Showing papers on "Excited state published in 1994"

Proton transfer in solution: Molecular dynamics with quantum transitions

Abstract: We apply ‘‘molecular dynamics with quantum transitions’’ (MDQT), a surface‐hopping method previously used only for electronic transitions, to proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom, including the solvent molecules, and treat the hydrogen motion quantum mechanically. We identify new obstacles that arise in this application of MDQT and present methods for overcoming them. We implement these new methods to demonstrate that application of MDQT to proton transfer in solution is computationally feasible and appears capable of accurately incorporating quantum mechanical phenomena such as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to represent the proton transfer reaction AH–B■A−–H+B in liquid methyl chloride, where the AH–B complex corresponds to a typical phenol–amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a stringent test of the theory. MDQT proves capable of treating both limits, as well as the intermediate regime. Up to four quantum states were included in this simulation, and the method can easily be extended to include additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this method is not perturbative, so trajectories can be continued after the barrier is crossed to follow the subsequent dynamics.

Spectroscopy of quantum levels in charge-tunable InGaAs quantum dots.

Abstract: Imbedding self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons. Because of their small diameters of about 20 nm the Coulomb charging energy is significantly smaller than the quantization energies. We extract energy spacings of about 41 meV between the $s$-like ground state and the first excited $p$-like state from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.

Spontaneous emission near the edge of a photonic band gap

Abstract: We study spontaneous emission near the edge of a photonic band gap. Instead of a simple exponential decay in the vacuum, spontaneous emission displays an oscillatory behavior. A single photon-atom bound dressed state exhibits a fractional steady-state atomic population on the excited state. For a three-level atom we evaluate the spectral splitting and subnatural linewidth of spontaneous emission. In the presence of N-1 unexcited atoms we show that the collective time scale factor is equal to ${\mathit{N}}^{\mathrm{\ensuremath{\varphi}}}$, where \ensuremath{\varphi}=2/3 for an isotropic band gap and \ensuremath{\varphi}=1 or 2 for anisotropic two-dimensional or three-dimensional band edges, respectively.

Analytic energy derivatives for ionized states described by the equation‐of‐motion coupled cluster method

Abstract: The theory for analytic energy derivatives of excited electronic states described by the equation‐of‐motion coupled cluster (EOM‐CC) method has been generalized to treat cases in which reference and final states differ in the number of electrons. While this work specializes to the sector of Fock space that corresponds to ionization of the reference, the approach can be trivially modified for electron attached final states. Unlike traditional coupled cluster methods that are based on single determinant reference functions, several electronic configurations are treated in a balanced way by EOM‐CC. Therefore, this quantum chemical approach is appropriate for problems that involve important nondynamic electron correlation effects. Furthermore, a fully spin adapted treatment of doublet electronic states is guaranteed when a spin restricted closed shell reference state is used—a desirable feature that is not easily achieved in standard coupled cluster approaches. The efficient implementation of analytic gradien...

Decay of ultraviolet-induced fiber bragg gratings

Abstract: We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium‐doped silica fiber. The decay is well characterized by a ‘‘power‐law’’ function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power‐law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated‐aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet‐induced refractive‐index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and keeping only the very stable portion of the index change.

Measurement of the size dependent hole spectrum in CdSe quantum dots

Abstract: We combine a new synthesis with transient optical hole burning to observe and assign the size evolution (19 to 115 \AA{} diameter, \ensuremath{\sigma}5%) of a series of excited states in CdSe quantum dots. We observe an avoided crossing around the spin orbit energy in the hole spectra for \ensuremath{\sim}65 \AA{} dots, indicating the importance of valence band complexities in the description of the excited states. Comparsion with dc Stark data shows that bleach spectra are consistent with localized carrier induced electric fields.

Metal—lingad and metal—metal coupling elements

Abstract: The electronic matrix element coupling a ground and charge-transfer excited state can be calculated from the energy intensity of the appropriate charge-transfer transition. An expression for the electronic coupling element widely used for this purpose is based on equations derived by Mulliken and Hush for an effective two-state model and is frequently assumed to be valid only in the perturbation limit. This expression is shown to be exact within a two-state model. Provided that overlap can be neglected and that the spectroscopic transition is polarized along the donor—acceptor axis, it can be applied to system ranging from those which are very weakly coupled to those which are very strongly coupled. Application of the Mulliken—Hush expression to (NH3)5RuL2+ complexes, for which metal—lingand backbonding is important, yields metal—lingand coupling elements of 5000–6000 cm−1 with pyridyl lingands (donor—acceptor separation 3.5 A), in very good agreement with estimates obtained from a molecular orbital analysis of the band energies. With use of the superexchange formalism, the metal—lingand coupling elements were used to calculate metal—metal coupling elements for binuclear mixed-valence complexes. Comparison of these values with those obtained from the Mulliken—Hush expression applied directly to the metal-to-metal charge-transfer transition yields agreement within a factor of two or better.

Time-resolved electron-temperature measurement in a highly excited gold target using femtosecond thermionic emission.

Abstract: We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11 000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission The fast relaxation times (15 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron temperatures

Reference interaction site model self-consistent field study for solvation effect on carbonyl compounds in aqueous solution

Abstract: In the previous study, Chem. Phys. Lett. 214, 391 (1993), we developed a new computational procedure for the solvation effect on the electronic structure of solute based upon the reference interaction site model (RISM) integral equation and the Hartree–Fock equation. The method enables us to calculate the solvent distribution and solute electronic wave functions simultaneously, which is free from such empirical parametrizations as appeared in the usual models based on the dielectric continuum picture. In the present article, we have applied the method to several carbonyl compounds in aqueous solution. The SPC model was used to describe the liquid water. The vertical n→π*, π→π*, and σ→π* transitions of formaldehyde are examined by the RISM‐self‐consistent field formalism coupled with the restricted Hartree–Fock approximation, and then the free energy calculation was performed for the excited state in order to estimate the contributions for the optical fluorescence spectra. The intramolecular energy turned out to give significant contribution to the bathochromic shift of fluorescence relative to the absorption in the liquid phase. Furthermore the importance of the structural effect of the functional group was discussed by the calculations of three more carbonyl compounds, acetaldehyde, acetone, and acrolein.

The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2

Abstract: The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm−1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm−1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO2.

Excited-State Intramolecular Proton Transfer in 2-(2-Hydroxyphenyl)benzimidazole and -benzoxazole: Effect of Rotamerism and Hydrogen Bonding

Abstract: An excited-state intramolecular proton transfer (ESIPT) process in 2-(2’-hydroxyphenyl) benzimidazole and -benzoxazole (HPBI and HBO, respectively) has been studied using steady-state and time-resolved emission spectroscopy at various temperatures and by semiempirical quantum chemical methods. For both of them two distinct ground-state rotamers I and I1 respectively responsible for the “normal” and the “tautomer” emission have been detected. In hydrocarbon solvents at room temperatureand at 77 K the tautomer emission predominates over the normal emission for both HPBI and HBO. This indicates that rotamer 11, responsible for the tautomer emission, is intrinsically stabler than rotamer I, which causes the normal emission. In alcoholic glass at 77 K for HPBI a dramatic enhancement of the normal emission is observed. It is suggested that due to the increased solvation, the more polar rotamer I becomes stabler than I1 for HPBI in alcohol and the substantial temperature variation is due to the change in the population of the two rotamers with temperature. From the detailed temperature variation in alcoholic medium the ground-state energy difference between rotamers I and I1 is determined. In dioxane-water mixtures it is observed that with the addition of water the quantum yield of the normal emission increases, which is ascribed to the inhibition of the ESIPT process due to the formation of an intermolecular hydrogen bond involving water. CNDO/S-CI calculations were performed optimizing thegroundstate geometry by the AM1 method. Details of the energy, dipole moment, and charge distribution of the rotamers in the ground state (SO) and the first excited singlet state (SI) and the barrier for the interconversion of I and I1 in SO, SI, and first excited triplet state are discussed. The calculation indicates that the barrier for the interconversion of the two rotamers is too high in the excited state (SI and TI) for free interconversion.

Spontaneous and induced atomic decay in photonic band structures

Abstract: We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either ‘bare’ or ‘dressed’ by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of non-decaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the ind...

Quantum size dependence of femtosecond electronic dephasing and vibrational dynamics in CdSe nanocrystals

Abstract: Femtosecond photon-echo techniques are used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe. Using three-pulse photon echoes, the modulation of the echo signal from the LO-phonon mode is effectively suppressed, and both the electronic dephasing and the couplings to lattice vibrations are probed directly. Detailed measurements are reported as a function of both nanocrystal size and temperature. The dephasing times vary from 85 fs in nanocrystals of 20-\AA{} diameter to 270 fs in 40-\AA{} crystals. These rates are determined by several dynamical processes, all of which depend sensitively on the size of the nanocrystal. The time scale of the trapping of the electronic excitation to surface states increases with increasing size. The coupling of the excitation to low-frequency vibrational modes is strongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between the electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a manifestation of coupling between the interior confined excitons and localized surface states, which destroys the spherical symmetry of the excited state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.

Optical probes of excited states in poly(p-phenylenevinylene).

Abstract: We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. We have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation.

Direct imaging of reptation for semiflexible actin filaments

Abstract: ACCORDING to the reptation model of polymer diffusion1, a polymer chain exhibits snake-like motion through the entangled mesh of surrounding molecules, in which the undulations of the chain are restricted to a tubelike region2 The reptation model can account for many of the dynamic properties of entangled polymer solutions and melts, and has received support from observations of block copolymer diffusion across an interface3; but reptative motion has not previously been imaged directly4,5 Here we report such a direct observation of reptation, obtained by video microscopy of fluorescently labelled single, semiflexible filaments of actin in a solution of unlabelled actin filaments From the restricted thermal undulations of these filaments we can measure the diameter of the confining tube, and we also observe the characteristic thermally excited sliding of the filament out of the end of the tube We find that the chain self-diffusion coefficient decreases approximately linearly as the filament length increases, in agreement with the reptation model

Photophysical and photosensitizing properties of benzoporphyrin derivative monoacid ring A (BPD-MA)

Abstract: The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O2 (1 delta g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (lambda approximately 688 nm, epsilon approximately 33,000 M-1 cm-1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, Es = 42.1 kcal mol-1, lifetime, tau f = 5.2 ns and fluorescence quantum yield, phi f = 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, tau T > or = 25 microseconds, an energy level, ET = 26.9 kcal mol-1 and the molar absorption coefficient is epsilon T = 26,650 M-1 cm-1 at 720 nm. A dramatic effect of oxygen on the fluorescence (phi f) and intersystem crossing (phi T) quantum yields has been observed. The BPD-MA presents rather high triplet (phi T = 0.68 under N2-saturated conditions) and singlet oxygen (phi delta = 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (phi Pb) of which was found to be on the order of 5 x 10(-5) in organic solvents.

Lifetimes for Radiative Charge Recombination in Donor-Acceptor Molecules

Abstract: In this paper we demonstrate that the marked solvent dependence of the rates kmd for radiative recombination in some donor (D)-bridge (B)-acceptor (A) molecules, which increase with decreasing solvent polarity (i.e., with increasing peak energy ( v ) for charge-transfer fluorescence), can be quantitatively accounted for in terms of a dominating contribution of (DBA)*-D+BAmixing, involving intensity borrowing from local (DBA)* electronic excitations. In these DBA molecules, the traditional two-level D+BA--DBA coupling scheme is inapplicable. The analysis of the ( v ) dependence of krad for a certain DBA in a series of solvents results in the (DBA)*-D+BAcouplings V , which are in good agreement with the V parameters extracted from oscillator strengths for charge-transfer absorption. The V parameters, which obey the relation V' 0: exp(-aN) (where N is the number of bonds in the bridge), determine the rates for nonradiative (DBA)* D+BAcharge separation and recombination from electronically excited states. The (DBA) *-D+BAmixing is maximized for the isolated, solvent-free DBA molecule. For the isolated molecules analyzed herein, the fraction of (DBA)* admixture within the charge-transfer state is -0.02, being even smaller for the solvated molecules.

Fully quantal initial‐state‐selected reaction probabilities (J=0) for a four‐atom system: H2(v=0, 1, j=0)+OH(v=0,1, j=0)→H+H2O

Abstract: Exact‐dynamics (six‐dimensional) quantum simulations of energy‐resolved initial‐state‐selected rearrangement reaction probabilities are presented for H2(v=0,1,j=0) +OH(v=0,1,j=0) →H+H2O, at J=0, using the time‐dependent reactive‐scattering formalism. A few narrow resonances appear at low reaction energies when the H2 is vibrationally excited, and are shown to be partially associated with the strong‐interaction region (in addition to the asymptotic reagents channel, where the potential has an unphysical well). Vibrational excitation of the OH bond is shown to exhibit little influence on the reaction probabilities. Together with similar results due to Zhang and Zhang (J. Chem. Phys., in press), these are the first initial‐state‐selected simulations of exact‐dynamics four‐atom molecular reactions.

Molecular-Level Electron Transfer and Excited State Assemblies on Surfaces of Metal Oxides and Glass

Abstract: A general procedure is described for the attachment to antimony-doped tin dioxide (Sn02:Sb), tin-doped indium oxide (In203:Sn), or glass surfaces of molecules with known electron transfer or excited state properties, e.g. [Ru(bpy)2(4,4’-(CO~H)zbpy)] (PF6)2 (bpy = 2,2’-bipyridine, 4,4’-(COzH)zbpy = 4,4’-dicarboxy-2,2’-bipyridine), based on the interaction between surface hydroxyls and carboxylic acid groups. Integrations of cyclic voltammetric waveforms on the metal oxide electrodes give maximum surface coverages of r 1 X 10-10 mol/cm2 for the ruthenium complex, which corresponds to a monolayer coverage. Atomic force microscope (AFM) measurements reveal that the metal oxide surfaces are highly roughened with root mean square roughnesses in the range 4-6.5 nm for tin oxide. The smaller organics, N-methyl-N-viologenpropanoic acid bis(hexafluorophosphate), [MVC02H] (PF6)2, and 1 OH-phenothiazine10-propanoic acid, PTZ-C02H, display similar surface coverages. Resonance Raman measurements on surfaces containing the ruthenium complex imply that attachment to SnO2, In203, and Ti02 is via an ester bond. For ,902, two modes of binding are suggested, a majority by a chelating carboxylato link and a minority by ester formation. Binding constants for surface attachment were measured in CH2Cl2 at 298 K by equilibration, which gave K = 8 X lo4 M-’ on both SnO2:Sb and InzO3:Sn. Surface molecular assemblies have been prepared containing [ Ru( bp y ) 2(4,4’(COzH)2bpy)] (PF6) 2 and [Os( bpy) 2( 4,4’(CO2H)2 bpy ) ] (PF6) 2, [ MVCO#)(PF6)2, and PTZ-CO2H. In these assemblies, separate waves are observed for the different redox couples at potentials near those found for surfaces containing only a single component. Emission decay of the metal-toligand charge transfer (MLCT) excited state of [Ru(bpy)2(4,4’-(CO~H)2bpy)](PF6)2 attached to the glass backings of metal oxide electrodes or to glass slides was found to be nonexponential with average lifetimes (( 7 ) ) from < 5 to 600 ns with CHzCl2 in the external solution. ( T ) increases as surface coverage decreases. There is evidence for excited stateground state interactions by a red-shift in the emission maximum as surface coverage increases. Emission decay remains nonexponential even on surfaces that are lightly covered. Emission is nearly completely quenched on the semiconductor surfaces, with ( T ) < 5 ns. The bound Ru(I1) emitters on glass were quenched by electron or energy transfer to the coattached quenchers [MV-C02H] (PF& FTZ-C02H, or [Os(bpy)2(4,4’-(C02H)zbpy)] (PF& suggesting that lateral electron and energy transfer can occur across the surface. Surface lifetime quenching also occurred in the presence of added 10-methyl-10-phenothiazene in the external CH2Cl2 solution. The kinetics of lifetime quenching did not follow Stern-Volmer kinetics but could be fit to a model in which there are both quenchable and unquenchable sites on the same surface.

Rate expressions for excitation transfer. II. Electronic considerations of direct and through–configuration exciton resonance interactions

Abstract: The electronic interactions which promote singlet–singlet and triplet–triplet electronic excitation (energy) transfer (EET) are investigated in detail. Donor and acceptor locally excited configurations, ψ1(A*B) and ψ4(AB*), respectively, are each allowed to mix with bridging ionic configurations, ψ2(A+B−) and ψ3(A−B+) to form the new donor and acceptor wave functions ΨR=ψ1+λψ2+μψ3 and ΨP=ψ4+μψ2+λψ3. Use of the latter wave functions leads to the establishment of the matrix element TRP= 〈ΨR‖H−E1‖ΨP〉≊T14−(T12T24+T 13T34)/A, with Tij=〈ψi‖H−E1‖ψj〉 and A=E2−E1, as the exciton resonance interaction term for EET. Introduction of the Mulliken approximation shows that the ‘‘direct’’ exciton resonance interaction term (T14) contributes primarily a Coulombic interaction, for singlet–singlet EET, while the ‘‘through–configuration’’ exciton resonance interaction term [−(T12T24+T13T34)/A] replaces the Dexter exchange integral (which is a component of H14) as the primary source of short‐range orbital overlap‐dependent EE...

Photophysical and photochemical properties of fullerenes

Abstract: On irradiation with visible and UV light, fullerenes C60 and C70 give high yields of their triplet states. Fluorescence is very weak from both, and phosphorescence has been observed only from C70. The triplet states are also formed efficiently by energy transfer from triplet sensitizers with energies above 35 kcal/mol. Triplet fullerenes transfer energy efficiently to oxygen, giving singlet molecular oxygen, but they react and quench singlet oxygen very inefficiently, making them excellent photosensitizers. The fullerenes C60 and C70 are easily reduced but very difficult to oxidize. Electron transfer to triplet C60 and C70 occurs readily from electron donors to produce the radical anions. C60 can also be oxidized by excited high-potential photosensitizers to the radical cation. Some dihydrofullerenes also give high yields of the triplet state and singlet oxygen on irradiation. Photoreaction of C60 with electron-rich compounds appears to be an efficient route to difunctionalized adducts.

Spectroscopic properties of aromatic dicarboximides. Part1.—N—H and N-methyl-substituted naphthalimides

Abstract: The photophysical properties of the N—H and N-methyl derivatives of 1,2-, 2,3- and 1,8-naphthalimides have been studied. The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 2b, 2.8 D for 3b and <2 D for 4b, 1 D ≈ 3.335 64 × 10–30 C m, and 2b, 3b and 4b are N-methyl-1,2- naphthalimide, N-methyl-2,3-napthalimide and N-methyl-1,8-naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 4b increases dramatically by three orders of magnitude compared with that of 2b; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, kf, increases. This difference is found to arise from the energy gap between the lowest1(π, π*) singlet excited state and the upper 3(n,π*) triplet state, which is of the order of 9 kcal mol–1 for 2b and less than 2 kcal mol–1 for 4b in acetonitrile solution. Protic solvents increase the energy difference between the n,π* and π,π* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 4b is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet–triplet annihilation process occurs with the N-methyl derivatives 3b and 4b which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

Ab initio molecular dynamics with excited electrons.

Abstract: A method to do ab initio molecular dynamics suitable for metallic and electronically hot systems is described. It is based on a density functional which is costationary with the finite-temperature functional of Mermin, with state being included with possibly fractional occupation numbers. Optimization with respect to density only, rather than states and occupation numbers, is necessary. As an illustration, the method is used to calculate structure and dynamics in dense hot hydrogen.

1.54‐μm photoluminescence from Er‐implanted GaN and AlN

Abstract: We report the observation of the 1.54‐μm luminescence of optically excited Er3+ in ion‐implanted epitaxially grown GaN and AlN films using below band‐gap excitation. The Er‐implanted layers were co‐implanted with oxygen. At room temperature, this luminescence for GaN grown on sapphire is nearly as intense as it is at 6 or 77 K and exhibits many resolved transitions between crystal‐field levels of the 4I13/2 first excited multiplet and the 4I15/2 ground multiplet.

Exciton dynamics in gaas quantum wells under resonant excitation

Abstract: We present a comprehensive investigation of the dynamics of resonantly excited nonthermal excitons in high-quality GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-well structures on picosecond time scales. The dynamics was investigated using the luminescence upconversion technique with two independently tunable, synchronized dye lasers, which allowed measurements of the time evolution of polarized resonant luminescence with 4-ps time resolution. We show that the evolution of excitons from the initial nonthermal distribution to the thermal regime is determined by three different physical processes: (1) the enhanced radiative recombination of the metastable two-dimensional exciton polaritons, (2) the spin relaxation of excitons, and (3) the momentum relaxation of excitons. We also show that these three processes have comparable rates, so that a unified model accounting for all important processes is essential for a correct analysis of the experimental results. Using such a unified model, we have determined the rates of these processes contributing to the initial relaxation of excitons as a function of quantum-well width, temperature, and applied electric field. Quantum confinement strongly influences the radiative recombination and spin relaxation of excitons, and our study provides significant insights into these processes in quantum wells. The measured radiative recombination rate is about a factor of 2 smaller than calculated theoretically. The electric field reduces the electron-hole overlap and hence reduces the spin-relaxation rate of excitons between the optically allowed \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1〉 states. The measured variation is in good qualitative agreement with a recent theory, but somewhat slower than predicted by the theory.

Time dependence of the laser-induced femtosecond lattice instability of Si and GaAs: Role of longitudinal optical distortions.

Abstract: We extend our previous analysis of the ultrafast laser-induced instability of the diamond structure of semiconductors by including longitudinal optical-phonon distortions in addition to the instability of the transverse acoustic phonons. Generally, longitudinal optical distortions enhance the instability of the transverse acoustic phonons, increasing the kinetic energy of the atoms and the final lattice temperature. These phonons make a transition to a centrosymmetric structure of GaAs with metallic properties possible. We present results for the time dependence of the instability of Si for the case where 15% of the valence electrons have been excited into the conduction band. Thus, already 100 fsec after the excitation of the plasma the atoms are displaced about 1 \AA{} from their equilibrium position and their kinetic energy has increased to approximately 0.4 eV. Collisions between the atoms then lead to a rapid melting of the crystal. These results are in good agreement with recent experiments performed on Si and GaAs.