Theoretical Study of Few Body Processes

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Collision physics is currently enjoying an incredibly exciting time as a consequence of recent new experimental advances. One of these advances is the COLTRIMS (cold-target recoil-ion momentum spectroscopy) method which can be used to obtain full three-dimensional pictures of a collision process. These 3D pictures contain very detailed information about the fundamental forces controlling nature on the atomic/molecular level and the challenge for theory is to see if our theoretical models can accurately predict what is observed experimentally. One of the most sensitive probes of the important fundamental forces is charged particle ionization of atoms/molecules. Problems of this type are called the few-body problem. For two particles, the problem is completely solved since we have exact quantum mechanical solutions. For three particles, there is no exact quantum mechanical solution so this is one of the important fundamental unsolved problems in physics. The only way to study the problem is to develop theoretical models and compare the predictions of the model with experimental measurements. Although significant progress has been made for understanding the 3-body problem, there is still much work to be done particularly for collisions with larger atoms and molecules, interactions that are important in numerous gas-phase processes in Nature and in applied fields of research. The 4-body problem is significantly more difficult than the 3-body problem and, so far, the amount of work done on the 4-body problem is limited. The purpose of this project is to theoretically examine several open questions in the field of charged particle ionization of atoms and molecules for both the 3-body and 4-body problems and for both electron-impact and heavy-particle impact. There are two types of theoretical approaches which can be used for these problems - perturbative and non-perturbative. Non-perturbative methods can be used for collisions with small atoms and small molecules and they are generally more accurate than non-perturbative. For larger atoms and molecules, the only approach that is currently feasible is the perturbative approach which is proposed for this work. Previously, the PI has shown that an approach called the 3-body distorted wave (3DW) for atoms or molecular 3-body distorted wave (M3DW) for molecules predicts results which are in very good agreement with experiment with experiment for some atoms (He, Ne) and some molecules (H2, N2), relatively good agreement with experiment with experiment for some atoms (Ar, Kr) and some molecules (NH3, CH4), qualitative agreement for some molecules (formic acid (CH2O2), pyrimidine (C4H4N2), tetrahydrofuran (THF C4H8O), tetrahydrofurfuran alcohol (THFA, C5H10O2), tetrahydropyran (THP, C5H10O), and 1,4-dioxane (C4H8O2)), and very poor agreement for some other molecules (SF6). One of the objectives of the proposed work is to improve the M3DW model for electron-impact collisions with molecules. There are fewer detailed experimental measurements for heavy-particle collisions than for electron-impact collisions and the theoretical models tend to be less sophisticated. Another objective of this work is to improve the theoretical models for both 3-body and 4-body collisions for heavy particles for both atoms and molecules.