Rotationally inelastic processes of C2− ( 2Σg+ ) colliding with He (¹ S) at low temperatures: ab initio interaction potential, state changing rates and kinetic modelling

Abstract

We discuss in detail the quantum rotationally inelastic dynamics of an important anion often discussed as a possible constituent of the interstellar medium (ISM) and in different environments of circumstellar envelopes: the C 2 − molecular ion. Its interaction forces with one of the most abundant atoms of the ISM, the neutral helium atom, are obtained for the first time using ab initio quantum chemistry methods. The overall angular anisotropy of the potential energy surface is analysed in order to link its features with the efficiency of transferring energy from the abundant He atoms to the internal rotational levels of this molecular anion. Calculations of the corresponding rotational state-to-state inelastic cross sections, for both excitation and de-excitation paths are obtained by using a multichannel quantum method. The corresponding inelastic rates at the temperatures of interest are determined and their role in distributing molecular states over the different populations of the rotational levels at the temperatures of that environment is discussed. These computed rates are also linked to the dynamical behaviour of the title molecule when confined in cold ion traps and made to interact with He as the common buffer gas, in preparation for state-selective photo-detachment experiments.

Beyond the helium buffer: 12C−2 rotational cooling in cold traps with H2 as a partner gas: interaction forces and quantum dynamics

“…The rate constants for C − 2 /p-H 2 (j = 0) calculated here and those for C − 2 /He which we have previously obtained in our earlier work [31] can be used now to check an approximation which is often employed in astrophysical modelling studies ( e.g. see: [64]), consisting in using the collisionally inelastic coefficients computed for the helium partner to further obtain those for H 2 as a partner by assuming the same value of their scattering crosssections.…”

“…In a series of recent works, we have investigated various inelastic processes of noble gas atoms colliding with C − 2 to assess their usefulness as buffer gases for cold-ion trapping experiments. Our simulations showed that thermalisation times of the rotational states of C − 2 with noble gases decreased in the order helium > neon > argon, due to the increasing interaction energy with atomic size and with dipole polarisability which are then yielding larger rate coefficients [31,32]. We have also calculated the rate coefficients for vibrationally inelastic collisional quenching of the anion's ν = 2 and ν = 1 levels [33], a step which can be useful in laser cooling experiments.…”

“…The scattering cross-sections for the corresponding transitions for C − 2 /He collisions, which we have previously calculated [31], are also shown in Figure 5 allowing comparisons to be made with those for C − 2 /p-H 2 (j = 0) system. Cross-sections for C − 2 colliding with molecular hydrogen turn out to be significantly larger than those for He, as anticipated in the previous discussion and clearly due to the stronger interaction in the former system compared with the latter.…”

“…As the most abundant isotopologue 12 C − 2 is a homonuclear diatomic molecule, it does not exhibit a pure ro-vibrational spectrum making its detection in emission difficult but transitions to and from low-lying excited electronic states may allow for the anions detection. The rotationally inelastic collisions considered here between C − 2 and H 2 serve as a complement to our earlier work on C − 2 /He collisions, the latter partner gas being the second most abundant gas in astronomical environments [31].…”

“…For the C − 2 /Ar system, the anion was treated as a pseudo-singlet ( 1 ) as the rate constants for the collisional cooling process were of primary interest [32]. This constraint simplifies the dynamics but does not greatly affect the accuracy of the calculations, as discussed previously in our earlier studies [31,63]. To compare the C − 2 /p-H 2 (j = 0) cross-sections to those of pseudo-singlet C − 2 /Ar, the relevant summed doublet cross-sections are used which are obtained by simply adding both cross-sections for the same final N state but different j state, for example σ 0.5→2.5 + σ 0.5→1.5 is compared to σ 0→2 for the pseudo-singlet case.…”