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.

We directly observe radiative charge transfer (RCT) in Ne clusters by dispersed vacuum-ultraviolet photon detection. The doubly ionized Ne 2+ -Ne n 1 -initial states of RCT are populated after resonant 1s-3p photoexcitation or 1s photoionization of Ne n clusters with n 2800 á ñ » . These states relax further producing Ne + -Ne + -Ne n 2 -final states, and the RCT photon is emitted. Ab initio calculations assign the observed RCT signal to the Ne 2p D Ne ninitial state, while transitions from other possible initial states are proposed to be quenched by competing relaxation processes. The present results are in agreement with the commonly discussed scenario, where the doubly ionized atom in a noble gas cluster forms a dimer which dissipates its vibrational energy on a picosecond timescale. Our study complements the picture of the RCT process in weakly bound clusters, providing information which is inaccessible by charged particle detection techniques.

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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 þ 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 being 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 about 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.