在Para -氫間質的分子過程的研究

Abstract

作為間質材料para-氫(pH2)已經引起許多人的注意，有以下幾個理由。首先，氫氣是最輕的分 子，從而導致大的零點能和隧道效應，這就是為什麼固體氫會被稱為量子固體。其次，pH2 在低溫 條件下J=0，具有球形對稱。這導致了小的分子間相互作用力，晶格常數大以及籠效應小。Momose 等測量CD4 分子在pH2 間質中的振動轉動光譜，發現P(1)峯過渡v4 的模式有一個寬0.015 cm- 1， 其中作為光譜v3 的模式有更廣泛的寬度。 據透露，一些光譜會和pH2 間質具有很大的雜質效應。舉例來說，少量的稀有氣體雜質會經由 pH2 禁止氣相中的誘導紅外線的吸收。Ortho-氫(oH2)的不純也起了重要的作用，這是因為oH2 是自 然存在的雜質，具有非零角動量J=1，從而導致四極矩。由於四極矩弱的長範圍相互作用，oH2 能 夠聚在一起，形成oH2 集群與摻雜客體分子。 雖然pH2 的量子性質提供了客體分子的有趣光譜，然而詳細的理論工作還尚未完成，在大多數 的情況下其實驗結果是與氣相譜做分配比較。我計劃發展一個從頭算理論來解決pH2 在超低溫條件 下的光譜和動力學問題。我會從一個pH2 二聚體及客體分子的量子化學計算開始。它也可能討論 oH2 雜質效應取代客體分子的oH2。在詳細分析這些二聚體，我會使用雙重絕熱近似法去考慮系統 的問題即客體分子被嵌入在一個固體pH2。由於在實驗中的分子動力學利用間質隔離光譜法，pH2 分子光譜具有極為重要的研究動態以及淵博知識。我將使用一個定量的理論去回答為什麼有些光譜 是狹窄的高峯而有些卻不是。

Para-hydrogen has attracted much attention as a matrix material. There are several reasons for this. First, hydrogen is the lightest molecule, which leads to large zero point energy and tunneling effect. This is why solid hydrogen is called quantum solid. Second, pH2 is in J=0 state in a low temperature limit having spherical symmetry. This leads to small intermolecular interactions, large lattice constant, and small cage effects. Momose et al. measured rovibrational spectra of CD4 molecule in pH2 matrix, and found that the peak of P(1) transition of v4 mode has a width of 0.015 cm-1, where as the spectra of v3 mode has broader width. It is revealed that some spectra with pH2 matrix have large impurity effects. For example, small amount of rare gas impurity induces IR absorption by pH2 that is forbidden in the gas phase. ortho-hydrogen (oH2) impurity also plays significant roles. This is because oH2 is naturally existing impurity having non-zero angular momentum J=1, leading to quadrupole moment. Due to the weak long range interaction with the quadrupole moment, oH2 can get together forming an oH2 cluster with the doped guest molecule. Even though quantum nature of pH2 provides interesting spectra of the trapped molecules, detailed theoretical work has not been done, and experimental results are assigned by the comparison with the gas phase spectra in most of the cases. I am planning to develop an ab initio theory to solve spectroscopic and dynamical problems with pH2 in ultra cold conditions. I am going to start from a quantum chemical calculation of a dimer that consists of a pH2 and the guest molecule. It is also possible to discuss the oH2 impurity effect by replacing the guest molecule by oH2. After the detailed analysis of these kinds of dimers, I will consider the system in which the guest molecule is embedded in a solid pH2 by using dual adiabatic approximation. Since the experiments on dynamics of molecules in matrix utilize spectroscopic method, profound knowledge of molecular spectra in pH2 has a crucial importance in studies of dynamics as well. I would like to answer using a quantitative theory why some spectra show extremely narrow peaks whereas others are not.