ULTRAFAST DYNAMICS OF ELECTRONICALLY EXCITED MOLECULES AND MATERIALS PROBED WITH NONLINEAR SPECTROSCOPY AND PHOTOIONIZATION

Many of the natural processes of molecules and atoms in chemical dynamics are on timescales in the femtosecond to attosecond range. Currently, many research goals focus on first observing and understanding these dynamics, then controlling them. A promising method for studying these different processes is nonlinear dynamics, specifically using ul?trafast, femtosecond lasers. The timescales of these lasers unlock the natural timescales of these dynamics enabling new discoveries. Nonlinear optical studies can provide information on dipole moments, population, and coherence in a molecular or atomic system, and explore how these properties evolve as a function of time. In this work an all optical method for studying these dynamics will be demonstrated in gas and solid systems. After an introduc?tion to ultrafast laser pulses in Chapter 1, the all optical studies of the nonlinear response of gaseous carbon dioxide will be discussed in Chapter 2. Expanding on this idea, Chapter 3 will present the work done with the same optical studies on solid state magnesium oxide crystals. We show in this chapter how the nonlinear dynamics may contain information about the band structure dynamics under intense laser light. In addition to the studies of the electronic dynamics of atoms and molecules using optical measurements, one can use the high intensity, ultrashort laser to photoionize molecules and atoms and study the ionized electrons or ions. Ionization has provided a large amount of information in atomic an molecular systems that provide complimentary information to the all-optical studies. Time-resolved photoelectron spectroscopy (TRPES) provides access to non-adiabatic dynamics that are not accessible otherwise. A simple experiment may measure the total yield of the ions or electrons born from ionization. If this yield is measured in a pump-probe scheme, this measurement is difficult since one cannot differentiate the different ions and electrons that come from different pathways of the molecule or atom. Using a velocity map imaging spectrometer one can ionize molecules and map the electron’s or ion’s initial momentum and energy to a detector and reconstruct the 3D distribution in the interaction region. Used in pump-probe schemes, a laser pulse can initialize the wavepacket to an excited state which can travel along the potential energy surfaces. Another laser pulse can then interact with the system and ionize the molecule and information about the potential energy surfaces can be obtained. Chapter 4 of this dissertation describes the construction of a velocity map imaging spectrometer and the initial data collected. The following is a list of manuscripts written while pursing this degree

Francis Walz, Siddhant Pandey, Liang Z. Tan, and Niranjan Shivaram, Electric field measurement of femtosecond time-resolved four-wave mixing signals in molecules, Opt. Express 30, 36065-36072 (2022) • Siddhant Pandey, Liang Z. Tan, Francis Walz, Varun Makhija, and Niranjan Shiv?aram, Ultrafast temporal phase-resolved nonlinear optical spectroscopy in the molec?ular frame, Optica 11, 776-781 (2024)

Siddhant Pandey, Francis Walz, Niranjan Shivaram Femtosecond Temporal Phase?Resolved Nonlinear Optical Spectroscopy in Molecules with Lock-in Enabled Phase Tracking, arXiv:2503.05986

Francis Walz, Shashank Kumar, Siddhant Pandey, Yuyan Zhong, Liang Tan, and Niranjan Shivaram (2024) Strong-field Driven Subcycle Band Modulation Measured with Electric Field Observables in Four-wave Mixing. In Preparation