Coherent dynamics via polarized parent/product photofragmentation spectroscopy: Experiment and theory.

摘要

The objective in quantum-state to quantum-state studies of photon initiated unimolecular reactions is to unveil the details of the potential energy surface and/or surfaces (PESs) that govern the molecular fragmentation process. The primary focus of this work involves the theoretical and experimental study of light induced unimolecular reactions of rotationally state-selected, polarized molecules. We demonstrate that state to state experiments can determine quantum mechanical photodissociation amplitudes (e.g., the phase and magnitude of wavefunctions in chemical reactions) within the context of a model independent theoretical framework. These photodissociation amplitudes are the elements of the transition, or Tˆ-matrix that connect the initial state of the parent molecule to the final states of the product molecules/atoms. They are of utmost importance since they are the most fundamental molecular parameters for the characterization of a photochemical reaction.; In addition to developing the angular momentum machinery that is needed to extract Tˆ-matrix elements from state-selected photodissociation experiments, we have also performed extensive experimental studies of nitrosyl chloride (CINO), an atmospherically relevant molecule. By using our formalism and experimentally probing this system we show that the Tˆ-matrix elements can be found directly from angle-integrated cross-sections and/or the orientation/alignment moments. In these studies we measured the product NO (X2Π, ν″ = 0, N) angle resolved and angle integrated angular momentum distributions produced from the photodissociation of CINO in the 285–355 nm region. The NO photofragments were probed with (1 + 1) REMPI coupled with time-of-flight (TOF) mass spectroscopy. The measured TOF spectra were analyzed using our theoretical framework to extract the Tˆ-matrix elements that govern the photodissociation of CINO in this wavelength region. This experimental work provided the first examples where this goal was achieved as a function of photolysis wavelength.; Ultimately, we show that once these amplitudes are known as a function of photolysis wavelength, they can be used to determine structural and dynamical information about dissociative excited PESs which govern the general process of bond formation. We further show that information about coherent processes that occur during the reaction can be measured. We demonstrate for the first time how our method allows for the experimental ability to extract sub-femtosecond time domain information from these amplitude measurements.