Showing papers on "Excited state published in 1990"

The Quantum Mechanics of Larger Semiconductor Clusters ("Quantum Dots")

Abstract: How can one understand the excited electronic states of a nanometer sized semiconductor crystallite, given that the crystallite structure is simply that of an excised fragment of the bulk lattice? This question is motivated by recent experiments on chemically synthesized "quantum crystallites," sometimes called "quantum dots," in which it is observed that the optical spectra are quite sensitive to size. For example, bulk crystalline CdSe is a semiconductor with an optical band gap at 690 nm, and continuous optical absorption at shorter wavelengths. However, 3540/~ diameter CdSe crystallites containing some 1500 atoms exhibit a series of discrete excited states with a lowest excited state at 530 nm (1-3). With increasing size, these states shift red and merge to form the optical absorption of the bulk crystal. Electron microscopy and Bragg X-ray scattering measurements show that these crystallites have the same structure and unit cell as the bulk semiconductor. Such changes have now been observed in the spectra of many different semiconductors. This phenomenon is a "quantum size effect" related to the development of the band structure with increasing crystallite size (4). Smaller crystallites behave like large molecules (e.g. polycyclic aromatic hydrocarbons) their spectroscopic and photophysical properties. They are true "clusters" that do not exhibit bulk semiconductor electronic properties. In this review

Quantum Zeno effect

Abstract: The quantum Zero effect is the inhibition of transitions between quantum states by frequent measurements of the state. The inhibition arises because the measurement causes a collapse (reduction) of the wave function. If the time between measurements is short enough, the wave function usually collapses back to the initial state. We have observed this effect in an rf transition between two $^{9}$${\mathrm{Be}}^{+}$ ground-state hyperfine levels. The ions were confined in a Penning trap and laser cooled. Short pulses of light, applied at the same time as the rf field, made the measurements. If an ion was in one state, it scattered a few photons; if it was in the other, it scattered no photons. In the latter case the wave-function collapse was due to a null measurement. Good agreement was found with calculations.

Quantum electrodynamics near a photonic band gap: Photon bound states and dressed atoms.

Abstract: It is shown that in dielectrics exhibiting a complete photonic band gap, quantum electrodynamics predicts the occurrence of bound states of photons to hydrogenic atoms. When the atomic transition frequency lies near a photonic band edge, the excited atomic level experiences an anomalous Lamb shift and splits into a doublet. One member of this doublet exhibits resonance fluorescence whereas the other level is dressed by the emission and reabsorption of near-resonant photons whose amplitude decays exponentially from the vicinity of the atom.

Photoelectron spectroscopy of metal cluster anions : Cu−n, Ag−n, and Au−n

Abstract: Negative ion photoelectron spectra of Cu−n, Ag−n(n=1–10), and Au−n (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu−2, Ag−2, and Au−2, the M2 X 1Σ+g←M−2 X 2Σ+u transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σ+u triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu−3 and Ag−3 spectra to the transition from the linear ground state of the anion (M−31Σ+g) to an excited linear state of the neutral (M3 2Σ+u). The Au−3 spectrum is attributed to a two‐photon process, photodissociation followed by photodetachment of the Au− or Au−2 fragment. For larger clusters, we measure the threshold and vertical detachment energies...

Efficiencies of photoinduced electron-transfer reactions: role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

Abstract: In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D A'-/D'+). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A'-/D'+ - A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer. By assuming a constant rate of separation, the rates of the return electron transfer process are obtained. These highly exothermic return electron transfer reactions (-AG,, = 2-3 eV) were found to be strongly dependent on the reaction exothermicity. The electron-transfer rates showed a marked decrease (ea. 2 orders of magnitude in this AG, range) with increasing exothermicity. This effect represents a clear example of the Marcus "inverted region". Semiquantum mechanical electron-transfer theories were used to analyze the data quantitatively. The electron-transfer rates were found also to depend upon the degree of charge delocalization within the ions of the pair, which is attributed to variations in the solvent reorganization energy and electronic coupling matrix element. Accordingly, mostly on the basis of redox potentials, one can vary the quantum yield of free-ion formation from a few percent to values approaching unity. Use of a strong donor with a strong acceptor to induce reactions based on electron transfer is likely to be inefficient because of the fast return electron transfer in the resulting low-energy ion pair. A system with the smallest possible driving force for the initial charge-separation reaction results in a high-energy, and therefore long-lived ion pair, which allows the desired processes to occur more efficiently. The use of an indirect path based on secondary electron transfer, a concept called "cosensitization", results in efficient radical-ion formation even when the direct path results in a very low quantum yield.

O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence.

Abstract: Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

Exciton versus band description of the absorption and luminescence spectra in poly(p-phenylenevinylene).

Abstract: The fluorescence spectra of poly(p-phenylenevinylene) have been investigated at 6 K employing site-selective laser spectroscopy. They delineate the existence of a well-defined energy in the tail of the absorption edge below which the fluorescence emission is quasiresonant with an excitation featuring a Stokes shift of 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ only. Above this localization threshold ${\ensuremath{ u}}_{\mathrm{loc}}$ spectral diffusion is observed with the emission independent of excitation yet carrying a significant polarization memory for excitation energies up to 1.9 eV in excess of ${\ensuremath{ u}}_{\mathrm{loc}}$. This is incompatible with the band picture involving photogeneration of an uncorrelated electron-hole pair, yet consistent with the concept of random walks of neutral excitations through an inhomogeneously broadened density of states. The chromophores are associated with a distribution of segments of the polymer chains along which the excitation is delocalized. Published results on time-resolved fluorescence support the exciton picture.

Production of hollow atoms by the excitation of highly charged ions in interaction with a metallic surface.

Abstract: The capture of many electrons by ${\mathrm{Ar}}^{17+}$ ions, at low velocity, near a metallic surface, has been studied. Multiexcited bound states with many electrons in the outermost shells (hollow atoms) have been observed. The surrounding of an ionic excited core by many outermost electrons greatly decreases the lifetimes of the states. This characteristic decrease explains the main striking features of the relaxation of the ions.

Shocked molecular solids: Vibrational up pumping, defect hot spot formation, and the onset of chemistry

Abstract: A model and detailed calculations are presented to describe the flow of energy in a shocked solid consisting of large organic molecules. The shock excites the bulk phonons, which rapidly achieve a state of phonon equilibrium characterized by a phonon quasitemperature. The excess energy subsequently flows into the molecular vibrations, which are characterized by a vibrational quasitemperature. The multiphonon up pumping process occurs because of anharmonic coupling terms in the solid state potential surface. Of central importance are the lowest energy molecular vibrations, or ‘‘doorway’’ modes, through which mechanical energy enters and leaves the molecules. Using realistic experimental parameters, it is found that the quasitemperature increase of the internal molecular vibrations and equilibration between the phonons and vibrations is achieved on the time scale of a few tens of picoseconds. A new mechanism is presented for the generation of ‘‘hot spots’’ at defects. Defects are postulated to have somewhat greater anharmonic coupling, causing the vibrational temperature in defects to briefly overshoot the bulk. The influence of the higher defect vibrational temperature on chemical reactivity is calculated. It is shown that even small increases in defect anharmonic coupling have profound effects on the probability of shock induced chemistry. The anharmonic defect model predicts a size effect. The defect enhanced chemical reaction probability is reduced as the particle size is reduced.

Femtosecond studies of the presolvated electron: An excited state of the solvated electron?

Abstract: Femtosecond studies of the formation, decay, and absorption spectra of the wet or presolvated electron are presented. The discovery of an isosbestic wavelength shows for the first time that the wet-to-solvated electron transition involves only two states. The model of the wet electron as an excited state of the solvated electron and the wet-to-solvated electron transition, viewed as a radiationless transition, is discussed. An ultrafast component of the visible absorption signal is believed to be ${\mathrm{H}}_{2}$${\mathrm{O}}^{+}$.

Theory of optically excited intrinsic semiconductor quantum dots

Abstract: The influence of the Coulomb interaction on one and two electron-hole-pair excitations in semiconductor quantum dots is analyzed. Using a numerical matrix-diagonalization scheme, the energy eigenvalues and the eigenfunctions of the relevant states are computed. Significant deviations from the strong-confinement approximation are observed. It is shown that the biexciton binding energy increases with decreasing dot size. This result is verified using third-order perturbation theory for small quantum dots. The optical properties of the quantum dots are computed, and it is shown that the Coulomb interaction significantly influences the allowed dipole transitions, causing increasing two-pair absorption on the high-energy side of the decreasing one-pair absorption. Surface-polarization effects are studied for quantum dots embedded in another dielectric medium.

Above-threshold dissociation of H+2 in intense laser fields.

Abstract: We present nonperturbative, time-independent calculations of the photodissociation rate of H{sup +}{sub 2} in intense laser fields. The energy distribution of the protons consists in a sequence of peaks evenly spaced by half the photon energy, all of equal width but of varying heights. They result from multiphoton absorption above the dissociation threshold, with equal sharing of the excess photon energy between H and H{sup +}. Surprisingly, the distribution of higher-energy peaks decreases with increasing intensity, due to stimulated free-free emission of the dissociating fragments. We also predict a sharp angular distribution of the protons along the electric field vector of a linearly polarized laser.

Biexcitons in semiconductor quantum dots

Abstract: Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.

Antenna effect in luminescent lanthanide cryptates: a photophysical study

Abstract: — Excited state emission and absorption decay measurements have been made on the cage-type cryptate complexes [M bpy.bpy.bpy]n+, where Mn+= Na+, La3+, Eu3+, Gd3+ or Tb3+ and [bpy.bpy.bpy] is a tris-bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1π-π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H2O or D2O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mn+= Na+, La3+ and Gd3+ the lowest excited state of the cryptate is a 3ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H2O solution and in the second-millisecond time scale at 77 K in MeOH-EtOH. For Mn+= Eu3+, the lowest excited state is the luminescent 5D0 Eu3+ level which in H2O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 103 s_1 at room temperature and 1.2 × 103 s−′ at 77 K. The relatively low efficiency of 5D0 population upon 1ππ* excitation is attributed to the presence of a ligand-to-metal charge transfer level through which 1ππ* decays directly to the ground state. For Mn+= Tb3+ the lowest excited state is the luminescent 5D4 Tb3+ level. The process of 5D4 population upon 1ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high-efficiency, activated back energy transfer from the 5D4 Tb3+ level to the 3ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm_1) and the intrinsically long lifetime of 5D4. At 77 K back energy transfer cannot take place and the 5D4 Tb3* level deactivates to the ground state with rate constant 5.9 × 102 s-′ (H2O solution). The relevance of these results toward the optimization of Eu3+ and Tb3+ cryptates as luminescent probes is discussed.

Intramolecular quenching of excited singlet states by stable nitroxyl radicals

Abstract: Absorbance and steady-state and time-resolved fluorescence measurements were employed to examine the mechanism(s) of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts. This work establishes the following: (1) the absorption and emission energies of the fluorophores are unaffected by the presence of the nitroxide substituent(s), and the residual emission that is observed from the adducts arises from the locally excited singlet of the fluorophore, not from charge recombination; (2) rate constants for intramolecular quenching by the nitroxides (k{sub q}) are high (10{sup 8}-10{sup 10} s{sup {minus}1}) and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching (>10{sup 8} s{sup {minus}1}) are observed over distances as great as 12 {angstrom}; (3) Foerster energy transfer does not contribute significantly to the quenching due to the low values for the spectral overlap integrals; (4) the K{sub q}'s do not increase proportionally to the solvent-dependent increases in the Dexter overlap integral, indicating that energy transfer by the Dexter mechanism is not responsible for the quenching; (5) the values of k{sub q} show no obvious correlation with the calculated free energies for photoinduced electron transfer, suggesting that this quenching pathway is also unimportant; (6) for hematoporphyrin-nitroxide adducts,more » which contain a fluorophore whose singlet energy is below that of the first excited state energy of the nitroxide (thus precluding energy transfer), significant rates of quenching are still observed.« less

Bond-selected bimolecular chemistry : H + HOD(4νOH)→OD + H2

Abstract: The reaction of HOD containing four quanta of O–H bond stretching vibration with H atoms produces OD fragments almost exclusively. Vibrational overtone excitation prepares HOD(4νOH) that reacts with H atoms formed in a microwave discharge. The endothermic reaction of water with hydrogen atoms does not occur for ground vibrational state water but proceeds at roughly the gas kinetic collision rate for the vibrationally excited molecule. The production of OD fragments from HOD(4νOH) in the reaction is at least two orders of magnitude more efficient than the production of OH, indicating very selective reaction of the vibrationally excited bond.

Ab initio calculation of the OH (X 2Π, A 2Σ+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Σ+) state

Abstract: The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 A, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities ...

Probing the transition state with negative ion photodetachment: the chlorine atom + hydrogen chloride and bromine atom + hydrogen bromide reactions

Abstract: The transition-state region for neutral hydrogen-transfer reactions can be probed by photodetaching the appropriate stable, hydrogen-bonded negative ion. This paper presents a detailed account of this method, in which the Cl + HCl and Br + HBr reactions are investigated by photoelectron spectroscopy of ClHCl{sup {minus}}, BrHBr{sup {minus}}, and the corresponding deuterated species. The photoelectron spectra exhibit resolved vibrational structure attributed to the unstable neutral (ClHCl) or (BrHBr) complex. The BrHBr{sup {minus}} and BrDBr{sup {minus}} spectra exhibit narrow (15-20 meV) peaks that are likely to result from reactive resonance states supported by the Br + HBr potential energy surface, as well as peaks that appear to be from an electronically excited state of the (BrHBr) complex. The BrHBr{sup {minus}} and BrDBr{sup {minus}} results have been analyzed to yield an effective collinear potential energy surface for the Br + HBr reaction.

Temperature and pressure dependence of ozone formation rates in the range 1-1000 bar and 90-370 K

Abstract: The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.

The influence of potential energy surface topologies on the dissociation of H2

Abstract: In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state‐to‐state cross sections in both the dissociated and the scattered flux. The potential energy surface is two‐dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule.

Plasma fluctuation measurements in tokamaks using beam‐plasma interactions

Abstract: High‐frequency observations of light emitted from the interactions between plasma ions and injected neutral beam atoms allow the measurement of moderate‐wavelength fluctuations in plasma and impurity ion densities. To detect turbulence in the local plasma ion density, the collisionally excited fluorescence from a neutral beam is measured either separately at several spatial points or with a multichannel imaging detector. Similarly, the role of impurity ion density fluctuations is measured using charge exchange recombination excited transitions emitted by the ion species of interest. This technique can access the relatively unexplored region of long‐wavelength plasma turbulence with k⊥ρi≪1, and hence complements measurements from scattering experiments. Optimization of neutral beam geometry and optical sightlines can result in very good localization and resolution (Δx≤1 cm) in the hot plasma core region. The detectable fluctuation level is determined by photon statistics, atomic excitation processes, and b...

Rotationally resolved ultraviolet spectrum of the benzene–Ar complex by mass‐selected resonance‐enhanced two‐photon ionization

Abstract: High resolution laser excitation was combined with the technique of mass‐selected two‐photon ionization via a resonant intermediate state to measure rotationally resolved UV spectra of benzene–Ar van der Waals clusters. When the second laser pulse in the two color experiment is delayed by 7 ns no line broadening due to the second ionizing absorption step is observed. Spectra of three vibronic bands in the S1 ←S0 transition of benzene (h6)–Ar and benzene (d6)–Ar were measured yielding a line spectrum with a linewidth of 130 MHz. Resolution is sufficient to demonstrate that no asymmetry splitting of the rotational lines occurs and the spectrum is to a high precision that of a symmetric rotor. A detailed analysis of the rotational structure yields an accurate set of rotational constants. We find that the Ar is located on the C6 rotational axis. Its distance from the benzene ring plane is 3.582 A in the electronic ground state and decreases by 59±3 mA in the electronically excited state due to the increased polarizability of the benzene molecule after electronic excitation.