Simulations of Molecular Strong-Field Ionization: Two-Color Orientation-Dependent Yields and Second Ionization as a Probe of Attosecond Charge Migration

Author

Denawakage D. N. N. M. Jayasinghe, Louisiana State University and Agricultural and Mechanical CollegeFollow

Degree

Doctor of Philosophy (PhD)

Department

Department of Chemistry

Document Type

Dissertation

Abstract

In this dissertation we present computer simulations of two related processes: molecules undergoing strong-field tunneling ionization (SFI) due to an intense external laser fields, and SFI-induced charge migration (CM), which is the coherent movement of positive electron-hole along a molecule, following ionization. We investigate the angle-dependent ionization of oriented polar molecules and demonstrate SFI as a probing technique of CM using bromoacetylene molecule as a test case via second ionization. In the first part of the dissertation, we present orientation-dependent two-color ionization of NO, OCS, and CH₃Br using real-time time-dependent density functional theory (RT-TDDFT) with auxiliary basis functions, a tuned range-separated hybrid functional, and a complex absorbing potential (CAP). The highest ionization yields are obtained at θ = 50^o for NO and θ = 120^o for both OCS and CH₃Br, where θ is the angle between the permanent molecular dipole moment and the positive field direction. In these molecules, the ionization yield is maximum when the force on the electrons points from O→N, O→S, and C→Br. In NO, the butterfly-shaped peaks clearly resemble the highest occupied molecular orbital (HOMO), whereas OCS and CH₃Br exhibits a more multichannel character to the ionization. These results were also used to validate weak-field asymptotic theory (WFAT) calculations.

In the second part of this dissertation, we use RT-TDDFT with a CAP to simulate how strong-field ionization can be used to probe CM in bromoacetylene (HCCBr). The CM is initiated using a constrained density functional theory (cDFT) sudden approximation for the first ionization, and the second ionization is caused by a single-color (CM frequency matched) laser pulse polarized perpendicular to the CM direction. We filter the initial density matrix to remove spurious high energy excitations caused by cDFT. We observe yield modulations as a function of time-delay between the first and second ionization that correlate with the location of the hole in the molecule. Specifically, ionization increases when the hole is on the triple bond (extra electrons on the bromine atom). These results suggest that SFI may be a viable approach for measuring attosecond CM.

Date

4-2-2025

Recommended Citation

Jayasinghe, Denawakage D. N. N. M., "Simulations of Molecular Strong-Field Ionization: Two-Color Orientation-Dependent Yields and Second Ionization as a Probe of Attosecond Charge Migration" (2025). LSU Doctoral Dissertations. 6743.
https://repository.lsu.edu/gradschool_dissertations/6743

Committee Chair

Lopata, Kenneth