Doubly Charged Carbon Dioxide Produced by Electron Impact with Molecular Clusters^†

Competition between chemical bonding and Coulombic repulsion in multi-charged molecules determines the stabilities or lifetimes of these molecular ions. Normally, multi-charged small molecules dissociate spontaneously, due to the overwhelming Coulombic repulsion. Such an effect was also recognized for highly charged micro-sized particles [1−3]. Doubly charged carbon dioxide (CO₂⁺⁺), as an interesting system, has been investigated extensively [3−9], since its first experimental evidence was reported in 1961 [10]. The following issues about CO₂⁺⁺ deserve attention: First, the lifetime values of metastable CO₂⁺⁺ vary markedly from less than one microsecond (μs) to over twenty μs [4−6], possibly due to the time observation windows or the ion production methods (e.g., photoionization, electron- or ion-impact ionization) [4−10]. Second, the vibrational spectra of CO₂⁺⁺ produced by photoionization were not reasonably assigned when assuming the linear molecular structures in low-lying states [5, 7], while the CO₂⁺⁺ structure could be nonlinear before the spontaneous fragmentation [8]. Third, twelve electronic states were predicted to lie in an energetic range of 10 eV above the ground state X³$\Sigma_{\mathrm{g}}^- $ (assuming the linear structure of the neutral) [7], and these excited states could contribute to the observation of CO₂⁺⁺.

On the other hand, irradiation-induced reactions of icy dust particles and grains in interstellar space and the planetary upper atmospheres can be mimicked with the laboratory experiments of electron-molecular cluster impacts [11−15], which is a recent topic in our group. As for doubly charged clusters (CO₂)_n⁺⁺, elaborate efforts were made to understand their dissociation and evaporation dynamics [3, 12−17]. The smallest doubly charged CO₂ cluster was (CO₂)₄₃⁺⁺ [16], while a recent study reported the observation of (CO₂)₃₀⁺⁺ which is close to the estimation of the Rayleigh limit, (CO₂)₂₆⁺⁺ [17]. Under the milder conditions (relative to the cold helium droplets [3] or the very high nozzle pressure 1.5 bar [16]), here we report an experiment-computation combined study of the CO₂⁺⁺ production by electron-impact ionization with a supersonic molecular beam of CO₂. We emphasize on potential correlations between CO₂⁺⁺ and the ionic clusters.

All measurements were carried out with our home-made apparatus [18]. The system comprising a low-energy electron source and a trochoidal electron monochromator [18] was replaced with a commercial electron gun (ELG-2/EGPS-1022, Kimball Physics) to produce the higher energy (tunable from 1 eV to 2000 eV) electrons without the energy monochromatization. In this work, the electron gun worked at incident energies of 100, 150 and 200 eV, respectively. Prior to the measurements, we calibrated the electron energy and found that its uncertainty was about 0.2 eV. The electron pulse width and the working frequency were 500 ns and 5000 Hz respectively. A continuous supersonic molecular beam was produced with a specially machined conical nozzle (diameter: 70 μm; cone angle: 30°). The cationic products under certain nozzle pressures (0.43, 0.70, 0.80, 0.90, 1.04, 1.12 bar) were pushed out of the collision region (about 2 mm×2 mm×2 mm), flew through a set of ion lenses, then were detected with the ion detector. This time-of-flight mass spectrometer was also used to record the mass spectra of the anionic clusters of CO₂ by low-energy electron impacts [11]. In this work, the ion lens voltages were switched for positive ion detection. The gas sample of CO₂ with a high purity (99.999 %) was used in the previous [11] and present experiments. In each measurement, the ion count of CO₂⁺ (parent ion) was about tens of thousands, while that of CO₂⁺⁺ ions was of hundreds or more.

The calculations were performed with the Gaussian 16 suit program [19]. The geometrical parameters of CO₂⁺⁺ and (CO₂)₂⁺⁺ in low-lying states were fully optimized at the second-order perturbation (MP2)/6-311+G(d) level [20], and their stabilities as the minima on potential energy surfaces were examined with harmonic vibrational frequency calculations. Then, the single-point energies (including the corrections of zero-point energy obtained at the MP2 level) of the stable conformers were obtained at the couple-clustered CCSD(T)/6-311+G(d) level [21]. In this work, the electronically ground and first-excited states of CO₂⁺⁺ and (CO₂)₂⁺⁺ were considered.

As shown in FIG. 1, CO₂⁺⁺ and (CO₂)_n⁺ (n = 2, 3, 4) are the minor yields at an electron energy 150 eV and under a nozzle pressure 0.90 bar. The appearance energy of CO₂⁺⁺ from CO₂ is 37.2 eV and that from the clusters could be reduced by 7 eV due to solvation effect [3−6]. Since the ion production by electron impact is very low at the energetic threshold, we do not aim to determine the threshold values by the electron-impact ionization. Moreover, we have not observed the doubly or triply charged clusters under the present nozzle pressures, possibly, because the neutral clusters are smaller than those reported previously [16]. The parent cation CO₂⁺ is assigned to the strongest peak in FIG. 1. One may speculate that a contribution of (CO₂)₂⁺⁺ to this peak is possible, since (CO₂)₂⁺⁺ and CO₂⁺ have the same mass-to-charge ratio. However, previous studies have indicated that small doubly charged clusters prefer quick fissions (with a fission time of less than 10 μs) [11, 19].

The insets of FIG. 1 indicate that the ion intensity of CO₂⁺⁺ is less than one percentage of that of CO₂⁺, but comparable to that of (CO₂)₂⁺ and slightly stronger than those of (CO₂)_n⁺ (n = 3, 4). Although we have not observed the doubly or triply charged clusters, the production of CO₂⁺⁺ from the unstable (CO₂)_n⁺⁺ is possible, namely, via a spontaneous fragmentation (CO₂)_n⁺⁺→(n–1)CO₂ + CO₂⁺⁺. The dynamic features of this process should exhibit distinct differences in comparison with those from the monomer CO₂: first, CO₂⁺⁺ could be the ionic core of the unstable (CO₂)_n⁺⁺, or the double charge should be confined to a dimer or trimer in the cluster; second, the spontaneous fragmentation of (CO₂)_n⁺⁺ lags behind the prompt production from the monomer, which may be a visible contribution with different flight times to the spectral peak of CO₂⁺⁺; third, CO₂⁺⁺ in excited states could undergo state-to-state transitions, which may also be visualized in its peak profile of the mass spectrum due to time lags or energy releases.

As shown in FIG. 2, we compare the spectral profiles of CO₂⁺⁺ recorded at a given electron energy of 150 eV and under different nozzle pressures. Usually, the molecule is colder and the cluster is larger in the beam which is produced under the higher nozzle pressure. Therefore, as shown in FIG. 2(a) (under the nozzle pressure of 0.43 bar), the molecular motions perpendicular to the translational direction result in a diffuser peak of CO₂⁺⁺. The colder and well-collimated molecules result in the sharper and stronger peak, as shown in FIG. 2(d) (0.90 bar). Some fine structures of the main peak in FIG. 2(f) may be attributed to different CO₂⁺⁺ sources: the molecular monomer, clusters, and excited-state species. The mechanisms proposed here deserve further validations in the future.

Besides the direct production of CO₂⁺⁺ from the monomer, we pay more attention to possible pathways from the clusters (CO₂)_n. We plot the production ratios between CO₂⁺⁺ and the ionic clusters (CO₂)_n⁺ (n = 2, 3, 4) in FIG. 3. In contrast to the experiments on helium dopant droplets [16, 17], here we do not observe the doubly charged clusters in the mass spectra. Even if those large clusters (CO₂)_n⁺⁺ [16, 17] are also produced in the present experiment, they may quickly decay to small singly charged clusters. Based on this, we believe that there are no direct links between (CO₂)_n⁺ and CO₂⁺⁺. However, information about the environments where CO₂⁺⁺ is produced can be tracked from the ratios shown in FIG. 3. In general, the ratios increase with the enhancement of the nozzle pressure, indicating that important contributions of the larger clusters to the CO₂⁺⁺ production. On the other hand, local maxima are found for the pressure of 0.90 bar at the electron impact energies of 150 and 200 eV, and 1.04 bar at 100 eV, implying specific dynamics of electron impact ionizations. A similar strategy was used to unveil the production competition between (CO₂)_n¯ and (CO₂)_n–1O¯ [11]. Comparatively, the production mechanisms of CO₂⁺⁺ are much more complicated than those of the anions.

To the best of our knowledge, only one theoretical study of CO₂⁺⁺ was reported, in which the geometrical parameters were not fully optimized and the ground state ${}^3\Sigma_{\mathrm{g}}^- $ with a linear structure (bond length of 1.207 Å) was proposed [7]. As listed in Table I, we confirm that the ground state is a triplet state (³Aʺ) with a nonlinear structure, based on full optimizations of the geometrical parameters. One C–O (bond length of 1.364 Å) is much longer than that of CO₂⁺ (bond length of 1.1779 Å [22]), implying that CO₂⁺⁺(³Aʺ) decomposes readily into CO⁺ + O⁺. However, if this weakly bound species is produced in the clusters, its survival possibility can benefit from the evaporation cooling [11]. The first excited state of CO₂⁺⁺ is ¹Δ_g with a linear structure and 3.20 eV above the ground state ³Aʺ. The transition ¹Δ_g→³Aʺ is spin-forbidden, so CO₂⁺⁺(¹Δ_g), as a metastable species, should contribute primarily to the CO₂⁺⁺ signals in the mass spectra. The vibrational frequencies predicted here could be helpful to re-assign the vibrational spectra observed previously [5, 7], but it is beyond the present work.

In contrast to CO₂⁺⁺, we find that the ground state of the doubly charged dimer C₂O₄⁺⁺ is ¹A_g, rather than a triplet state. Our calculations indicate that its lowest triplet state is a repulsive state (leading to two CO₂⁺ or CO₂ + CO₂⁺⁺). The carbon-carbon bond length of C₂O₄⁺⁺(¹A_g) is shorter than that in alkanes (1.53–1.55 Å [22]), indicating a high stability of C₂O₄⁺⁺. Therefore, this species seems to contribute the peak of CO₂⁺ (due to the same mass-charge ratio m/z = 44) in FIG. 1. However, its Coulombic explosion to two CO₂⁺ is highly favored in energetics [11, 19]. In FIG. 4, we depict the molecular structure of C₂O₄⁺⁺(¹A_g) and its energetic correlations with two dissociation limits, CO₂(¹Σ_g) + CO₂⁺⁺(¹Δ_g) and CO₂(¹Σ_g) + CO₂⁺⁺(³Aʺ). Since C₂O₄⁺⁺(¹A_g) → CO₂(¹Σ_g) + CO₂⁺⁺(³Aʺ) is a spin-forbidden process, it is preferable to produce CO₂⁺⁺(¹Δ_g) from C₂O₄⁺⁺(¹A_g) in high-lying vibrational states. Moreover, once a triplet-state transient C₂O₄⁺⁺ is produced by electron-impact double ionization with molecular clusters, CO₂⁺⁺ (³Aʺ) could be produced and survive in the vaporization cooling.

The production of CO₂⁺⁺ by electron collisions with a molecular beam of CO₂ is reinvestigated here. The mass spectra obtained under different experimental conditions, namely, the electron impact energies and the nozzle pressures, are compared, indicating multiple ion sources of CO₂⁺⁺. Based on ab initio calculations, we find the correlations between CO₂⁺⁺ and C₂O₄⁺⁺ and suggest that these two doubly charged ions have different contributions to the mass spectra. These doubly charged molecules deserve further exploration, particularly, their productions from large molecular clusters.