Abstract

Several radicals of importance in combustion chemistry are not easily detected by the well-developed optical methods of laser-induced fluorescence (LIF), Raman scattering techniques (including CARS), and absorption or emission spectroscopy. Recently resonance-enhanced multiple photon ionization (REMPI) has been successfully used for the detection of several of these species including H, O, C, CH₃, C₂O, and CH₂. We report here the first REMPI detection of the HCO radical via the two-photon resonant excitation of low-lying Rydberg states followed by nonresonant photoionization. In our experiments the XeCl excimer laser was used for the photolysis of acetaldehyde (CH₃CHO) at 308 mm. A second laser, an excimer-pumped tunable dye laser, was used for REMPI ionization of HCO radicals at various time delays following the photolysis pulse. REMPI spectra attributable to HCO were observed for wavelengths in the 380-400-nm range. The ionization signal was found to vary as the second power of the laser intensity (n = 1.97 ± 0.07). The analogous REMPI (blue-shifted) spectrum of DCO was also recorded following photolysis of CD₃CDO at 308 nm. Confirmation of the identity of the REMPI absorber was obtained when the identical DCO spectrum was observed following photolysis of CH₃CDO. A partial vibrational and rotational analysis of the observed spectra suggests that the observed REMPI corresponds to resonant excitation of the 3p-Rydberg state(s) predicted by ab initio calculations.

© 1985 Optical Society of America