous initial and final rovibrational states of the hydrogen molecule, and the dependence of the vibrational excitation cross section on the rovibrational initial target state has been investigated. The vibrational excitation cross sections are in very good agreement with measurements for the transitions $v=0\ensuremath{\rightarrow}1$ and $v=0\ensuremath{\rightarrow}2$, while for higher vibrational channels the agreement is less satisfactory. However, the oscillatory structures in $v=0\ensuremath{\rightarrow}4$ vibrational excitation and higher channels predicted by Domcke and collaborators and measured by Allan are described by the present calculation, in very good agreement with the experimental data. A detailed analysis of the origin of the oscillations has been performed. It is shown that the oscillations can be qualitatively understood within the so-called boomerang model of Herzenberg. The resonance contribution to the vibrationally elastic scattering $(v\ensuremath{\rightarrow}v)$ is also discussed. It is found that this cross section is dominated at low energies by the resonance contribution, as predicted by Schulz. The calculated integral vibrational excitation cross sections generally are in good agreement with other theoretical data obtained by different approaches. A comprehensive study of the effect of isotopic substitution has been performed, and an inverse isotope effect in vibrational excitation has been found for certain vibrational levels of the target.

Isotope effects in vibrational excitation and dissociative electron attachment of DCl and DBr

The results of calculations of vibrational excitation and dissociative electron attachment cross-sections of DCl and DBr are reported. The calculations are based on the nonlocal resonance model for electron-HCl/HBr scattering. The cross-sections for many initial rovibrational target states were calculated both for the hydrogenated and the deuterated compounds. The calculations reveal an unexpected result: the vibrational excitation cross-section of the deuterated molecule may in some cases be (significantly) larger than that of the hydrogenated compound. This effect is observed when the target molecule is initially excited to a vibrational state the energy of which is close to the threshold of dissociative attachment. Rotational excitation of the target molecule plays a similar role. Isotope effects in dissociative electron attachment are also discussed.

Computing resonance positions, widths, and cross sections via the Feshbach-FanoR-matrix method: Application to potential scattering

The general Feshbach-Fano R-matrix procedure proposed recently by Nestmann [J. Phys. B 31, 3929 (1998)] makes it possible to construct resonance metastable states of transient molecular ions and their coupling elements to the background scattering continuum. These quantities are needed for the study of nuclear dynamics in the framework of the nonlocal resonance model. The performance of this approach is carefully studied and its properties analyzed in the field of potential scattering. An improvement of the Nestmann procedure which makes the calculation more stable and robust is proposed.

Experimental and Theoretical Evidence for Long-Lived Molecular Hydrogen AnionsH2−andD2−

The existence of (metastable) molecular hydrogen anions H2(-), D2(-), and H3(-) is demonstrated. These anion species were produced by sputtering of TiH2 and TiD2 targets with Cs+ ions and were identified by accelerator mass spectrometry. From the respective flight times through the spectrometer, lifetimes for H2(-) and D2(-) of at least 3 micros and 4 micros, respectively, can be inferred. Theoretical calculations within the nonlocal resonance model predict the existence of highly rotationally excited anions with lifetimes in the micros range. It is proposed that in sputtering molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.

Dissociative electron attachment and vibrational excitation ofH2by low-energy electrons: Calculations based on an improved nonlocal resonance model

An improved nonlocal resonance model proposed by \ifmmode \check{C}\else \v{C}\fi{}\'{\i}\ifmmode \check{z}\else \v{z}\fi{}ek, Hor\'a\ifmmode \check{c}\else \v{c}\fi{}ek, and Domcke [J. Phys. B 31, 2571 (1998)] is used for the calculation of cross sections of electron dissociative attachment and vibrational excitation of molecular hydrogen by the impact of low-energy electrons in the range of $^{2}\ensuremath{\Sigma}_{u}^{+}$ resonance. The model is based on ab initio data and takes full account of the nonlocality of the effective potential for the nuclear motion. The dissociative attachment cross sections and rate constants are calculated for all target states $(v,J)$ of relevance and compared with other theoretical and experimental data. It is found that the present dissociative attachment cross section calculated under the conditions of the experiment carried out by Schulz and Asundi reproduces the larger of the two values proposed by them, i.e.--$2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$. A detailed discussion of the dissociative attachment cross section as a function of the vibrational and rotational target states is given. Very narrow peaks, with a width of $1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, are observed in the dissociative attachment cross section for large values of the orbital quantum number $J$. These structures are interpreted as shape resonances in ${\mathrm{H}}^{\ensuremath{-}}+\mathrm{H}$ collision dynamics. It is shown that for large values of $J$ rotational excitation of the hydrogen molecule enhances the dissociative attachment more efficiently than vibrational excitation. The largest dissociative attachment cross section of $28.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ is obtained for $v=1$ and $J=29$. The process of vibrational excitation will be discussed in a separate paper.

Vibrational excitation of hydrogen fluoride by low-energy electrons: theory andexperiment

Vibrational excitation (VE) of HF by low-energy electrons has been investigatedexperimentally and theoretically. A new nonlocal resonance model has beenconstructed based on ab initio calculations of the coupling between a discrete stateand continuum states. VE and resonant elastic cross sections have been calculatedfor a set of initial vibrational states of the molecular target. New high-resolutionmeasurements of VE cross sections for the transitions v = 0 → 1,..., 4 havebeen carried out. The calculated cross sections are in good agreement with theexperimental data, indicating that the mechanisms responsible for the richthreshold structures found in the collision cross sections of HF are wellunderstood.