The photoinitiated unimolecular decomposition of formaldehyde via the H+HCO radical channel has been examined at energies where the S0 and T1 pathways both participate. The barrierless S0 pathway has a loose transition state (which tightens somewhat with increasing energy), while the T1 pathway involves a barrier and therefore a tight transition state. The product state distributions which derive from the S0 and T1 pathways differ qualitatively, thereby providing a means of discerning the respective S0 and T1 contributions. Energies in excess of the H+HCO threshold have been examined throughout the range 1103⩽E†⩽2654 cm−1 by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S0 dominates at the low end of the energy range. Here, T1 participation is sporadic, presumably due to poor coupling between zeroth-order S1 levels and T1 reactive resonances. These T1 resonances have small decay widths because they lie below the T1 barrier. Alternatively, at the high end of the energy range, the T1 pathway dominates, though a modest S0 contribution is always present. The transition from S0 dominance to T1 dominance occurs over a broad energy range. The most reliable value for the T1 barrier (1920±210 cm−1) is given by the recent ab initio calculations of Yamaguchi et al. It lies near the center of the region where the transition from S0 dominance to T1 dominance takes place. Thus, the present results are consistent with the best theoretical calculations as well as the earlier study of Chuang et al., which bracketed the T1 barrier energy between 1020 and 2100 cm−1 above the H+HCO threshold. The main contribution of the present work is an experimental demonstration of the transition from S0 to T1 dominance, highlighting the sporadic nature of this competition.

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A reaction complex is formed from a van der Waals dimer precursor, HBr⋅I2, and is monitored with picosecond time resolution using standard pump–probe spectroscopy. The reaction is initiated in a slightly attractive region of an excited electronic state with insufficient energy to fragment and will eventually undergo an internal conversion to a lower electronic state via electronic to vibration energy transfer. A resulting product, highly vibrationally excited molecular I2, is monitored by resonance enhanced multiphoton ionization (REMPI) combined with time of flight mass spectroscopy. The HBr constituent of the precursor HBr⋅I2 is photodissociated at 220 nm. The H-atom departs instantaneously, allowing the remaining electronically excited Br(2P1/2) to form a collision complex, (BrI2)*, in a restricted region along the Br+I2 reaction coordinate determined by the precursor geometry. The evolution of this complex is probed in real time by tuning the probe to the REMPI line of the I atom: 298 nm. The resulting transients include I2+ and I+, with lifetimes of 55(±5) and 40(±5) ps, respectively. Similar results are obtained for initiation from DBr⋅I2, with risetimes of 43(±5) and 29(±5) ps measured for the I2+ and I+ transients, respectively. The originally formed (BrI2)* does not have enough internal energy to dissociate directly, but must undergo an internal conversion to a lower electronic state in order to continue to reactants or products. An isotope effect is also detected and explained with a simple kinetics model that is consistent with mechanism described above. Temporal discrepancies in the risetimes of I2+ and I+ imply that either the ground state process is also being observed or that differing vibrational states of the I2 product are formed at differing rates and detected with differing efficiencies.

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