High resolution exploration of f-wave emission in ro-vibrational autoionisation in H2

氢分子是宇宙中最简单、含量最多的中性分子。由于其相对简单，它是实验和理论研究的自然起点。它还通过在燃料电池中用于环境友好型能源生产而获得技术重要性。涉及氢分子的一个基本过程是紫外光的光电离，光子从分子中射出一个电子。然而，尽管分子很简单，但它的光电离光谱并不简单和平滑，如果光子以直接的方式电离分子，就会预料到这一点。相反，它被丰富的尖锐结构所支配，这是因为它更有可能在两步过程中发生光电离。在这里，高海拔的中性H2首先被光子的吸收激发。然后，这个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。除了作为主导过程外，自电离也很重要，因为它还涉及电子运动与分子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。这两步过程的激发和衰减路径可以通过观察发射光电子的能量和产率来研究。此外，通过测量发射的光电子的角行为，可以研究光电子和分子离子之间交换的角动量。实际上，这幅图变得更加复杂，因为在室温下，氢分子的旋转能级不止一个。光电离光谱变得相当复杂，更重要的是，它使实验与理论联系起来变得更加困难。以前的实验通常是H2的基态振动的几个旋转水平的总和。这种旋转选择实验数据的缺乏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来消除这个障碍，我们把99%以上的目标H2分子放在一个单一的旋转水平上。我们设计并制造了一个电子分光仪，使我们能够识别最终离子状态的单个旋转水平，并测量光电子的角行为。我们还获得了紫外线光源（位于意大利Elettra的GasPhase光束线）的许可，该光源具有必要的光谱分辨率，可以分离出中间中性状态的单个旋转水平。将这些结合在一起，可以对分子中的光电离进行全面的研究，以确定(1)初始目标态，(2)中间态和(3)最终离子态的旋转状态。最后一个能力是我们可以用D2同位素代替H2。这稍微改变了分子的能级，并且可以揭示当核间分离的变化改变了光电离过程的动力学时隐藏的跃迁。因此，所提出的实验将为这个基本分子系统中的光电离提供一个完整而清晰的图像。

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 its 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 also involves interactions between electronic motion and the molecular motions of rotation and vibration, which 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 investigate 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 remove this road block where we put more than 99% of the target H2 molecules into a single rotational level. We have designed and built an electron spectrometer that allows us to identify individual rotational levels of the final ionic state and to measure the angular behaviour of the photoelectrons. We have also been granted access to an ultra violet light source (the GasPhase beamline at Elettra, Italy) 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 (1) the initial target state, (2) the intermediate state and (3) 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.