Dissociative recombination of NO+: Dynamics of the X 1Σ+ and a 3Σ+ electronic states

Abstract

We have studied the dissociation dynamics of NO ϩ ions in their ground, X 1 ⌺ ϩ , and first excited metastable, a 3 ⌺ ϩ states, induced by the capture of electrons of variable collision energy in the dissociative recombination ͑DR͒ process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO ϩ (X 1 ⌺ ϩ ,vϭ0) ions dissociate dominantly to the N( 2 D) ϩO( 3 P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N( 4 S)ϩO( 1 D) dissociation limit contributes 0͑Ϯ2͒% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3 ⌺ ϩ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3 ⌺ ϩ state is found to be 730͑Ϯ50͒ ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1 ⌺ ϩ and the a 3 ⌺ ϩ states. It is found that vibrational relaxation via infrared radiation is faster for NO ϩ (a 3 ⌺ ϩ ,vϾ0) ions than the electronic decay of these metastable-state ions to the ground state.

Electron energy-dependent product state distributions in the dissociative recombination of O2+

“…15 Collision-induced dissociation and charge-transfer reactions between the ions and the rest gas in the ring take place throughout the ring. This process limits the lifetime of the ion beam.…”

“…17,18 The toroidal region, i.e., the region where the electron beam merges into and demerges out of the ion beam, provides a collision region in which the electron collision energy is much larger than in the remainder of the electron cooler. 15 This toroidal effect is corrected for, 19,20 as is the electron space-charge effect, which affects the electron energy due to the presence of positive ions trapped in the space charge of the electron beam.…”

“…Although heavy-ion storage-ring 2D experiments do not have sufficient resolving power for precise angular distributions, dissociations with the molecular axis parallel to the collision velocity will show up clearly as nonisotropic. 15 The lack of agreement between our observed isotropy and the predicted angular dependencies 25 deserves further attention. These theoretical predictions agree with the rules derived by Dunn 35 for electron-impact processes and with the predictions of O'Malley and Taylor 36 for dissociative attachment.…”

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

“…Details of CRYRING and the imaging technique employed can be found elsewhere in the literature. 13,48,49 Here, only the main and most relevant features will be outlined. The ion source described earlier was integrated into the injectionline apparatus.…”

Dissociative recombination of the weakly bound NO-dimer cation: Cross sections and three-body dynamics

“…We also note that dissociation towards O * ͑ 1 D͒ +N͑ 4 S͒, which is energetically and spin allowed in the dimer ion, is not observed in the DR of the monomer ion in accord with the conservation of electron spin during the DR reaction. 16 Figure 1͑a͒ summarizes the reactions ͑1a͒-͑1g͒. The three-body-reaction channels are given on the left-hand side and the two-body-reaction channels on the right-hand side.…”