Alexander I. Kuleff

Multielectron wave-packet propagation: General theory and application

An ab initio method for multielectron wave-packet propagation in relatively large systems is presented. It allows the description of ultrafast electron dynamics processes before the coupling with the nuclear motion becomes important. The method is applied to the amino acid glycine for the investigation of the migration of hole charge following the ionization of the system. Two different mechanisms of ultrafast charge migration are identified and discussed. It is shown that the electron correlation can be the driving force for the charge-transfer dynamics in glycine.

Site- and energy-selective slow-electron production through intermolecular Coulombic decay

Irradiation of matter with light tends to electronically excite atoms and molecules, with subsequent relaxation processes determining where the photon energy is ultimately deposited and electrons and ions produced. In weakly bound systems, intermolecular Coulombic decay (ICD) enables very efficient relaxation of electronic excitation through transfer of the excess energy to neighbouring atoms or molecules that then lose an electron and become ionized. Here we propose that the emission site and energy of the electrons released during this process can be controlled by coupling the ICD to a resonant core excitation. We illustrate this concept with ab initio many-body calculations on the argon–krypton model system, where resonant photoabsorption produces an initial or ‘parent’ excitation of the argon atom, which then triggers a resonant-Auger-ICD cascade that ends with the emission of a slow electron from the krypton atom. Our calculations show that the energy of the emitted electrons depends sensitively on the initial excited state of the argon atom. 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.Low-energy electrons (LEEs) are known to be effective in causing strand breaks in DNA. Recent experiments show that an important direct source of LEEs is the intermolecular Coulombic decay (ICD) process. Here we propose a new cascade mechanism initiated by core excitation and terminated by ICD and demonstrate its properties. Explicit calculations show that the energies of the emitted ICD-electrons can be controlled by selecting the initial atomic excitation. The properties of the cascade may have interesting applications in the fields of electron spectroscopy and radiation damage. Initiating such a cascade by resonant X-ray absorption from a high-Z element embedded in a cancerous cell nucleus, ICD will deliver genotoxic particles locally at the absorption site, increasing in that way the controllability of the induced damage. When embedded in a medium, electronically excited atoms and molecules efficiently decay radiationlessly by transferring their excess energy to the neighboring species in the environment and ionizing them, creating in that way low-energy electrons (LEEs) and radical cations. This process is known as intermolecular Coulombic decay (ICD) [1]. Since its discovery in 1997 [1], the ICD has been successfully investigated in a variety of systems [2]. It usually proceeds on a femtosecond timescale and becomes faster the more neighbors are present, dominating most of the competing relaxation processes. Experimental investigation of ICD in water dimers [3] found the rate of this process to be so large as to completely suppress the proton transfer in the inner-valence ionized water molecules. As a result of ICD, two intact water cations are produced by the consecutive Coulomb

Ultrafast correlation-driven electron dynamics

Exposing molecules to ultrashort laser pulses creates electronic wave packets, and therefore, triggers pure electron dynamics in the excited or ionized system. In the case of ionization, these dynamics may manifest as a migration of the initially created localized hole throughout the system and were termed charge migration. Here, we review the theoretical foundation and the most important results obtained in the study of the charge migration phenomenon, as well as give some perspectives for the directions the future studies could go.

Tracing ultrafast interatomic electronic decay processes in real time and space.

Advances in laser pump-probe techniques open the door for observations in real time of ultrafast electronic processes. Particularly attractive is the visualization of interatomic processes where one can follow the energy transfer from one atom to another. The interatomic Coulombic decay (ICD) provides such a process which is abundant in nature. A wave packet propagation method now enables us to trace fully ab initio the electron dynamics of the process in real time and space, taking into account all electrons of the system and their correlations. The evolution of the electronic cloud during the ICD process in NeAr following Ne2s ionization is computed and analyzed. The process takes place on a femtosecond time scale, and a surprisingly strong response is found already in the attosecond regime.

Charge migration following ionization in systems with chromophore-donor and amine-acceptor sites

The ultrafast charge migration following outer-valence ionization in three different but related molecules, namely, 2-phenylethyl-N,N-dimethylamine (PENNA), and its butadiene (MePeNNA) and ethylene (BUNNA) derivates, is studied in detail. The molecules have different chromophore-donor sites, but nearly identical amine-acceptor sites. The results show that the charge migration process depends strongly on the particular donor site, varying from ultrafast migration of the charge from the donor to the acceptor site (4 fs for MePeNNA) to no migration at all (for BUNNA). The influence of the geometrical structure of the molecule on the charge migration is also investigated. It is shown that energetically closely lying conformers may exhibit dramatically different charge migration behaviors. The basic mechanism of the charge migration process in the studied molecules is analyzed in detail and is demonstrated to be due to electron correlation and relaxation effects.

Core Ionization Initiates Subfemtosecond Charge Migration in the Valence Shell of Molecules

After the ionization of a valence electron, the created hole can migrate ultrafast from one end of the molecule to another. Because of the advent of attosecond pulse techniques, the measuring and understanding of charge migration has become a central topic in attosecond science. Here, we pose the hitherto unconsidered question whether ionizing a core electron will also lead to charge migration. It is found that the created hole in the core stays put, but in response to this hole interesting electron dynamics takes place which can lead to intense charge migration in the valence shell. This migration is typically faster than that after the ionization of a valence electron and transpires on a shorter time scale than the natural decay of the core hole by the Auger process, making the subject very challenging to attosecond science.

Attosecond Hole Migration in Benzene Molecules Surviving Nuclear Motion.

Hole migration is a fascinating process driven by electron correlation, in which purely electronic dynamics occur on a very short time scale in complex ionized molecules, prior to the onset of nuclear motion. However, it is expected that due to coupling to the nuclear dynamics, these oscillations will be rapidly damped and smeared out, which makes experimental observation of the hole migration process rather difficult. In this Letter, we demonstrate that the instantaneous ionization of benzene molecules initiates an ultrafast hole migration characterized by a periodic breathing of the hole density between the carbon ring and surrounding hydrogen atoms on a subfemtosecond time scale. We show that these oscillations survive the dephasing introduced by the nuclear motion for a long enough time to allow their observation. We argue that this offers an ideal benchmark for studying the influence of hole migration on molecular reactivity.

Ultrafast charge migration following valence ionization of 4-methylphenol: jumping over the aromatic ring.

Electronic many-body effects alone can be responsible for the migration of a positive charge created upon ionization in molecular systems. Here, we report an ultrafast charge migration taking place after valence ionization of the molecule 4-methylphenol. The results obtained by a fully ab initio methodology show that the positive charge localized initially on the methyl group can migrate to the hydroxyl group in less than 2 fs jumping over the whole aromatic ring.

Electron Correlation in Real Time

Electron correlation, caused by the interaction among electrons in a multielectron system, manifests itself in all states of matter. A complete theoretical description of interacting electrons is challenging; different approximations have been developed to describe the fundamental aspects of the correlation that drives the evolution of simple (few-electron systems in atoms/molecules) as well as complex (multielectron wave functions in atoms, molecules, and solids) systems. Electron correlation plays a key role in the relaxation mechanisms that characterize excited states of neutral or ionized atoms and molecules populated by absorption of extreme ultraviolet (XUV) or X-ray radiation. The dynamics of these states can lead to different processes such as Fano resonance and Auger decay in atoms or interatomic Coulombic decay or charge migration in molecules and clusters. Many of these relaxation mechanisms are ubiquitous in nature and characterize the interaction of complex systems, such as biomolecules, adsorbates on surfaces, and hydrogen-bonded clusters, with XUV light. These mechanisms evolve typically on the femtosecond (1 fs=10(-15) s) or sub-femtosecond timescale. The experimental availability of few-femtosecond and attosecond (1 as=10(-18) s) XUV pulses achieved in the last 10 years offers, for the first time, the opportunity to excite and probe in time these dynamics giving the possibility to trace and control multielectron processes. The generation of ultrashort XUV radiation has triggered the development and application of spectroscopy techniques that can achieve time resolution well into the attosecond domain, thereby offering information on the correlated electronic motion and on the correlation between electron and nuclear motion. A deeper understanding of how electron correlation works could have a large impact in several research fields, such as biochemistry and biology, and trigger important developments in the design and optimization of electronic devices.

Intermolecular Coulombic decay in small biochemically relevant hydrogen-bonded systems.

Intermolecular Coulombic decay (ICD) is a very fast and efficient relaxation pathway of ionized and excited molecules in environment. The ICD and related phenomena initiated by inner-valence ionization are explored for H(2)O···HCHO, H(2)O···H(2)CNH, H(2)O···NH(3), NH(3)···H(2)O, H(2)O···H(2)S, H(2)S···H(2)O, and H(2)O···H(2)O (p-donor···p-acceptor). This set of small hydrogen-bonded systems contains seven types of hydrogen bonding, which are typical for biochemistry, and thus its investigation provides insight into the processes that can take place in living tissues. In particular, an estimate of the ICD in biosystems interacting with water (their usual medium) is made. This decay mode is expected to be a source of low-energy electrons proven to be of extreme genotoxic nature. For the purpose of our study, we have used high-precision ab initio methods in optimizing the geometries and computing the single- and double-ionization spectra of formaldehyde-, formaldimine-, ammonia-, hydrogen sulfide-, and water-water complexes. The energy range of the emitted ICD electrons, as well as the kinetic energy of the dissociating ions produced by ICD, is also reported.