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We report the observation of the radiative decay of singly charged noble gas ground-state ions embedded in heterogeneous van der Waals clusters. Electron-photon coincidence spectroscopy and dispersed photon spectroscopy are applied to identify the radiative charge transfer from Kr atoms to a Ne_{2}^{+} dimer, which forms after single valence photoionization of Ne atoms at the surface of a NeKr cluster. This mechanism might be a fundamental decay process of ionized systems in an environment.

Energy and charge transfer processes play an important role in many fundamental reactions in chemistry, biochemistry, and even technology. If an entity that is part of a larger system is photoexcited, its energy will dissipate, for example, by rearrangement of electron density in a large molecule or by photon emission (fluorescence). Here, we report the experimental observation of free electrons from a heterogeneous van der Waals cluster, in which some sites act as electron emitters receiving their energy efficiently from other "antenna" sites that are resonantly excited in the UV range. By complementing electron spectroscopy with fluorescence detection, we can directly observe that electron emission via this mechanism completely quenches fluorescence once the channel opens. We suggest this mechanism to be important for both quenching of fluorescence as well as resonantly enhancing free electron production in a variety of systems.

The authors are sorry to report that the content between Figure 3 and the title 2.4. [...]., Competing Interests: The authors declare no conflicts of interest.

The detection of a single photon is the most sensitive method for sensing of photon emission. A common technique for single photon detection uses microchannel plate arrays combined with photocathodes and position sensitive anodes. Here, we report on the combination of such detectors with grating diffraction spectrometers, constituting a low-noise wavelength resolving photon spectroscopy apparatus with versatile applicability. We recapitulate the operation principle of such detectors and present the details of the experimental set-up, which we use to investigate fundamental mechanisms in atomic and molecular systems after excitation with tuneable synchrotron radiation. Extensions for time and polarization resolved measurements are described and examples of recent applications in current research are given.

Despite its importance, the structure and dynamics of liquid water are still poorly understood in many apsects. Here, we report on the observation of optical fluorescence upon soft X-ray irradiation of liquid water. Detection of spectrally resolved fluorescence was achieved by a combination of the liquid microjet technique and fluorescence spectroscopy. We observe a genuine liquid-phase fluorescence manifested by a broad emission band in the 170-340 nm (4-7 eV) photon wavelength range. In addition, another narrower emission near 300 nm can be assigned to the fluorescence of OH (A state) in the gas phase, the emitting species being formed by Auger electrons escaping from liquid water. We argue that the newly observed broad-band emission of liquid water is relevant in search of extraterrestrial life, and we also envision the observed electron-ejection mechanism to find application for exploring solutes at liquid-vapor interfaces.