Mixture Of Para Research Articles

C++H2( j)→CH++H: The effect of reagent rotation on the integral cross section in the threshold region

Integral cross sections for the reaction of C+ with hydrogen have been determined as a function of translational and rotational energy. Measurements were made with a guided beam apparatus, using a temperature variable scattering cell or alternatively a nozzle beam and different ortho–para mixtures of H2. The detailed results are in good agreement with statistical phase space calculations provided that the nuclear spins are assumed to be completely decoupled (frozen spin approximation). One consequence of fundamental interest is that H2( j=1) is less reactive than H2( j=0) in the threshold region. Further, the theory predicts an oscillatory structure for the kinetic energy dependence of the reaction cross section which has been observed for the first time. The calculated cross sections are used to predict reliable rate coefficients for H2 in selected rotational ( j=0–7) states. An analytical approximation of these results allows a simple calculation of rate coefficients for nonequilibrium rotational populations as can be found, e.g., in interstellar space.

Observations on the use of a thermal conductivity cell to measure the para hydrogen concentration in a mixture of para and ortho hydrogen gas

This paper describes the use of a thermal conductivity cell to measure the para hydrogen concentration in a mixture of para and ortho hydrogen gas. The device was calibrated over its entire range and the output compared with the theoretically expected form. The output of this device had been previously assumed to be linear, this assumption can lead to systematic errors of up to 5.6% in the determination of the para hydrogen concentration. A simple calibration procedure is described.

Heat capacities of solid ortho—para mixtures of deuterium in the ordered state

The heat capacities of three ortho—para mixtures of solid deuterium in the rotationally ordered state as a function of temperature as well as the order—disorder transition temperature, T c( x) for several compositions have been measured. The rotational heat capacities for all three mixtures, x (para or J = 1 mole fraction) = 0.69 9, 0.81 9 and 0.92 1 per mole of J = 1 species can be represented by a single function in terms of the reduced temperatures, T c( x)/ T, which to a good approximation is given by ▪, where Γ 0 eff(1)/ k = 1.0 deg is the effective electric quadrupolar coupling constant for the anisotropic interactions in pure para D 2 and T c(1) = 4.05 degrees the order—disorder transition temperature. It is shown that this correlation follows from the assumption that both the libron energies and band widths of the libron modes scale with composition in a manner identical to the transition temperatures, namely ▪.

Nuclear Spin—Lattice Relaxation in Ortho—Para Mixtures of Solid and Liquid Deuterium

A comprehensive study of the nuclear spin—lattice relaxation in the hexagonal close packed phase of solid deuterium and the liquid is presented. All of the measurements covering the temperature range 2.5–20.3°K and for para concentration 2%–94% were carried out using a pulsed technique, the pulse sequence being saturation −τ—90° at 8.0 MHz. It is shown that all of the experimental data can be accounted for on the basis of a kinetic model which assumes three principal relaxation mechanisms for the transfer of spin energy to the lattice from the two spin systems I=1, J=1 (para-D2) and I=2, J=0 (ortho-D2). These are (1) the modulation of the intramolecular spin—spin interactions by the anisotropic electric quadrupolar intermolecular interactions which represents the most important process for the direct relaxation of para-D2, (2) the intermolecular spin—spin interaction modulated by the self-diffusion in the solid or liquid which represents the only direct process for relaxation of the ortho-D2, and (3) the adiabatic cross relaxation between the two spin systems, the indirect process for the relaxation of the ortho-D2 dependent on the extent of coupling between the two different spins. This varies from the extreme of strong coupling in the rigid solid, where the self-diffusion is negligible, to weak coupling in the high-temperature solid and liquid where thermal motion is significant. A quantitative explanation of the experimental results for the hexagonal close packed solid is given in terms of existing theories for the mechanisms mentioned above. For the rigid solid regime (up to approximately 14°K), where the relaxation is determined entirely by the first mechanism, this requires that an effective electric quadrupole coupling constant in the solid of from 0.55 to 0.62Γ 0 be the coupling constant for the isolated pair. In the diffusion dominated regime, 14°K to the triple point all three mechanisms play an important role. Excellent agreement between theory and experiment is obtained for the same Γ eff, a self-diffusion coefficient D0=9.05× 10−2cm2/sec and an activation energy Ea=667 cal/mole, except for solids of very low para concentration. For the latter D0=2.0× 10−2cm2/sec and Ea=608 cal/mole. The theory predicts the observed frequency dependence of the spin—lattice relaxation in the diffusion-dominated regime. In the liquid state a spin—lattice relaxation time can be separately identified for each spin system. Whereas the para-D2 (I=1, J=1) results can be accounted for on a model which assumes mechanism (1) modified by the thermal motion in the liquid, no simple explanation is possible for the ortho-D2 data.

Título en español.

Resumen en español.