Following a Photochemical Reaction of Br2 using Photoion and Photoelectron Momentum Spectroscopy

Abstract

When a chemical bond is breaking, the valence electrons within the bond undergo rearrangement, causing a corresponding rearrangement and separation of the atoms within a molecule. This process couples the motion of valence electrons and atoms, making it essential to concurrently visualize both in order to comprehend chemical reactions. However, obtaining complete information about these two different dynamics using a single observable presents a challenge. Over the past two decades, attosecond science has emerged as a means to generate multiple observables from a molecule by subjecting it to an intense femtosecond laser pulse, effectively disassembling it into electrons and ions. Additionally, the momentum distributions of photoions and photoelectrons contain intricate structures that offer valuable insights into the dynamic behavior of ions and electrons. This thesis explores the techniques for observing the atomic and valence electronic dynamics in the photochemical reaction of Br2 by examining the momentum distributions of ions and electrons. We first look at the electron momentum distribution and its subcycle interference structures. Here, we introduce a method for disentangling subcycle interference structure from non-adjacent quarter cycles by using orthogonal two-color laser Fields. Then we introduce a semiclassical model for simulating both atomic and molecular photoelectron spectra.. For atomic ionization, various of interference structures are comprehensively studied. In particular, we investigate the origin of fan-like interference structure by employing a time-variable soft-core Coulomb potential. For molecular ionization, our focus is on the photoionization of neutral Br2. We simulate photoelectron momentum distributions by taking into account various dynamic factors, such as molecular orbital structures, the ionization potential, and the internuclear distance. Furthermore, we conduct a comparative analysis of the electron momentum distributions between molecular Br2 and atomic Br, aiming to elucidate the underlying reasons for their discrepancies. At last, we will present our Br2 pump-probe experiment, which encompasses the experimental techniques and results. We employed a cold target recoil ion momentum spectroscopy (COLTRIMS) apparatus to record the momentum spectra of ions and electrons from the ionization of dissociating neutral Br2 molecules. When observing ions, we investigate the molecular orbital evolution during single ionization and the atomic separation during Coulomb explosion. In the case of electron observation, we analyze the electron momentum distributions stemming from single ionization and compare them with our simulation results. Our combined observations of ions and electrons provide a complete view of the photochemical reaction of Br2. We identify distinct transition stages, involving both molecular orbital evolution and structural transformation.