Showing papers on "Excited state published in 2000"

Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems

Abstract: We show theoretically with the simplest possible model that the intensity of an upconversion luminescence that is excited by the sequential absorption of n photons has a dependence on absorbed pump power P, which may range from the limit of Pn down to the limit of P1 for the upper state and less than P1 for the intermediate states. The two limits are identified as the cases of infinitely small and infinitely large upconversion rates, respectively. In the latter case, the dependence of luminescence intensities from intermediate excited states on pump power changes with the underlying upconversion and decay mechanisms. In certain situations, energy transfer upconversion and excited-state absorption can be distinguished by the measured slopes. The competition between linear decay and upconversion in the individual excitation steps of sequential upconversion can be analyzed. The influence of nonuniform distributions of absorbed pump power or of a subset of ions participating in energy-transfer upconversion is investigated. These results are of importance for the interpretation of excitation mechanisms of luminescent and laser materials. We verify our theoretical results by experimental examples of multiphoton-excited luminescence in Cs3Lu2Cl9:Er3+, Ba2YCl7:Er3+, LiYF4:Nd3+, and Cs2ZrCl6:Re4+.

Fast quantum gates for neutral atoms

Abstract: We propose several schemes for implementing a fast two-qubit quantum gate for neutral atoms with the gate operation time much faster than the time scales associated with the external motion of the atoms in the trapping potential. In our example, the large interaction energy required to perform fast gate operations is provided by the dipole-dipole interaction of atoms excited to low-lying Rydberg states in constant electric fields. A detailed analysis of imperfections of the gate operation is given.

Ab Initio Multiple Spawning: Photochemistry from First Principles Quantum Molecular Dynamics

Abstract: The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously. Quantum mechanical effects in the nuclear dynamics are included, especially the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states. The AIMS method makes it possible to describe photochemistry from first principles molecular dynamics, with no empirical parameters. We describe the method and present the application to two molecules of interest in organic photochemistryethylene and cyclobutene. We show that the photodynamics of ethylene involves both covalent and ionic electronic excited states and the return to the ground state proceeds through a pyramidalized geometry. For the photoinduced ring opening of cyclobutene, we show that the disrotatory motion predicted by the Woodward−Hoffmann rules is established within the first 50 fs after optical excitation.

Fast Evaluation of Geometries and Properties of Excited Molecules in Solution: A Tamm-Dancoff Model with Application to 4-Dimethylaminobenzonitrile

Abstract: We present a method to evaluate ab initio energy, wave function, and gradient of a solvated molecule in an electronically excited state. In particular, this paper extends the Polarizable Continuum ...

Inverted Electron-Hole Alignment in InAs-GaAs Self-Assembled Quantum Dots

Abstract: New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.

Trapping an atom with single photons

Abstract: The creation of a photon-atom bound state was first envisaged for the case of an atom in a long-lived excited state inside a high-quality microwave cavity. In practice, however, light forces in the microwave domain are insufficient to support an atom against gravity. Although optical photons can provide forces of the required magnitude, atomic decay rates and cavity losses are larger too, and so the atom-cavity system must be continually excited by an external laser. Such an approach also permits continuous observation of the atom's position, by monitoring the light transmitted through the cavity. The dual role of photons in this system distinguishes it from other single-atom experiments such as those using magneto-optical traps, ion traps or a far-off-resonance optical trap. Here we report high-finesse optical cavity experiments in which the change in transmission induced by a single slow atom approaching the cavity triggers an external feedback switch which traps the atom in a light field containing about one photon on average. The oscillatory motion of the trapped atom induces oscillations in the transmitted light intensity; we attribute periodic structure in intensity-correlation-function data to 'long-distance' flights of the atom between different anti-nodes of the standing-wave in the cavity. The system should facilitate investigations of the dynamics of single quantum objects and may find future applications in quantum information processing.

How exactly did the universe become neutral

Abstract: We present a re—ned treatment of H, He I, and He II recombination in the early universe. The diUer- ence from previous calculations is that we use multilevel atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are (1) allowing excited atomic level populations to depart from an equilibrium distribution, (2) replacing the total recombination coefficient with recombi- nation to and photoionization from each level calculated directly at each redshift step, and (3) correct treatment of the He I atom, including the triplet and singlet states. We —nd that is approximately 10% smaller at redshifts than in previous calcu- x e (4 n e /n H ) (800 lations, as a result of the nonequilibrium of the excited states of H that is caused by the strong but cool radiation —eld at those redshifts. In addition, we —nd that He I recombination is delayed compared with previous calculations and occurs only just before H recombination. These changes in turn can aUect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements, such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation —eld, produce negligible change to the ionization fraction. The lower at low z found in this work aUects the abundances of H molecular and ionic species by 10%¨25%. x e However, this diUerence is probably not larger than other uncertainties in the reaction rates. Subject headings: atomic processescosmic microwave backgroundcosmology: theory ¨ early universe

Ultrafast electron localization dynamics following photo-induced charge transfer

Abstract: Molecular dynamics occurring in the earliest stages following photo-induced charge transfer were investigated. Femtosecond time-resolved absorption anisotropy measurements on [Ru(bpy) 3 ] 2+ , where bpy is 2,2′-bipyridine, reveal a time dependence in nitrile solutions attributed to initial delocalization of the excited state over all three ligands followed by charge localization onto a single ligand. The localization process is proposed to be coupled to nondiffusive solvation dynamics. In contrast, measurements sampling population dynamics show spectral evolution associated with wave packet motion on the excited state surface that is independent of solvent. The results therefore reveal two important contributions to the evolution of charge transfer states in condensed phase, one that is strongly coupled to the surrounding environment and another that follows a potential internal to the molecule.

Hidden symmetries in the energy levels of excitonic 'artificial atoms'

Abstract: Quantum dots1,2,3,4,5,6,7 or ‘artificial atoms’ are of fundamental and technological interest—for example, quantum dots8,9 may form the basis of new generations of lasers The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure) Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states

Quantum Chemical Study of Photoinjection Processes in Dye-Sensitized TiO2 Nanoparticles

Abstract: Results from quantum chemical INDO/S-CI calculations of the absorption properties of anatase TiO2 nanoparticles sensitized by catechol and benzoic acid are reported. Experimental observations that catechol causes a strong shift in the TiO2 absorption threshold, while benzoic acid does not, are explained from the calculations which indicate that only catechol introduces electronic levels in the TiO2 band gap. The 420 nm excitation in the TiO2 + catechol system responsible for the shift is seen to be a charge-transfer excitation from the catechol π HOMO orbital to Ti(3d) levels at the bottom of the conduction band. A direct photoinjection scheme which does not involve excited states of the adsorbate is proposed to account for the electron injection process.

Sequential Energy and Electron Transfer in an Artificial Reaction Center: Formation of a Long-Lived Charge-Separated State

Abstract: A novel molecular triad, representing an artificial reaction center, was synthesized via linking a fullerene moiety to an array of two porphyrins (i.e., a zinc tetraphenyl porphyrin (ZnP) and a free base tetraphenyl porphyrin (H2P)). In this ZnP−H2P−C60 triad, the ZnP performs as an antenna molecule, transferring its singlet excited state energy to the energetically lower lying H2P. In benzonitrile, this energy transfer (k = 1.5 × 1010 s-1) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of H2P to yield ZnP−H2P•+−C60•- and subsequently ZnP•+−H2P−C60•- with rate constants of 7.0 × 109 s-1 and 2.2 × 109 s-1, respectively. The final charge-separated state, formed in high yield (0.4), gives rise to a remarkable lifetime of 21 μs in deoxygenated benzonitrile and decays directly to the singlet ground state. In contrast, in nonpolar toluene solutions the deactivation of the porphyrin chromophores (ZnP and H2P) takes place via singlet−singlet energy transfer l...

Effect of the Solvent Environment on the Spectroscopic Properties and Dynamics of the Lowest Excited States of Carotenoids

Abstract: The spectroscopic properties and dynamics of the lowest excited singlet states of peridinin, fucoxanthin, neoxanthin, uriolide acetate, spheroidene, and spheroidenone in several different solvents have been studied by steady-state absorption and fast-transient optical spectroscopic techniques. Peridinin, fucoxanthin, uriolide acetate, and spheroidenone, which contain carbonyl functional groups in conjugation with the carbon−carbon π-electron system, display broader absorption spectral features and are affected more by the solvent environment than neoxanthin and spheroidene, which do not contain carbonyl functional groups. The possible sources of the spectral broadening are explored by examining the absorption spectra at 77 K in glassy solvents. Also, carotenoids which contain carbonyls have complex transient absorption spectra and show a pronounced dependence of the excited singlet state lifetime on the solvent environment. It is postulated that these effects are related to the presence of an intramolecul...

Fluorescence correlation spectroscopy reveals fast optical excitation-driven intramolecular dynamics of yellow fluorescent proteins

Abstract: Fast excitation-driven fluctuations in the fluorescence emission of yellow-shifted green fluorescent protein mutants T203Y and T203F, with S65G/S72A, are discovered in the 10−6–10−3-s time range, by using fluorescence correlation spectroscopy at 10−8 M. This intensity-dependent flickering is conspicuous at high pH, with rate constants independent of pH and viscosity with a minor temperature effect. The mean flicker rate increases linearly with excitation intensity for at least three decades, but the mean dark fraction of the molecules undergoing these dynamics is independent of illumination intensity over ≈6 × 102 to 5 × 106 W/cm2. These results suggest that optical excitation establishes an equilibration between two molecular states of different spectroscopic properties that are coupled only via the excited state as a gateway. This reversible excitation-driven transition has a quantum efficiency of ≈10−3. Dynamics of external protonation, reversibly quenching the fluorescence, are also observed at low pH in the 10- to 100-μs time range. The independence of these two bright–dark flicker processes implies the existence of at least two separate dark states of these green fluorescent protein mutants. Time-resolved fluorescence measurements reveal a single exponential decay of the excited state population with 3.8-ns lifetime, after 500-nm excitation, that is pH independent. Our fluorescence correlation spectroscopy results are discussed in terms of recent theoretical studies that invoke isomerization of the chromophore as a nonradiative channel of the excited state relaxation.

Femtosecond Excited-State Dynamics of an Iron(II) Polypyridyl Solar Cell Sensitizer Model

Abstract: Time-resolved electronic absorption spectroscopy on a ∼100 fs time scale has been used to study excited-state dynamics in an FeII polypyridyl complex. [Fe(tren(py)3)]2+, where tren(py)3 is tris(2-pyridylmethyliminoethyl)amine, forms a 1MLCT excited state upon irradiation at 400 nm and is known from previous studies to undergo relaxation to a low-lying ligand-field state having S = 2. Static absorption measurements on the low-spin parent complex and a high-spin analogue have been used to identify spectroscopic signatures for the S = 0 and S = 2 ligand-field states, respectively. Comparison of these data with femtosecond and nanosecond differential absorption spectra establishes that the net ΔS = 2 intersystem crossing is essentially complete in well under 1 ps. Spectroelectochemistry on [Fe(tren(py)3)]2+ has also been used to find an absorption feature characteristic of the initially formed 1MLCT state at λ ≳ 600 nm. Analysis of single-wavelength kinetics data in this spectral region reveals that the charg...

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

Abstract: Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO2 system, a very fast electron injection from the excited dye to the conduction band of TiO2 is expected. Our measurements show an injection time τinj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fas...

Effect of Axial Substitution on the Optical Limiting Properties of Indium Phthalocyanines

Abstract: Nonlinear absorption, refraction, and optical limiting by a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described. The absorption cross sections and temporal evolution of the low-lying excited states are also reported. A large nonlinear absorption that increased with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be dominated by a strong excited state absorption from an orientationally averaged triplet state. We derive and experimentally confirm the relation between the molecular absorption cross sections and the fluence-dependent nonlinear absorption coefficients. The effective nonlinear refraction on the nanosecond time scale was reduced because the electronic contribution to the nonlinear refractive index was of the opposite sign from the thermal contribution. An optical limiter using the new material, p-(trifluoromethyl)phenylindium(III) tetra-tert-butylphthalocyanine [(t-...

Strong exciton-erbium coupling in Si nanocrystal-doped SiO2

Abstract: Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ~1 excited Er ion in its vicinity.

The theory of two-electron atoms: between ground state and complete fragmentation

Abstract: Since the first attempts to calculate the helium ground state in the early days of Bohr-Sommerfeld quantization, two-electron atoms have posed a series of unexpected challenges to theoretical physics. Despite the seemingly simple problem of three charged particles with known interactions, it took more than half a century after quantum mechanics was established to describe the spectra of two-electron atoms satisfactorily. The evolution of the understanding of correlated two-electron dynamics and its importance for doubly excited resonance states is presented here, with an emphasis on the concepts introduced. The authors begin by reviewing the historical development and summarizing the progress in measuring the spectra of two-electron atoms and in calculating them by solving the corresponding Schr\"odinger equation numerically. They devote the second part of the review to approximate quantum methods, in particular adiabatic and group-theoretical approaches. These methods explain and predict the striking regularities of two-electron resonance spectra, including propensity rules for decay and dipole transitions of resonant states. This progress was made possible through the identification of approximate dynamical symmetries leading to corresponding collective quantum numbers for correlated electron-pair dynamics. The quantum numbers are very different from the independent particle classification, suitable for low-lying states in atomic systems. The third section of the review describes modern semiclassical concepts and their application to two-electron atoms. Simple interpretations of the approximate quantum numbers and propensity rules can be given in terms of a few key periodic orbits of the classical three-body problem. This includes the puzzling existence of Rydberg series for electron-pair motion. Qualitative and quantitative semiclassical estimates for doubly excited states are obtained for both regular and chaotic classical two-electron dynamics using modern semiclassical techniques. These techniques set the stage for a theoretical investigation of the regime of extreme excitation towards the three-body breakup threshold. Together with periodic orbit spectroscopy, they supply new tools for the analysis of complex experimental spectra.

Charge-transfer correction for improved time-dependent local density approximation excited-state potential energy curves: Analysis within the two-level model with illustration for H2 and LiH

Abstract: Time-dependent density-functional theory (TDDFT) is an increasingly popular approach for calculating molecular excitation energies However, the TDDFT lowest triplet excitation energy, ωT, of a closed-shell molecule often falls rapidly to zero and then becomes imaginary at large internuclear distances We show that this unphysical behavior occurs because ωT2 must become negative wherever symmetry breaking lowers the energy of the ground state solution below that of the symmetry unbroken solution We use the fact that the ΔSCF method gives a qualitatively correct first triplet excited state to derive a “charge-transfer correction” (CTC) for the time-dependent local density approximation (TDLDA) within the two-level model and the Tamm-Dancoff approximation (TDA) Although this correction would not be needed for the exact exchange–correlation functional, it is evidently important for a correct description of molecular excited state potential energy surfaces in the TDLDA As a byproduct of our analysis, we sh

Atomic Data for Resonance Absorption Lines. II. Wavelengths Longward of the Lyman Limit for Heavy Elements

Abstract: This compilation extends the 1991 listing of atomic data by Morton to the heavier stable elements from germanium to bismuth Technetium, thorium, and uranium are added because they can live long enough to be astrophysically detectable The tabulation emphasizes resonance lines, ie, lines the lower level of which is the ground state, or an excited fine-structure state of the ground term, and is restricted to wavelengths longward of the H I Lyman limit at 91175 A This paper has attempted to review all data published by mid-1999 and includes some later material The tables contain the best data available to the author on level designations, ionization potentials, vacuum and air wavelengths, lower and upper energy levels, statistical weights, transition probabilities, natural damping constants (reciprocal lifetimes), oscillator strengths, and the often-used combinations of log gf and log λf All ion stages with classified lines are included Individual components resulting from isotope shift and hyperfine structure are listed explicitly for Rb I, Cs I, Hg I and Hg II, Ti II, and Pb II The accompanying text provides references, explanations for the critical selection of data, and notes indicating where new measurements or calculations are needed This compilation should be particularly useful in the analysis of interstellar and quasar absorption lines and other astrophysical sites where the density of particles and radiation is low enough to excite only the lowest atomic levels The data also are relevant to the study of stellar atmospheres, particularly those with enhanced abundances of heavy elements

An investigation of the F+H2 reaction based on a full ab initio description of the open-shell character of the F(2P) atom

Abstract: Expanding on an earlier Communication [M. H. Alexander, H.-J. Werner, and D. E. Manolopoulos, J. Chem. Phys. 109, 5710 (1998)], we present here the full framework for the quantum treatment of reactions of the fluorine atom with molecular hydrogen. This involves four potential energy surfaces (PESs) and two, coordinate-dependent spin–orbit interaction terms, all of which were fitted to the results of ab initio calculations. Quantum scattering calculations, based on a time-independent method formulated in hyperspherical coordinates, were carried out to determine initial and final state-resolved reactive cross sections, for reaction of F in its ground (2P3/2) and excited (2P1/2) spin–orbit state with H2 in j=0 and j=2(pH2) and j=1(oH2). The overall reactivity of the excited state of F, which can occur only through nonadiabatic transitions, is found to be small, at most 25% of the reactivity of the ground spin–orbit state, which is adiabatically allowed. In addition, when compared with results of earlier calculations, based on a single, electronically adiabatic, PES, our calculations show that even fine details of the dynamics of the F+H2 reaction will be well described by calculations on a single PES. The contribution of the excited spin–orbit state can be seen most clearly in the formation of HF products in the v=3 vibrational manifold, which are nearly thermoneutral (or even slightly endoergic) in the reaction of ground-state F atoms. The cross section for the near resonant electronic-rotational process [F*+H2(j=0)→F+H2(j=2)] is found to be large, in confirmation of earlier work.

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Abstract: Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).

The photoprotector mechanism of mycosporine-like amino acids. Excited-state properties and photostability of porphyra-334 in aqueous solution.

Abstract: The photostability and photophysical parameters of an aqueous solution of the mycosporine-like amino acid (MAA) porphyra-334 have been determined. The excited-singlet state lifetime, measured by time-correlated single photon counting, was 0.4 ns. Laser flash photolysis experiments at 355 nm did not show any transient species. The triplet state of porphyra-334 was sensitized by triplet-triplet energy transfer. The T-T absorption spectrum was determined and the maximal absorption coefficient at 440 nm was estimated to be 1 x 10(4) M(-1) cm(-1). In this way an upper limit for the intersystem crossing quantum yield was determined. The very low quantum yield of fluorescence (phiF = 0.0016) and triplet formation (phiT < 0.05) together with a photodecomposition quantum yield of 2-4 x 10(-4), in the absence and the presence of oxygen respectively, can be explained by a very fast internal conversion process. These results support the photoprotective role assigned to this MAA in living systems.

Conical intersections induced by repulsive 1πσ* states in planar organic molecules: malonaldehyde, pyrrole and chlorobenzene as photochemical model systems

Abstract: Ab initio calculations of reaction paths and potential-energy profiles of excited states are reported for the model systems malonaldehyde, pyrrole and chlorobenzene. In all three cases, optically dark singlet states of πσ * character have been found, which are repulsive with respect to an appropriate reaction coordinate. The resulting surface crossings with 1 ππ * states and/or the electronic ground state, which are symmetry allowed for the planar systems, are converted into conical intersections by out-of-plane modes of appropriate symmetry. It is argued that conical intersections of this type are a common phenomenon in planar organic molecules with heteroatoms and may dominate the photochemistry of these systems.