Ultrafast electronic relaxation pathways of the molecular photoswitch quadricyclane.

The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.

Manipulating hydrogen atoms using permanent magnets: Characterisation of a velocity-filtering guide.

A Halbach array composed of 12 permanent magnets in a hexapole configuration is employed to deflect hydrogen atoms as they exit a Zeeman decelerator. The ability to preferentially manipulate H atoms is very useful, as there are currently very few techniques that are appropriate for purifying a beam of H atoms from precursor molecules (such as molecular hydrogen or ammonia), seed gases, and other contaminant species. The extent to which hydrogen atoms are deflected by a single Halbach array when it is tilted or shifted off the main beam axis is characterised experimentally and interpreted with the aid of a simple mathematical model. A radical beam filter is subsequently introduced, where four Halbach arrays arranged in series serve to deflect H atoms away from the main beam axis and around skimming blades; all other components of the incoming beam are blocked by the blades and are thus not transmitted through the magnetic guide. The properties of the guide, as established by experimental measurements and complemented by detailed simulations, confirm that it is a highly effective beam filter-successfully generating a pure and velocity-selected beam of H atoms.