Femtosecond Dynamics of Excited-State Evolution in

Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2'-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in approximately 300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.     

Time-resolved dynamics in N2O4 probed using high harmonic generation

The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.     

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene

Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.     

Conditional dynamics of interacting quantum dots

Conditional quantum dynamics, where the quantum state of one system controls the outcome of measurements on another quantum system, is at the heart of quantum information processing. We demonstrate conditional dynamics for two coupled quantum dots, whereby the probability that one quantum dot makes a transition to an optically excited state is controlled by the presence or absence of an optical excitation in the neighboring dot. Interaction between the dots is mediated by the tunnel coupling between optically excited states and can be optically gated by applying a laser field of the right frequency. Our results represent substantial progress toward realization of an optically effected controlled-phase gate between two solid-state qubits.     

Optical signatures of coupled quantum dots

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots. This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing.     

Charge migration in DNA: ion-gated transport

Electron hole (radical cation) migration in DNA, where the quantum transport of an injected charge is gated in a correlated manner by the thermal motions of the hydrated counterions, is described here. Classical molecular dynamics simulations in conjunction with large-scale first-principles electronic structure calculations reveal that different counterion configurations lead to formation of states characterized by varying spatial distributions and degrees of charge localization. Stochastic dynamic fluctuations between such ionic configurations can induce correlated changes in the spatial distribution of the hole, with concomitant transport along the DNA double helix. Comparative ultraviolet light-induced cleavage experiments on native B DNA oligomers and on ones modified to contain counterion (Na(+))-starved bridges between damage-susceptible hole-trapping sites called GG steps show in the latter a reduction in damage at the distal step. This reduction indicates a reduced mobility of the hole across the modified bridge as predicted theoretically.     

Signatures of H2CO photodissociation from two electronic states

Even in small molecules, the influence of electronic state on rotational and vibrational product energies is not well understood. Here, we use experiments and theory to address this issue in photodissociation of formaldehyde, H2CO, to the radical products H + HCO. These products result from dissociation from the singlet ground electronic state or the first excited triplet state (T1) of H2CO. Fluorescence spectra reveal a sudden decrease in the HCO rotational energy with increasing photolysis energy accompanied by substantial HCO vibrational excitation. Calculations of the rotational distribution using an ab initio potential energy surface for the T1 state are in very good agreement with experiment and strongly support dominance of the T1 state in the dynamics at the higher photolysis energies.     

Conformational dynamics in a dipeptide after single-mode vibrational excitation

The dynamics of conformational isomerization are explored in a methyl-capped dipeptide, N-acetyl-tryptophan methyl amide (NATMA), using infrared-ultraviolet (IR-UV) hole-filling and IR-induced population transfer spectroscopies. IR radiation selectively excites individual NH stretch vibrational fundamentals of single conformations of the molecule in the early portions of a gas-phase expansion, and then this excited population is collisionally recooled into its conformational minima for subsequent conformation-specific detection. Efficient isomerization is induced by the IR excitation that redistributes population between the same conformations that have population in the absence of IR excitation. The quantum yields for transfer of the population into the various conformational minima depend uniquely on which conformation is excited and on which NH stretch vibration is excited within a given conformation.     

Architecture of a charge-transfer state regulating light harvesting in a plant antenna protein

Energy-dependent quenching of excess absorbed light energy (qE) is a vital mechanism for regulating photosynthetic light harvesting in higher plants. All of the physiological characteristics of qE have been positively correlated with charge transfer between coupled chlorophyll and zeaxanthin molecules in the light-harvesting antenna of photosystem II (PSII). We found evidence for charge-transfer quenching in all three of the individual minor antenna complexes of PSII (CP29, CP26, and CP24), and we conclude that charge-transfer quenching in CP29 involves a delocalized state of an excitonically coupled chlorophyll dimer. We propose that reversible conformational changes in CP29 can "tune" the electronic coupling between the chlorophylls in this dimer, thereby modulating the energy of the chlorophyll-zeaxanthin charge-transfer state and switching on and off the charge-transfer quenching during qE.     

Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.     

Direct electrochemical measurements inside a 2000 angstrom thick polymer film by scanning electrochemical microscopy

An extremely small, conically shaped Pt microelectrode tip (with a radius of 30 nanometers) and the precise positioning capabilities of the scanning electrochemical microscope were used to penetrate a thin (200 nanometers) polymer film and obtain directly the standard potential and kinetic parameters of an electrode reaction within the film. The thickness of the film was determined while it was immersed in and swollen by an electrolyte solution. The film studied was the perfluorosulfonate Nafion containing Os(bpy)(3)(2+) (bpy, 2,2'-bipyridine) cast on an indium tin oxide surface. The steady-state response at the ultramicroelectrode allowed direct determination of the rate constant for heterogeneous electron transfer K(o) and the diffusion coefficient D without complications caused by transport in the liquid phase, charge exchange at the liquid-polymer interface, and resistive drop.     

Quantum phase transition in organic charge-transfer complexes

A phase transition in an organic charge-transfer complex, which originates from the neutral-ionic valence instability, can be tuned toward zero kelvin with use of external pressure or chemical modification as a control parameter. The phase diagram and observed dielectric behaviors are typical of quantum paraelectricity, yet this zero-kelvin transition point namely, the quantum critical point, accompanies large quantum fluctuation of the molecular charge, as demonstrated by the molecular vibrational mode spectra. The result indicates that the pi-electron transfer between donor and acceptor molecules is coupled with the zero-point lattice dynamics around the quantum critical point.     

Molecular/Organic ferromagnets

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.     

Protein dynamics control the kinetics of initial electron transfer in photosynthesis

The initial electron transfer dynamics during photosynthesis have been studied in Rhodobacter sphaeroides reaction centers from wild type and 14 mutants in which the driving force and the kinetics of charge separation vary over a broad range. Surprisingly, the protein relaxation kinetics, as measured by tryptophan absorbance changes, are invariant in these mutants. By applying a reaction-diffusion model, we can fit the complex electron transfer kinetics of each mutant quantitatively, varying only the driving force. These results indicate that initial photosynthetic charge separation is limited by protein dynamics rather than by a static electron transfer barrier.     

Spin-charge separation and localization in one dimension

We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained by tunneling between two parallel wires in an GaAs/AlGaAs heterostructure while varying electron density. We observed two spin modes and one charge mode of the coupled wires and mapped the dispersion velocities of the modes down to a critical density, at which spontaneous localization was observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that was not observed. The measured spin velocity was smaller than theoretically predicted.     

Hydrated electron dynamics: from clusters to bulk

The electronic relaxation dynamics of size-selected (H2O)n-/(D2O)n[25 eaq-(s(dagger)) internal conversion lifetime.     

Nitrogenase complexes: multiple docking sites for a nucleotide switch protein

Adenosine triphosphate (ATP) hydrolysis in the nitrogenase complex controls the cycle of association and dissociation between the electron donor adenosine triphosphatase (ATPase) (Fe-protein) and its target catalytic protein (MoFe-protein), driving the reduction of dinitrogen into ammonia. Crystal structures in different nucleotide states have been determined that identify conformational changes in the nitrogenase complex during ATP turnover. These structures reveal distinct and mutually exclusive interaction sites on the MoFe-protein surface that are selectively populated, depending on the Fe-protein nucleotide state. A consequence of these different docking geometries is that the distance between redox cofactors, a critical determinant of the intermolecular electron transfer rate, is coupled to the nucleotide state. More generally, stabilization of distinct docking geometries by different nucleotide states, as seen for nitrogenase, could enable nucleotide hydrolysis to drive the relative motion of protein partners in molecular motors and other systems.     

The dynamic energy landscape of dihydrofolate reductase catalysis

We used nuclear magnetic resonance relaxation dispersion to characterize higher energy conformational substates of Escherichia coli dihydrofolate reductase. Each intermediate in the catalytic cycle samples low-lying excited states whose conformations resemble the ground-state structures of preceding and following intermediates. Substrate and cofactor exchange occurs through these excited substates. The maximum hydride transfer and steady-state turnover rates are governed by the dynamics of transitions between ground and excited states of the intermediates. Thus, the modulation of the energy landscape by the bound ligands funnels the enzyme through its reaction cycle along a preferred kinetic path.     

In-plane spectral weight shift of charge carriers in YBa2Cu3O6.9

The temperature-dependent redistribution of the spectral weight of the CuO2 plane-derived conduction band of the YBa2Cu3O6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01- to 5.6-electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states.     

Nonadiabatic interactions in the Cl + H2 reaction probed by ClH2- and ClD2- photoelectron imaging

The degree of electronic and nuclear coupling in the Cl + H2 reaction has become a vexing problem in chemical dynamics. We report slow electron velocity-map imaging (SEVI) spectra of ClH2- and ClD2-. These spectra probe the reactant valley of the neutral reaction potential energy surface, where nonadiabatic transitions responsible for reactivity of the Cl excited spin-orbit state with H2 would occur. The SEVI spectra reveal progressions in low-frequency Cl.H2 bending and stretching modes, and are compared to simulations with and without nonadiabatic couplings between the Cl spin-orbit states. Although nonadiabatic effects are small, their inclusion improves agreement with experiment. This comparison validates the theoretical treatment, especially of the nonadiabatic effects, in this critical region of the Cl + H2 reaction, and suggests strongly that these effects are minor.