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Showing papers on "Excited state published in 2007"


Journal ArticleDOI
TL;DR: Femtosecond transient absorption studies indicate that the rate constant for electron transfer from the thermalized s-state of CdSe quantum dots increases with decreasing particle size, which can be easily modulated by controlling the particle size.
Abstract: Electron injection from excited CdSe quantum dots into TiO2 nanoparticles can be easily modulated by controlling the particle size. Femtosecond transient absorption studies indicate that the rate constant for electron transfer from the thermalized s-state of CdSe quantum dots increases with decreasing particle size. The energy difference between the conduction bands of the two semiconductor systems acts as a driving force for the electron transfer in the normal Marcus region. An increase in the interparticle electron transfer rate constant by 3 orders of magnitude (from ∼107 to 1010 s-1) has been achieved by decreasing the CdSe particle diameter from 7.5 to 2.4 nm.

815 citations


Journal ArticleDOI
22 Nov 2007-Nature
TL;DR: It is experimentally demonstrated that a change in conformation of LHCII occurs in vivo, which opens a channel for energy dissipation by transfer to a bound carotenoid, suggesting that this is the principal mechanism of photoprotection.
Abstract: Under conditions of excess sunlight the efficient light-harvesting antenna found in the chloroplast membranes of plants is rapidly and reversibly switched into a photoprotected quenched state in which potentially harmful absorbed energy is dissipated as heat, a process measured as the non-photochemical quenching of chlorophyll fluorescence or qE. Although the biological significance of qE is established, the molecular mechanisms involved are not. LHCII, the main light-harvesting complex, has an inbuilt capability to undergo transformation into a dissipative state by conformational change and it was suggested that this provides a molecular basis for qE, but it is not known if such events occur in vivo or how energy is dissipated in this state. The transition into the dissipative state is associated with a twist in the configuration of the LHCII-bound carotenoid neoxanthin, identified using resonance Raman spectroscopy. Applying this technique to study isolated chloroplasts and whole leaves, we show here that the same change in neoxanthin configuration occurs in vivo, to an extent consistent with the magnitude of energy dissipation. Femtosecond transient absorption spectroscopy, performed on purified LHCII in the dissipative state, shows that energy is transferred from chlorophyll a to a low-lying carotenoid excited state, identified as one of the two luteins (lutein 1) in LHCII. Hence, it is experimentally demonstrated that a change in conformation of LHCII occurs in vivo, which opens a channel for energy dissipation by transfer to a bound carotenoid. We suggest that this is the principal mechanism of photoprotection.

815 citations


Journal ArticleDOI
16 Feb 2007
TL;DR: This work reports an almost ideal realization of Wheeler's delayed-choice gedanken experiment with single photons allowing unambiguous which-way measurements.
Abstract: We report an almost ideal realization of wheeler's "delayed-choice" experiment where the light pulses are true single photons, allowing unambiguous which-way measurements. The clock-triggered single-photon source at the heart of the experiment, previously developed for quantum key distribution, is based on the pulsed, optically excited photoluminescence of a single N-V colour centre in a diamond nanocrystal. This system, which consists in a substitutional nitrogen atom (N) associated to a vacancy (V) in an adjacent lattice site of the diamond crystalline matrix, has an unsurpassed efficiency and photostability at room temperature.

582 citations


Journal ArticleDOI
TL;DR: It is demonstrated that the intermolecular hydrogen bonds between coumarin 102 (C102) and hydrogen-donating solvents are strengthened in the early time of photoexcitation to the electronically excited state by theoretically monitoring the stretching modes of C=O and H-O groups.
Abstract: To study the early time hydrogen-bonding dynamics of chromophore in hydrogen-donating solvents upon photoexcitation, the infrared spectra of the hydrogen-bonded solute-solvent complexes in electronically excited states have been calculated using the time-dependent density functional theory (TDDFT) method. The hydrogen-bonding dynamics in electronically excited states can be widely monitored by the spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds. In this study, we have demonstrated that the intermolecular hydrogen bonds between coumarin 102 (C102) and hydrogen-donating solvents are strengthened in the early time of photoexcitation to the electronically excited state by theoretically monitoring the stretching modes of C=O and H-O groups. This is significantly contrasted with the ultrafast hydrogen bond cleavage taking place within a 200-fs time scale upon electronic excitation, proposed in many femtosecond time-resolved vibrational spectroscopy experiments. The transient hydrogen bond strengthening behaviors in excited states of chromophores in hydrogen-donating solvents, which we have demonstrated here for the first time, may take place widely in many other systems in solution and are very important to explain the fluorescence-quenching phenomena associated with some radiationless deactivation processes, for example, the ultrafast solute-solvent intermolecular electron transfer and the internal conversion process from the fluorescent state to the ground state.

554 citations


Journal ArticleDOI
TL;DR: The spectral tunability of this antenna effect is demonstrated and it is shown that maximum enhancement is achieved when the emission frequency is red-shifted from the surface plasmon resonance of the particle.
Abstract: The fluorescence from a single molecule can be strongly enhanced near a metal nanoparticle acting as an optical antenna. We demonstrate the spectral tunability of this antenna effect and show that maximum enhancement is achieved when the emission frequency is red-shifted from the surface plasmon resonance of the particle. Our experimental results, using individual gold and silver particles excited at different laser-frequencies, are in good agreement with an analytical theory which predicts a different spectral dependence of the radiative and non-radiative decay rates.

532 citations


Journal ArticleDOI
TL;DR: It is shown that the EIT spectra allow direct optical detection of electric field transients in the gas phase, and measurements of the fine structure splitting of the nd series up to n=96 are extended.
Abstract: We demonstrate coherent optical detection of highly excited Rydberg states (up to n=124) using electromagnetically induced transparency (EIT), providing a direct nondestructive probe of Rydberg energy levels. We show that the EIT spectra allow direct optical detection of electric field transients in the gas phase, and we extend measurements of the fine structure splitting of the nd series up to n=96. Coherent coupling of Rydberg states via EIT could also be used for cross-phase modulation and photon entanglement.

520 citations


Journal ArticleDOI
TL;DR: A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM).
Abstract: A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra in solution, and it can be used for studying different relaxation time regimes. SS and conventional linear response (LR) models have been compared by computing the emission energies for different benchmark systems (formaldehyde in water and three coumarin derivatives in ethanol). Special attention is given to the influence of dynamical solvation effects on LR geometry optimizations in solution. The results on formaldehyde point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents and/or weak transitions. The computed emission energies for coumarin derivatives are very close to their experimental counterparts, pointing out the importance of a proper treatment of nonequilibrium solvent effects on both the excited and the ground state energies. The availability of SS-PCM/TD-DFT models for the study of absorption and emission processes allows for a consistent treatment of a number of different spectroscopic properties in solution.

426 citations


Journal ArticleDOI
TL;DR: The water-soluble, near-IR-emitting DNA-encapsulated silver nanocluster presented herein exhibits extremely bright and photostable emission on the single-molecule and bulk levels, and the nonpower law blinking kinetics suggest that these very small species may be alternatives to much larger and strongly intermittent semiconductor quantum dots.
Abstract: The water-soluble, near-IR-emitting DNA-encapsulated silver nanocluster presented herein exhibits extremely bright and photostable emission on the single-molecule and bulk levels. The photophysics have been elucidated by intensity-dependent correlation analysis and suggest a heavy atom effect of silver that rapidly depopulates an excited dark level before quenching by oxygen, thereby conferring great photostability, very high single-molecule emission rates, and essentially no blinking on experimentally relevant time scales (0.1 to >1,000 ms). Strong antibunching is observed from these biocompatible species, which emit >109 photons before photobleaching. The significant dark-state quantum yield even enables bunching from the emissive state to be observed as a dip in the autocorrelation curve with only a single detector as the dark state precludes emission from the emissive level. These species represent significant improvements over existing dyes, and the nonpower law blinking kinetics suggest that these very small species may be alternatives to much larger and strongly intermittent semiconductor quantum dots.

422 citations


Journal ArticleDOI
TL;DR: The state of the art of the DFT description of charge transfer electronic excited states of (mostly) d6 transition metal complexes is presented and discussed in this article, where a brief theoretical background places DFT amongst quantum-chemical techniques and discusses the approximations involved.

411 citations


Journal ArticleDOI
TL;DR: Compared to conventional HF-based CIS(D), the method is more robust in electronically complex situations due to the implicit account of static correlation effects by the GGA parts and the (D) correction often works in the right direction.
Abstract: Double-hybrid density functionals are based on a mixing of standard generalized gradient approximations (GGAs) for exchange and correlation with Hartree-Fock (HF) exchange and a perturbative second-order correlation part (PT2) that is obtained from the Kohn-Sham (GGA) orbitals and eigenvalues. This virtual orbital-dependent functional (dubbed B2PLYP) contains only two empirical parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (ac), respectively. Extensive testing has recently demonstrated the outstanding accuracy of this approach for various ground state problems in general chemistry applications. The method is extended here without any further empirical adjustments to electronically excited states in the framework of time-dependent density functional theory (TD-DFT) or the closely related Tamm-Dancoff approximation (TDA-DFT). In complete analogy to the ground state treatment, a scaled second-order perturbation correction to configuration interaction with...

399 citations


Journal ArticleDOI
TL;DR: It is shown that, under harmonic approximation by considering the Duschinsky rotation effect, the molecular fluorescence properties can be quantitatively calculated from first principles coupled with the correlation function formalism for the internal conversion.
Abstract: It is a highly desirable but difficult task to predict the molecular fluorescence quantum efficiency from first principles. The molecule in the excited state can undergo spontaneous radiation, conversion of electronic energy to nuclear motion, or chemical reaction. For relatively large molecules, it is impossible to obtain the full potential energy surfaces for the ground state and the excited states to study the excited-state dynamics. We show that, under harmonic approximation by considering the Duschinsky rotation effect, the molecular fluorescence properties can be quantitatively calculated from first principles coupled with our correlation function formalism for the internal conversion. In particular, we have explained the peculiar fluorescence behaviors of two isomeric compounds, cis,cis-1,2,3,4-tetraphenyl-1,3-butadiene and 1,1,4,4-tetraphenyl-butadiene, the former being nonemissive in solution and strongly emissive in aggregation or at low temperature, and the latter being strongly emissive in solution. The roles of low-frequency phenyl ring twist motions and their Duschinsky mode mixings are found to be crucial, especially to reveal the temperature dependence. As an independent check, we take a look at the well-established photophysics of 1,4-diphenylbutadiene for its three different conformers. Both the calculated radiative and nonradiative rates are in excellent agreement with the available experimental measurements.

Journal ArticleDOI
TL;DR: The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.
Abstract: Intense multiphoton electron emission is observed from sharp (approximately 20 nm radius) metallic tips illuminated with weak 100-pJ, 7-fs light pulses. Local field enhancement, evidenced by concurrent nonlinear light generation, confines the emission to the tip apex. Electrons are emitted from a highly excited nonequilibrium carrier distribution, resulting in a marked change of the absolute electron flux and its dependence on optical power with the tip bias voltage. The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.

Journal ArticleDOI
TL;DR: It is demonstrated that the intermolecular hydrogen bond C=O...H-O between fluorenone and methanol molecules is significantly strengthened in the electronically excited-state upon photoexcitation of the hydrogen-bonded FM-MeOH complex, which can be used to explain well all the spectral features of fluore None chromophore in alcoholic solvents.
Abstract: The time-dependent density functional theory (TDDFT) method was performed to investigate the excited-state hydrogen-bonding dynamics of fluorenone (FN) in hydrogen donating methanol (MeOH) solvent. The infrared spectra of the hydrogen-bonded FN-MeOH complex in both the ground state and the electronically excited states are calculated using the TDDFT method, since the ultrafast hydrogen-bonding dynamics can be investigated by monitoring the vibrational absorption spectra of some hydrogen-bonded groups in different electronic states. We demonstrated that the intermolecular hydrogen bond C=O...H-O between fluorenone and methanol molecules is significantly strengthened in the electronically excited-state upon photoexcitation of the hydrogen-bonded FM-MeOH complex. The hydrogen bond strengthening in electronically excited states can be used to explain well all the spectral features of fluorenone chromophore in alcoholic solvents. Furthermore, the radiationless deactivation via internal conversion (IC) can be facilitated by the hydrogen bond strengthening in the excited state. At the same time, quantum yields of the excited-state deactivation via fluorescence are correspondingly decreased. Therefore, the total fluorescence of fluorenone in polar protic solvents can be drastically quenched by hydrogen bonding.

Journal ArticleDOI
TL;DR: In this article, a dark matter candidate with an excited state 1-2 MeV above the ground state was proposed, which may be collisionally excited and deexcites by e{sup +}e{sup -} pair emission.
Abstract: We propose a dark matter candidate with an 'excited state' 1-2 MeV above the ground state, which may be collisionally excited and deexcites by e{sup +}e{sup -} pair emission. By converting its kinetic energy into pairs, such a particle could produce a substantial fraction of the 511 keV line observed by the International Gamma-Ray Astrophysics Laboratory/SPI in the inner Milky Way. Only a small fraction of the dark matter candidates have sufficient energy to excite, and that fraction drops sharply with galactocentric radius, naturally yielding a radial cutoff, as observed. Even if the scattering probability in the inner kpc is <<1% per Hubble time, enough power is available to produce the {approx}3x10{sup 42} pairs per second observed in the galactic bulge. We specify the parameters of a pseudo-Dirac fermion designed to explain the positron signal, and find that it annihilates chiefly to e{sup +}e{sup -} and freezes out with the correct relic density. We discuss possible observational consequences of this model.

Journal ArticleDOI
TL;DR: It is shown that a true minimum on the bright S2 electronic state is responsible for the first step that occurs on a femtosecond time scale, and it is suggested that subsequent barrier crossing to the minimal energy S2/S1 conical intersection isresponsible for the picosecond decay.
Abstract: The reaction dynamics of excited electronic states in nucleic acid bases is a key process in DNA photodamage. Recent ultrafast spectroscopy experiments have shown multicomponent decays of excited uracil and thymine, tentatively assigned to nonadiabatic transitions involving multiple electronic states. Using both quantum chemistry and first principles quantum molecular dynamics methods we show that a true minimum on the bright S2 electronic state is responsible for the first step that occurs on a femtosecond time scale. Thus the observed femtosecond decay does not correspond to surface crossing as previously thought. We suggest that subsequent barrier crossing to the minimal energy S2/S1 conical intersection is responsible for the picosecond decay.

Journal ArticleDOI
TL;DR: This work presents proof that the emission from a strongly-coupled QD- microcavity system is dominated by a single quantum emitter.
Abstract: We observe antibunching in the photons emitted from a strongly coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single-photon source operating in the strong coupling regime, with a Purcell factor of 61+/-7 and quantum efficiency of 97%.

Journal ArticleDOI
TL;DR: It is shown that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot and second-order correlation measurements further confirm nonclassical light emission.
Abstract: We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, $g(\ensuremath{\tau})$, as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to $13.3\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$. Second-order correlation measurements further confirm nonclassical light emission.

Journal ArticleDOI
TL;DR: A closer inspection of the model calculations, which reproduce the experimental findings, reveals that the term Bose-Einstein condensation of three alpha particles must not be taken too literally.
Abstract: The first excited 0(+) state in 12C (Hoyle state) has been predicted to be a dilute self-bound gas of bosonic alpha particles, similar to a Bose-Einstein condensate. To clarify this conjecture, precise electron scattering data on form factors of the ground state and the transition to the Hoyle state are compared with results of the fermionic molecular dynamics model, a microscopic alpha-cluster model, and an alpha-cluster model with reduced degrees of freedom (in the spirit of a Bose-Einstein condensed state). The data indicate clearly a dilute density with a large spatial extension of the Hoyle state. A closer inspection of the model calculations, which reproduce the experimental findings, reveals that the term Bose-Einstein condensation of three alpha particles must not be taken too literally.

Journal ArticleDOI
TL;DR: To interpret the photophysics of 1 and 4 in both the solid and solution states, the "2-conformations with 2-spin-states model (2C x 2S model)" is proposed.
Abstract: Studies on synthesis, structures, and photophysics have been carried out for a series of luminescent copper(I) halide complexes with the chelating ligand, 1,2-bis[diphenylphosphino]benzene (dppb). The complexes studied are halogen-bridged dinuclear complexes, [Cu(mu-X)dppb]2 (X = I (1), Br (2), Cl (3)), and a mononuclear complex, CuI(dppb)(PPh3) (4). These complexes in the solid state exhibit intense blue-green photoluminescence with microsecond lifetimes (emission peaks, lambdamax = 492-533 nm; quantum yields, Phi = 0.6-0.8; and lifetimes, tau = 4.0-10.4 mus) at 298 K. In 2-methyltetrahydrofuran (2mTHF) solutions at 298 K, only 1 and 4 show weaker emission (Phi = 0.009) with shorter lifetimes (tau = 0.35 and 0.23 mus) and red-shifted spectra (lambdamax = 543 and 546 nm). The emission in the solid state originates from the (M + X)LCT excited state with a distorted-tetrahedral conformation, in which emissive excited states, 1(M + X)LCT and 3(M + X)LCT, are in equilibrium with an energy difference of approximately 2 kcal/mol. On the other hand, the complexes in the 2mTHF solutions emit from the MLCT excited state with an energetically favorable flattened conformation in the temperature range of 298-130 K. The flattened geometry with equilibrated 1MLCT and 3MLCT states has a nonradiative rate at least 2 orders of magnitude larger than that of the distorted-tetrahedral geometry, leading to a much smaller emission quantum yield (Phi = 0.009) at 298 K. Since the flattening motion is markedly suppressed below 130 K, the emission observed in 2mTHF below 130 K is considered to occur principally from the (M + X)LCT state with a distorted-tetrahedral geometry. To interpret the photophysics of 1 and 4 in both the solid and solution states, we have proposed the "2-conformations with 2-spin-states model (2C x 2S model)". The electroluminescence device using (1) as a green emissive dopant showed a moderate EL efficiency; luminous efficiency = 10.4 cd/A, power efficiency = 4.2 lm/W at 93 cd/m(2), and maximum external quantum efficiency = 4.8%.

Journal ArticleDOI
04 Oct 2007-Nature
TL;DR: This work demonstrates a lasing effect with a single artificial atom—a Josephson-junction charge qubit—embedded in a superconducting resonator.
Abstract: A lasing effect with a single artificial atom (a Josephson-junction charge qubit) that is embedded in a superconducting resonator is demonstrated, making use of the property that such artificial atoms are strongly and controllably coupled to resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons. Solid-state superconducting circuits1,2,3 are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments4,5,6,7,8 it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom—a Josephson-junction charge qubit9—embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons.

Journal ArticleDOI
TL;DR: A simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction is proposed, which concludes that the ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band can be described by the Fermi golden rule, despite its high value.
Abstract: The ultrafast relaxation of aqueous iron(II)−tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at ∼600 nm, which decays in ≤20 fs, and a weak emission at ∼660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and ...

Journal ArticleDOI
TL;DR: Density functional theory (DFT) calculations were able to reproduce and rationalize the experimental redox and excited-states properties of the Ir complexes under study.
Abstract: A series of Ir(III)-based heteroleptic complexes with phenylpyridine (ppy) and 2-(5-phenyl-4H-[1,2,4]triazol-3-yl)-pyridine (ptpy) derivatives as coordinating ligands has been characterized by a number of experimental and theoretical techniques. Density functional theory (DFT) calculations were able to reproduce and rationalize the experimental redox and excited-states properties of the Ir complexes under study. The introduction of fluorine and trifluoromethyl substituents is found not only to modulate the emission energy but also often to change the ordering of the lowest excited triplet states and hence their localization. The lowest triplet states are best characterized as local excitations of one of the chromophoric ligands (ppy or ptpy). The admixture of metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) character is small and strongly depends on the nature of the excited state; their role is, however, primordial in defining the radiative decay rate of the complexes. T...

Journal ArticleDOI
TL;DR: The implementation of the time-dependent theory for the fitting of experimental spectra and the simulation of model spectra as well as the quantum mechanical calculation of the model parameters is discussed, showing the best agreement that has been obtained with quantum chemical methods for this problem.
Abstract: A general method for the simulation of absorption (ABS) and fluorescence band shapes, resonance-Raman (rR) spectra, and excitation profiles based on the time-dependent theory of Heller is discussed. The following improvements to Heller’s theory have been made: (a) derivation of new recurrence relations for the time-dependent wave packet overlap in the case of frequency changes between the ground and electronically excited states, (b) a new series expansion that gives insight into the nature of Savin’s preresonance approximation, (c) incorporation of inhomogeneous broadening effects into the formalism at no additional computational cost, and (d) derivation of a new and simple short-time dynamics based equation for the Stokes shift that remains valid in the case of partially resolved vibrational structure. Our implementation of the time-dependent theory for the fitting of experimental spectra and the simulation of model spectra as well as the quantum mechanical calculation of the model parameters is discuss...

Journal ArticleDOI
TL;DR: This work demonstrates fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin by exploiting the spontaneous decay rate of the excited state.
Abstract: Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order ${10}^{9}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, which is set by the spontaneous decay rate of the excited state.

Journal ArticleDOI
TL;DR: In this paper, a review on the photochemistry of aryl halides is presented, with emphasis on the recent progress in photodissociation dynamics at 266 nm by using photofragment translational spectroscopy.
Abstract: In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266 nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266 nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed.

Journal ArticleDOI
TL;DR: A scaled quantum mechanics/molecular mechanics potential is used that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited-state evolution of bovine Rh.
Abstract: The primary event that initiates vision is the photoinduced isomerization of retinal in the visual pigment rhodopsin (Rh). Here, we use a scaled quantum mechanics/molecular mechanics potential that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited-state evolution of bovine Rh. The analysis of a 140-fs trajectory provides a description of the electronic and geometrical changes that prepare the system for decay to the ground state. The data uncover a complex change of the retinal backbone that, at ≈60-fs delay, initiates a space saving “asynchronous bicycle-pedal or crankshaft” motion, leading to a conical intersection on a 110-fs time scale. It is shown that the twisted structure achieved at decay features a momentum that provides a natural route toward the photoRh structure recently resolved by using femtosecond-stimulated Raman spectroscopy.

Journal ArticleDOI
22 Jun 2007-Science
TL;DR: This work measured the relative efficiencies of vibration and translation in promoting the gas-phase reaction of CHD3 with the Cl atom to form HCl and CD3 and observed that C–H stretch excitation is no more effective than an equivalent amount of translational energy in raising the overall reaction efficiency.
Abstract: The influence of vibrational excitation on chemical reaction dynamics is well understood in triatomic reactions, but the multiple modes in larger systems complicate efforts toward the validation of a predictive framework. Although recent experiments support selective vibrational enhancements of reactivities, such studies generally do not properly account for the differing amounts of total energy deposited by the excitation of different modes. By precise tuning of translational energies, we measured the relative efficiencies of vibration and translation in promoting the gas-phase reaction of CHD3 with the Cl atom to form HCl and CD3. Unexpectedly, we observed that C–H stretch excitation is no more effective than an equivalent amount of translational energy in raising the overall reaction efficiency; CD3 bend excitation is only slightly more effective. However, vibrational excitation does have a strong impact on product state and angular distributions, with C–H stretch-excited reactants leading to predominantly forward-scattered, vibrationally excited HCl.

Journal ArticleDOI
TL;DR: An example of electronic structure-driven tuning of the excited-state properties is presented, thus opening the way to a combined theoretical and experimental strategy for the design of new iridium(III) phosphors with specific target characteristics.
Abstract: We report a combined experimental and theoretical study on cationic Ir(III) complexes for OLED applications and describe a strategy to tune the phosphorescence wavelength and to enhance the emission quantum yields for this class of compounds. This is achieved by modulating the electronic structure and the excited states of the complexes by selective ligand functionalization. In particular, we report the synthesis, electrochemical characterization, and photophysical properties of a new cationic Ir(III) complex, [Ir(2,4-difluorophenylpyridine)2(4,4'-dimethylamino-2,2'-bipyridine)](PF(6)) (N969), and compare the results with those reported for the analogous [Ir(2-phenylpyridine)2(4,4'-dimethylamino-2,2'-bipyridine)](PF(6)) (N926) and for the prototype [Ir(2-phenylpyridine)2(4,4'-tert-butyl-2,2'-bipyridine)](PF(6)) complex, hereafter labeled N925. The three complexes allow us to explore the (C/\N) and (N/\N) ligand functionalization: considering N925 as a reference, we investigate in N926 the effect of electron-releasing substituents on the bipyridine ligand, while in N969, we investigate the combined effect of electron-releasing substituents on the bipyridine ligand and the effect of electron-withdrawing substituents on the phenylpyridine ligands. For N969 we obtain blue-green emission at 463 nm with unprecedented high quantum yield of 85% in acetonitrile solution at room temperature. To gain insight into the factors responsible for the emission color change and the different quantum yields, we perform DFT and TDDFT calculations on the ground and excited states of the three complexes, characterizing the excited-state geometries and including solvation effects on the calculation of the excited states. This computational procedure allows us to provide a detailed assignment of the excited states involved in the absorption and emission processes and to rationalize the factors determining the efficiency of radiative and nonradiative deactivation pathways in the investigated complexes. This work represents an example of electronic structure-driven tuning of the excited-state properties, thus opening the way to a combined theoretical and experimental strategy for the design of new iridium(III) phosphors with specific target characteristics.

Journal ArticleDOI
TL;DR: This study confirms an indirect electron injection mechanism for Ru(II) dyes onTiO2 and indicates a remarkable effect of dye protonation on the electronic properties of N719-sensitized TiO2 nanoparticles.
Abstract: We performed fully first principles quantum mechanical calculations of the ground- and excited-state properties of the [cis-(NCS)2-Ru(II)-bis(2,2‘-bipyridine-4,4‘-dicarboxylate)] dye, N719, adsorbed onto a model TiO2 nanoparticle. Our study confirms an indirect electron injection mechanism for Ru(II) dyes on TiO2 and indicates a remarkable effect of dye protonation on the electronic properties of N719-sensitized TiO2 nanoparticles. We find that two different electron injection mechanisms (adiabatic and nonadiabatic) may be present in DSSCs employing dyes carrying a different number of protons. Despite such differences, the absorption spectra corresponding to strongly and weakly coupled dye/TiO2 excited states are remarkably similar, so that a discrimination of the two electron injection regimes does not appear to be feasible based on inspection of the absorption spectra.

Journal ArticleDOI
TL;DR: A detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented, showing that it is important to consider the effects of the linker structure on both energy relaxation and electronic coupling in bichromophoric molecules.
Abstract: Bichromophoric molecules can support two spatially separated excited states simultaneously and thus provide novel pathways for electronic state relaxation Exciton fission, where absorption of a single photon leads to two triplet states, is a potentially useful example of such a pathway In this paper, a detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented Their spectroscopy is analyzed in terms of a three-state kinetic model in which the singlet excited state can fission into a triplet pair state, which in turn undergoes recombination on a time scale longer than the molecule's radiative lifetime This model allows us to fit both the prompt and delayed fluorescence decay data quantitatively The para-phenylene linked bis(tetracene) molecules 1,4-bis(tetracen-5-yl)benzene (1) and 4,4‘-bis(tetracen-5-yl)biphenylene (2) show intramolecular exciton fission with yields of ∼3%, whereas no delayed fluorescence is observed for tetracene or the meta-linked molecu