Martin Fournier

Reaction Rate and Isomer-Specific Product Branching Ratios of C2H + C4H8: 1-Butene, cis-2-Butene, trans-2-Butene, and Isobutene at 79 K

The reactions of C2H radicals with C4H8 isomers 1-butene, cis-2-butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-reaction rate constants are obtained by measuring the formation rate of the reaction product species as a function of the reactant density under pseudo-first-order conditions. The rate constants are (1.9 ± 0.5) × 10(-10), (1.7 ± 0.5) × 10(-10), (2.1 ± 0.7) × 10(-10), and (1.8 ± 0.9) × 10(-10) cm(3) s(-1) for the reaction of C2H with 1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene compare well to values measured previously at 103 K using C2H chemiluminescence. Photoionization spectra of the reaction products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross sections, these fits provide isomer-resolved product branching fractions. The reaction between C2H and 1-butene yields (65 ± 10)% C4H4 in the form of vinylacetylene and (35 ± 10)% C5H6 in the form of 4-penten-1-yne. The cis-2-butene and trans-2-butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the isobutene reaction yields (26 ± 15)% 3-penten-1-yne, (35 ± 15)% 2-methyl-1-buten-3-yne, and (39 ± 15)% 4-methyl-3-penten-1-yne. The branching fractions reported for the C2H and butene reactions indicate that these reactions preferentially proceed via CH3 or C2H3 elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the formation of cyclic species.

Nuclear Spin Symmetry Conservation in 1H216O Investigated by Direct Absorption FTIR Spectroscopy of Water Vapor Cooled Down in Supersonic Expansion

We report the results of an experimental study related to the relaxation of the nuclear spin isomers of the water molecule in a supersonic expansion. Rovibrational lines of both ortho and para spin isomers were recorded in the spectral range of H2O stretching vibrations at around 3700 cm-1 using FTIR direct absorption. Water vapor seeded in argon, helium, or oxygen or in a mixture of oxygen and argon was expanded into vacuum through a slit nozzle. The water vapor partial pressure in the mixture varied over a wide range from 1.5 to 102.7 hPa, corresponding to a water molar fraction varying between 0.2 and 6.5%. Depending on expansion conditions, the effect of water vapor clustering was clearly seen in some of our measured spectra. The Boltzmann plot of the line intensities allowed the H2O rotational temperatures in the isentropic core and in the lateral shear layer probed zones of the planar expansion to be determined. The study of the OPR, i.e., the ratio of the ortho to para absorption line intensities as a function of Trot, did not reveal any signs of the OPR being relaxed to the sample temperature. In contrast, the OPR was always conserved according to the stagnation reservoir equilibrium temperature. The conservation of the OPR was found irrespective of whether water molecule clustering was pronounced or not. Also, no effect of the paramagnetic oxygen admixture enhancing OPR relaxation was observed.

Diversification and agrarian change under environmental constraints in rural China: Evidence from a poor township of Beijing municipality

This article illustrates the impact of changes related to market reforms and environmental policies on the economic structure in rural China by providing a comparative analysis of several villages in a poor township in Beijing municipality. Two main concomitant phenomena are affecting agricultural and non-agricultural choices in the studied area. First, the introduction of market mechanisms is encouraging local population to engage in new activities that are closer to local comparative advantages. Second, rural households are facing new constraints in the form of environmental protection measures, which have weakened traditional insurance channels provided by forest resources and cattle stock. Drawing on household-level survey data and interviews with village heads conducted in ten villages of Labagoumen township in December 2003, this article analyzes households decisions in response to market reforms and environmental constraints. We find large disparities both between villages and households in the diversification process and discuss the reasons of observed inertia in the region, most households still heavily relying on corn production.