Doctor of Philosophy (PhD)
Strong-field ionization and the resulting electronic dynamics are important for a range of processes such as high harmonic generation, photodamage, charge resonance enhanced ionization, and ionization-triggered charge migration. Modeling ionization dynamics in molecular systems from first-principles can be challenging due the large spatial extent of the wavefunction which stresses the accuracy of basis sets, and the intense fields which require non-perturbative time-dependent electronic structure methods. In this dissertation, we develop a time dependent density functional theory approach which uses a Gaussian-type orbital (GTO) basis set to capture strong-field ionization rates and dynamics in atoms and small molecules. This involves propagating the electronic density matrix in time with a time-dependent laser potential and a spatial non-Hermitian complex absorbing potential (CAP) which is projected onto an atom-centered basis set to remove ionized charge from the simulation. For the density functional theory (DFT) functionals we use are tuned range-separated LC-PBE* and LC-PBE0*, which have the correct asymptotic 1/r form of the potential and a reduced delocalization error compared to traditional DFT functionals. Ionization rates are computed for hydrogen, molecular nitrogen, and iodoacetylene under various field frequencies, intensities, and polarizations (angle-dependent ionization), and the results shown to quantitatively agree with time-dependent Schrodinger equation and strong-field approximation calculations. This tuned DFT with GTOs method opens the door to predictive all-electron time-dependent density functional theory (TDDFT) simulations of ionization and ionization-triggered dynamics in molecular systems using tuned range-separated hybrid functionals.
Moreover, the ionization probability of carbonyl sulfide (OCS) and methyl halides (CH3X, X = F,Cl,Br) molecules in intense linearly polarized 800-nm laser pulses as a function of the angle between the molecular axis and the laser polarization are calculated. We compute these same angular-dependent strong-field ionization yields for this family of molecules using the same approach described above. For OCS, single and double ionization yields are calculated. For the single-ionization case, experimental measurements agree well with TDDFT calculations and with previous experiments. For double ionization, the agreement between experiment and theory is less compelling, reflecting the substantial challenges of computing double-ionization yields using TDDFT methods.
Sissay, Adonay, "Simulations of Strong-Field Ionization in Molecules Using Tuned Range-Separated Time-Dependent Density Functional Theory" (2019). LSU Doctoral Dissertations. 4998.
Available for download on Sunday, June 21, 2026