Near-threshold rotational excitation of molecular ions by electron impact

Abstract

Abstract.New cross sections for the rotational excitation of H + 3 by electrons are calculated ab initio at low impact energies. The validity of the adiabatic-nuclei-rotation (ANR) approximation, combined with R-matrix wavefunctions, is assessed by comparison with rovibrational quantum defect theory calculations based on the treatment of Kokoouline and Greene (Phys. Rev. A 68 012703 2003). Pure ANR excitation cross sections are shown to be accurate down to threshold, except in the presence of large oscillating Rydberg resonances. These resonances occur for transitions with ∆J = 1 and are caused by closed channel effects. A simple analytic formula is derived for averaging the rotational probabilities over such resonances in a 3-channel problem. In accord with the Wigner law for an attractive Coulomb field, rotational excitation cross sections are shown to be large and finite at threshold, with a significant but moderate contribution from closed channels.

On the chemistry and distribution of HOC$\mathsf{^+}$ in M 82

“…This has been shown to be valid down to threshold for molecular ions, provided a simple "Heaviside correction" is applied to excitation cross sections. Full details can be found in Faure et al (2006). Note that closed-channel effects are neglected here as these are expected to be small for strongly polar ions (see Faure et al 2006).…”

“…Full details can be found in Faure et al (2006). Note that closed-channel effects are neglected here as these are expected to be small for strongly polar ions (see Faure et al 2006). Furthermore, in contrast to the calculations of Faure & Tennyson (2001) restricted to the lowest 3 rotational levels, we here have considered rotational transitions between levels with J ≤ 20 (i.e.…”

Rotational Cooling ofHD+Molecular Ions by Superelastic Collisions with Electrons

“…Recently, however, evidence for a rotational quantum number dependence of the vibrational SEC rates in H þ 2 has been obtained [5], and first calculations of vibrational SEC cross sections for H þ 2 have been performed within MQDT [6] including rotational couplings, which approach the measured SEC rate coefficients already much more closely. The present experiment was designed to investigate SEC between rotational states built on the vibrational ground state of HD þ in order to elucidate the role of rotational angular momenta in the SEC process and to provide an experimental basis for a more detailed comparison with ongoing theoretical developments [6][7][8].Although rotational cooling by SEC in storage ring experiments has been conjectured before (see, e.g., [9]), this is the first quantitative measurement of rotational SEC rates for a molecular ion.The experiment uses the DR process to probe the time evolution of the population P J ðtÞ of the molecular rotational states J when merging rotationally hot HD þ ðv; JÞ ions circulating in a storage ring with cold electrons of the same velocity as the ions. Since HD þ is known to relax to the v ¼ 0 ground state within a few 100 ms [10] and the relative ion-electron energies E are smaller than the energy difference between two adjacent vibrational and rotational states, the only inelastic collisions which can take place thereafter are rotational SECs, i.e., HD þ ð0; JÞ þ e À ðE ¼ 0Þ !…”

“…Recently, however, evidence for a rotational quantum number dependence of the vibrational SEC rates in H þ 2 has been obtained [5], and first calculations of vibrational SEC cross sections for H þ 2 have been performed within MQDT [6] including rotational couplings, which approach the measured SEC rate coefficients already much more closely. The present experiment was designed to investigate SEC between rotational states built on the vibrational ground state of HD þ in order to elucidate the role of rotational angular momenta in the SEC process and to provide an experimental basis for a more detailed comparison with ongoing theoretical developments [6][7][8].…”

“…These calculations followed those performed on H þ 2 [18] at a fixed HD þ bond length of 2:0a 0 except that they were carried out in C 1v symmetry with a shifted center of mass which gave a dipole moment of 0:85D. Note that the cross sections did not need to be corrected for threshold effects because the ANR approximation was shown to be valid down to threshold [8]. As observed previously for ions with moderate dipole moments (&1D) [7], cross sections with ÁJ ¼ À1 were found to be significantly smaller than those with ÁJ ¼ À2.…”