<i>T</i> 1 barrier height, <i>S</i>1–<i>T</i>1 intersystem crossing rate, and <i>S</i>0 radical dissociation threshold for H2CO, D2CO, and HDCO

Abstract

Vacuum UV laser-induced fluorescence has been used to detect the atomic products in the photofragmentation of H2CO, D2CO, and HDCO. The dissociation is shown to occur on both the ground (S0) and the first triplet (T1) potential surfaces. The T1 exit barrier height (2.9–6.0 kcal/mol) and the S1→T1 intersystem crossing rate near the T1 barrier top (4×107 s−1) have been determined experimentally. The threshold energy for the radical dissociation channel on the S0 surface, 86.57±0.16 kcal/mol, has also been measured accurately by rovibronically resolved photolysis. The best values for the heat of formation (9.99±0.19 kcal/mol) and the C–H bond enthalpy (15.69±0.19 kcal/mol) of HCO radical have been derived from the measured threshold energies as well.