ExoMol molecular line lists – XXVII. Spectra of C2H4

Abstract

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...

The ExoMol Atlas of Molecular Opacities

“…Only one temperature is plotted since, despite containing 60 billions line, the line list is only complete up to 700 K. However, it is likely that ethylene will decompose at higher temperatures. Although not shown here, the main features of the ethylene spectrum show an unusually small sensitivity to temperature [75]. Rey et al [108] have also computed a far-infrared ethylene line list.…”

“…Cross sections generated using ExoMol line lists for methane, 10to10 line list [53], silane [72], phosphine [56], ammonia [79] and ethylene [75]. Figure 2 shows cross sections for a number of other hydrides: methane, silane, phosphine, ammonia and ethylene.…”

The infrared spectrum of PF3and analysis of rotational energy clustering effect

“…Variational ro-vibrational calculations were carried out using the TROVE program. The TROVE methodology is well documented [72][73][74][75][76] and has been applied to a variety of molecules, mostly as part of the ExoMol project [7,65,69,73,[77][78][79][80][81][82][83][84][85]. Only the specific details relevant to this work on PF 3 will be discussed here.…”

“…This was carried out for the SO 3 molecule where up to J = 130 was required for 800 K. For PF 3 , even higher values of J would be required but this can be carried out using special methods. These include saving the Hamiltonian matrix to disc and diagonalizing using external parallel programs or making use of diagonalizers specially developed for large matrices [46,85].…”

Symmetry Adaptation of the Rotation-Vibration Theory for Linear Molecules

“…The aim of the present work is to illustrate how D ∞h symmetry can be implemented into general nuclear-motion programs. As an example we use TROVE [5,6], a numerical variational method to solve for the ro-vibrational (rotational-vibrational) spectra of (small to medium) general polyatomic molecules, which has been used to simulate the hot spectra of various polyatomic molecules [7][8][9][10][11][12][13][14][15][16][17][18][19] as part of the ExoMol project [20,21], a database of ab initio spectra of molecules of astrophysical importance. Since calculations of this type are based on electronic properties of the molecule, primarily the potential energy and dipole moment surfaces, obtained by solving the electronic Schrödinger equation, one can claim that such calculations are also rovibronic.…”