Reaction Kinetics at Very Low Temperatures Measured Using a Pulsed Laval Nozzle System

A pulsed Laval nozzle apparatus for the study of the 'kinetics of reactions at low temperatures has been designed, built and characterised. The System makes use of a convergent-divergent shaped Laval nozzle, which results in cooling of the gas flow, do~ , to 70 K. The experimental setup includes a, main chamber in which two pulsed solenoid valves deliver gas to a pre-expansion reservoir from where the gas flows through the Laval nozzle and out into the low pressure regime in the main chamber. For kinetic studies, a pulsed laser photolysis ~ laser induced fluorescence method was used to measure rate coefficients of reactions ofOH at 86 ± 7 K and 69 ± 6 K, with tertiary butyl hydroperoxide (t-BuOOH) used as an OH precursor when required. The Laval nozzles were characterised using two methods; impact pressure measurements and rotational spectroscopy, the temperature measurements from both of which agreed within error. Measurements of rate coefficients for collisional quencQing ofNO (A2L+, v'= 0) by O2 and NO and OH (A2L+, v' = 0) by O2 and N2, were found to agree well with predictions from previpus experimental data and a model containing contributions from the multipole attractive forces model and Harpoon mechanism. Measurements of rate coeffic~ents for reactions of OH with ethene (k1), propyne (k2), acetylene (k3) and benzene (k4) were also carried out. The rate coefficients for these reactions' were found to be k1 = (2.12 ± 0.12) x 10-11 and (3.22 ± 0.46) x 10-11 cm3 molecule-1 set, at 86 K and 69 K, respectively, k2 = (3.11 ± 0.09) x 10-12, (5.02 ± 0.11) x 10-12 and (5.08 ± 0.65) x 10-12 cm3 molecule-1S-l, at 298 K, 86 K and 69 K, respectively, and the upper limit for k3 was assigned to be < 2.4 x 10-12 cm3 molecule-1S-l at 69 K, 14 was unable to be measured due to interference from the benzene sensitized decomposition of the t-BuOOH OH precursor. Reactions of the acetyl radical with O2 (ks) and OH with acetone (k<,) and methyl ethyl ketone, MEK (k7), were also studied, with ks = (5.87 ± 0.39) x 10,12 and (9.21 ± 1.24) x 10-12 cm3 molecule-1 S'l at 86 K and 69 K, respectively. Measurements of the rate coefficients of k<, and k7 were carried out using two different OH sources; reaction of the acetyl radical with O2 and photolysis of t-BuOOH. Curved bimolecular plots, observed mainly at 69 K, indicated formation of acetone and MEK dimers as well as clusters with t-BuOOH. Lower limits of rate coefficients, when acetyl + O2 was used as the OH source, were measured as k<, = (5.67.± 0.32) x 10-11 and (3.29 ±0.59) x 10-11 cm3 molecule-l s-t, k7 = (8.01 ± 1.17) x 10-11 and (5.59 ±0.83) x 10-11 cm3 molecule-l S-l at 86 K and 69 K, respectively, these showed a large incre~se in the rate coefficient from the reported room temperature values and both rate coefficients agreed well with those measUred with t-BuOOH as the OH precursor. However, calculations to determine the effect and extent of clustering in the system indicate that the actual value of the rate coefficient for k<, at 69 K could be up to 85% highe.~ than the value reported here.

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