Experimental and theoretical characterization of competing decay-channels in molecular hydrogen

Diatomic hydrogen H2 is the most abundant molecular species in the universe and it is the only molecule that is known to fluoresce when excited above its ionization energy. This decay by fluorescence competes with slow ionization and dissociation decays which take place on the nanosecond scale. In astrophysics this emission is important as it contributes significantly to the extreme ultraviolet emission spectra of H2, observed for instance in outer planet atmospheres.

In a coorperation with Prof. Michele Glass-Maujean from the Laboratoire de physique moléculaire pour l’atmosphère et l’astrophysique, the absolute cross sections for the competing decay channels fluorescence, dissociation, and ionization of photon-induced superexcited H2 molecular levels have been measured by irradiating an H2 sample with vacuum ultraviolet photons provided by the undulator beam line U125/2-10m-NIM at BESSY II (Berlin). The exciting photons were dispersed by a 10 m-normal-incidence monochromator resulting in a spectral resolution of 0.0012 nm. The fluorescence radiation was recorded using a visible and a VUV sensitive detector.

Good agreement is found with first principles calculations carried out by multichannel quantum defect theory. The calculations by M. Glass-Maujean reproduce the balance between the competing decay processes as well as its substantial level-to-level evolution.

Contact: Philipp Reiß



Photon-photon and photon-electron coincidence measurements

The investigation of atoms and molecules via the emission of photons, electrons and ions after selective excitation with synchrotron radiation offers extensive possibilities for the study of quantum mechanical effects that originate from the interaction of the electrons in the respective systems.

In addition to fluorescence wavelength- or as the case may be energy- and angle resolving measurements of photons and electrons that have been used for decades, the option to measure these processes time resolved gives another tool for a more thorough examination of the decay dynamics in these systems.

In coincidence measurements, several photons or particles emitted by a single atom or molecule are detected in order to either assign them to a single cascading decay process or to study the dynamics of multiply excited systems that emit several particles at once with a specific correlation.

With the currently performed photon-photon coincidence measurements the lifetimes of excited states in hydrogen atoms that are created by the decay of a doubly excited hydrogen molecule are determined in dependency of the particle density. This is expected to give information on the degree of quantum mechanical entanglement of these atoms and the influence of collisions on this entanglement.

Contact: Philipp Reiß

Photon-photon coincidences of hydrogen molecules in superexcited states

Even with hydrogen being the most abundant and investigated molecule in the universe, there are still various properties not yet fully explained. One example for this is the fluorescence detected when excited above the ionization threshold. This emission is very important for astrophysics as it contributes significantly to the extreme ultraviolet emission and absorption spectra of H2 observed in outer planet atmosphere or interstellar clouds.

This behavior is attributed to the regime of superexcited states that lie beyond the ground states of the respective molecular ions. For these superexcited states there are several competing decay channels possible, one being the dissociation into two neutral hydrogen atoms. These fragments are usually still excited and will then decay by atomic fluorescence in the very well-known Lyman or Balmer series. By using a photon-photon coincidence set-up, it is possible to study this process exclusively.

The correlation of the neutral fragments gives rise to effects like a strong angular anisotropy of the fluorescence and there are several hints at lifetime fluctuations of the excited atomic states. Future experiments will include not only time- but also angle- resolved measurements in order to gather information on the direction of emission of the two photons with respect to each other.

Contact: Philipp Schmidt

Investigation of the Interatomic Coulombic Decay via fluorescence spectroscopy

Decay paths of excited atoms and atomic assemblies (e.g. molecules, clusters) are of fundamental interest in atomic and molecular physics. One of these mechanisms is the Interatomic Coulombic Decay (ICD). In weakly bound systems, excited sites can decay by transferring the deposited energy to a neighboring component. The process has been predicted in the late 1990’s and was observed first experimentally by ion-ion-electron coincidence experiments in noble gas clusters. After more precise investigations the mechanism (ICD) was found to be a fundamental physical process in many bound systems and was proven experimentally several times. 

In general, after ICD the system can still be in an excited state and subsequently decay by the emission of fluorescence which might not occur in an atomic system. Fluorescence spectroscopy should therefore be a possible detection scheme. Since there is in principle no vacuum needed to detect fluorescence, it has also advantages compared to the present detection principles, namely ion and electron spectroscopy. This promotes the investigation of biological relevant systems like liquids or even tissue, where ICD is believed to be also an important process. 

First preliminary experiments on fluorescence spectroscopy of noble gas clusters were performed. Several experiments are planned to find the first proof of ICD via fluorescence measurement. An elegant opportunity offer mixed clusters, e.g. NeXe: after excitation of one kind of atoms (Ne) due to ICD fluorescence of the cluster partner (Xe) should be observable.

Contact: Andreas Hans



Circular Dichroism in the fluorescence emission of chiral molecules


Structures differing from their own mirror images are called chiral. At a first glance, a chiral object and its mirror image (the so-called enantiomers) look quite the same, but it is possible to show that neither any translation nor rotation will bring them to conformity.

Organic compounds are often chiral due to their complex carbon backbone. Moreover, both enantiomers of one molecule have different physical or chemical properties in many cases. This ranges from different odors to crucially different reactions of biological systems to chiral agents. 

Therefore, it is of great importance to develop simple and fast analysis methods which can distinguish between the enantiomers of a chiral compound. Many techniques are based on circular dichroism (CD), i.e. the different absorption of left and right circularly polarized light. Until now fluorescence emission after an inner shell excitation step has not been used as a probe for chirality. When exciting inner shell electrons to a delocalized orbital, it is possible that dichroism appears during the relaxation of the molecule or its fragments via the emitted fluorescence photons.

The aim of this project is to characterize prototypical chiral molecules like Fenchone and Limonene for CD-investigations regarding their fluorescence. As to that excitation, relaxation, and fragmentation channels after inner shell excitations will be extensively investigated by means of fluorescence spectrometry and complementary methods. It is of particular interest to achieve insights in the mechanisms, which carry information about the molecule’s chirality through its different states during the process.

This project is part of the LOEWE initiative ELCH.

Contact: Benjamin Kambs