UC Irvine

Velocity map ion imaging (VMI) is a powerful experimental technique used to investigate the photodissociation dynamics of molecules following the absorption of light. This technique provides details on photofragment speed and angular distributions, giving insight into complex photochemical dynamics. This thesis will focus on three distinct cases: Chapter 2) UV dissociation of a neutral molecule, CHI2Cl, Chapter 3) UV dissociation of an ion, CH3CHO+, and Chapter 4) IR vibrational overtone predissociation of a van der Waals complex, (HCl)2.

The near-UV photochemistry of CHI2Cl has been explored over a range of wavelengths. Results show that photolysis of CHI2Cl forms CHICl + I with the majority of available energy being partitioned into the CHICl internal degrees of freedom. Complementary high-level MRCI calculations, including spin-orbit coupling, were performed to characterize the electronically excited states, confirming that repulsive states of A″ symmetry with highly mixed singlet/triplet character are responsible for the CHI2Cl absorption spectrum and dynamics. The reaction between the photoproduct CHICl with O2 was also investigated using broadband transient absorption spectroscopy, with results demonstrating the production of chlorinated formaldehyde oxide, CHClOO, also known as a chlorinated Criegee intermediate.

To produce CH3CHO+, single-photon vacuum ultraviolet (VUV) light was used to ionize the neutral molecule. Four major photofragments were observed following the UV dissociation of CH3CHO+: CH3+, CH4+, HCO+, CH3CO+. Photofragment yield spectroscopy was used to determine branching fractions, with HCO+ found to be the dominant fragment. All four photofragments are formed with distinct dynamics. The most surprising result is perhaps formation of fast-moving CH4+ with a distinctly anisotropic angular distribution that is indicative of prompt dissociation. CH3+ appears to originate from secondary fragmentation of CH3CO+ (triple fragmentation) at longer wavelengths, but is attenuated at higher energy as isomers of CH3CHO+ become energetically accessible.

Vibrational predissociation of (HCl)2 following excitation of both the free (ν1) and bound (ν2) HCl stretches via the first overtone have been investigated using velocity map imaging and IR action spectroscopy. Predissociation products have been identified in high-J levels of both ν=2 and ν=1. Ion imaging measurements of the HCl predissociation product reveal a weaker HCl–HCl bond dissociation energy than previously suggested. Correlated product pair distributions indicate the dynamics are dependent on the vibrational mode excited. Excitation of the 2ν1 mode results in HCl fragments formed with a wider range of J-levels populated compared to the more tightly localized rotational distribution observed following 2ν2 excitation.