Lopez Dominguez, Jesus Alberto (2015-05). Theory of Molecular Photoionization: From Vibrational Dependent Processes to the Effects of Rotational Motion in Ionizing Non-Linear Molecules on MFPADs and RFPADs. Doctoral Dissertation.
Thesis
In this work, I present a series of studies in the field of molecular photionization and in a more general way, in the electron-molecule scattering processes area. It takes as a starting point the study of well known molecular features in the photionization of small molecules and reviews the usual approach to compare experimental and theoretical cross sections and branching ratios. I then suggest a different theoretical model which enables a better frame of reference for the comparison. This method is based on the logarithmic derivative of the cross section. Also, a theoretical approach has been taken to study the effect that rotational motion has on the dynamics of photoionization between ionization and fragmentation, a series of equations are derived to compute the 3D recoil frame photoelectron angular distributions for non-linear molecules in the case where the axial-recoil approximation breaks down. The main concepts and ideas relevant to most of the work presented are introduced at the beginning of this work, even those that are part of the standard literature in textbooks on the subject of scattering and collision theory, but being of fundamental importance to the later developments of this thesis, are treated in a way pertaining to the approaches taken in the analysis of results and in the elaboration of the new theoretical findings presented in the rest of this work. I introduced what I call the electronic factor, which is a Franck-Condon factor that provides a common ground to compare experimental and theoretical branching ratios. For that purpose, two approaches are taken, one considering first an expansion of the matrix element of the dipole operator -> u up to first order terms and second, an extension where we also assume harmonic oscillator functions and the same frequencies in the initial and final vibrational states. Later on this methodology has been applied and analyzed for highly symmetric linear molecules and for a less symmetric polyatomic molecule with encouraging results. I also present photoionization studies on different target molecules, where the different symmetries, number and type of atoms, and other properties derived from these, of the chosen targets, allowed for a theoretical elaboration on a diverse number of specific and more general ideas relative to molecular photoionization. Some of the ideas explored are the effects that Cooper minima play on the deviation of Franck-Condon behavior and how the contribution of individual partial waves are expressed in the total experimental (or theoretical) cross section. As a mean of comparison between theory and experiment in this context, the electronic factor, F, derived previously is used. Effects of symmetry and complexity of the molecules in these studies are discussed. Finally, I present studies of the effects of rotational motion on non-linear molecules undergoing an ionization and later fragmentation. For this purpose, the previously developed ideas for linear and diatomic molecules are extended into the general case which allows one to treat a wider range of molecular targets incorporating any symmetry type and limited only by the computational resources available to treat big systems. To demonstrate its usefulness we compute MFPADs for core C 1s photoionization of CH4. I include the rotational motion by letting the pre-ionizing meta stable state to have lifetimes from ? = 0 ps to ? ps, evidencing a better agreement with experimental results than previous theoretical predictions where the axial-recoil approximation was assumed.
