A computational study of collisions relevant to cyanogen, carbon dioxide and helium in astrophysical environments

Abstract

Collisions are very important in determining the abundances of the di erent species that exist in our world. Atoms, molecules, ions, electrons and even photons, have their concentrations dictated by the collisions they undergo. In this work, three types of collisions are studied. The photodissociation, collision with photons, of cyanogen (CN) is studied in chapter 1. Cross sections were computed for the 2 +(4) X 2 +, 2 +(5) X 2 +, 2 (4) X 2 +, and 2 (5) X 2 + electronic transitions. Partial and LTE cross sections were evaluated for all rovibrational levels (v00N00) of the ground electronic state of CN, X 2 +, and over a large wavelength range of astrophysical interest. The results will allow for the calculation of reliable CN photodissociation rates for astrophysical environments with varying radiation and dust properties. The rotational quenching of CO2 by collisions with He is the subject of Chapters 3 and 4. CO2 was treated as a rigid rotor in the ground state. If CO2 is initially in the rotational state speci ed by the rotational quantum number jo, cross sections for transitions to all possible lowerð€€€j rotational states, for a wide range of relative collision energy, were calculated. In Chapter 4, the emphasis is on rotational quenching of CO2 by collisions with He in the ultracold regime. Complex scattering lengths and elastic and inelastic rotational quenching cross sections have been computed for carbon dioxide, with rotational excitation j as high as 200, due to ultracold 4He collisions. It is predicted that the ratio of the elastic to inelastic cross section, or gure-of-merit, is su ciently large that highly rotationally excited CO2 could be a viable candidate for cooling and trapping. In chapter 5, new potential energy curves were used to calculate the cross section for excitation transfer from the He(23S) atoms to the He(11S) gas, the di usion of He(23S) atoms in He(11S) gas, and the total elastic cross sections for the collision of He(11S) and He(23S) atoms. The results were compared to published results and a good agreement is observed. The elastic cross sections for the collision of He(11S) and He(33S) were calculated for the first time.