The CH-3Σ+ Anion: Inelastic Rate Coefficients from Collisions with He at Interstellar Conditions

Abstract

We present accurate ab initio calculations on several properties of a gas-phase system of interest in the interstellar medium (ISM), where the title molecular anion has been often surmised but not yet confirmed by observations. The CH^-3Σ⁺ constitutes the smallest term in the series of longer anionic polyynes which have been observed in the ISM (e.g., C₄H^- and several others). Hence, its dynamical behavior in collision with He atoms, one of the most abundant atoms in that environment, can provide quantitative indicators on the changes which can occur in the rotational state population of the title anion when driven by this collision dynamics. We therefore report an accurate evaluation of the full potential energy surface (PES) which acts between the molecular anion in its ground vibrational state and the He atom. The relevant inelastic scattering cross sections and the corresponding inelastic rate coefficients are then computed within a quantum treatment of the collisions. We find that the fairly small values of the final inelastic rate coefficients indicate state-changing processes by collisions to be inefficient paths for modifying the rotational state populations of this anion and therefore to aid its possible observation from direct radiative emission in the microwave region.

Figure 1

Behavior of the partial excess charge on each of the atoms in the anion as a function of the internuclear distance. Notice the asymptotic values where the negative charge moved entirely on the C atom

Figure 2

Locations and values of the lowest three vibrational w.f.s with their corresponding turning points.

Figure 3

Energy for the first six rotational states of the ν = 0 vibrational states (right) and their Boltzmann distribution when treating the title anion as a pseudo-singlet rotor.

Figure 4

Spatial distribution of the interaction potential energy around the molecular anion. See main text for further details.

Figure 5

Multipolar expansion coefficients for the computed PES of the present study. See main text for further details.

Figure 6

Computed state-to-state rotationally inelastic cross sections, treating the molecular anion either as a full triplet rotor (dotted lines) or as a pseudo-singlet rotor (continuous lines). We report for comparison five different de-excitation processes with Δj = 1 from the lowest 5 levels. See main text for further comments.

Figure 7

Computed state-to-state rotationally inelastic cross sections, treating the molecular anion as a pseudo-singlet rotor. The upper-left panel reports excitation processes with Δj = 1 from the lowest 5 levels. The lower-left panel shows those with Δj = 2 transitions. The de-excitation cross sections are presented in the upper- and lower-right panels.

Figure 8

Computed state-to-state rotationally inelastic rate coefficients, treating the molecular anion as a pseudo-singlet rotor. The upper-left panel reports excitation processes with Δj = 1 from the lowest 5 levels. The lower-left panel shows the excitations rates with Δj = 2 transitions. The de-excitation rate coefficients are presented in the upper- and lower-right panels of the figure.

Figure 9

Comparing the computed state-changing rate coefficients between CH^– (thick lines) and CN^– (thin lines). The data of the former anion are from the present calculations while those of the latter are from our earlier work. The two upper panels report rotational excitation processes, while the lower two panels show de-excitation processes.

Figure 10

Computed state-changing rate coefficients for CH^– (red sticks) and CN^– (blue sticks). The data of the former anion are from the present calculations, while those of the latter are from our earlier work. The two panels report rotational de-excitation processes at two different temperatures and for the lowest four excited rotational states of the two molecular systems.