Large ions in premixed benzene-oxygen flames

Abstract

The large, non-aliphatic ions in benzene-oxygen flames can be subdivided into three groups: polycyclic aromatic hydrocarbon ions (PAH ions), oxygenated PAH ions (oxo-PAH ions) and polyhedral carbon ions. They occur both positively and negatively charged. Oxo-PAH ions of either sign and negative PAH ions are not formed in acetylene flames. The oxo-PAH− appear first and occupy a mass range up to 500 u, those with an even number of C atoms dominating the mass distribution. They show a weak growth but mainly decompose to PAH−. The even-numbered oxo-PAH− give odd-numbered PAH− which do not grow any further. Positive oxo-PAH ions peak later, occupying a mass range up to about 450 u. Among these the even-numbered species prevail. They decompose to PAH+ which continue to grow up to more than 103 u. The positive even-numbered oxo-PAH ions can be considered as protonated phenol and its PAH analoga, while the odd-numbered oxo-PAH+ are regarded as oxo-phenalenylium and its higher benzo-condensed analoga. The oxo-PAH− are taken as phenolate plus higher analoga and as phenalenate plus analogous species, respectively. Negative polyhedral ions (C50−, C60−) have recently been detected in non-sooting benzene flames. An additional source for small negative polyhedral ions other than small soot particles is discussed. It is concluded that negative polyhedral ions are formed from neutral species and gain their charge later. No such conclusion can be drawn for the positive polyhedral ions as long as the respective neutral molecules are unknown.