Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph.D.)--Bowling Green State University, 2009. / Document formatted into pages; contains xvii, 186 p. : ill. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 274-296). Available also in a digital version from Dissertation Abstracts.

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The threshold potentials were determined for the excitation energy necessary for low energy electrons to induce chemical reactions of carbon monoxide, and of nitric oxide adsorbed on a gold surface. The reactions were studied as a function of temperature (100 °C to 200 °C) and pressure (1.83 x 10-5 to 6.40 x 10-4 torr). The electron source was a thorium oxide coated iridium filament which was heated by a current between 1A and 3A to keep thermal distribution of the electrons to less than 0.4 ev. The reaction surface was a polycrystalline evaporated film prepared by subliming gold onto a stainless steel mesh support. Mass analysis was done by quadrupole mass spectrometry in a flow system. The general results of the research is that a technique has been developed to study the products of reactions of excited state atoms or molecules on metal surfaces. The electron-impact excitation method is an alternative to photochemical and other methods.

Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge separation mechanism should occur at all donor-acceptor interfaces. My results show that entropy facilitates charge separation and indicate that the density of acceptor states should be a design consideration when constructing organic solar cells.

Electric field studies, electrochromism, were used to obtain excited-state data for analogous divalent sulfur compounds. The sulfides investigated were dimethyl sulfide and small cyclic sulfides including the three to six member ring compounds. The excited-state dipole moments and polarizabilities are reported for the first s, p, and d Rydberg absorption bands which occur in the near vacuum ultraviolet region from 230 to 170 nm. The excited-state data are interpreted in terms of the particular excited-state (s, p, or d) for the molecules and the bending differences due to the presence of the ring and the number of atoms in the ring. The next section describes the use of electrochromism to investigate the pressure effect of argon, carbon tetrafluoride and sulfur hexafluoride on the spectra for molecular Rydberg states.

A series of picosecond excite-probe experiments was performed on various concentrations of aqueous and ethanolic solutions of rhodamine B in order to determine the existence of dimerization in those solutions. The goals of the research presented in this dissertation were twofold. Initially, various techniques of time-resolved spectroscopy were to be employed to further characterize the ground and excited-state molecular properties of the aqueous RB dimer. The information obtained, and the techniques developed in that study would then be utilized in an effort to secure evidence which would support or refute the claims of rhodamine B dimerization in an ethanolic solution.

N-methyl-6-hydroxyquinolinium (NM6HQ) is a powerful excited-state proton donor, exhibiting a huge pKa drop from 7.2 in the ground state to -7 in the excited state. The zwitterionic nature of the proton transfer product encourages intramolecular electron transfer away from the hydroxyl moiety to the distal ring, allowing for a large pKa jump in the excited state. This process is reversible, making the NM6HQ salts powerful transient superacids. We have investigated the excited-state proton transfer (ESPT) from NM6HQ salts to various basic solvents (alcohols, DMSO). A model has been developed that adequately describes the ion-dipole interactions in the ESPT geminate-recombination process. Our studies have shown that the counterion plays a large role in the ESPT. Likewise, initiation of cationic polymerization is controlled by the counterion. NM6HQ perfluoroalkylsulfonates appear to be the first molecules reported which are capable of initiating aliphatic epoxide polymerization at room temperature through a proton transfer mechanism.