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Papers by Gaetan VanGyseghem

When radiation interacts with a molecule, several processes may occur. One of these, photoionizat... more When radiation interacts with a molecule, several processes may occur. One of these, photoionization, is the topic of this study. The system is broken down into two nuclei and N electrons, the wavefunctions of which are entirely codependent. Operating under the Born-Oppenheimer approximation :,2 allows for the nuclear and electronic wavefunctions to be separable, which greatly simplifies the situation. The task, then, is to freeze the nuclei and find the electronic wavefunctions. These fixed-nuclei wavefunctions are found, in this study, using the UK molecular R-Matrix (UKRmol) suite of codes 3,4,5,6,7,8 , which solves for the wavefunctions separately near the nuclei and far from the nuclei, then matches the solutions smoothly at the boundary of these regions. The Born-Oppenheimer approximation , which allows such a separation of nuclear and electronic motion, is, however, only applicable when the electronic motion is very fast compared to the nuclear motion. Near the nuclei, the electron's kinetic energy is great, and this approximation is sufficient. When a low-energy electron is far from the nuclei, its speed is significantly decreased, and the Born-Oppenheimer approximation breaks down. To account for this, the quantum defects, which characterize the complicated inner region, i.e., the region in which the Born-Oppenheimer approximation is valid, on the boundary, are extracted from the frozen-nuclei results and used to form an unphysical reactance matrix K(R) and dipole matrix ⇀ d (R). From these, a vibrational frame transformation is applied, and a more prudent result is achieved. The primary focus of this study is the photoabsorption of molecular nitrogen (N 2) near the first ionization threshold. Beginning with an existing photoionization model 9 , a selfconsistent field (SCF) representation and a configuration-interaction (CI) representation have been treated using the UKRmol suite of codes. This has been used to construct photoionization cross sections σ P I , as well as R-matrices R, physical and unphysical reactance matrices K (phys) and K as functions of energy, and dipole matrices ⇀ d :0,::,:2. From the reactance matrices, the eigenquantum defects µ were extracted and plotted against energy. These eigenquantum defects or reactance matrices are needed as a function of internuclear separation R to enact a vibrational frame transformation via potential energy curves, which were constructed for both N 2 and N + 2 .

Bulletin of the American Physical Society, 2015

Submitted for the DAMOP16 Meeting of The American Physical Society Near-Threshold, Vibrationally-... more Submitted for the DAMOP16 Meeting of The American Physical Society Near-Threshold, Vibrationally-Resolved Photoionization of Molecular Nitrogen GAETAN VANGYSEGHEM, THOMAS GORCZYCA, Western Michigan University, CONNOR BALLANCE, Queen’s University Belfast — Photoionization of molecular nitrogen (N2) is investigated near the first ionization threshold using an R-matrix, multi-channel quantum defect theory (MQDT) approach. Building on an existing fixed-nuclei R-matrix photoionization model [M. Tashiro, J. Chem. Phys. 132, 134306, (2010)], which, in turn, is built on the UKRmol suite of codes, photoionization cross sections, as well as scattering and dipole matrices, are computed in the Born-Oppenheimer approximation. By varying the internuclear separation, potential energy curves have been constructed for the N2 and N + 2 states and compared to quantum chemistry calculations. Using these fixed-nuclei potential energy curves, and corresponding vibronic eigenenergies and eigenfunctions, a...

Bulletin of the American Physical Society, Jun 10, 2015

When radiation interacts with a molecule, several processes may occur. One of these, photoionizat... more When radiation interacts with a molecule, several processes may occur. One of these, photoionization, is the topic of this study. The system is broken down into two nuclei and N electrons, the wavefunctions of which are entirely codependent. Operating under the Born-Oppenheimer approximation :,2 allows for the nuclear and electronic wavefunctions to be separable, which greatly simplifies the situation. The task, then, is to freeze the nuclei and find the electronic wavefunctions. These fixed-nuclei wavefunctions are found, in this study, using the UK molecular R-Matrix (UKRmol) suite of codes 3,4,5,6,7,8 , which solves for the wavefunctions separately near the nuclei and far from the nuclei, then matches the solutions smoothly at the boundary of these regions. The Born-Oppenheimer approximation , which allows such a separation of nuclear and electronic motion, is, however, only applicable when the electronic motion is very fast compared to the nuclear motion. Near the nuclei, the electron's kinetic energy is great, and this approximation is sufficient. When a low-energy electron is far from the nuclei, its speed is significantly decreased, and the Born-Oppenheimer approximation breaks down. To account for this, the quantum defects, which characterize the complicated inner region, i.e., the region in which the Born-Oppenheimer approximation is valid, on the boundary, are extracted from the frozen-nuclei results and used to form an unphysical reactance matrix K(R) and dipole matrix ⇀ d (R). From these, a vibrational frame transformation is applied, and a more prudent result is achieved. The primary focus of this study is the photoabsorption of molecular nitrogen (N 2) near the first ionization threshold. Beginning with an existing photoionization model 9 , a selfconsistent field (SCF) representation and a configuration-interaction (CI) representation have been treated using the UKRmol suite of codes. This has been used to construct photoionization cross sections σ P I , as well as R-matrices R, physical and unphysical reactance matrices K (phys) and K as functions of energy, and dipole matrices ⇀ d :0,::,:2. From the reactance matrices, the eigenquantum defects µ were extracted and plotted against energy. These eigenquantum defects or reactance matrices are needed as a function of internuclear separation R to enact a vibrational frame transformation via potential energy curves, which were constructed for both N 2 and N + 2 .

Bulletin of the American Physical Society, 2015

Submitted for the DAMOP16 Meeting of The American Physical Society Near-Threshold, Vibrationally-... more Submitted for the DAMOP16 Meeting of The American Physical Society Near-Threshold, Vibrationally-Resolved Photoionization of Molecular Nitrogen GAETAN VANGYSEGHEM, THOMAS GORCZYCA, Western Michigan University, CONNOR BALLANCE, Queen’s University Belfast — Photoionization of molecular nitrogen (N2) is investigated near the first ionization threshold using an R-matrix, multi-channel quantum defect theory (MQDT) approach. Building on an existing fixed-nuclei R-matrix photoionization model [M. Tashiro, J. Chem. Phys. 132, 134306, (2010)], which, in turn, is built on the UKRmol suite of codes, photoionization cross sections, as well as scattering and dipole matrices, are computed in the Born-Oppenheimer approximation. By varying the internuclear separation, potential energy curves have been constructed for the N2 and N + 2 states and compared to quantum chemistry calculations. Using these fixed-nuclei potential energy curves, and corresponding vibronic eigenenergies and eigenfunctions, a...

Bulletin of the American Physical Society, Jun 10, 2015