Max Planck Institut für Dynamik und Selbstorganisation, Göttingen, Germany.
J Chem Phys. 2010 Feb 14;132(6):064501. doi: 10.1063/1.3274509.
Up to 16 parahydrogen and orthodeuterium molecules have been assembled around an OCS carbonyl sulfide chromophore molecule inside the pure (4)He and mixed (4)He(3)He droplets at temperatures of 0.38 and 0.15 K, respectively. The infrared spectra of the resulting complexes exhibit a sequence of rotationally resolved vibrational nu(3) bands in the vicinity of 2060 cm(-1), which are sufficiently separated to assign them to clusters with specific numbers of attached molecules for n=1-16. The present article contains the first complete analysis of the spectra for n=2-8 and a full documentation of the results for n=8-15 briefly described in a short report [Europhys. Lett. 83, 66008 (2008)]. Distinct rotational Q-branches are observed for all OCS-(o-D(2))(n) clusters at the He droplet temperatures of 0.38 K and 0.15 K, indicating that the (o-D(2))(n) shell rotates nearly freely about the molecular OCS axis. In the case of OCS-(p-H(2))(n) at 0.38 K, the Q-branch is seen for most n, with the exception of n=5, 6 and n=12. At 0.15 K, the Q-branch has disappeared for all n>or=11, indicating that the axial rotations are no longer active. Previously, the absence of a Q-branch for n=5 and 6 was explained by the high group symmetry of the bosonic p-H(2) rigid (donut) rings around the OCS molecule. This model, however, fails in explaining the disappearance of the Q-branch for n>or=11. In essential agreement with recent path-integral Monte Carlo calculations, the observed phenomenon is attributed to the onset of superfluidity in the multiring p-H(2) shell and the related permutations of bosonic p-H(2) molecules. A floppy shell model, which accounts for the effect of tunneling and exchange of molecules within the clusters, is able to explain the postulated superfluid behavior of the p-H(2) shell at low temperatures. Within this model the activation of states of low axial symmetry is responsible for the appearance of the Q-branch at higher temperatures.
多达 16 个仲氢和氘分子已在纯 (4)He 和混合 (4)He(3)He 液滴中围绕 OCS 羰基硫发色团分子组装，温度分别为 0.38 和 0.15 K。所得复合物的红外光谱在 2060 cm(-1) 附近表现出一系列旋转分辨的振动 ν(3) 带，它们的间隔足够大，可以将其分配给具有特定附着分子数的 n=1-16 的簇。本文包含了对 n=2-8 的光谱的首次完整分析，并完整记录了在之前的一篇短文中简要描述的 n=8-15 的结果[Europhys. Lett. 83, 66008 (2008)]。在 0.38 K 和 0.15 K 的 He 液滴温度下，所有 OCS-(o-D(2))(n) 簇都观察到了明显的旋转 Q 分支，表明 (o-D(2))(n) 壳在 OCS 分子轴周围几乎自由旋转。在 0.38 K 下的 OCS-(p-H(2))(n) 情况下，除了 n=5、6 和 n=12 之外，大多数 n 都可以看到 Q 分支。在 0.15 K 下，对于所有 n≥11，Q 分支都消失了，这表明轴向旋转不再活跃。之前，n=5 和 6 时没有 Q 分支的情况被解释为 OCS 分子周围的 bosonic p-H(2)刚性（甜甜圈）环的高群对称性。然而，该模型在解释 n≥11 时 Q 分支消失的现象时失败了。与最近的路径积分蒙特卡罗计算基本一致，观察到的现象归因于多环 p-H(2)壳中的超流性的出现以及 bosonic p-H(2)分子的相关排列。能够解释低温度下 p-H(2)壳超流行为的软壳模型，考虑了分子在团簇内的隧道和交换效应。在这个模型中，低轴向对称性状态的激活负责在较高温度下出现 Q 分支。
