Photoionisation of rotationally cooled Hydrogen beyond the Born-Oppenheimer approximation.

氢分子是宇宙中最简单、最丰富的中性分子。由于其相对简单，它是实验和理论研究的自然起点。通过将其用于燃料电池以环保地生产能源，它也获得了技术上的重要性。涉及分子氢的一个基本过程是紫外光的光电离，其中一个光子从分子中射出一个电子。然而，尽管分子很简单，但观察到光致电离光谱并不像光子以直接方式电离分子所预期的那样简单和平滑。相反，它被丰富的尖锐结构所主导，这是因为它更有可能在两个步骤的过程中发生光致电离。在这里，氢的高位中性态首先被光子的吸收所激发。然后，这种状态随着光电子的发射而衰变为分子离子H2+，这一过程被称为自电离。除了占主导地位的过程外，自动电离也很重要，因为它涉及电子运动与分子的旋转和振动运动之间的相互作用，而这在直接光电离中是不会发生的。通过观察抛射光电子的能量和产额，可以研究这两步过程中涉及的激发和衰变路径。此外，通过测量发射的光电子的角行为，可以确定光电子和分子离子之间交换的角动量。在实践中，这幅图变得更加复杂，因为在室温下，氢分子的不止一个转动能级被填充。因此，光致电离光谱变得更加复杂，更重要的是，这使得实验与理论的联系变得更加困难。以前的实验一般都是对H2基态的几个转动能级的求和。缺乏循环选择的实验数据仍然是这一基本系统理论发展的障碍。我们已经开发了实验技术，将99%以上的目标H2分子置于单个转动能级中。我们已经建造了一台电子能谱仪，使我们能够识别最终离子状态的各个旋转能级，并测量光电子的角行为。我们还可以使用具有必要光谱分辨率的紫外光光源来分离中间中性态的各个转动能级。综合起来，可以全面研究分子中定义明确的旋转态：初始目标态、中间态和最终离子态的光致电离。最后一种能力是，我们可以用同位素D2取代H2。这略微改变了分子能级，并可以揭示原本被隐藏的跃迁，而核间分离的变化改变了光致电离过程的动力学。因此，拟议的实验将提供这一基本分子系统中的光致电离的完整和清晰的图景。

Molecular hydrogen is the simplest and most abundant neutral molecule in the Universe. Due to its relative simplicity it is the natural starting point for both experimental and theoretical studies. It is also gaining technological importance through its use in fuel cells for the environmentally-friendly production of energy. A fundamental process involving molecular hydrogen is photoionisation by ultra violet light where a photon ejects an electron from the molecule. However, despite the simplicity of the molecule it is observed that the photoionisation spectrum is not simple and smooth, as would be expected if the photon ionised the molecule in a direct manner. Instead it is dominated by a wealth of sharp structure which arises because it is much more likely for photoionisation to occur in a two-step process. Here a high-lying, neutral state of H2 is first excited by absorption of a photon. This state then decays to the molecular ion H2+ with the emission of a photoelectron in a process called autoionisation. Apart from being the dominant process, autoionisation is important because it involves interactions between electronic motion and the molecular motions of rotation and vibration that do not occur in direct photoionisation. The excitation and decay paths involved in this two-step process can be investigated by observing the energies and yields of the ejected photoelectrons. Furthermore, by measuring the angular behaviour of the emitted photoelectron it is possible to determine the angular momentum exchanged between the photoelectron and the molecular ion. In practice this picture becomes more complicated because more than one rotational level of the hydrogen molecule is populated at room temperature. The photoionisation spectra are then considerably more complex, and more importantly, it makes it much more difficult to connect experiment with theory. Previous experiments have generally been a sum over several rotational levels of the ground vibrational state of H2. This lack of rotationally-selected experimental data remains a road block to the theoretical development of this fundamental system. We have developed experimental techniques to put more than 99% of the target H2 molecules into a single rotational level. We have built an electron spectrometer that allows us to identify individual rotational levels of the final ionic state and measure the angular behaviour of the photoelectrons. We also have access to an ultra violet light source with the necessary spectral resolution to isolate individual rotational levels of the intermediate neutral states. Brought together this enables a comprehensive study of photoionisation in a molecule for well defined rotational states in: the initial target state, the intermediate state and the final ion state. A final capability is that we can substitute H2 by the isotope D2. This slightly shifts the molecular energy levels and can reveal transitions that are otherwise hidden while the change in inter-nuclear separation modifies the dynamics of the photoionisation process. The proposed experiments would thus provide a complete and clear picture of photoionisation in this fundamental molecular system.