Book Description

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 CH 2I2 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.