氦纳米液滴中氖团簇的同位素富集
Isotope enrichment in neon clusters grown in helium nanodroplets
摘要：
氖簇离子Ne $_s^+$生长在预电离，质荷比选择氦纳米滴（HND）揭示了强烈的富集的重同位素$^{22}$ Ne，这取决于簇的大小s和实验条件。对于小尺寸的富集比以前报道的裸氖簇生长在喷嘴扩张，随后电离。这种浓缩可以追溯到氖原子在碰撞池中的大量蒸发，该碰撞池用于从HND中剥离氦。推导出了团簇离子中$^{22}$ Ne的富集与气相中相应的耗尽因子$F$之间的关系。因此发现的值为$F$是在良好的协议与理论表达式，有关同位素分馏两相平衡的原子气体的德拜温度。此外，从F计算出的两种同位素之间的零点能量的差异与在谐波近似下包括核量子效应的氖团簇离子的理论研究相当吻合。另一个拟合参数提供了对观测到的Ne $_s^+$的前体的尺寸s $_i$的估计。该值与通过模拟Ne $_s^+$的生长估计的尺寸以及从其他实验数据推导的下限和上限令人满意地一致。另一方面，在中性HND中生长的氖簇随后通过电子轰击电离，根本没有统计学上显著的同位素富集。这一发现表明，嵌入在HND的集群的电离诱导解离的程度是相当小的比裸露的集群。
英文摘要：
Neon cluster ions Ne$_s^+$ grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope $^{22}$Ne that depends on cluster size s and the experimental conditions. For small sizes the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of $^{22}$Ne in the cluster ion and its corresponding depletion factor $F$ in the vapor phase. The value thus found for $F$ is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size s$_i$ of the precursor of the observed Ne$_s^+$. The value is in satisfactory agreement with the size estimated by modeling the growth of Ne$_s^+$, and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than for bare clusters.
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