UV-Absorption Spectra of the Radical Transients Generated from the 193-nm Photolysis of Allene, Propyne and 2-Butyne
Askar Fahr, Allan H. Laufer
The 193 nm photochemistry of allene (H2C=C=CH2), propyne (H3C-C CH) and 2-butyne (H3C-C C-CH3) have been examined and the UV spectral region between 220 nm and 350 nm has been surveyed for detection of transient species generated from the photolysis of these molecules. An emphasis of the study has been on the identification of the isomeric nature of the C3H3 radical, that is propargyl (H2CCCH) or propynyl (H3CCC), which can be produced from the 193 nm photolysis of these molecules. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples were employed for identification of the final photodissociation products. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of transient absorption features centered at about 243 nm and 330 nm regions. The observed transient absorption spectra, following the 193 nm photolysis of allene, consist of a strong absorption band around 240 nm and a weaker band in the 330 nm region with a relative intensity of about 8/1. These features originate primarily from propargyl radicals and the observed spectrum is in good agreement with the known spectrum of H2CCCH radical in the 240 nm region.Propyne and 2-butyne are structurally similar. However, it is very likely that the methyl (CH3) and propynyl (CH3C C) radicals are the primary photodissociation products of 2-butyne. This was confirmed by the GC/MS product analysis of photolyzed 2-butyne/He mixtures, indicating the formation of C2H6 (formed from the combination of CH3 radicals), and a number of C6H6 and C4H6 isomers formed from self- and cross- reactions of C3H3 and CH3 radicals. The spectra obtained from the 193 nm photolysis of both HCCCH3 and CH3CCCH3 molecules were nearly identical consisting of two relatively broad bands centered at about 240 nm and 320 nm regions with a relative intensity of about 2/1, respectively. The origin of these absorption features are primarily from propynyl (H3CCC) radicals. Similarity of the spectra of transient specie(s) following the 193 nm photolysis of propyne and 2-butyne also suggest that propynyl radicals are the primary photodissociation product of propyne.