Knowledge Bank

Two methods of exciting the spectra of free radicals are described. The first is, essentially, a photochemical method. A vapour is irradiated with short-wavelength (Schumann region) radiation. In this frequency range one quantum is sufficiently energetic to dissociate the molecule into two radicals or atoms, one of which is electronically excited. The spectrum of the radical or atom is then observed as a fluorescence. The second method consists of a high-frequency (electrodeless) discharge in which the conditions of power input, pressure, and rate of flow have been adjusted to give mild excitation such that the spectra of secondary (diatomic) decomposition products are not predominant. The Ethylene Flame Bands, attributed by their discoverer to the radical HCO, have been observed in fluorescence in formaldehyde, and a few of the stronger members of the system have been excited in a discharge in formaldehyde. These observations support the original assignment which has recently been criticized. A band system lying in the blue and near ultraviolet can be observed in fluorescence in formic acid. This was originally attributed to the HCO radical, but the present work has shown that it is more probably due to the $HCO2$ radical. The band system can also be excited in the discharge in formic acid with much higher intensity. A new band system excited in the discharge in fast flowing acetylene is briefly described. The emitter is not known; but it is, fairly certainly, polyatomic.