Self-reaction of hydroxyl radicals, OH + OH → H(2)O + O (1a) and OH + OH → H(2)O(2) (1b), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-834 K temperature and 1-100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O((1)D), produced in photolysis of N(2)O at 193 nm, with H(2)O. The temporal behavior of OH radicals was monitored via transient absorption of light from a dc discharge in H(2)O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study combined with the literature data indicate that the rate constant of reaction 1a, associated with the pressure independent component, decreases with temperature within the temperature range 298-414 K and increases above 555 K. The pressure dependent rate constant for (1b) was parametrized using the Troe expression as k(1b,inf) = (2.4 ± 0.6) × 10(-11)(T/300)(-0.5) cm(3) molecule(-1) s(-1), k(1b,0) = [He] (9.0 ± 2.2) × 10(-31)(T/300)(-3.5±0.5) cm(3) molecule(-1) s(-1), F(c) = 0.37.

Unimolecular dissociation of formyl radical, HCO → H + CO (1), was studied using pulsed laser photolysis coupled to transient UV−vis absorption spectroscopy. One-pass UV absorption, multipass UV absorption, and cavity ring down spectroscopy in the red spectral region were used to monitor temporal profiles of the HCO radical. A heatable high-pressure flow reactor of a new design was employed. Reaction 1 was studied over a buffer gas (He) pressure range 0.8−100 bar and a temperature range 498−769 K. Formyl radicals were prepared by pulsed photolysis of acetaldehyde and propionaldehyde (308 nm, XeCl excimer laser, 320 nm, doubled dye laser). In addition to formyl radicals monitored at 230 and 613.8 nm, methyl radicals were monitored via absorption at 216.5 nm. The initial concentrations of free radicals were varied between 7 × 1010 and 8 × 1013 molecules cm-3. The obtained second-order rate constant at 1 bar is k 1(He) = (0.8 ± 0.4) × 10-10 exp(−66.0 ± 3.4 kJ mol-1/RT) cm3 molecule-1 s-1 (498−769 K). The low-pressure data of this study were combined with those from a high-temperature shock tube study and the low-temperature data on the reverse reaction to yield k 1(He) = (0.60 ± 0.14) × 10-10 exp(−64.2 ± 1.4 kJ mol-1/RT) cm3 molecule-1 s-1 over an extended temperature range, 298−1229 K. The dissociation rate constants measured in this work are lower than previously reported by a factor of 2.2 at the highest temperature of our measurements and a factor of 3.5 at the low end. Our experimental data indicate a pressure dependence of the second-order rate constant for the dissociation of formyl radical (1), which is attributed to pressure falloff expected from the theory of isolated resonances.

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N(2)O at 193.3 nm, with H(2)O. Temporal profiles of CH(3) were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H(2)O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, k(1) = (1.20 ± 0.20) × 10(-10)(T/300)(-0.49) cm(3) molecule(-1) s(-1).

The design and operational characteristics of the Novosibirsk free electron laser facility are described. Selected experiments in the terahertz range carried out recently at the user stations are surveyed in brief.