Controlling the outcomes of chemical reactions has long been a goal of chemistry. It is central to optimising reaction yields and product selectivity. Multi-dimensional spectroscopy has been used to study the photolysis of formaldehyde in a molecular beam. This has allowed the absolute quantum yields of all three possible reactions:
(i) HCHO → H + HCO (barrierless radical dissociation)
(ii) HCHO → H2 + CO (roaming reaction to molecular products)
(iii) HCHO → H2 + CO (reaction via a traditional transition state)
to be measured simultaneously, as a function of the initially excited state. These experiments showed that the initially prepared state can alter the reaction branching ratios by up to a factor of two. For example, in two experiments where the excitation energy is different by only 5 cm-1 (0.05 nm), the branching ratios of the three processes (i):(ii):(iii) changed from 18%:37%:45% to 9%:11%:80%.
Detailed theoretical modelling showed that there are both shape and quantum resonances in this reactive system. When HCHO absorbs a photon just above the H + HCO radical dissociation energy, the departing H-atom can be trapped behind an angular momentum barrier leading to a shape resonance, which couples, for example, via tunnelling, to the H + HCO channel. Alternatively, it can couple with the H2 + CO channel, forming a Feshbach resonance. Specifically, the observed experimental results were found to be due to a multi-dimensional quantum Feshbach resonance; it is this resonance, in an isolated HCHO molecule, that changes the chemical outcome of the reaction by a factor of two.
The features of the reactive potential energy surface that lead to both shape and Feshbach resonances are general and the possible implications of these results will be discussed.