Session M8: Focus Session: Interatomic Coulombic Decay

ICD and its exploration by short, intense and coherent light pulses

How does a microscopic system like an atom or a small molecule get rid of the excess electronic energy it has acquired, for instance, by absorbing a photon? If this microscopic system is isolated, the issue has been much investigated and the answer to this question is more or less well known. But what happens if our system has neighbors as is usually the case in nature or in the laboratory? In a human society, if our stress is large, we would like to pass it over to our neighbors. Indeed, this is in brief what happens also to the sufficiently excited microscopic system. A new mechanism of energy transfer has been theoretically predicted and verified in several exciting experiments. This mechanism seems to prevail ``everywhere'' from the extreme quantum system of the He dimer to water and even to quantum dots. The transfer is ultrafast and typically dominates other relaxation pathways. To exploit the high intensity of laser radiation, we propose to select frequencies at which single-photon absorption is of too low energy and two or more photons are needed to produce states of an atom that can undergo interatomic Coulombic decay (ICD) with its neighbors. ICD is an extremely efficient decay mechanism for excited systems which are embedded in environment. For the Ne2 dimer it is explicitly demonstrated that the proposed multiphoton absorption scheme is much more efficient than schemes used until now, which rely on single-photon absorption. Extensive calculations on Ne2 show how the low-energy ICD electrons and Ne$+$ pairs are produced for different laser intensities and pulse durations. At higher intensities the production of Ne$+$ pairs by successive ionization of the two atoms becomes competitive and the respective emitted electrons interfere with the ICD electrons. It is also shown that a measurement after a time delay can be used to determine the contribution of ICD even at high laser intensity. The study can provide a hint how the energy deposited by a FEL on one site in a medium can be transferred fast to the surrounding. Work on ICD can be found on the ICD Bibliography: http://www.pci.uni-heidelberg.de/tc/usr/icd/ICD.refbase.html   

Time dependence of ICD

We will discuss experimental studies of ICD in van der Vaals dimers of rare gas atoms and small molecules using the COLTRIMS technique. The talk will cover ICD after resonant Auger excitation (Nature 505, 664 (2014)) and two studies unveiling the time dependence of ICD in the energy (PRL 111, 233004 (2013)) and in the time domain (PRL 111, 093401 (2013)). A new technique to make ultrafast movies without the use of short pulses will be discussed.   

Electron relaxation in quantum dot and quantum well systems by the ICD mechanism

Electron relaxation in quantum dot (QD) and quantum well (QW) systems has a significant impact on QD and QW optoelectronic devices such as lasers, photodetectors, and solar cells. Several different fundamental relaxation mechanisms are known. We focus here on inter-coulombic decay (ICD) mechanism. In 2011 we have shown that the electron relaxation in a quantum dot dimer due to the ICD mechanism is on a picoseconds timescale (PRB \textbf{83}, 113303) and therefore IR QD detectors based on ICD seems to be feasible. Here we discuss the possibility to observe electron relaxation in QWs. In QWs the effective mass of the electron is not continuous, and can affect the lifetime of the ICD process. In order for the ICD to be the dominant decay mechanism, it must prevail over all other possible competitive decay processes. We have found in our setup that the ICD lifetime is on the timescale of picoseconds. An enhancement of the ICD process occurs when the ionized electron temporarily trapped in a shape-type resonance in the continuum. An experiment based on our findings is currently in progress. In this talk another possibility to observe the ICD phenomenon in two coupled QWs is proposed, by transferring an electron through a two coupled quantum wells structure populated by only one electron. An enhancement in the electron transmission would be obtained when the energy of the incoming electrons allows them to be temporarily trapped inside one of the two quantum wells via the ICD mechanism.   

Interatomic Coulombic decay in nanodroplets

Interatomic (molecular) Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process for excited atoms or molecules embedded in a chemical environment. Via ICD, the excited system can get rid of the excess energy, which is transferred to one of the neighbors and ionize it. ICD produces two charged particles next to each other and thus leads to Coulomb explosion. Kinetic energy distribution of the ionic fragments gives information on the dynamics of the decay process. From the theoretical point of view general quantum mechanical equations for describing the decay processes and the subsequent fragmentations are known but are only applicable for rather small systems. During the presentation, a semiclassical approach for modeling ICD and the subsequent fragmentations will be presented. This approach involves a classical treatment for the nuclear motion while retaining a quantum description for the electron dynamics. Such approach has low computational costs and can be used to study much larger systems. Comparison of the results from semiclassical and from quantum mechanical calculations will be shown for simple systems, demonstrating the good performance of the semiclassical method. Results on ICD in nanodroplets will finally be reported.   

New resonances from the coherence of Auger and intercoulombic (ICD) processes in the photoionization of endohedral fullerenes

Considering the photoionization of Ar@C$_{\mathrm{60}}$, we predict resonant femtosecond decays of both Ar and C$_{\mathrm{60}}$ vacancies through the continua of atom-fullerene hybrid final states. The resulting resonances emerge from the interference between simultaneous autoionizing and intercoulombic decay (ICD) processes [1]. For Ar 3s$\to $np excitations, these resonances are far stronger than the Ar-to-C$_{\mathrm{60}}$ resonant ICDs, while for C$_{\mathrm{60}}$ excitations they are strikingly larger than the corresponding Auger features. The results indicate the power of hybridization to enhance decay rates, and modify lifetimes and line profiles. These decays are also likely to exist generally in the ionization of molecules, nano-dimers and -polymers, and fullerene onions that support hybridized electrons as well. A jellium based time-dependent local density approximation (TDLDA) [2], with the Leeuwen and Baerends exchange-correlation functional to produce accurate asymptotic behavior, is employed to calculate the dynamical response of the system to the photon field. [1] M.H. Javani, J.B. Wise, R. De, M.E. Madjet, S.T. Manson, and H.S. Chakraborty, \underline {arXiv:1312.2144}~[physics.atm-clus]; [2] M.E. Madjet, T. Renger, D.E. Hopper, M.A. McCune, H.S. Chakraborty, J.-M. Rost, and S.T. Manson, \textit{Phys. Rev. A} 81, 013202 (2010).   

Resonant Auger-ICD cascade: A way to control slow-electron production in a medium

The recently proposed cascade [1] initiated by core excitation and terminated by intermolecular Coulombic decay (ICD) will be presented and its properties discussed. If core-excited species are embedded in an environment they decay very efficiently by the following cascade mechanism: a resonant Auger decay takes place in the initially excited species leaving the resulting ion in excited states of sufficient energy allowing the ion to continue to decay via ICD ionizing the environment. It will be shown that in complex media this cascade allows for a control over both the site of the initial excitation and the energy of the ICD-electrons emitted in the final step. Our calculations show that the energy of the emitted electrons depends sensitively on the parent excited state. The incident energy can thus be adjusted both to produce the initial excitation in a chosen atom and to realize an excitation that will result in the emission of ICD electrons with desired energies. These properties of the decay cascade might have consequences for fundamental and applied radiation biology and could be of interest in the development of new spectroscopic techniques. [1] K. Gokhberb, P. Kolorenc, A. I. Kuleff, and L. S. Cederbaum, Nature 505, 661 (2014).