UC Irvine

Chapter 1 outlines the focus of this thesis, understanding the mechanism of breaking a

chemical bond following absorption of light.

In Chapter 2 the design, construction and calibration of a new velocity-map direct current

slice ion imaging (VMI) time-of-flight mass spectrometer is described. Wavelength tunable pulsed lasers are used to selectively pump (dissociate) a target molecule and probe (ionize) the fragments. Combing the techniques allows correlated photofragment quantum state distributions to be explored.

Chapter 3 investigates the near-UV photodissociation dynamics of CH2I2 using ion imaging

over a range of excitation wavelengths. Ground state I(2P3/2) and spin-orbit excited I*(2P1/2) atoms were probed using 2+1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Analysis of the ion images shows that, regardless of iodine spin-orbit state, ~20% of the available energy is partitioned into translation ET indicating that the CH2I co-fragment is formed highly internally excited. A refined C–I bond dissociation energy of D0 = 2.155±0.008 eV is determined.

In Chapter 4 the photoproducts of OCS after UV excitation have been followed with photofragment excitation spectroscopy (PHOFEX), using REMPI to state-selectively monitor S(1D) and S(3P2,1,0) products while the pump wavelength was scanned. Probing the major S(1D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast spectra obtained probing S(3P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C–S stretch, following direct excitation to quasi-bound singlet and triplet states. The results confirm a recent theoretical prediction that direct excitation to the 23A” state can occur in OCS.

In Chapter 5 ion imaging measurements of CH3 fragments from photolysis of CH3CHO reveal multiple pathways to the same set of products. By systematically exploring product formation over a timescale of picoseconds to nanoseconds, and wavelengths between 265-328 nm, an evolving picture of the dynamics is found. Evidence to suggest that the three-body CH3+CO+H pathway remains closed at all wavelengths is presented.