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.

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.

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 1

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.

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An ultrafast mechanism belonging to the family of interatomic Coulombic decay (ICD) phenomena is proposed. When two excited species are present, an ultrafast energy transfer can take place bringing one of them to its ground state and ionizing the other one. It is shown that if large homoatomic clusters are exposed to an ultrashort and intense laser pulse whose photon energy is in resonance with an excitation transition of the cluster constituents, the large majority of ions will be produced by this ICD mechanism rather than by two-photon ionization. A related collective-ICD process that is operative in heteroatomic systems is also discussed.PACS numbers: 31.70. Hq, 32.80.Rm, 32.80.Wr The rapid development during the last decades of very intense light sources with extreme short pulse duration opened a new era in the study of radiation-matter interaction. Studying the interaction of intense fields with matter brought to the discovery of a whole plethora of new physical phenomena, like high-harmonic generation, above-threshold ionization, or tunneling ionization, to name only a few. In the same time, the progress in generating extremely short pulses gave the scientific community a powerful tool to monitor and control the electron dynamics in atomic and molecular systems and to study processes that take place on a time scale in which the electronic motion is still disentangled from the slower nuclear dynamics (for recent reviews see, e.g., Refs. [1, 2]). A number of free-electron lasers are in operation today providing extremely bright, coherent, and ultrashort pulses in the VUV regime. Exposed to such highly intense pulses, atomic and molecular systems will absorb a large amount of photons triggering various dynamical effects. In this letter we will restrict ourselves to situations where the single-photon energy in the pulse is not high enough to directly ionize the system. It is well known that even in this case the system can be ionized by a multiphoton ionization mechanism. The multiphoton ionization (MPI) results from the ability of quantum systems to absorb several and even many photons, whose individual energies are insufficient to ionize the system. The combined energy of the absorbed photons, though, suffices to eventually eject one or many electrons from the system. During the last decade the MPI has been intensively studied also in composite systems, like clusters, employing the new powerful laser sources (for a review see, e.g. Ref.[3]). However, little attention was paid to other mechanisms that can lead to a multiple ionization in an atomic or molecular cluster irradiated by an intense laser pulse.In this letter we aim at discussing a hitherto unrecognized mechanism for producing ionized species in homoatomic or homomolecular clusters exposed to an intense laser pulse, which in many cases can be by far One of the constituents of the system de-excites transferring the energy to the neighbor which uses it to emit its excited electron.the dominating one. For simplicity we will consider an atomic cluste...

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.