Investigation of unknown quantum states by using a wideband passive spectroscopy at terahertz frequency region

我们应用了实验室太赫兹频段的被动光谱技术，对这种难以获得精确能级结构的分子进行了研究。我们选择了甲酸甲酯作为研究对象。众所周知，该分子具有低能扭转模式，并具有复杂的光谱特征。光谱分配正在进行中，观察到了大量未分配的谱线。在给定的温度和压力下，我们测量了分子的选定基态谱线的绝对光谱强度。我们用这个结果估算了分子在我们观察到的温度下的“有效配分函数”，并发现这个值与由高水平从头计算得出的能量结构是一致的。目前的信息将有助于我们通过天文观测获得星际空间中丰富的甲酸甲酯。

We have applied a laboratory passive spectroscopy at terahertz frequency region to the molecule whose precise energy level structure is hard to obtain. We chose methyl formate for the present study. The molecule is known to have low energy torsional modes and to give complex spectral features. The spectral assignment is underway and plenty of unassigned spectral lines are observed. We measured absolute spectral intensity of the selected lines of the molecule in the ground states at given temperature and pressure. We estimated an "effective partition function" of the molecule at our observed temperature with this result, and found that the value is consistent with the calculated energy structure derived by a high-level ab initio calculation. The present information will help us to obtain an abundance of the methyl formate in interstellar space by astronomical observation.