UV absorption cross sections between 230 and 350 nm and pressure dependence of the photolysis quantum yield at 308 nm of CF3CH2CHO

Abstract

Ultraviolet (UV) absorption cross sections of CF₃CH₂CHO were determined between 230 and 350 nm by gas-phase UV spectroscopy. The forbidden n → π* transition was characterized as a function of temperature (269–323 K). In addition, the photochemical degradation of CF₃CH₂CHO was investigated at 308 nm. The possible photolysis channels are: CF₃CH₂ + HCO(R1a), CF₃CH₃ + CO(R1b), and CF₃CH₂CO + H (R1c). Photolysis quantum yields of CF₃CH₂CHO at 308 nm, Φ_λ=308nm, were measured as a function of pressure (25–760 Torr of synthetic air). The pressure dependence of Φ_λ=308nm can be expressed as the following Stern–Volmer equation: 1/Φ_λ=308nm = (4.65 ± 0.56) + (1.51 ± 0.04) × 10⁻¹⁸ [M] ([M] in molecule cm⁻³). Using the absorption cross sections and the photolysis quantum yields reported here, the photolysis rate coefficient of this fluorinated aldehyde throughout the troposphere was estimated. This calculation shows that tropospheric photolysis of CF₃CH₂CHO is competitive with the removal initiated by OH radicals at low altitudes, but it can be the major degradation route at higher altitudes. Photodegradation products (CO, HC(O)OH, CF₃CHO, CF₃CH₂OH, and F₂CO) were identified and also quantified by Fourier transform infrared spectroscopy. CF₃CH₂C(O)OH was identified as an end-product as a result of the chemistry involving CF₃CH₂CO radicals formed in the OH + CF₃CH₂CHO reaction. In the presence of an OH-scavenger (cyclohexane), CF₃CH₂C(O)OH was not detected, indicating that channel(R1c) is negligible. Based on a proposed mechanism, our results provide strong evidences of the significant participation of the radical-forming channel(R1a).