The energy dependence of CO(v,J) produced from H2CO via the transition state, roaming, and triple fragmentation channels

Abstract

The dynamics of CO production from photolysis of HCO have been explored over a 8000 cm energy range (345 nm-266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S←S absorption band, covering almost 8000 cm of excitation energy. The S radical and TS molecular channels are the most important over this energy range. The absolute quantum yield of roaming is fairly constant ∼5% at all energies. The T radical channel is important (20%-40%) between 1500 and 4000 cm above the H + HCO threshold, but becomes unimportant at higher energy. Triple fragmentation increases rapidly above its threshold reaching a maximum of 5% of the total product yield at the highest energy.

Zero-point energy conservation in classical trajectory simulations: Application to H2CO

“…Because three rather than two products are formed, triple fragmentation is entropically favoured and will dominate, at the expense of trajectories forming H 2 + CO, as photolysis energy increases. This is seen both experimentally 83,84 and in Table III. Indeed, triple fragmentation was the dominant outcome for the V + ZPE trajectories at all but the lowest photolysis energy.…”

“…This, and previous near-threshold observations of triple fragmentation, was attributed to tunneling rather than dissociation of "hot" H 2 molecules formed via roaming. 83,84 The number of classical trajectories forming triple fragmentation products, for each simulation type, is shown in Table III. Here, at the lower photolysis energies, approximately 20% of V trajectory simulations form H 2 products with sufficient internal energy to dissociate.…”

“…Here, at the lower photolysis energies, approximately 20% of V trajectory simulations form H 2 products with sufficient internal energy to dissociate. It is therefore possible that some of the near-threshold triple fragmentation observed experimentally 83,84 may have been from barrierless dissociation of "hot" H 2 in the 1000 cm 1 window below the triple fragmentation barrier. Because three rather than two products are formed, triple fragmentation is entropically favoured and will dominate, at the expense of trajectories forming H 2 + CO, as photolysis energy increases.…”

“…Indeed roaming reactions were first identified in the photodissociation of H 2 CO, and this system has been well studied both experimentally and theoretically. [3][4][5][6]46,[71][72][73][74][75][76][77][78][79][80][81][82][83][84] Roaming in H 2 CO can be considered intra-molecular abstraction of the formyl H atom within the van der Waals region of the H + HCO radical dissociation channel. Because radical dissociation is barrierless, the PES in this region of configuration space is relatively flat and the potential energy is close to the radical dissociation threshold.…”

Hamiltonian flow over saddles for exploring molecular phase space structures

“…of the product molecules. For example, particle imaging techniques [5] and especially the velocity map imaging (VMI) version, combined with advanced spectroscopic methods, such as multidimensional resonance-enhanced multiphoton ionization [6] and fast Fourier transform infrared spectroscopy [7], have revealed unprecedented details of chemical reactions at the microscopic level.…”