The distribution of rotational and vibrational energy in HCO produced by the O(P-3)+C2H4 reaction has been measured using laser-induced fluorescence detection via the (B) over tilde (2)A'-(B) over tilde (2)A' transition. Over a detection wavelength range of 248-290 nm, our experiments have shown that HCO is formed in both the ground state and in at least six vibrationally excited states with up to two quanta of energy in the C-O stretch and the bending mode. Dispersed fluorescence experiments were conducted to positively assign all of the HCO vibrational bands. The experiments confirmed that many bands, including the (B) over tilde (000)-(X) over tilde (000) band, are affected by overlap with other HCO bands. Spectral modeling was used to separate the contributions of overlapping HCO (B) over tilde-(X) over tilde bands and to determine a nascent HCO rotational temperature of similar to600 K, corresponding to similar to6% of the total energy from the O(P-3)+C2H4 reaction. HCO vibrational distributions were determined for two different average collision energies and were fit with vibrational temperatures of 1850+/-80 K and 2000+/-100 K, corresponding to similar to15% of the total energy. The observed Boltzmann distribution of vibrational energy in HCO indicates that HCO and CH3 are formed by the dissociation of an energized intermediate complex. (C) 2004 American Institute of Physics.