Photoinitiated H2CO unimolecular decomposition: Accessing H+HCO products via S-0 and T-1 pathways

The photoinitiated unimolecular decomposition of formaldehyde via the H + H CO radical channel has been examined at energies where the S-0 and T-1 path ways both participate. The barrierless S-0 pathway has a loose transition s tate (which tightens somewhat with increasing energy), while the T-1 pathwa y involves a barrier and therefore a tight transition state. The product st ate distributions which derive from the S-0 and T-1 pathways differ qualita tively, thereby providing a means of discerning the respective S-0 and T-1 contributions. Energies in excess of the H + HCO threshold have been examin ed throughout the range 1103 less than or equal to E dagger less than or eq ual to 2654 cm(-1) by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S -0 dominates at the low end of the energy range. Here, T-1 participation is sporadic, presumably due to poor coupling between zeroth-order S-1 levels and T-1 reactive resonances. These T-1 resonances have small decay widths b ecause they lie below the T-1 barrier. Alternatively, at the high end of th e energy range, the T-1 pathway dominates, though a modest S-0 contribution is always present. The transition from S-0 dominance to T-1 dominance occu rs over a broad energy range. The most reliable value for the T-1 barrier ( 1920 +/- 210 cm(-1)) is given by the recent ab initio calculations of Yamag uchi It lies near the center of the region where the transition from S-0 do minance to T-1 dominance takes place. Thus, the present results are consist ent with the best theoretical calculations as well as the earlier study of Chuang , which bracketed the T-1 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 S-0 to T-1 dominance, highlighting the sporadic nature of this competition. (C) 2000 American In stitute of Physics. [S0021-9606(00)01306-4].