Departament de Ciència de Materials i Química Física and IQTC, Universitat de Barcelona, Martí i Franquès, 1-11, 08028 Barcelona, Spain.
Phys Chem Chem Phys. 2019 Sep 25;21(37):21007-21021. doi: 10.1039/c9cp00952c.
The rotational energy relaxation (RER) of a molecule X2(j,mj) in a 4He superfluid nanodroplet [HeND or (4He)N; T = 0.37 K] has been investigated using a hybrid quantum dynamics approach recently proposed by us. As far as we know, this is the first theoretical study about rotational relaxation inside HeNDs, and here several (real and hypothetical) isotopes of H2 have been examined, in order to analyze the influence of the rotational constant Be of these fast rotors on the dynamics. The structure of the nanodroplet practically does not change during the RER process, which approximately takes place according to a cascade mechanism j → j - 2; j - 2 → j - 4; …; 2 → 0, and mj is conserved. The results are consistent with the very scarce estimated experimental data available. The lifetime of an excited rotational state (≈1.0-7.6 ns) increases when: (a) Be increases; (b) j increases; and (c) N decreases (above N = 100 there is a small influence of N on the lifetime). This also applies to the global relaxation time and transition time. The analysis of the influence of the coupling between the j and j - 2 rotational states (due to the X2-helium interaction) and the X2 angular velocity on the lifetime and related properties has been helpful to better understand the dynamics. In contrast to the RER results, for the vibrational energy relaxation (VER) in HeNDs, when the quantum number v increases a decrease is observed in the lifetime of the excited vibrational state. This difference can be interpreted taking into account that RER and VER are associated with very different types of motion. Besides, in VER the intermediate excited states show metastability, differing from the RER case. We hope that the present study will encourage more studies to be developed on the RER dynamics in HeNDs, a basic, interesting and difficult to study physical phenomenon about which we still know very little.
最近，我们使用一种混合量子动力学方法研究了 4He 超流纳米液滴[HeND 或 (4He)N；T = 0.37 K]中 X2(j,mj)分子的转动能量弛豫（RER）。据我们所知，这是首次对 HeND 内的转动弛豫进行理论研究，在此我们检验了几种（真实和假设的）H2 同位素，以分析这些快速转子的转动常数 Be 对动力学的影响。在 RER 过程中，纳米液滴的结构几乎不变，该过程大致按照级联机制 j → j - 2；j - 2 → j - 4；...；2 → 0 进行，同时保持 mj 守恒。结果与现有的非常有限的估计实验数据一致。当：(a) Be 增加；(b) j 增加；和(c) N 减少（当 N > 100 时，N 对寿命的影响较小）时，激发的转动态的寿命（≈1.0-7.6 ns）会增加。这也适用于全局弛豫时间和跃迁时间。分析 j 和 j - 2 转动态之间的耦合（由于 X2-氦相互作用）和 X2 角速度对寿命和相关性质的影响，有助于更好地理解动力学。与 RER 结果相反，在 HeND 中的振动能量弛豫（VER）中，当量子数 v 增加时，激发振动态的寿命会减少。这种差异可以通过考虑到 RER 和 VER 与非常不同类型的运动相关联来解释。此外，在 VER 中，中间激发态表现出亚稳态，与 RER 情况不同。我们希望本研究将鼓励更多关于 HeND 中 RER 动力学的研究，这是一种基本、有趣且难以研究的物理现象，我们对此仍知之甚少。
