Two-photon resonance REMPI detection of the formyl radical

The resonance enhanced multiple photon ionization (REMPI) spectrum of HCO has been observed for dye laser wavelengths from 373 to 427 nm. Long progressions in the bending vibration have been observed for excitation of two electronic states, tentatively identified as the 3p 2Σ+ and 3p 2Π Rydberg states. Estimates are obtained for the rotational constants, vibrational frequencies, anharmonicity constants, and electronic term values for the two states. Additional strong REMPI signals were obtained in the 418–426 nm region, tentatively assigned to the 3s 2Σ+ Rydberg state. A single pulse detection limit for HCO of 1013 cm−3 was measured which suggests the feasibility of the REMPI detection of HCO at the part-per-million level in atmospheric pressure hydrocarbon flames.