PHOTOIONIZATION EXPERIMENTS IN A SUPERSONIC MOLECULAR BEAM
LIVERMAN, MARK GREGORY
Doctor of Philosophy
The uses of resonance enhanced two-photon ionization (R2PI) for studying unimolecular energy redistribution of molecules in a supersonic molecular beam is explored. The R2PI process relies on initially exciting a molecule to a state that is stable with respect to the up-pumping rate of an ionizing laser field. For such situations, the ion signal produced is proportional to the population of the intermediate state. It can therefore be used in place of the observation of emitted light for measuring the population of an excited state. This is particularly useful for states that do not radiate. The pulsed valve used to create the molecular beam is described in detail. Several experiments, undertaken to characterize the types of information that might be obtained, are described. From these it was determined that the R2PI technique has a detection efficiency of almost 100%. In addition, at laser fluences less than those required to saturate the first, resonant step, only the parent ion is observed. The technique has been shown to be useful for recording electronic absorption spectra with 1 cm('-1) precision, both for molecules with long singlet lifetimes (longer than the 4 nsec laser pulse width) and for molecules with very short singlet lifetimes (up to three orders of magnitude shorter than the laser pulse width). The ionization threshold for a large polyatomic molecule, naphthalene, has been determined to a precision greater than previously reported. A two-laser experiment is also described in which the decay of unperturbed singlet and unrelaxed triplet states are observed. This experiment can easily be extended to provide sufficient information, with the addition of a single independently determined quantity, to completely characterize the intramolecular energy redistribution in molecules.