Andreas Hans

Observation of Enhanced Chiral Asymmetries in the Inner-Shell Photoionization of Uniaxially Oriented Methyloxirane Enantiomers

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Detecting ultrafast interatomic electronic processes in media by fluorescence

Interatomic coulombic decay (ICD), a radiationless transition in weakly bonded systems, such as solutes or van der Waals bound aggregates, is an effective source for electrons of low kinetic energy. So far, the ICD processes could only be probed in ultra-high vacuum by using electron and/or ion spectroscopy. Here we show that resonant ICD processes can also be detected by measuring the subsequently emitted characteristic fluorescence radiation, which makes their study in dense media possible.

Determination of absolute cross sections for cluster-specific decays

Fluorescence spectrometry is used to determine absolute cross sections of cluster-specific decay processes following photon excitation. Absolute values for selected processes in partially condensed supersonic jets are determined using atomic benchmarks. This method is applicable to a wide variety of processes in clusters in which a photon is emitted. In a proof-of-principle experiment, absolute cross sections of processes in two prototype systems were measured: (I) inner-valence excitation and ionization of Ar clusters and (II) resonant interatomic Coulombic decay (rICD) in Ne clusters. Example II is the first measurement of absolute cross sections of ICD whatsoever.

Extreme Ultraviolet to Visible Dispersed Single Photon Detection for Highly Sensitive Sensing of Fundamental Processes in Diverse Samples

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.

Angle-Resolved Auger Spectroscopy as a Sensitive Access to Vibronic Coupling.

In the angle-averaged excitation and decay spectra of molecules, vibronic coupling may induce the usually weak dipole-forbidden transitions by the excitation intensity borrowing mechanism. The present complementary theoretical and experimental study of the resonant Auger decay of core-to-Rydberg excited CH_{4} and Ne demonstrates that vibronic coupling plays a decisive role in the formation of the angle-resolved spectra by additionally involving the decay rate borrowing mechanism. Thereby, we propose that the angle-resolved Auger spectroscopy can in general provide very insightful information on the strength of the vibronic coupling.

Circular Dichroism in Fluorescence Emission Following the C 1s→π* Excitation and Resonant Auger Decay of Carbon Monoxide

Dichroism in angle-resolved spectra of circularly polarized fluorescence from freely-rotating CO molecules was studied experimentally and theoretically. For this purpose, carbon monoxide in the gas phase was exposed to circularly polarized soft X-ray synchrotron radiation. The photon energy was tuned across the C 1s→π* resonant excitation, which decayed via the participator Auger transition into the CO+ A 2Π state. The dichroic parameter β1 of the subsequent CO+ (A 2Π → X 2Σ+) visible fluorescence was measured by photon-induced fluorescence spectroscopy. Present experimental results are explained with the ab initio electronic structure and dynamics calculations performed by the single center method. Our results confirm the possibility to perform partial wave analysis of the emitted photoelectrons in closed-shell molecules.

Soft X-ray induced ultraviolet fluorescence emission from bulk and interface of a liquid water microjet

Tremendous progress has been made in the research on the structure and dynamics of liquids due to the development of advanced experimental techniques such as liquid microjets, enabling investigations on volatile samples in ultrahigh vacuum environments. The spectroscopy of charged particles, e.g. photoelectron or Auger electron spectroscopy on liquids, is an established field by now. Here, we report on the successful application of a fluorescence spectrometer to measure optical emission spectra from liquids irradiated with soft X-ray synchrotron radiation.

Investigation of environmental effects in prototypical noble gas clusters using fluorescence spectroscopy

Recently, it was demonstrated, that non-local relaxation processes in noble gas clusters can be identified and quantitatively characterized by fluorescence measurements [1, 2]. The detection of photons therefore provides an advantageous method for the investigation of environmental effects in weakly bound systems. Using electron and ion spectroscopy, often in coincidence experiments, numerous variants of interatomic Coulombic decay (ICD), electron transfer mediated decay (ETMD) and radiative charge transfer (RCT) were discovered and extensively investigated [3]. Noble gas clusters are regarded as prototype systems for the investigation of these process, because they can easily be produced in a controlled way.

Lyman series emission of valence and inner-shell excited gaseous H2O

We present emission cross section functions for Lyman series fluorescence emission after photoexcitation of water molecules with monochromatized synchrotron radiation as function of the exciting-photon energy. Both the valence and inner-shell excitation energy range of the water molecule are covered. The results are compared to literature and contribute to complete the model of dissociative relaxation dynamics of excited small molecules.

Excitation-energy resolved fluorescence spectra of hydrogen molecules in the regime of singly excited molecular states

We have measured the emission spectrum of gaseous hydrogen molecules in the far ultraviolet spectral range over the large interval of excitation energies between 11 eV and 17 eV. This regime is governed by singly excited electronic states below the ionization threshold. While these features have been already investigated separately in the past, the completeness of the present measurements allows a very intuitive representation of the participating molecular effects and their respective correlations. It also contains the first complete investigation of the Condon diffraction bands visible for hydrogen through transitions into the ground state continuum.