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Nanoscale semiconductors with isolated spin impurities have been touted as promising materials for their potential use at the intersection of quantum, spin, and information technologies. Electron paramagnetic resonance (EPR) studies of spins in semiconducting carbon nanotubes have overwhelmingly focused on spins more strongly localized by sp3-type lattice defects. However, the creation of such impurities is irreversible and requires specific reactions to generate them. Shallow charge impurities, on the other hand, are more readily and widely produced by simple redox chemistry, but have not yet been investigated for their spin properties. Here, we use EPR to study p-doped (6,5) semiconducting single-wall carbon nanotubes (s-SWNTs) and elucidate the role of impurity–impurity interactions in conjunction with exchange and correlation effects for the spin behavior of this material. A quantitative comparison of the EPR signals with phenomenological modeling combined with configuration interaction electronic structure calculations of impurity pairs shows that orbital overlap, combined with exchange and correlation effects, causes the EPR signal to disappear due to spin entanglement for doping levels corresponding to impurity spacings of 14 nm (at 30 K). This transition is predicted to shift to higher doping levels with increasing temperature and to lower levels with increasing screening, providing an opportunity for improved spin control in doped s-SWNTs. [ABSTRACT FROM AUTHOR]

Spontaneous emission and internal conversion rates are calculated within harmonic approximations and compared to the results obtained within the semi-classical extended thawed Gaussian approximation (ETGA). This is the first application of the ETGA in the calculation of internal conversion and emission rates for real molecular systems, namely, formaldehyde, fluorobenzene, azulene, and a dicyano-squaraine dye. The viability of the models as black-box tools for prediction of spontaneous emission and internal conversion rates is assessed. All calculations were done using a consistent protocol in order to investigate how different methods perform without previous experimental knowledge using density functional theory (DFT) and time-dependent DFT (TD-DFT) with B3LYP, PBE0, ωB97XD, and CAM-B3LYP functionals. Contrasting the results with experimental data shows that there are further improvements required before theoretical predictions of emission and internal conversion rates can be used as reliable indicators for the photo-luminescence properties of molecules. We find that the ETGA performs rather similar to the vertical harmonical model. Including anharmonicities in the calculation of internal conversion rates has a moderate effect on the quantitative results in the studied systems. The emission rates are fairly stable with respect to computational parameters, but the internal conversion rate reveals itself to be highly dependent on the choice of the spectral line shape function, particularly the width of the Lorentzian function, associated with homogeneous broadening. [ABSTRACT FROM AUTHOR]

The theoretical prediction of the rates of nonradiative processes in molecules is fundamental in assessing their emissive properties. In this context, global harmonic models have been widely used to simulate vibronic spectra as well as internal conversion rates and to predict photoluminescence quantum yields. However, these simplified models suffer from the limitations that are inherent to the harmonic approximation and can have a severe effect on the calculated internal conversion rates. Therefore, the development of more accurate semiclassical methods is highly desirable. Here, we introduce a procedure for the calculation of nonradiative rates in the framework of the time-dependent semi-classical Extended Thawed Gaussian Approximation (ETGA). We systematically investigate the performance of the ETGA method by comparing it to the adiabatic and vertical harmonic methods, which belong to the class of widely used global harmonic models. Its performance is tested in potentials that cannot be treated adequately by global harmonic models, beginning with Morse potentials of varying anharmonicity followed by a double well potential. The calculated radiative and nonradiative internal conversion rates are compared to reference values based on exact quantum dynamics. We find that the ETGA has the capability to predict internal conversion rates in anharmonic systems with an appreciable energy gap, whereas the global harmonic models prove to be insufficient. [ABSTRACT FROM AUTHOR]

We investigate the photodynamics of the 2-methylallyl radical by femtosecond time-resolved photoelectron imaging. The experiments are accompanied by field-induced surface hopping dynamics calculations and the simulation of time-resolved photoelectron intensities and anisotropies, giving insight into the photochemistry and nonradiative relaxation of the radical. 2-methylallyl is excited at 236 nm, 238 nm, and 240.6 nm into a 3p Rydberg state, and the subsequent dynamics is probed by multiphoton ionization using photons of 800 nm. The photoelectron image exhibits a prominent band with considerable anisotropy, which is compatible with the result of theory. The simulations show that the initially excited 3p state is rapidly depopulated to a 3s Rydberg state, from which photoelectrons of high anisotropy are produced. The 3s state then decays within several 100 fs to the D1 (nπ) state, followed by the deactivation of the D1 to the electronic ground state on the ps time scale. [ABSTRACT FROM AUTHOR]

We present a combination of time-dependent density functional theory with the quantum mechanical/molecular mechanical approach which can be applied to study nonadiabatic dynamical processes in molecular systems interacting with the environment. Our method is illustrated on the example of ultrafast excited state dynamics of indole in water. We compare the mechanisms of nonradiative relaxation and the electronic state lifetimes for isolated indole, indole in a sphere of classical water, and indole + 3H2O embedded in a classical water sphere. In the case of isolated indole, the initial excitation to the S2 electronic state is followed by an ultrafast internal conversion to the S1 state with a time constant of 17 fs. The S1 state is long living (>30 ps) and deactivates to the ground state along the N-H stretching coordinate. This deactivation mechanism remains unchanged for indole in a classical water sphere. However, the lifetimes of the S2 and S1 electronic states are extended. The inclusion of three explicit water molecules opens a new relaxation channel which involves the electron transfer to the solvent, leading eventually to the formation of a solvated electron. The relaxation to the ground state takes place on a time scale of 60 fs and contributes to the lowering of the fluorescence quantum yield. Our simulations demonstrate the importance of including explicit water molecules in the theoretical treatment of solvated systems. [ABSTRACT FROM AUTHOR]

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics 'on the fly' in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S2 over S1 to the ground state S0 of furan have been revealed for the first time. Upon initial excitation of the S2 state which has π-π* character, a nonadiabatic transition to the S1 state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores. [ABSTRACT FROM AUTHOR]

We present an approach for the simulation of time resolved photoelectron spectra based on the combination of the ab initio nonadiabatic molecular dynamics ”on the fly” with the Stieltjes imaging method utilizing discrete neutral states above the ionization limit for the approximate description of the ionization continuum. Our approach has been implemented in the framework of the time-dependent density functional theory and has been applied to interrogate the ultrafast internal conversion between the S2 and S1 states in pyrazine. The simulations reveal that, parallel to the S2→S1 internal conversion, a change in the dominant ionization process (S2→D1 versus S1→D0) occurs on the time scale of 20 fs such that no significant change in the photoelectron kinetic energy distribution is observed. The presented results are in full agreement with the experimental results presented in the accompanying paper [Suzuki et al., J. Chem. Phys. 132, 174302 (2010)] and provide an insight into the interplay between the nonradiative relaxation and the photoionization process in pyrazine as reflected in the time resolved photoelectron spectrum. Our approach represents a general tool for the investigation of ultrafast photoionization processes in complex systems and thus can be used to investigate the ultrafast femtochemistry of complex molecular systems including all degrees of freedom. [ABSTRACT FROM AUTHOR]

Nanoscale hybrid systems are a new class of molecular aggregates that offer numerous new possibilities in materials design. Diamondoid thiols are promising nanoscale building blocks for such hybrid systems. They allow the incorporation of functional groups and the investigation of their effects on the unique materials’ properties of diamondoids. Here we combine experimental data with ab initio theory to explore the optical properties of diamondoid thiols and their dependence on size and shape. Agreement between theoretically and experimentally obtained absorption spectra allows the identification of the nature of the optical transitions that are responsible for some photophysical and photochemical processes. We show that the optical properties of diamondoid thiols in the deep UV regime depend on the functionalization site but are largely size independent. Our findings provide an explanation for the disappearance of diamondoid UV photoluminescence upon thiolation for smaller diamondoids. However, our theoretical results indicate that for larger diamondoid thiols beyond the critical size of six diamondoid cages the lowest energy transitions are characterized by diamondoidlike states suggesting that UV luminescence may be regained. [ABSTRACT FROM AUTHOR]

Femtosecond time resolved photoelectron spectroscopy in combination with direct ab initio molecular dynamics “on the fly” based on density functional theory has been used to study the relaxation dynamics of optically excited states in small mass selected anionic gold clusters (Aun-; n=5–8). The nature of the dynamics strongly depends on the cluster size and structure. Oscillatory wavepacket motion (Au5-), a long lived excited state (Au6-), as well as photoinduced melting (Au7-,Au8-) is observed in real time. This illustrates nonscalable properties of excited states in clusters in the size regime, in which each atom counts. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the size and structure selective absorption properties of cationic silver cluster–tryptophan Trp–Agn+ (n=2–5,9) hybrid systems supported by photofragmentation experiments. Our time-dependent density functional theory calculations provide insight into the nature of excitations in interacting nanoparticle-biomolecule subunits and allow to determine characteristic spectral features as fingerprints of two different classes of structures: charge solvated and zwitterionic. Moreover, different types of charge transfer transitions have been identified. Charge transfer from π system of tryptophan to silver cluster occurs for charge solvated structures while charge transfer from silver to the NH3+ group takes place for zwitterionic structures. This has been confirmed by experimentally measured photofragmentation channels and molecular dynamics simulations. Our findings provide fundamental insight into the structure- and size-dependent mechanism responsible for the enhanced absorption and emission in nanoparticle-biomolecular hybrid systems. [ABSTRACT FROM AUTHOR]

The interactions between small gold oxide cluster anions, Au2,3On- (n=1–5), and CO were investigated in a fast-flow reactor mass spectrometer, and experimental results were verified with a guided ion beam mass spectrometer. Density functional calculations along with molecular dynamics simulations were also utilized to explain the experimental findings. From these studies, we show that, for the interactions between AumOn- and CO, each atom counts. With the addition of a single gold atom, it is observed that association of CO and replacement of O2 by CO become the dominant reaction channels as opposed to CO oxidation. We also present results that show that the oxidation of CO takes place only in the presence of a peripheral oxygen atom. However, this condition is not always sufficient. Furthermore, the association of CO onto AumOn- follows a general qualitative rule based on the relationship between the energy of the cluster lowest unoccupied molecular orbital and the binding energy of CO. [ABSTRACT FROM AUTHOR]

Bimetallic silver-gold clusters offer an excellent opportunity to study changes in metallic versus "ionic" properties involving charge transfer as a function of the size and the composition, particularly when compared to pure silver and gold clusters. We have determined structures, ionization potentials, and vertical detachment energies for neutral and charged bimetallic Ag[sub m]Au[sub n][3 ≤ (m+n) ≤5] clusters. Calculated VDE values compare well with available experimental data. In the stable structures of these clusters Au atoms assume positions which favor the charge transfer from Ag atoms. Heteronuclear bonding is usually preferred to homonuclear bonding in clusters with equal numbers of hetero atoms. In fact, stable structures of neutral Ag[sub 2]Au[sub 2], Ag[sub 3]Au[sub 3], and Ag[sub 4]Au[sub 4] clusters are characterized by the maximum number of hetero bonds and peripheral positions of Au atoms. Bimetallic tetramer as well as hexamer are planar and have common structural properties with corresponding one-component systems, while Ag[sub 4]Au[sub 4] and Ag[sub 8] have 3D forms in contrast to Au[sub 8] which assumes planar structure. At the density functional level of theory we have shown that this is due to participation of d electrons in bonding of pure Au[sub n] clusters while s electrons dominate bonding in pure Ag[sub m] as well as in bimetallic clusters. In fact, Au[sub n] clusters remain planar for larger sizes than Ag[sub m] and Ag[sub n]Au[sub n] clusters. Segregation between two components in bimetallic systems is not favorable, as shown in the example of Ag[sub 5]Au[sub 5] cluster. We have found that the structures of bimetallic clusters with 20 atoms Ag[sub 10]Au[sub 10] and Ag[sub 12]Au[sub 8] are characterized by negatively charged Au subunits embedded in Ag environment. In the latter case, the shape of Au[sub 8] is related to a pentagonal bipyramid capped by one atom and contains three exposed negatively charged Au atoms. They might... [ABSTRACT FROM AUTHOR]

The absorption spectra of Ag[sub 5–8] have been determined in the framework of the linear response equation-of-motion coupled cluster method and related techniques employing 11-electron relativistic effective core potential. In these treatments electron correlation effects for 11 electrons per atom are included, providing an accurate description of excited states of silver clusters. The calculations of transition energies and oscillator strengths have been carried out in a large energy interval for the stable structures and for the isomeric forms higher in energy. This allowed us to investigate the influence of structural properties on the spectroscopic patterns and to determine the role of d-electrons. Inclusion of d-electrons in the correlation treatment is mandatory to obtain accurate values for transition energies, but the excitations of s-electrons are primarily responsible for the spectroscopic patterns. They are characterized by the interference phenomena known in molecular spectroscopy which lead to a small number of intense and a large number of weak resonances. The calculated absorption spectra for the stable structures provide accurate predictions of the optical response properties in the gas phase and at the zero temperature. Since for neutral silver clusters the experimental data in the gas phase are not yet available, we also calculated spectra for deformed structures which model the influence of the environment such as rare-gas atoms, solid Ar-matrix or He-droplet. Comparison of our results with available experimental data permits us to identify structural properties responsible for the recorded spectral features. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an efficient method for the simulation of time-resolved photoelectron imaging (TRPEI) spectra in polyatomic molecules. Our approach combines trajectory-based molecular dynamics that account for non-adiabatic effects using surface hopping, with an approximate treatment of the photoionization process using Dyson orbitals as initial and Coulomb waves as final electron states. The method has been implemented in the frame of linear response time-dependent density functional theory. As an illustration, we simulate time- and energy-resolved anisotropy maps for the furan molecule and compare them with recent experimental data [T. Fuji, Y.-I. Suzuki, T. Horio, T. Suzuki, R. Mitric, U. Werner, and V. Bonacˇic-Koutecký, J. Chem. Phys. 133, 234303 (2010)]. Our method can be generally used for the interpretation of TRPEI experiments allowing to shed light into the fundamental photochemical processes in complex molecules. [ABSTRACT FROM AUTHOR]