Physics

Considering the photoionization of Ar@C60 and Kr@C60 endofullerenes, the decay of C60 innershell excitations through the outershell continuum of the confined atom via the inter-Coulombic decay (ICD) pathway is detailed. Excitations to atom-C60 hybrid states demonstrate coherence between ICD and electron-transfer mediated decay (ETMD). This should be the dominant above-threshold decay process for a variety of confined systems, and the strength of these resonances is such that they should be amenable for study by photoelectron spectroscopy.

Absorption cross-section spectra for gold nanoparticles were calculated using fully quantum Stochastic Density Functional Theory and a classical Finite-Difference Time Domain (FDTD) Maxwell solver. Spectral shifts were monitored as a function of size (1.3-3.1 nm) and shape (octahedron, cubeoctahedron, and truncated cube). Even though the classical approach is forced to fit the quantum TDDFT at 3.1nm, at smaller sizes there is a significant deviation as the classical theory is unable to account for peak splitting and spectral blue shifts even after quantum spectral corrections. We attribute the failure of classical methods at predicting these features to quantum effects and low density of states in small nanoparticles. Classically, plasmon resonances are modeled as collective conduction electron excitations, but at small nanoparticle size these excitations transition to few or even individual conductive electron excitations, as indicated by our results.

In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg n (n=2--5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg 2 ) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.

In this research, we employ accurate time-dependent density functional calculations for ultrashort laser spectroscopy of nitrogen molecule. Laser pulses with different frequencies, intensities, and durations are applied to the molecule and the resulting photoelectron spectra are analyzed. It is argued that relative orientation of the molecule in the laser pulse significantly influence the orbital character of the emitted photoelectrons. Moreover, the duration of the laser pulse is also found to be very effective in controlling the orbital resolution and intensity of photoelectrons. Angular resolved distribution of photoelectrons are computed at different pulse frequencies and recording times. By exponential growth of the laser pulse intensity, the theoretical threshold of two photons absorption in nitrogen molecule is determined.

Electron ionization of helium droplets doped with cesium or potassium results in doubly and, for cesium, triply charged cluster ions. The smallest observable doubly charged clusters are C s 2+ 9 and K 2+ 11 ; they are a factor two smaller than reported previously. The size of potassium dications approaches the Rayleigh limit nRay for which the fission barrier is calculated to vanish, i.e. their fissilities are close to 1. Cesium dications are even smaller than nRay, implying that their fissilities have been significantly overestimated. Triply charged cesium clusters as small as C s 3+ 19 are observed; they are a factor 2.6 smaller than previously reported. Mechanisms that may be responsible for enhanced formation of clusters with high fissilities are discussed.

The fluorescence of silicon clusters is reviewed. Atomic clusters of silicon have been at the focus of research for several decades because of the relevance of size effects for material properties, the importance of silicon in electronics and the potential applications in bio-medicine. To date numerous examples of nanostructured forms of fluorescent silicon have been reported. This article introduces the principles and underlying concepts relevant for fluorescence of nanostructured silicon such as excitation, energy relaxation, radiative and non-radiative decay pathways and surface passivation. Experimental methods for the production of silicon clusters are presented. The geometric and electronic properties are reviewed and the implications for the ability to emit fluorescence are discussed. Free and pure silicon clusters produced in molecular beams appear to have properties that are unfavourable for light emission. However, when passivated or embedded in a suitable host, they may emit fluorescence. The current available data show that both quantum confinement and localised transitions, often at the surface, are responsible for fluorescence. By building silicon clusters atom by atom, and by embedding them in shells atom by atom, new insights into the microscopic origins of fluorescence from nanoscale silicon can be expected.

Vibrationally resolved photoionization spectra of RbHe exciplexes forming on He nanodroplets are recorded using femtosecond pump-probe spectroscopy with amplitude-shaped probe pulses. The time-evolution of the spectra reveals an exciplex formation time ~10ps followed by vibrational relaxation extending up to >1ns. This points to an indirect, time-delayed desorption process of RbHe off the He surface.

We theoretically analyze angle-resolved photo-electron spectra (ARPES) generated by the interaction of C 60 with intense, short laser pulses. In particular, we focus on the impact of the carrier-envelope phase (CEP) onto the angular distribution. The electronic dynamics is described by time-dependent density functional theory, and the ionic background of $\csixty$ is approximated by a particularly designed jellium model. Our results show a clear dependence of the angular distributions onto the CEP for very short pulses covering only very few laser cycles, which disappears for longer pulses. For the specific laser parameters used in a recent experiments, a very good agreement is obtained. Furthermore, the asymmetry is found to depend on the energy of the emitted photoelectrons. The strong influence of the angular asymmetry of electron emission onto the CEP and pulse duration suggests to use this sensitivity as a means to analyze the structure of few-cycle laser pulses.

Small cationic fullerene fragments, produced by electron impact ionization of C60, were mass-selected and accumulated in cryogenic Ne matrixes. Optical absorption spectroscopy of these fragments with up to 18 carbon atoms revealed linear structures. Considering the recent discovery of fullerenes in Space and the very strong absorptions of long linear carbon clusters both in the UV-Vis and IR spectral regions, these systems are good candidates to be observed in Space. We present laboratory data, supported by quantum-chemical calculations and discuss the relevance of long carbon chains for astronomy.

It is shown that a molecule can dissociate by the energy transferred from a remote neighbor. This neighbor can be an excited neutral or ionic atom or molecule. If it is an atom, the transferred energy is, of course, electronic and in the case of molecules it can also be vibrational. Explicit examples are given which demonstrate that the transfer can be highly efficient at distances where there is no bonding between the transmitter and the dissociating molecule.