Inst. Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
Phys Chem Chem Phys. 2011 May 28;13(20):9655-66. doi: 10.1039/c1cp20426b. Epub 2011 Apr 14.
Isotope exchange in low pressure cold plasmas of H(2)/D(2) mixtures has been investigated by means of mass spectrometric measurements of neutrals and ions, and kinetic model calculations. The measurements, which include also electron temperatures and densities, were performed in a stainless steel hollow cathode reactor for three discharge pressures: 1, 2 and 8 Pa, and for mixture compositions ranging from 100% H(2) to 100% D(2). The data are analyzed in the light of the model calculations, which are in good global agreement with the experiments. Isotope selective effects are found both in the surface recombination and in the gas-phase ionic chemistry. The dissociation of the fuel gas molecules is followed by wall recycling, which regenerates H(2) and D(2) and produces HD. Atomic recombination at the wall is found to proceed through an Eley-Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. The best fit probabilities for Eley-Rideal abstraction with H and D are: γ(ER H) = 1.5 × 10(-3), γ(ER D) = 2.0 × 10(-3). Concerning ions, at 1 Pa the diatomic species H(2)(+), D(2)(+) and HD(+), formed directly by electron impact, prevail in the distributions, and at 8 Pa, the triatomic ions H(3)(+), H(2)D(+), HD(2)(+) and D(3)(+), produced primarily in reactions of diatomic ions with molecules, dominate the plasma composition. In this higher pressure regime, the formation of the mixed ions H(2)D(+) and HD(2)(+) is favoured in comparison with that of H(3)(+) and D(3)(+), as expected on statistical grounds. The model results predict a very small preference, undetectable within the precision of the measurements, for the generation of triatomic ions with a higher degree of deuteration, which is probably a residual influence at room temperature of the marked zero point energy effects (ZPE), relevant for deuterium fractionation in interstellar space. In contrast, ZPE effects are found to be decisive for the observed distribution of monoatomic ions H(+) and D(+), even at room temperature. The final H(+)/D(+) ratio is determined to a great extent by proton (and deuteron) exchange, which favours the enhancement of H(+) and the concomitant decrease of D(+).
利用质谱测量中性粒子和离子以及动力学模型计算，研究了 H(2)/D(2)混合物在低压冷等离子体中的同位素交换。这些测量包括电子温度和密度，是在一个不锈钢空心阴极反应器中进行的，放电压力分别为 1、2 和 8 Pa，混合物组成范围从 100%H(2)到 100%D(2)。数据根据模型计算进行分析，该计算与实验结果具有很好的整体一致性。在表面复合和气相离子化学中都发现了同位素选择效应。燃料气体分子的离解伴随着壁再循环，这会再生 H(2)和 D(2)并产生 HD。在壁上的原子复合被发现是通过 Eley-Rideal 机制进行的，其中吸附原子与气相 D 原子的反应具有优先性。Eley-Rideal 抽取与 H 和 D 的最佳拟合概率为：γ(ER H)=1.5×10(-3)，γ(ER D)=2.0×10(-3)。关于离子，在 1 Pa 时，由电子碰撞直接形成的双原子物种 H(2)(+)、D(2)(+)和 HD(+)在分布中占主导地位，而在 8 Pa 时，主要通过双原子离子与分子反应产生的三原子离子 H(3)(+)、H(2)D(+)、HD(2)(+)和 D(3)(+)占主导地位。在这种较高的压力下，与 H(3)(+)和 D(3)(+)相比，混合离子 H(2)D(+)和 HD(2)(+)的形成更有利，这是基于统计原因的预期。模型结果预测，在测量精度范围内，对于生成具有更高氘化程度的三原子离子的偏好非常小，这可能是室温下星际空间中氘分馏相关的显著零点能效应(ZPE)的残余影响。相比之下，ZPE 效应对于观察到的单原子离子 H(+)和 D(+)的分布具有决定性作用，即使在室温下也是如此。最终的 H(+)/D(+)比值在很大程度上取决于质子(和氘核)交换，这有利于 H(+)的增强和 D(+)的相应减少。
