Introduction

 

Chemical reactions and modifications of molecular structure are driven by energy. Often, this energy exists initially in the form of electronic excitations. The relaxation of electronically excited states is therefore a key question in chemistry and molecular biology. A number of relaxation mechanisms are now clear, some of them even after decades of research. Examples are transitions between different electronic states at conical intersections, the tunnelling of electrons between functional groups in large molecules and energy transport via excitons. The topic of this Research Unit is the investigation of a novel, recently discovered mechanism for the transformation of electronic energy created by excitation or ionization with UV radiation and far beyond, or energetic particles. We intend to form a strongly integrated Research Unit consisting both of the groups who have predicted and experimentally demonstrated this effect and groups who have a particular expertise which is necessary to elucidate its full relevance for chemistry, chemical physics and biochemistry.

In the last few years this relaxation process has become known as Intermolecular or Interatomic Coulombic Decay (ICD) [1]. The term designates an electronic decay process occurring in weakly bonded systems, for example in many liquids. Via ICD, electronic excitation energy is transferred into kinetic energies carried by a continuum electron, and the creation of two positive ions. The conditions for the occurrence of ICD are very general; it is expected they are met in many systems and situations. So far, most experiments have been carried out on van der Waals cluster of atoms and small molecules.

 


Fig.1: Sketch of Interatomic Coulombic Decay in a Ne dimer. Step 1: Photo- ionization of a Ne 2s electron. Step 2: Relaxation of a Ne 2p electron, energy transfer and ionization of the neighbouring atom. Step 3: Coulomb explosion of the system (from Ref. 2).


Since these systems are weakly bonded it is meaningful to discuss their electronic structure by referring to the electronic states of their constituents. The first step in ICD is the creation of a single electronic vacancy, the energy of which is located above the double ionization threshold of the weakly bonded system as a whole. An example is the ionization of an inner valence level in water. Although such processes have been investigated for a long time, it has been predicted only in 1997 that neighbouring water molecules could be involved in the relaxation of such a vacancy [1]. In the Intermolecular Coulombic Decay process, this primary vacancy is filled by an electron from a less strongly bound orbital, and at the same time the energy released in this transition is transferred to a neighbouring molecule, where it leads to ejection of an electron. This energy transfer is a consequence of electron correlation. The leading term in an expansion of the transition matrix element can be seen as the exchange of a virtual photon. Thus, the decay process does not require a strong – or indeed any – chemical bond nor any overlap of the wavefunctions between the two units involved. ICD has even been seen in He dimers [13],[14]. In this respect, ICD is somewhat similar to Förster resonant energy transfer. Intermolecular Coulombic Decay is not a resonant process, however, and is therefore expected to occur under far more general conditions than Förster energy transfer.


After ICD, two singly positively charged ions remain. These strongly repel each other, which – at least in small systems, e.g. dimers (Fig. 1) – leads to Coulomb explosion. In extended systems one rather expects structural transformation due to the potential energy of the system.


After the prediction of ICD [1] on theoretical grounds the first experimental evidence was obtained in 2003 [3]. One year later, an elegant experiment on the Ne dimer (Fig. 1) proved the occurrence of the effect beyond any reasonable doubt [4]. Meanwhile, more than 200 publications have appeared on the topic. The three groups who participated in this pioneering work are the initiators of the present proposal for a Research Unit.

 


Fig. 2: Energy balance in ICD of a water tetramer. In a monomer, the Double Ionization Potential (DIP) is above the single ionization energy of the inner valence 2a1 (O 2s) orbital (left-hand side). In a tetramer, various states with two vacancies at adjacent water monomers can be formed. These states are lower in energy compared to the monomer DIP because they contain less Coulomb repulsion energy between the two vacancies. This enables autoionization of the 2a1 vacancy.


ICD is similar to Auger decay in that it is an autoionization of a vacancy state. Auger decay, however, starts from deep inner shell vacancies in atoms or molecules and, as a result, the chemical environment of the vacancy is less important for the decay spectrum. As a matter of fact, Auger decay can easily proceed in isolated atoms or molecules, while ICD is purely an environment effect.


Putting ICD in a broader context, it bridges the gap between fundamental research on the correlated motion of electrons and nuclei and more applied research, for example, on the influence of low kinetic energy electrons in radiation chemistry. These topics are currently receiving huge attention both on the national and international level. In the Research Unit we expect to contribute not only exciting experiments on the ultrafast dynamics of the electronic charge cloud but also new ideas directed towards new fields of applications.


In the last year ICD in water was experimentally demonstrated [11],[12]. This underscores the great practical relevance of further research on this process. It is proposed that a Research Unit be established in order to answer key questions in connection with our understanding of ICD:

 

  1. Which physical and chemical parameters have the strongest influence on the ICD process?

     

  2. In which systems will ICD occur?

     

  3. In which areas of chemistry is ICD of importance? How can it be employed as a method in chemical research?

     

  4. Does ICD have biochemical implications, e.g. in the production of radiation damage?

     

  5. What is the time evolution of ICD?

     

ICD has emerged as a hot topic being pursued in many laboratories world-wide. Besides Germany Japan is one of the centres. The groups of Kiyoshi Ueda (Sendai University), Akira Yagishita (Photon Factory), Norio Saito (AIST) and Ken-Ichi Ito (UVSOR) all work on ICD, mostly in rare gas clusters. Some of the applicants have close collaborations and joint publications with at least some of these Japanese groups. European groups working on ICD are based in France, Italy and Sweden.

 

At the same time, time-resolved experiments on ICD are under way or have been proposed by several groups in the US, like Steve Leone group (LBNL Berkeley), Dan Newmark (UCB), Kapteyn/Murnane group (Boulder) or Arvinder Sandhu (University of Arizona). Time-resolved ICD experiments are used as part of the scientific case for several proposals for new light sources in the US and Canada.