Photochemistry of trimethyl arsine at 193 nm

Abstract

The photodecomposition of trimethyl arsine results in fragments/products that exhibit numerous UV overlapping absorptions. Nevertheless, quantum yields of CH₃, intramolecular C₂H₆, As(CH₃) and As(CH₃)₂ production can be obtained as 1.25, 0, 0.25 and 0.75, respectively, employing both quantitative spectroscopic detection of CH₃ at 216.4 nm and GC-mass spectral analysis of isotopically labelled ethane end-products. The isotopic experiments employed photolysis of mixtures of As(CH₃)₃ and (CD₃)₂CO, the latter providing the actinometric standard. Two absorptions attributed to As(CH₃)₂ were monitored by kinetic spectroscopy and a schematic mechanism describing the system is proposed. Some distinguishing features of the mechanism include: (1) the quantum yields appear to be independent of pressure, (2) rates of reaction of CH₃ with arsenic-containing fragments are slower than that of CH₃ combination, (3) a metastable absorption attributed to As(CH₃)₂ was observed, the rate constant for its quenching by helium to the ground state was measured.