Showing papers on "Excited state published in 1996"

Quantum Rabi oscillation: A direct test of field quantization in a cavity.

Abstract: We have observed the Rabi oscillation of circular Rydberg atoms in the vacuum and in small coherent fields stored in a high Q cavity. The signal exhibits discrete Fourier components at frequencies proportional to the square root of successive integers. This provides direct evidence of field quantization in the cavity. The weights of the Fourier components yield the photon number distribution in the field. This investigation of the excited levels of the atom-cavity system reveals nonlinear quantum features at extremely low field strengths.

Electronic Energy Transfer in CdSe Quantum Dot Solids

Abstract: We demonstrate electronic energy transfer between close packed quantum dots using cw and time resolved photoluminescence. Optically clear and thin, close packed quantum dot solids were prepared from mixtures of small and large CdSe quantum dots (38.5 and 62 \AA{}, $\ensuremath{\sigma}l4.5%$). Quenching of the luminescence (lifetime) of the small dots accompanied by enhancement of the luminescence (lifetime) of the large dots is consistent with long-range resonance transfer of electronic excitations from the more electronically confined states of the small dots to the higher excited states of the large dots.

Ultra-fast excited state dynamics in green fluorescent protein: multiple states and proton transfer.

Abstract: The green fluorescent protein (GFP) of the jellyfish Aequorea Victoria has attracted widespread interest since the discovery that its chromophore is generated by the autocatalytic, posttranslational cyclization and oxidation of a hexapeptide unit. This permits fusion of the DNA sequence of GFP with that of any protein whose expression or transport can then be readily monitored by sensitive fluorescence methods without the need to add exogenous fluorescent dyes. The excited state dynamics of GFP were studied following photo-excitation of each of its two strong absorption bands in the visible using fluorescence upconversion spectroscopy (about 100 fs time resolution). It is shown that excitation of the higher energy feature leads very rapidly to a form of the lower energy species, and that the excited state interconversion rate can be markedly slowed by replacing exchangeable protons with deuterons. This observation and others lead to a model in which the two visible absorption bands correspond to GFP in two ground-state conformations. These conformations can be slowly interconverted in the ground state, but the process is much faster in the excited state. The observed isotope effect suggests that the initial excited state process involves a proton transfer reaction that is followed by additional structural changes. These observations may help to rationalize and motivate mutations that alter the absorption properties and improve the photo stability of GFP.

Vibrational Energy Flow in Highly Excited Molecules: Role of Intramolecular Vibrational Redistribution

Abstract: A pedagogical overview of intramolecular vibrational redistribution (IVR) phenomena in vibrationally excited molecules is presented. In the interest of focus and simplicity, the topics covered deal primarily with IVR in the ground electronic state, relying on examples from the literature to illustrate key points. The experimental topics discussed attempt to sample systematically three different energy regimes on the full potential surface corresponding to (i) “low”, e.g., moderate- to high-resolution vibrational spectroscopies, (ii) “intermediate”, e.g., stimulated emission pumping and high overtone spectroscopies, and (iii) “high”, e.g., photofragment/predissociation dynamical spectroscopies. The interplay between experiment and theory is highlighted here because it has facilitated enormous advances in the field over the past decade.

Environmental Photochemistry on Semiconductor Surfaces: Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light

Abstract: Photosensitized degradation of a textile azo dye, Acid Orange 7, has been carried out on TiO2 particles using visible light Mechanistic details of the dye degradation have been elucidated using diffuse reflectance absorption and FTIR techniques Degradation does not occur on Al2O3 surface or in the absence of oxygen The dependence of the dye degradation rate on the surface coverage shows the participation of excited dye and TiO2 semiconductor in the surface photochemical process Diffuse reflectance laser flash photolysis confirms the charge injection from the excited dye molecule into the conduction band of the semiconductor as the primary mechanism for producing oxidized dye radical The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the oxidized dye radical and the photoinjected electrons Diffuse reflectance FTIR was used to make a tentative identification of reaction intermediates and end products of dye degradation

Synthesis of Size-Selected, Surface-Passivated InP Nanocrystals

Abstract: Quantum-confined InP nanocrystals from 20 to 50 A in diameter have been synthesized via the reaction of InCl3 and P(Si(CH3)3)3 in trioctylphosphine oxide (TOPO) at elevated temperatures. The nanocrystals are highly crystalline, monodisperse, and soluble in various organic solvents. Improved size distributions have been obtained by size-selectively reprecipitating the nanocrystals. The UV/vis absorption spectra of the particles show the characteristic blue shift of the band gap of up to 1 eV due to quantum confinement, a moderately well-resolved first excitonic excited state, and, in some cases, the resolution of a higher excited state. Structurally, the nanocrystals are characterized with powder X-ray diffraction and transmission electron microscopy. Raman spectroscopy reveals TO and LO modes near the characteristic bulk InP positions as well a surface mode resulting from finite size. The Raman line widths, line positions, and relative intensities are all size-dependent . X-ray photoelectron spectroscopy ...

Dynamics of molecules and chemical reactions

Abstract: Spectra, rates, and intranolecular dynamics quantum mechanical studies of molecular spectra and dynamics picturing quantized intranolecular vibrational energy flow: action diffusion, localization, and scaling canonical Van Vleck perturbation theory and its application to studies of highly vibrationally excited states of polyatomic molecules quantum molecular dynamics on grids time-dependent quantum dynamics for gas-phase and gas-surface reactions new methods for use in scattering calculations: the spectral projection method and the stabilization method time-independent wave-packet-distributed approximating functional approach to quantum dynamics computational spectroscopy of the transition state theory: rigorous quantum approaches for determining chemical reaction rates the geometric phase in reaction dynamics nondiabatic transitions: beyond Born-Oppenheimer application of semiclassical dynamics to chemical reactions nonlinear classical dynamics and unimolecular reactions the classical trajectory approach to reaction dynamics theory of activated rate processes.

Influence of Nearly Resonant Light on the Scattering Length in Low-Temperature Atomic Gases.

Abstract: We develop the idea of manipulating optically the scattering length $a$ in low-temperature atomic gases. If the incident light is close to resonance with one of the bound $p$ levels of electronically excited molecules, virtual radiative transitions of a pair of interacting atoms to this level can significantly change the value and even reverse the sign of $a$. The decay of the gas due to photon recoil and due to photoassociation can be minimized by selecting the frequency detuning and the Rabi frequency. Our calculations show the feasibility of optical manipulations of trapped Bose condensates through a light-induced change in the mean field interaction between atoms, which is illustrated for ${}^{7}\mathrm{Li}$.

Multichannel Rydberg spectroscopy of complex atoms

Abstract: Multichannel atomic spectra frequently exhibit such extraordinary visual complexity that they appear at first glance to be uninterpretable. The present review discusses how to unravel such spectra through the use of theoretical multichannel spectroscopy to extract the key dynamical implications. Moreover, this class of techniques permits a quantitative prediction or reproduction of experimental spectra for some of the more challenging atomic systems under investigation. It is shown that multichannel spectroscopy marries the techniques of multichannel quantum-defect theory to the eigenchannel $R$-matrix method (or related methods). It has long been appreciated that multichannel quantum-defect theory can successfully use a collision-theory framework to interpret enormously complicated Rydberg spectra. However, the capabilities of multichannel quantum-defect theory have increased dramatically during the past decade, through the development of nearly ab initio methods for the calculation of the short-range scattering parameters that control the interactions of closed and open channels. In this review, emphasis is given to the alkaline-earth atoms, for which many different observables have been successfully compared with experiment over broad ranges of energy and resolution. Applications of the method to describe the photoionization spectra of more complex open-shell atoms are also discussed. [S0034-6861(96)00504-1]

A refined H3 potential energy surface

Abstract: In evaluating some low temperature (T<1000 K) thermal rate coefficients for inelastic rotational excitation of H2 by H atoms, Sun and Dalgarno have found a marked sensitivity to the potential energy surface adopted for the calculations. We have investigated the origin of the discrepancies between previous H3 potential energy surfaces and have developed a refined surface which addresses these concerns. New quasiclassical trajectory calculations of cross sections for low energy rotational excitation are reported. The refined surface is based on 8701 ab initio energies, most newly computed for this purpose. It has the same functional form as our earlier (BKMP) surface, but since the fit of the parameters is more fully constrained than for any previous surface it is a more accurate representation. The refined surface matches the ab initio energies with an overall rms error of 0.27 mEh (i.e., 0.17 kcal/mol) and a maximum absolute deviation of 6.2 mEh (for a very compact high energy equilateral triangle conformation). For ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.18 mEh and the maximum absolute deviation is 1.7 mEh. The refined surface is compared critically to four previous surfaces, including the DMBE surface of Varandas et al., in several respects: Legendre expansion coefficients; the interaction region for low energy rotational excitation; near the collinear saddle point; near conical intersections of the ground and first excited state surfaces; the van der Waals well; and compact geometries. We have also compared new first excited state ab initio energies for 1809 conformations with corresponding predictions from the DMBE surface.

Energy and Photoinduced Electron Transfer in Porphyrin−Fullerene Dyads

Abstract: Time-resolved fluorescence and absorption techniques have been used to investigate energy and photoinduced electron transfer in a covalently linked free-base porphyrin−fullerene dyad and its zinc analog. In toluene, the porphyrin first excited singlet states decay in about 20 ps by singlet−singlet energy transfer to the fullerene. The fullerene first excited singlet state is not quenched and undergoes intersystem crossing to the triplet, which exists in equilibrium with the porphyrin triplet state. In benzonitrile, photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene competes with energy transfer. The fullerene excited singlet state is also quenched by electron transfer from the porphyrin. Overall, the charge-separated state is produced with a quantum yield approaching unity. This state lives for 290 ps in the free-base dyad and 50 ps in the zinc analog. These long lifetimes suggest that such dyads may be useful as components of more complex light-harvesting systems.

Density-functional theory for excited states

Abstract: A Kohn-Sham formalism for the treatment of excited states within the density-functional theory (DFT) is presented. DFT exchange and correlation energy functionals for excited states are defined. Explicit expressions for these functionals are derived by generalizing a recent DFT perturbation theory. A computational scheme for the treatment of excited states within the DFT is suggested. Differences of Kohn-Sham eigenvalues are shown to be well-defined approximations for excitation energies. Correction terms to these approximations are presented. Perturbation theory expansions for band gaps are discussed. \textcopyright{} 1996 The American Physical Society.

Dynamics of Bimolecular Photoinduced Electron-Transfer Reactions

Abstract: From the accumulated results of several research groups over the last 25 years, it is clear that photoinduced electron-transfer reactions have significantly broadened the scope of organic photochemistry.1 The fundamental mechanistic principle is that when quenching of an excited state via electron transfer is sufficiently exothermic, the reaction occurs at or close to the diffusion-controlled limit (kdiff). In polar solvents, where most reactions are carried out, the primary intermediate is a geminate radical-ion pair, A•-/D•+ (eq 1).3 Return electron transfer within the

Gain and differential gain of single layer InAs/GaAs quantum dot injection lasers

Abstract: We present gain measurements and calculations for InAs/GaAs quantum dot injection lasers. Measurements of the modal gain and estimation of the confinement factor by transmission electron microscopy yield an exceptionally large material gain of 6.8(±1)×104 cm−1 at 80 A cm−2. Calculations including realistic quantum dot energy levels, dot size fluctuation, nonthermal coupling of carriers in different dots, and band filling effects corroborate this result. A large maximum differential gain of 2×10−12 cm2 at 20 A cm−2 is found. The width of the gain spectrum is determined by participation of excited quantum dot states. We record a low transparency current density of 20 A cm−2. All experiments are carried out at liquid nitrogen temperature.

Electron Interactions with CF4

Abstract: Carbon tetrafluoride (CF4) is one of the most widely used components of feed gas mixtures employed for a variety of plasma‐assisted material‐processing applications. It has no stable excited states and, in a plasma environment, is an ideal source of reactive species, especially F atoms. To assess the behavior of CF4 in its use in manufacturing semiconductor devices and other applications, it is necessary to have accurate information about its fundamental properties and reactions, particularly its electronic and ionic interactions and its electron collision processes at low energies (<100 eV). In this article we assess and synthesize the available information on the cross sections and/or the rate coefficients for collisional interactions of CF4 with electrons. Assessed information is presented on: (i) cross sections for electron scattering (total, momentum, elastic differential, elastic integral, inelastic), electron‐impact ionization (total, partial, multiple, dissociative), electron‐impact dissociation (...

Electronic Excitation Transfer in the LH2 Complex of Rhodobacter sphaeroides

Abstract: Ultrafast fluorescence upconversion measurements were carried out on the peripheral (LH2) light harvesting antenna complex of Rhodobacter sphaeroides isolated in the detergents N-octyl-β-d-glucopyranoside and lauryl dimethylamine oxide. The B800 and B850 bands were excited in separate experiments, and the B850 emission was detected in each case. We make use of the recently determined crystal structure of a purple bacterial LH2 complex to simulate our data and to calculate the exciton level structure of the B850 aggregate. The B800 to B850 excitation transfer occurs with a 650 fs time constant. The depolarization of B850 emission follows a wavelength dependent, biexponential decay with time constants 50−90 and 400−500 fs. We can reproduce the non-exponentiality of the depolarization by assuming incoherent hopping between dimeric sites with a 250 cm-1 full width at half-maximum (fwhm) Gaussian distribution of site energies (inhomogeneity). We calculate the homogeneous hopping time between dimers in B850 to ...

Exciton Delocalization Length in the B850 Antenna of Rhodobacter sphaeroides

Abstract: The properties of elementary excited states in the B850 band of the peripheral light-harvesting antenna (LH2) of the photosynthetic purple bacterium Rhodobacter sphaeroides has been studied at room temperature by means of femtosecond transient absorption experiments combined with computer simulations. Polarized pump−probe kinetics have a fast component of 100 and 65 fs for the anisotropic and isotropic decays, respectively. Direct numerical simulations show that for incoherent hopping-like excitation transfer in the B850 ring of 18 Bchl a molecules at room temperature the fast component of the anisotropy decay is 3 times longer than the corresponding component of the isotropic decay, strongly suggesting that delocalized exciton states are involved in the observed dynamics. To estimate the coherence length of the exciton we have measured absorption difference spectra of LH2 from 810 to 880 nm 2 ps after the excitation into the B800 band with 75 fs laser pulses. Exciton calculations where also monomeric dou...

Multistep Photochemical Charge Separation in Rod-like Molecules Based on Aromatic Imides and Diimides

Abstract: A series of intramolecular triads with linear, rod-like structures has been developed that undergo very efficient two-step electron transfer following direct excitation of a chromophore possessing a charge transfer (CT) excited state. The CT state of 4-aminonaphthalene-1,8-imide (ANI), produced by direct excitation of the chromophore, has about 70% of a negative charge transferred from the amine to the imide. Attachment of aniline (An) and p-methoxyaniline (MeOAn) donors to ANI by means of a piperazine bridge results in linear dyads, An-ANI and MeOAn-ANI, that undergo rapid electron transfer in about 10-11 s to give a >99% yield of the ion pairs, An+-ANI- and MeOAn+-ANI-, in which the charges are separated by 7.7 A. The formation and decay of these ion pairs can be monitored directly by transient absorption spectroscopy. Further attachment of a 1,8:4,5-naphthalenediimide (NI) electron acceptor to the imide group of ANI using a 2,5-dimethylphenyl spacer results in triads An-ANI-NI and MeOAn-ANI-NI. Excitat...

The Microwave Spectrum of trans-2,3-Dimethyloxirane in Torsional Excited States

Abstract: Abstract The microwave spectrum of trans-2,3-dimethyloxirane (CH3CHOCHCH3) in the excited tor-sional states υ17 = 1 and υ33 = 1 has been measured in the range from 8 to 26 GHz and assigned. An analysis of internal rotation splittings of the observed rotational transitions was performed using the internal axis method (or \"combined axis method\") with a newly developed program accounting for the top-top coupling. The threefold hindering potential V3 and the direction cosines λ g,i of the internal rotation axes i with respect to the principal inertia axes g are in a good agreement with the ground state values. Additionally, the sixfold hindering parameter V6 was found to be - 0.2600(12) kJ/mol. The value of the parameter V′12 describing the top-top coupling in the potential function (via V′12 sin 3 τ1 sin 3 τ2), was determined to -0.4240(6) kJ/mol.

The important role of heteroaromatics in the design of efficient second-order nonlinear optical molecules: Theoretical investigation on push-pull heteroaromatic stilbenes

Abstract: First hyperpolarizabilities of a large number of push−pull substituted conjugated systems with heteroaromatic spacers have been calculated. The static, nonresonant components were computed at the ab initio level (4-31G basis) using the coupled perturbed Hartree−Fock approach and at the AM1 level employing the finite field method. Sum-over-states procedure has also been used with the AM1/CI method to compute β0 and β at an excitation energy of 1.17 eV. The computed β values at the various levels are reasonably similar and exhibit the same trends. The largest values are obtained with a donor on pyrrole and an acceptor on thiophene or thiazole. The variations do not always inversely follow the order of delocalization energies of the heterocyclic rings. The trends in the dipole moment changes and transition energies between the ground and first excited charge-transfer state primarily determine the variations in the computed β values.

Circular Dichroism and Circular Polarization of Photoluminescence of Highly Ordered Poly{3,4-di[(S)-2-methylbutoxy]thiophene}

Abstract: Conformationally disordered poly{3,4-di[(S)-2-methylbutoxy]thiophene} is prepd via monomer polymn by anhyd FeCl3 Optically active ordered polymer is prepd by cooling of the polymer to -30 Deg In the aggregated phases a splitting of the excited state into two exciton levels occurs

Ultranarrow Spectral Lines via Quantum Interference.

Abstract: We show that ultrasharp spectral lines, with widths 2 orders of magnitude below the natural width, may be produced in the resonance fluorescence of a V-type three-level atom excited by a single-mode laser field when the dipole moments are nearly parallel. The smaller the splitting of the excited doublet, the narrower the line. This effect is due to quantum interference between the two transition pathways.

QM/MMpol: A Consistent Model for Solute/Solvent Polarization. Application to the Aqueous Solvation and Spectroscopy of Formaldehyde, Acetaldehyde, and Acetone

Abstract: We present a hybrid quantum mechanical/molecular mechanical theory that consistently treats many-body polarization effects in the MM region (QM/MMpol). The method described here is based on our previous theory for single-point ground and excited states. Here we extend the formalism to include analytical gradients, making the approach useful for molecular dynamics (MD) simulations. Central to our development is a consistent treatment of the interaction of the MM polarizable dipoles with the full QM wave function in a fashion that allows for energy conservation in the MD. We present implementation details for NDDO semiempirical QM Hamiltonians of the MNDO type. We also present an analysis of the solvent and substituent effects on the spectroscopic blue shift of the n−π* electronic excited state of a series of carbonyl-containing solutes: formaldehyde, acetaldehyde, and acetone. We use the AM1 semiempirical Hamiltonian to treat the solute and the POL1 polarizable model for the solvent. For formaldehyde, we ...

Vibrational energy relaxation of HOD in liquid D2O

Abstract: Molecular Dynamics simulation is used to study the vibrational relaxation of the first excited state of the O–H stretch for HOD dissolved in D2O. The technique applied is based on a Landau–Teller type formula, in which the solvent contribution is computed classically, while the quantum nature of the solute enters through the transition moments of the molecular normal modes. The experimental result for the relaxation time (≊8 ps) is accounted for, and the pathway to the ground state is determined. The relaxation proceeds through a sequence of intramolecular transitions initially facilitated by the solute internal anharmonicities. In particular, the anharmonicity allows an initial and rate‐determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step is precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, including the solute. Relaxation by Coriolis coupling plays a minor but non‐negligible role, while the centrifugal coupling contribution to the relaxation is negligible.

Time-resolved spectroscopy of wild-type and mutant Green Fluorescent Proteins reveals excited state deprotonation consistent with fluorophore-protein interactions

Abstract: Recently steady-state and picosecond time-resolved absorption and fluorescence spectroscopy on the Green Fluorescent Protein (GFP) have been interpreted by a mechanism where the key process is an excited state deprotonation of the chromophore (M. Chattoraij, B.A. King, G.U. Bublitz and S.G. Boxer, Proc. Natl. Acad. Sci. USA, 93 (1996) 8362–8367). Such a conclusion was borne out by the mirror image of the picosecond decay of the protonated species RH∗ in the blue and the concomitant picosecond rise of the green fluorescence of the deprotonated fluorophore R−∗ as well as the significant slowing of both kinetic features upon deuteration. We report similar experiments confirming this mechanism. The results of ultrafast spectroscopy on wild-type GFP together with two important mutants combined with the recent crystal structures are shown to shed more light on the interplay between absorption and emission phenomena in GFP. Beyond some differences with previous results pertaining, for instance, to the assignment of vibronic progressions in absorption spectra and the temperature dependence of excited state deprotonation, several new features have been identified. These concern the deprotonated ground state R− in equilibrium as well as the excited state RH∗. In particular, we have studied the distributed fluorescence kinetics in the time and frequency domain, excited state absorption features observed in femtosecond time-resolution, and the dependence of excited state proton transfer kinetics on the aggregational state of the protein.

High resolution uv spectroscopy of phenol and the hydrogen bonded phenol/water cluster

Abstract: The S1←S0 000 transitions of phenol and the hydrogen bonded phenol(H2O)1 cluster have been studied by high resolution fluorescence excitation spectroscopy. All lines in the monomer spectrum are split by 56±4 MHz due to the internal rotation of the −OH group about the a axis. The barrier for this internal motion is determined in the ground and excited states; V2″=1215 cm−1, and V2′=4710 cm−1. The rotational constants for the monomer in the ground state are in agreement with those reported in microwave studies. The excited state rotational constants were found to be A′=5313.7 MHz, B′=2620.5 MHz, and C′=1756.08 MHz. The region of the redshifted 000 transition of phenol(H2O)1 shows two distinct bands which are 0.85 cm−1 apart. Their splitting arises from a torsional motion which interchanges the two equivalent H atoms in the H2O moiety of the cluster. This assignment was confirmed by spin statistical considerations. Both bands could be fit to rigid rotor Hamiltonians. Due to the interaction between the overal...