Knowledge Bank

The electronic absorption spectra of HCO and DCO free radicals have been obtained by the flash photolysis of $CH3CHO$ and $CH3CDO$, using absorption path lengths up to 20 meters. Two band systems have been found. The first lies in the region $7400A-5000A$ and consists of a regular progression of five or six bands which show simple P, Q, R structures under high resolving power. Analysis reveals that the bands arise from a nonlinear lower state to a linear upper state. Rotational constants for these states have been evaluated and bond lengths and angles calculated. Further diffuse bands of this system were found at low dispersion indicating predissociation of the linear upper state. The second band system has been found only for DCO and consists of a moderately strong band near 7600A and a weaker band near 7200A. The band structure under high resolving power is quite complex and analysis reveals that the band is due to a perpendicular type transition between two nonlinear states. The rotational constants of these states, however, are different from those of the states giving rise to the first band system. A search for the corresponding band system of HCO at both high and low dispersion has not yet proved successful. A calculated band structure, however, shows a close similarity to some unidentified diffuse bands near 7500A found in the spectrum of the planet Uranus. The nature of the four observed electronic states will be discussed.