Final Report Summary - ICD (Intermolecular Coulombic decay and control of photoinduced processes in physics, chemistry, and biology)
The major goals of the project were 1) to explore the potential of the ICD phenomena in systems of physical, chemical, and biological interest, and 2) to develop strategies and methods to exploit the uncovered potential of the ICD. These objectives were fully accomplished and the investigation of ICD and related phenomena is now a rapidly growing field of research. Stimulated by our results, a number of experimental groups from all over the world have performed experiments and are now actively working on studying and exploiting the different aspects of the various ICD processes.
During the project, our team has developed a palette of theoretical and computational methods and techniques allowing high-quality calculations for obtaining static and dynamic properties of the ICD processes in different systems ranging from quantum dots, through van-der-Waals clusters, to hydrogen-bonded molecules. Using these tools, we were able to elucidate various aspects of the ICD processes and predict new mechanisms from the ICD family. The collaborations with several experimental groups were strongly intensified giving rise to a series of spectacular experiments on the subject, confirming the enormous potential of the ICD and related phenomena. Inspired by these results, other experimental groups entered the field and performed and are preparing new experiments on the ICD and related phenomena, making the ICD a blooming new field of research.
Several key results need to be specifically mentioned. The groundbreaking experiments performed by the groups of R. Dörner (Frankfurt) and U. Hergenhehn (Berlin) confirmed that the ICD takes place in water (the natural environment of every biological system) and that it is extremely efficient de-excitation mode leading to emission of low-energy electrons (LEEs). The LEEs are proven to be extremely important for causing damages to living tissues. In order to gain deeper insight into the process in water, we performed a detailed study of the ICD of singly and doubly ionized water dimers. We showed also that the ICD following Auger decay in water is an additional source of genotoxic low-energy electrons.
We continued our study on ICD relaxation mechanisms in small systems representing the most common types of hydrogen bonding between water and biochemically relevant molecules. Our results show that the inner-valence ionization of water triggers ICD in all of the studied species, while in the majority of them the ICD can be triggered also after the ionization of the system itself. This important study showed that the ‘building blocks’ of the biomolecules, and therefore the biomolecules themselves, can take part in an ICD process, producing LEEs and highly reactive free radicals locally, increasing in this way the probability for inducing further damages to the biosystem. These results can have far reaching consequences. By adding a suitable constituent opening the ICD channel one may produce LEE locally on the probe that may subsequently damage some undesirable part of the biosystem.
We made a step further in this direction by showing a possibility to control the ICD process demonstrating that the ICD efficiency can be regulated by protonation or deprotonation, i.e. be regulating the pH value of the environment.
We also proposed a new cascade mechanism initiated by core excitation and terminated by ICD which allows for a control over both, the site of the initial energy deposition and the energy of the ICD-electron. The demonstrated properties of this resonant-Auger–ICD (RA-ICD) cascade may have very interesting applications in the field of electron spectroscopy and radiation damage. In particular, by initiating such RA-ICD cascade via resonant X-ray absorption from a high-Z element embedded in the nucleus of a cancerous cell, the ICD process will deliver genotoxic LEEs and radical cations locally at the absorption site, increasing in that way the controllability of the induced damage. These results attracted a considerable attention from several experimental groups from Germany, Japan, and Italy, which already performed experiments confirming the predicted efficiency and properties of the RA-ICD process. New experiments are in preparation.
In parallel, we continued to investigate the fundamental aspects of the ICD mechanism. Here we have to mention our study on the phenomenon in, probably, the most extreme quantum system – the helium dimer, investigated experimentally by the group of R. Dörner (Frankfurt). The impact of the nuclear dynamics on the ICD process was studied demonstrating that the ICD spectroscopy can be used for imaging vibrational wave functions.
We also showed that when weakly bound clusters are irradiated with strong monochromatic light below the ionization threshold, the ICD mechanism between excited species can lead to ionization very efficiently. Motivated by our predictions, several large-scale experiments were carried out in the newly opened seeded-FEL facility in Trieste (FERMI) and SCSS FEL facility in Japan. All these experiments fully confirmed our predictions.
We have to mention also several important experiments performed in a close collaboration with our group. Very recently the first experimental confirmations of the electron-transfer-mediated decay (ETMD, a process from the family of ICD phenomena) were done by the groups of K. Ueda (Sendai, Japan) and U. Hergenhahn (Berlin) using different techniques. In addition, a new process, the so-called 3-electron ICD, was discovered and soon after confirmed experimentally by the group of Ueda. An important step towards the experimental study of the ICD process in dense media was done in the group of A. Ehresmann (Kassel, Germany), demonstrating that ICD can be efficiently detected by measuring the subsequently emitted characteristic fluorescence. The latter may open the door for experimental studies of the ICD phenomena in biosystems in their natural environment, where the techniques using detection of charged particles have very limited application.
Before concluding, we would like to emphasize another two important results of our efforts. In line with our objectives, we demonstrated that the ICD can be an efficient quencher of molecular photoionization, which may open the door for designing schemes for control of photoinduced processes. Such a possibility has an enormous potential and may have far reaching consequences. We have shown also that ETMD provides a particularly efficient neutralization pathway for multiply charged ions embedded in an environment. The mechanism is general and expected to accompany the Auger decay of atoms and molecules in weakly bound environment, and, therefore, to play an important role in radiation biology.
Finally, we would like to note that the results obtained until now represent a breakthrough in the field. They clearly show that the ICD phenomena are ubiquitous in nature and studying them opens new horizons for exploring and exploiting their enormous potential.
During the project, our team has developed a palette of theoretical and computational methods and techniques allowing high-quality calculations for obtaining static and dynamic properties of the ICD processes in different systems ranging from quantum dots, through van-der-Waals clusters, to hydrogen-bonded molecules. Using these tools, we were able to elucidate various aspects of the ICD processes and predict new mechanisms from the ICD family. The collaborations with several experimental groups were strongly intensified giving rise to a series of spectacular experiments on the subject, confirming the enormous potential of the ICD and related phenomena. Inspired by these results, other experimental groups entered the field and performed and are preparing new experiments on the ICD and related phenomena, making the ICD a blooming new field of research.
Several key results need to be specifically mentioned. The groundbreaking experiments performed by the groups of R. Dörner (Frankfurt) and U. Hergenhehn (Berlin) confirmed that the ICD takes place in water (the natural environment of every biological system) and that it is extremely efficient de-excitation mode leading to emission of low-energy electrons (LEEs). The LEEs are proven to be extremely important for causing damages to living tissues. In order to gain deeper insight into the process in water, we performed a detailed study of the ICD of singly and doubly ionized water dimers. We showed also that the ICD following Auger decay in water is an additional source of genotoxic low-energy electrons.
We continued our study on ICD relaxation mechanisms in small systems representing the most common types of hydrogen bonding between water and biochemically relevant molecules. Our results show that the inner-valence ionization of water triggers ICD in all of the studied species, while in the majority of them the ICD can be triggered also after the ionization of the system itself. This important study showed that the ‘building blocks’ of the biomolecules, and therefore the biomolecules themselves, can take part in an ICD process, producing LEEs and highly reactive free radicals locally, increasing in this way the probability for inducing further damages to the biosystem. These results can have far reaching consequences. By adding a suitable constituent opening the ICD channel one may produce LEE locally on the probe that may subsequently damage some undesirable part of the biosystem.
We made a step further in this direction by showing a possibility to control the ICD process demonstrating that the ICD efficiency can be regulated by protonation or deprotonation, i.e. be regulating the pH value of the environment.
We also proposed a new cascade mechanism initiated by core excitation and terminated by ICD which allows for a control over both, the site of the initial energy deposition and the energy of the ICD-electron. The demonstrated properties of this resonant-Auger–ICD (RA-ICD) cascade may have very interesting applications in the field of electron spectroscopy and radiation damage. In particular, by initiating such RA-ICD cascade via resonant X-ray absorption from a high-Z element embedded in the nucleus of a cancerous cell, the ICD process will deliver genotoxic LEEs and radical cations locally at the absorption site, increasing in that way the controllability of the induced damage. These results attracted a considerable attention from several experimental groups from Germany, Japan, and Italy, which already performed experiments confirming the predicted efficiency and properties of the RA-ICD process. New experiments are in preparation.
In parallel, we continued to investigate the fundamental aspects of the ICD mechanism. Here we have to mention our study on the phenomenon in, probably, the most extreme quantum system – the helium dimer, investigated experimentally by the group of R. Dörner (Frankfurt). The impact of the nuclear dynamics on the ICD process was studied demonstrating that the ICD spectroscopy can be used for imaging vibrational wave functions.
We also showed that when weakly bound clusters are irradiated with strong monochromatic light below the ionization threshold, the ICD mechanism between excited species can lead to ionization very efficiently. Motivated by our predictions, several large-scale experiments were carried out in the newly opened seeded-FEL facility in Trieste (FERMI) and SCSS FEL facility in Japan. All these experiments fully confirmed our predictions.
We have to mention also several important experiments performed in a close collaboration with our group. Very recently the first experimental confirmations of the electron-transfer-mediated decay (ETMD, a process from the family of ICD phenomena) were done by the groups of K. Ueda (Sendai, Japan) and U. Hergenhahn (Berlin) using different techniques. In addition, a new process, the so-called 3-electron ICD, was discovered and soon after confirmed experimentally by the group of Ueda. An important step towards the experimental study of the ICD process in dense media was done in the group of A. Ehresmann (Kassel, Germany), demonstrating that ICD can be efficiently detected by measuring the subsequently emitted characteristic fluorescence. The latter may open the door for experimental studies of the ICD phenomena in biosystems in their natural environment, where the techniques using detection of charged particles have very limited application.
Before concluding, we would like to emphasize another two important results of our efforts. In line with our objectives, we demonstrated that the ICD can be an efficient quencher of molecular photoionization, which may open the door for designing schemes for control of photoinduced processes. Such a possibility has an enormous potential and may have far reaching consequences. We have shown also that ETMD provides a particularly efficient neutralization pathway for multiply charged ions embedded in an environment. The mechanism is general and expected to accompany the Auger decay of atoms and molecules in weakly bound environment, and, therefore, to play an important role in radiation biology.
Finally, we would like to note that the results obtained until now represent a breakthrough in the field. They clearly show that the ICD phenomena are ubiquitous in nature and studying them opens new horizons for exploring and exploiting their enormous potential.