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arXiv:2606.11352

Nonadiabatic Dynamics and Rotational Coupling in $\mathrm{HeH^+}$ Dissociative Recombination and Resonant Ion-Pair Formation

Sifiso Musa Nkambule, Malibongwe Tsabedze, Oscar N. Mabuza, Mbuso K. Matfunjwa

physics.atm-clusquant-phSubmitted Jun 9, 2026• Announced Jun 11, 2026• Revised Jun 28, 2026 (v2)
v1 · Jun 9, 2026v2 · Jun 28, 2026
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Abstract

We present a time-dependent wave-packet study of dissociative recombination (DR) and resonant ion-pair (RIP) formation in $\mathrm{HeH^+}$ isotopologues. Nuclear dynamics are treated on a manifold of 23 coupled electronic states of $^2Σ$, $^2Π$, and $^2Δ$ symmetries, including rotational couplings between different symmetries. The results reveal that inclusion of a large manifold of resonant states and rotational couplings significantly enhances the DR cross section relative to earlier theoretical studies. In the diabatic representation, $^2Σ$ states dominate the recombination dynamics, while in the adiabatic representation, $^2Π$ and $^2Δ$ states contribute significantly at low collision energies. For RIP formation, two different diabatization schemes yield systematically larger cross sections than previous models, highlighting the sensitivity of ion-pair production to electronic coupling structure. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. Thermal rate coefficients are computed over $10^{2}$ to $2\times 10^4$ K thermal electron temperatures. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. The results are compared with rotational-state-resolved experimental and theoretical results. The present results highlight the importance of multistate coupling and rotational interactions in electron-driven fragmentation processes relevant to primordial and astrophysical plasmas.

Comments
11 pages and 8 figures. arXiv admin note: text overlap with arXiv:2301.03893

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Nonadiabatic Dynamics and Rotational Coupling in HeH+\mathrm{HeH^+}HeH+ Dissociative Recombination and Resonant Ion-Pair Formation
We present a time-dependent wave-packet study of dissociative recombination (DR) and resonant ion-pair (RIP) formation in HeH+\mathrm{HeH^+}HeH+ isotopologues. Nuclear dynamics are treated on a manifold of 23 coupled electronic states of 2Σ^2Σ2Σ, 2Π^2Π2Π, and 2Δ^2Δ2Δ symmetries, including rotational couplings between different symmetries.
The results reveal that inclusion of a large manifold of resonant states and rotational couplings significantly enhances the DR cross section relative to earlier theoretical studies. In the diabatic representation, 2Σ^2Σ2Σ states dominate the recombination dynamics, while in the adiabatic representation, 2Π^2Π2Π and 2Δ^2Δ2Δ states contribute significantly at low collision energies. For RIP formation, two different diabatization schemes yield systematically larger cross sections than previous models, highlighting the sensitivity of ion-pair production to electronic coupling structure. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. Thermal rate coefficients are computed over 10210^{2}102 to 2×1042\times 10^42×104 K thermal electron temperatures.
Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. The results are compared with rotational-state-resolved experimental and theoretical results.
The present results highlight the importance of multistate coupling and rotational interactions in electron-driven fragmentation processes relevant to primordial and astrophysical plasmas.
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Thermal collisions between potassium Rydberg atoms with intermediate values of quantum number n and target molecules that capture free low-energy electrons are investigated. The velocity, binding energy, and angular distributions of heavy-Rydberg ion-pair states formed through such collisions are measured. The experimental data are analyzed with the aid of semiclassical Monte Carlo simulations to investigate the dynamics of electron attachment to a variety of target species. Multiple reaction channels involving short- and long-lived intermediates are seen for electron capture by 1,1,1-C2Cl3F3 and CBrCl3 (where dissociation of CBrCl3 intermediates involves production of Br- and Cl- ions). Electron attachment to BrCN produces long-lived BrCN-* intermediates whose dissociation results in the creation of rotationally-hot CN- anions. Electron capture by Fe(CO)5 leads to formation of Fe(CO)5-* intermediates which can fragment producing Fe(CO)4- anions, either directly or after partial redistribution of the excess energy within the intermediate has occurred. Electron transfer in Rydberg atom collisions with C2Cl4 provide no evidence of formation of long-lived C2Cl4 ions, only Cl- ions are seen. Moreover, strong Rydberg atom scattering is observed following collisions of K(12p) Rydberg states with CH3NO2. This finding is consistent with a theory which predicts that collisions between Rydberg atoms with intermediate values of n and polar molecules can lead to strong resonant quenching of Rydberg states, i.e., n and l changing, through formation of short-lived K+...CH3NO2- ion-pair states. Finally, the improvements in sensitivity to electron capture reaction dynamics that might be achieved using different Rydberg species and a crossed molecular beam are examined. Simulations are performed to demonstrate that the use of a hyperthermal helium Rydberg atomic beam and a crossed beam of target molecules such as CCl4, BrCN and CF3I can provide new insights into dissociative electron attachment reactions. The calculations show that not only can much detailed information on translational energy release distributions and intermediate lifetimes be obtained, but also on the electron radial probability density distributions in the Rydberg atoms themselves.
A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T\ensuremath{\sim}{10}^{\ensuremath{-}8}$ K. A global six-dimensional H2{}_{2}2​-H2{}_{2}2​ potential-energy surface is adopted from a previous work [Boothroyd et al., J. Chem. Phys. 116, 666 (2002)]. State-resolved integral cross sections ${\ensuremath{\sigma}}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}({\ensuremath{\varepsilon}}_{\text{kin}})$ of different quantum-mechanical rotational transitions $ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}$ in the HD molecules and corresponding state-resolved thermal rate coefficients ${k}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}(T)$ have been computed. Additionally, for comparison, H2{}_{2}2​+H2{}_{2}2​ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.
Full-dimensional quantum scattering calculations are reported for ro-vibrational transitions in HD+HD collisions using a highly accurate interaction potential for the H2_22​-H2_22​ system. Several near-resonant ro-vibrational transitions are identified that conserve the overall rotational angular momentum and nearly conserve the internal energy of the collision partners. Key anisotropic terms that drive the rotational transitions and angular momentum partial waves that contribute to low energy resonant features in the energy dependence of the cross sections are identified. The computed results are in agreement with total cross sections reported in previous experimental results, including resonant features in the energy dependence of the cross section. In particular, low-energy cross sections show a strong resonant feature associated with an l=3l=3l=3 partial wave in the incident channel. Rate coefficients for several inelastic rotational and ro-vibrational transitions are reported for temperatures ranging from 0.10.10.1 K to 200200200 K and they display a maximum between 111 K-101010 K reflecting the important contributions from the l=3l=3l=3 shape resonance that occurs around 2.5 K.
We present a kinetic model based on coupled rate-equation simulations that accounts for state-detailed rate processes in the evolution of a molecular Rydberg gas to plasma. Calculations support a steady-state picture of Rydberg predissociation and its contribution to plasma dissipation. Inelastic collisions of electrons with Rydberg states efficiently transfer energy from and to the thermal bath of free electrons, giving rise to quasi equilibria of relatively high temperature. Dissipative effects that remove low- n population change the time course of this heating. Including statistically sampled channels of Rydberg predissociation with n - and ℓ-dependent lifetimes slows the relaxation to quasi equilibrium and retards the initial electron temperature increase. At later times, the dissociative loss of low- n population displaces this quasi equilibrium leading to continuous electron heating. Dissociation products appear at a rate that is controlled initially by the rate of Rydberg relaxation to quasi equilibrium. Thereafter, the overall rate at which the total dissociation consumes neutral molecules determines the decay rate for all levels. Simulations predict a total predissociation rate that is smaller than the initial rate of direct electron–ion dissociative recombination. At later times, residual predissociation displays an apparent first-order decay with a rate constant that fits well with experimental observations.
We report close-coupling (CC) quantum dynamics calculations for collisional excitation/de-excitation of the lowest four rotational levels of OD − and of OH − interacting with Rb atoms in a cold ion trap. The calculations are carried out over a range of energies capable of yielding the corresponding rates for state-changing events over a rather broad interval of temperatures which cover those reached in earlier cold trap experiments. They involved sympathetic cooling of the molecular anion through a cloud of laser-cooled Rb atoms, an experiment which is currently being run again through a Heidelberg–Innsbruck collaboration. The significance of isotopic effects is analysed by comparing both systems and the range of temperatures examined in the calculations is extended up to 400 K, starting from a few mK. Both cross sections and rates are found to be markedly larger than in the case of OD − /OH − interacting the He atoms under the same conditions, and the isotopic effects are also seen to be rather significant at the energies examined in the present study. Such findings are discussed in the light of the observed trap losses of molecular anions.