A newly calculated ab initio potential energy surface is used to compute collision-driven state-changing cross sections and rate coefficients over a range from 5 to 100 K for CN−(1Σ), the smallest anion detected in the interstellar medium, interacting with He, an abundant species in this environment. We compare our presently computed rate coefficients with those previously published for the similar and important systems CN–He, CN-H2, and CN−–H2 to illustrate the broader network of inelastic, state-changing processes for these four systems. We also discuss the size-scaling effects that occur when changing partners from He to H2. We further analyze the differences in size between collision-driven rate coefficients when going from neutral CN to its anion. All the present results are discussed in detail, to provide accurate and realistic data for chemical networks that wish to include the CN− anion in their modeling of astrochemical environments.

Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH−(1Σ), MgH−(1Σ), and C2H−(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.

A new line list for ethylene, 12 C 2 1 H 4 is presented. The line list is based on high level ab initio potential energy and dipole moment surfaces. The potential energy surface is refined by fitting to experimental energies. The line list covers the range up to 7000 cm −1 (1.43 µm) with all ro-vibrational transitions (50 billion) with the lower state below 5000 cm −1 included and thus should be applicable for temperatures up to 700 K. A technique for computing molecular opacities from vibrational band intensities is proposed and used to provide temperature dependent cross sections of ethylene for shorter wavelength and higher temperatures. When combined with realistic band profiles (such as the proposed three-band model), the vibrational intensity technique offers a cheap but reasonably accurate alternative to the full ro-vibrational calculations at high temperatures and should be reliable for representing molecular opacities. The C 2 H 4 line list, which is called MaYTY, is rmade available in electronic form from the CDS ExoMol XXVII: Line list for Ethylene 3 In this work we present new ab initio potential energy and dipole moment surfaces for ethylene and use them to compute a line list for elevated temperatures as part of the ExoMol database project (Tennyson & Yurchenko 2012; Tennyson et al. 2016). We name this line list MaYTY. Compared to the line list of Rey et al. (2016) we slightly increase the applicable frequency range and include many more weak transitions (50 billion here compared to 60 million previously) which are important for total opacity. Rovibrational energy levels were computed variationally using a refined PES with the TROVE program suite (Yurchenko et al. 2007; Yachmenev & Yurchenko 2015). Ethylene is the first 6 atom molecule in the Exomol database and the largest for which we have computed a line list so far. We also propose a new procedure for computing molecular opacities from vibrational transition moments only. Similar J = 0 approaches are very common in simulating spectra of large polyatomic molecules (Jornet-Somoza et al. 2012), where either very simple band profiles (e.g. Lorentzian) or sophisticated functional forms (such as the narrow band approach of Consalvi & Liu (2015)) are used. Here we develop a three-band model based on three fundamental bands of C 2 H 4 (one parallel and two perpendicular), which also represent its three dipole moment components. This J = 0-effort approach has allowed us to significantly extend the temperature as well as the frequency range of our line list and should be also useful for larger polyatomic molecules.The paper is organised as follows: In Section 2 we give details of our PES and DMS along with our variational calculations and how transition intensities were calculated. In Section 3 we give details of the MaYTY line list and compare with experimental data. The new procedure for generating opacities from vibrational band intensities is discussed in Section 4. We present conclusions in Sections 5. METHODS Potential Energy Surface and RefinementA...

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.

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