Research in this laboratory involves generating elusive Criegee intermediates in a pulsed supersonic expansion and utilizing various spectroscopic techniques to study their photo-induced chemistry and dynamics. UV-vis excitation of Criegee intermediates induces a strong π*←π transition that is associated with the carbonyl oxide group. Absorption is followed by rich excited state dynamics involving multiple coupled excited state potentials and yields both O ³P and O ¹D products.

Schematic representation of the VMI apparatus

Velocity map imaging (VMI) is a powerful technique for studying the dynamics of molecular photodissociation processes. This laboratory extends the VMI technique to study the photodissociation dynamics of Criegee intermediates. Following UV-vis excitation, the O-atom products are ionized by 2 +1 resonance enhanced multi-photon ionization and velocity mapped to a spatially sensitive detector. The resultant 2-D VMI images are mathematically reconstructed into 3-D Newton spheres, which reveal the product velocity and angular distributions. Conservation of momentum allows for analysis of the polyatomic co-product’s internal energy.

A sample image of O ¹D generated following UV excitation of CH₂OO, and the corresponding total kinetic energy release distribution (TKER).

This technique has been applied to various alkyl-substituted Criegee intermediates and has allowed us to show absorption of Criegee intermediates result in prompt dissociation across the whole UV-vis region and significantly internally excited products are produced. CH₂OO