| Popis: |
Velocity map ion imaging was used to probe the H2 + CO channel in HCHO photolysis. The dependence of the roaming to TS branching ratio to the angular momentum (J, Ka, Kc) of the parent HCHO was measured. For Ka = 0, roaming is heavily favoured for odd J < 5. For J > 5, the ratio is constant. For Ka = 1, low J demonstrated a uniformly higher ratio of roaming to TS photolysis, before also becoming constant for J > 5. For Ka ≥ 2, these preferences are no longer present. It is concluded that photolysis of HCHO at 298 K in the atmosphere is capable of roaming, and that the roaming to TS ratio would be the ≈ 20% photolysis branching ratio established in the literature. Continuous-wave cavity ring-down spectroscopy (cw-CRDS) was employed to investigate photolysis of HCHO at 298 K in the presence of O2 in search of HO2 radicals formed from the reaction of O2 with hot-H2 formed from roaming. HO2 production was observed following HCHO photolysis via the 2143 state, as expected, from radical photolysis. HO2 was also observed from the 2161 vibronic band, in violation of the established thermochemistry of HCHO. The pressure dependence of HO2 formation via 2161 was also different to 2143, pointing to a different reaction mechanism. The reaction of highly-excited H2 from roaming photolysis of HCHO with O2 was postulated as the new mechanism. Further cw-CRDS experiments were performed for photolysis of HCHO via the lower-lying S1 2141 and 41 levels. These levels lie below the roaming threshold, but anomalous HO2 production was still observed. This required the postulation of a new reaction mechanism beyond that of radical or roaming photolysis. The new mechanism invokes the abstraction of a hydrogen atom from a photoexcited, vibrationally-hot HCHO molecule by an O2 molecule prior to photolysis. Spectroscopic and pressure dependent measurements were made to establish that this process occurs throughout the S1 state of HCHO. The process was named atmospheric photothermal oxidation (APTO). Fourier-transform infrared spectroscopy was used to establish the viability of APTO under atmospheric pressure conditions. Pressure, wavelength and O2/N2 mixing ratio dependent measurements of the quantum yield of HCHO APTO were made. The quantum yield of APTO was found to increase with O2 partial pressure, establishing it as an atmospherically relevant mechanism. Microcanonical master equation modelling was used to determine a rate constant for the abstraction step of APTO. |
