The Effects of Homogeneous and Heterogeneous Solvation on the Femtosecond Dynamics of Atmosperhically Relevant Clusters

Open Access
Author:
Bianco, Nicholas Joseph
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 02, 2011
Committee Members:
  • Albert Welford Castleman Jr., Dissertation Advisor
  • Albert Welford Castleman Jr., Committee Chair
  • John V Badding, Committee Member
  • Miriam Arak Freedman, Committee Member
  • James Kasting, Committee Member
Keywords:
  • spectroscopy
  • pump-probe
  • decay
  • femtosecond
  • dynamics
  • clusters
  • water
  • formic acid
  • sulfur dioxide
  • time-of-flight
  • ultraviolet
Abstract:
In this thesis, we present a detailed investigation on the relaxation dynamics of the sulfur dioxide cluster system, (SO2)n, n = 1 to 6. We utilized the pump-probe technique to excite to the C (2 1A′) state of the molecule with a pump of 200 nm and a probe of 400 nm. The results from this one photon pump to the C state are compared to previous findings of two-photon pumping to the same state. Specifically, the addition of more SO2 molecules to the cluster does not affect the relaxation dynamics, suggesting that self-solvation does not influence these electronic energy levels of the potential energy surface of SO2 to an appreciable extent. However, the probe photon order demonstrates that clustering frustrates the appearance of molecular fragments, demonstrating a strongly cohesive cluster with high binding energy. This also suggests that the energy of the C state shifts lower upon clustering, thereby requiring additional photons for ionization. Therefore, growth in cluster size does not create an environment that suppresses the electronic energy levels of the potential energy surface of SO2, and as such, we observe the prompt dissociation of SO2. Finally, under very intense laser illumination, clusters are observed to undergo Coulomb explosion, resulting in the appearance of charge states for atoms of the heterogeneously composed cluster requiring nearly identical sequential ionization potentials. In addition to the SO2 cluster studies, formic acid-water cluster studies were performed utilizing 800 nm pump and 400 nm probe. These mixed cluster species demonstrate a mono-exponential decay that increases in time with increasing cluster size via either component. Another experiment performed while studying formic acid-water clusters involves the Water-Gas Shift reaction. Since the mid 1800’s the reverse water gas shift reaction has been tested. It has been known for a long time that adding strong acids, such as hydrochloric, to a beaker of formic acid will catalyze the decarbonylation of formic acid in the bulk phase, thereby producing carbon monoxide. Here we demonstrate that this reaction can occur on a molecular scale. Further, we show that excitation by a UV laser can speed the reaction to occur on a sub-picosecond time scale. Additionally, we have demonstrated quantum control by selectively enhancing the production of H2O and CO upon the addition of HCl, and upon the addition of water clusters we produce H2 and CO2 gas. We find that in the absence of steric hindrance of a bulk liquid (where this reaction is known to occur), the application of ultraviolet light allows the reaction to occur on a femtosecond scale.