The hydrogen bond of water from the perspective of soft X-ray spectroscopy

Its importance for life and its unusual properties keep water within the focus of ongoing research; this focus especially applies to water in the liquid phase. Scientists agree that the hydrogen-bond network, which is formed by interactions between the water molecules, is key for understanding the anomalies of water. However, a better understanding of the structure of this network, as well as its dynamics, must yet be established. Soft X-ray spectroscopy allows the investigation of the local electronic structure of water by probing the occupied and unoccupied valence molecular orbitals. In this Focus Review, we present soft-X-ray-based techniques, their development in terms of liquid spectroscopy, and recent studies on the hydrogen-bond network of liquid water.