Showing papers on "Excited state published in 1984"

Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state

Abstract: We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular‐like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrodinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron‐hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.

Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production

Abstract: The lowest charge‐transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O‐34) of ion pair dissociation This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials

Size effects in the excited electronic states of small colloidal CdS crystallites

Abstract: This paper reports experimental studies of the development of bulk optical properties as a function of crystallite size for the inorganic direct gap semiconductor CdS. Small crystallites are synthesized via colloidal chemical techniques, and their optical properties are studied in situ at extreme dilution. The crystallites are characterized via high resolution transmission electron microscopy. Direct images show (111) lattice planes, and establish the crystallite structures as close to those of excised fragments of bulk CdS (zinc‐blende cubic). Large crystallites (> 100 A average diameter) show an optical absorption, in colloidal solution, close to that of bulk crystalline material. However, small crystallites of 30 A average diameter show a large blue shift (∼0.8 eV) in absorption edge (effective band gap), and an intensification of edge absorption relative to absorption at higher energy regions. These observations can be understood as quantum size effects resulting from confinement of an electron and hole in a small volume. 40 A average size crystallites show a smaller shift (∼0.25 eV), and corresponding changes in their fluorescence, and resonance Raman excitation, spectra.

A linear response, coupled‐cluster theory for excitation energy

Abstract: Expressions for static and dynamic properties in coupled-cluster (CC) theory are derived. In the static case, using diagrammatic techniques, it is shown how consideration of orbital relaxation effects in the theory introduces higher-order correlation effects. For the dynamic case, excitation energy expressions are obtained without consideration of orbital relaxation effects and shown to be equivalent to an equation of motion (EOM) approach subject to a coupled-cluster ground-state wave function and an excitation operator consisting of single and double excitations. Illustrative applications for excited states of ethylene are reported.

Theory of fluorescence depolarization in macromolecules and membranes

Abstract: A comprehensive formalism is developed to describe the decay of the fluorescence emission anisotropy r(t) in macroscopically isotropic systems where both excited state and orientational dynamics contribute to the depolarization. It is shown how energy transfer, heterogeneity, and interconversion of excited states with different emission characteristics as well as both overall and internal reorientation can be treated in a unified way. Limits when the state and orientation dynamics are uncoupled and when the interconversion of the states is either much slower or much faster than the irreversible decay rates, are considered. A systematic treatment of the influence of internal motions is presented. First, the geometry of the transition dipoles is explicitly ‘‘factored out’’ and general expressions for r(t) are obtained for several cases including when the motion occurs about a fixed axis and an axis which in turn can ‘‘wobble’’ about a director. The initial and long‐time behavior of r(t) is examined and then, a variety of dynamical models (e.g., discrete jumps, free and restricted Langevin motion about an axis, diffusive motion of an axis in an orienting potential) are used to obtain the time dependence of the relevant correlation functions which appear in the above general expressions. In this way, one can obtain r(t) for a large class of models. Of particular interest is an approximate analytic expression for r(t), valid for any orientation of the transition dipoles and restricting potential, of a cylindrical probe in a membrane. The influence of collective (hydrodynamic) fluctuations of the membrane director are also considered.

Energy levels of Wannier excitons in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Energy levels of Wannier excitons in a quantum-well structure consisting of a single slab of GaAs sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated with the use of a variational approach. Owing to lowering of symmetry along the axis of growth of this quantum-well structure and the presence of energy-band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed, leading to two exciton systems, namely, the heavy-hole exciton and the light-hole exciton. The values of the binding energies of the ground state and of a few lowlying excited states of these two exciton systems are calculated as a function of the size of the GaAs quantum well for several values of the heights of the potential barriers and their behavior is discussed. The results thus obtained are also compared with the available experimental data. The reliability of the various approximations made in this calculation is discussed.

Quantum electrodynamics near an interface. II.

Abstract: A quantum-mechanical linear-response formalism is used to calculate the frequency shift and lifetime of an excited atom near an arbitrary flat interface. The results depend on the frequency-dependent atom and field susceptibilities, and in the vicinity of an interface can be expressed in terms of the appropriate Fresnel reflection coefficients; the contributions from surface excitations are easily identified. As examples, we consider an atom above a metal and a dielectric waveguide.

Intramolecular vibrational relaxation and spectra of CH and CD overtones in benzene and perdeuterobenzene

Abstract: A theoretical model is presented for the vibrational dynamics of highly excited CH and CD overtones in benzene and perdeuterobenzene. The origin, path, and time scale for the overtone relaxation are described. The critical near resonant interaction responsible for the energy flow from an excited CH(D) oscillator to the ring is a Fermi resonance coupling, identified by Sibert, Reinhardt, and Hynes [Chem. Phys. Lett. 92, 455 (1982)]. Quantum overtone spectra are calculated both from time independent and time dependent perspectives and good qualitative agreement is found with the experimental overtone spectra of Reddy, Heller, and Berry [J. Chem. Phys. 76, 2814 (1982)]. Some expected consequences for future experiments on benzene and related systems are indicated.

Determination of plasma-ion velocity distribution via charge-exchange recombination spectroscopy

Abstract: Spectroscopy of line radiation from plasma impurity ions excited by charge-exchange recombination reactions with energetic neutral beam atoms is rapidly becoming recognized as a powerful technique for measuring ion temperature, bulk plasma motion, impurity transport, and more exotic phenomena such as fast alpha particle distributions. In particular, this diagnostic offers the capability of obtaining space- and time-resolved ion temperature and toroidal plasma rotation profiles with relatively simple optical systems. Cascade-corrected excitation rate coefficients for use in both fully stripped impurity density studies and ion temperature measurements have been calculated to the principal ..delta..n = 1 transitions of He+, C/sup 5 +/, and O/sup 7 +/ with neutral beam energies of 5 to 100 keV/amu. A fiber optically coupled spectrometer system has been used on PDX to measure visible He/sup +/ radiation excited by charge exchange. Central ion temperatures up to 2.4 keV and toroidal rotation speeds up to 1.5 x 10/sup 7/ cm/s were observed in diverted discharges with P/sub INJ/ less than or equal to 3.0 MW.

Photo-Chemistry of Colloidal Metal Sulfides 8. Photo-Physics of Extremely Small CdS Particles: Q-State CdS and Magic Agglomeration Numbers

Abstract: Extremely small CdS particles were prepared in propanol−2 solution at − 78°C and in aqueous solution in the presence of sodium hexametaphosphate at room temperature. These colloids are colorless. Their UV absorption spectra exhibit several maxima. Aging of the colloids is accompanied by intensity variations in the absorption maxima and by a shift of the onset of absorption to longer wavelengths. These small CdS particles hardly possess semiconductor properties (Q-state CdS). A semi-classical treatment of the energies of an electron-hole pair in these particles yielded the wavelengths of their absorption spectra. At the small particle sizes used, the first excited state was reached by photon-absorption in the UV, and the second excited state was generally not reached at all. The various maxima in the absorption spectra are explained in terms of a size distribution of the colloids with preferential agglomeration numbers. Reasons for the formation of such a structured size distribution are given. The fluorescence and fluorescence excitation spectra of the small particles were also investigated. Particles below a certain size have only one broad fluorescence band at a much longer wavelengths than the onset of absorption. As the particle size increases, this band is shifted towards longer wavelengths, and finally an additional rather sharp band appears at the threshold of absorption. CdS colloids in the Q-state can be made which fluoresce as desired anywhere between the red and the blue. Also reported are the first experiments in which the preparation of Q-type CdS in the solid state is achieved by evaporating the solvent from the colloidal solutions.

Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF3H

Abstract: The absorption spectrum of trifluoromethane has been recorded between 900 and 14 000 cm−1 with resolutions between 0.004 and 0.5 cm−1 (pressure broadened). 22 bands were assigned as arising from the interacting CH stretching and bending manifolds, which account for most of the absorption in the overtone region. The results can be understood quantitatively with an effective, tridiagonal many‐level Fermi resonance Hamiltonian. The experimental and theoretical results are summarized in Table II. The Hamiltonian is given in Table III and shows a very large stretching–bending interaction constant ‖ksbb‖=106 cm−1, which is even larger than the diagonal anharmonic constant for the stretching vibration ‖x′ss‖=62 cm−1. This leads to extensive vibrational redistribution between stretching and bending motions at high levels of excitation. The time dependent redistribution is calculated with the spectroscopic Hamiltonian. A rotational analysis is presented for some of the bands involved in the Fermi resonance. The ef...

Nuclear and electron dynamics in the photodissociation of water

Abstract: The photodissociation of water in its first absorption band is studied by photolyzing H2O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (2Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H2O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H2O. The spin distribution is almost statistical. A strong J dependent Λ‐doublet population inversion is found from cold H2O, but there is no inversion from warm H2O. The inversion provides a possible pump mechanism for the astronomical OH maser and is simply explained by approximate symmetry conservation. The orientation of the unpaired pπ lobe in OH in the upper Λ‐doublet state is measured to be perpendicular to the OH rotation plane. The J dependence of the inversion is explained by Λ‐doublet mixing in OH and quantitatively described in terms of the singly occupied pπ‐lobe in the excited water and the orientation of the corresponding singly occupied pπ‐lobe in OH. The alignment of OH is measured by polarizing both lasers. The large polarization effects are strongly dependent upon J and also upon the temperature of H2O. It is shown that the dependence is related both to Λ‐doublet mixing and hyperfine structure of OH. For the cold H2O the data indicate, despite the strong J dependence of both polarization and Λ‐doublet inversion, a completely planar dissociation process. It is shown that due to Λ‐doublet mixing the transition moment of Π molecules has a J dependent angle relative to the OH rotation plane which approaches the high J limit at the same rate that the molecule shifts from Hund’s case (a) to case (b). The model for the J dependence of the Λ‐doublet population and the polarization is important for chemical reactions, surface scattering and other processes where Π molecules are analyzed with LIF.

Identification of the SiCC radical toward IC +10216 - The first molecular ring in an astronomical source

Abstract: In the radio spectrum of the envelope of the evolved carbon star IRC +10216, the fraction of lines from exotic molecules seldom or never observed in the terrestrial laboratory is exceptionally high. At least 20 lines have not been identified, and it is not known whether one, two, or a number of new molecules are involved. It is shown in the present investigation that nine of the previously unidentified lines in IRC +10216 are produced by SiCC, a radical long known to exist in stellar atmospheres. The true ground-state geometry of this radical has only been obtained recently on the basis of an elegant two-photon ionization experiment in a supersonic molecular beam. Information regarding the molecular geometry and lower rotational levels of SiCC is presented in a graph. The intensities of the SiCC lines confirm the assignments, yield new data on the temperature and density in the envelope of IRC +10216, and indicate that the amount of SiCC there is substantial.

New mode of IR detection using quantum wells

Abstract: A new mode of IR detection using photoemission from a single quantum well is proposed and optimization of the device performance by the proper choice of parameters is discussed. Despite the very thin device structures, theoretical calculations show large absorption at wavelengths near cutoff. The largest photoemissive response is found by adjusting the well parameters so that an excited virtual state lies just above threshold.

Visible spectroscopy of jet‐cooled SiC2: Geometry and electronic structure

Abstract: SiC2 has been prepared in a supersonic molecular beam by laser vaporization of a silicon carbide rod within a pulsed supersonic nozzle. Rotational analysis of the 0‐0 band of the well‐known 4980 A band system of this molecule reveals that, contrary to previous assumptions, the molecule is triangular in both the ground and excited electronic states. In both states the molecule is of C2V symmetry with a C–Si–C angle between 40° and 41°. The correct assignment of the spectrum is A ′B2←x ′A1. The carbon–carbon bond length is 1.25 A in the ground state, suggesting that the molecule is best understood as a silicon atom bound to the side of a triply bonded C2 fragment. The optical transition moment is polarized along the b axis of the molecule which is parallel to the carbon–carbon bond axis. In the A 1B2 excited state the carbon–carbon bond opens up to 1.30 A consistent with a π*←π excitation of the carbon–carbon triple bond. The silicon–cargon distance is measured to be 1.81 A in the x 1A1 state, lengtheni...

Circularly polarized conical patterns from circular microstrip antennas

Abstract: A method is presented for generating circularly polarized conical patterns from circular microstrip antennas. These antennas are excited at higher order modes and require different feed arrangements for different mode excitations. It is determined that the peak direction of the conical pattern can be varied over a wide angular range. Modal expansion technique is employed to calculate the radiation patterns of these antennas.

Self-consistent relativistic band structure of the noble metals

Abstract: The authors present results of relativistic band structure calculations for the noble metals Cu, Ag and Au. Particular effort was focused on good-quality self-consistent charge densities and on well converged excited bands (up to 30 eV above the Fermi level) to serve present needs of photoemission analysis within that energy regime. The calculations rest on Kohn-Sham-type relativistic one-particle equations with a local exchange-correlation potential either of the form derived by Hedin and Lundqvist (1971) or to the Xalpha form suggested by Slater. In using the latter potential best agreement with the experiments could be obtained by assigning alpha the values 0.70, 0.82 and 0.85 for Cu, Ag and Au, respectively. Self-consistency effects caused by relaxation of the core states were accounted for fully. The authors' findings do not support results of recent (differently performed) calculations by other authors, using the same (or similar) one-particle equations, who generally obtain d bands which are bound too weakly and whose widths come out too large. In contrast, they even find good agreement with spin-orbit effects observed in the photoemission spectra of Cu.

Excited states at metal surfaces and their non-radiative relaxation

Abstract: ?We present a general overview of the spectroscopy and relaxation dynamics of vibrational and electronic excitations of molecules and atoms adsorbed on metal surfaces. We discuss briefly some experimental tools, in particular, electron energy loss spectroscopy, and present experimental illustrations of rotational, vibrational, and electronic excitations of adsorbates. For excitations at surfaces, new effective nonradiative decay paths are opened involving, for example, excitation of electron-hole pairs and phonons. For excited molecules not in direct contact with the metal substrate (or where this overlap can be neglected), the nonradiative quenching can only result from the interaction between the oscillating electric field of the excited molecule and the metal. We first discuss the "classical theory" of this coupling and point out its limitations. We then present an improved theory and compare its predictions with experimental data. For excited molecules in direct contact with the substrate, electron transfer between the molecule and the metal can often occur, and this leads in general to strongly nonadiabatic processes. We illustrate this type of decay path for both vibrational and electronic excitations. Finally, multiphonon and phase relaxation of vibrations at surfaces are briefly discussed.

Analysis of Seismic Body Waves Excited by the Mount St. Helens Eruption of May 18, 1980

Abstract: Seismic body waves which were excited by eruption of Mt St Helens, and recorded by the Global Digital Seismographic Network (GDSN) stations are analyzed to determine the nature and the time sequence of the events associated with the eruption The polarity of teleseismic P waves (period 20 sec) is identical at six stations which are distributed over a wide azimuthal range This observation, together with a very small S to P amplitude ratio (at 20 sec), suggests that the source is a nearly vertical single force that represents the counter force of the eruption The time history of the vertical force suggests two distinct groups of events, about two minutes apart, each consisting of several subevents with a duration of about 25 sec The magnitude of the force is approximately 26 to the 17th power dyne this vertical force is in contrast with the long period (approximately 150 sec) southward horizontal single force which was determined by a previous study and interpreted to be due to the massive landslide

Selective perturbation of ligand field excited states in polypyridine ruthenium(II) complexes

Abstract: A new bipyridine-type ligand which allows the selective perturbation of ligand field (LF) excited-state energies is introduced. Any metal-to-ligand charge-transfer (MLCT) excited states occur at nearly the same energy as in the bipyridine complex. The syntheses of the corresponding ruthenium(II) complexes and their photophysical properties are reported. The results clearly support a current model for the excited-state behavior of polypyridine complexes in which the lowest MLCT decays by thermal population of a higher lying LF state. This method of selective LF-state perturbation nicely complements the perturbation of MLCT states achieved by modification of the ligand's reduction potential.

Dimers of aromatic molecules: (Benzene)2, (toluene)2, and benzene–toluene

Abstract: The optical absorption spectra of the first excited singlet states of the benzene, toluene, and toluene–benzene dimers, created in a supersonic molecular jet, are reported. The absorption spectra are detected through two‐color time of flight mass spectroscopy; this method eliminates fragmentation of dimers and higher clusters and the dimer spectra are uniquely observed. The benzene dimer observed in this experiment is suggested to have a parallel stacked and displaced configuration of C2h symmetry. Both the toluene and toluene–benzene dimers have two configurations: parallel stacked and displaced [based on (benzene)2] and perpendicular. (Benzene)2, (toluene)2, and toluene–benzene form excimers in the excited state for the parallel stacked displaced configurations. The transformation of (benzene)2 to the excimer takes place at the 00 with a ∼0 cm−1 barrier while the excimer is formed for toluene–benzene with a ∼900 cm−1 barrier. An exciton analysis of the (benzene)2 000 and 610 yields M12, the excitation e...

Approximate account of the connected quadruply excited clusters in the coupled-pair many-electron theory

Abstract: In nondegenerate systems, the tetraexcited clusters are well approximated by products of disconnected pair clusters and the connected quadruply excited component is negligible. In contrast, when the reference state becomes quasidegenerate with the lowest biexcited configuration(s), the connected quadruply excited clusters become very important. To extend the applicability of the coupled-pair many-electron theory to such situations, we approximate the connected tetraexcited contribution in the form suggested by the unrestricted Hartree-Fock-type wave function, or one of its projected versions, such as the alternant molecular-orbital method. We show that the incorporation of the connected quadruply excited clusters into the coupled-pair equations effectively cancels certain nonlinear terms, originating from disconnected quadruple excitations, so that the resulting equations are very similar (up to a numerical factor) to the approximate coupled-pair theory, in which only those nonlinear terms which factorize with respect to the hole-electron pairs are considered. This fact shows in turn why various approximate coupled-pair approaches can often provide better results than the full coupled-pair many-electron theory.

The electromagnetic theory of surface enhanced spectroscopy

Abstract: Surface enhanced spectroscopy (SES) was born in 1 974 with the measure­ ment of Raman spectra of molecules adsorbed on a roughened electrode surface (la-e). Given the smallness of the Raman cross section, the detection of a Raman signal should have generated some excitement. This did not happen, but was delayed until 1977 when Jeanmaire & van Duyne (2) and Albrecht & Creighton (3) showed that the rough silver surface enhances the Raman cross section by a factor ranging between 104 and 106. This started a great outpouring (4a-i) of theoretical and experimental work, whose central theme is to understand how the presence of a solid surface modifies the spectroscopic and photochemical properties of a molecule located nearby. We divide the effects of the surface in two categories: electromagnetic, which can be described by solving Maxwell's equations, and chemical, which belong to quantum chemistry. The enhancement of the local laser field due to the polarization of the surface is an example of an elec­ tromagnetic effect. The appearance of a new excited state caused by chemisorption, leading to an enhancement of the Raman cross section through resonance Raman scattering (which would not be expected on the basis of the gas phase properties of the molecule), is an example of a chemical effect. While there is no doubt that chemisorption modifies the optical response of adsorbed molecules, the magnitude of the modification is still a subject of controversy. We don't know whether we should expect large modifications for all molecules, or for a small class (e.g. those with 11: orbitals); or whether

Magneto-optical determination of exciton binding energy in GaAs- Ga 1 − x Al x As quantum wells

Abstract: The binding energy of the exciton in GaAs quantum wells confined within Ga-Al-As is determined by the observation of the different behavior of the ground state and the excited states of excitonic transitions of different subbands with excitation spectroscopy in magnetic fields. An increase in the binding energy with decreasing well thickness is found with values higher than theoretically expected. This discrepancy is explained by an experimentally determined higher reduced mass than that used in the theoretical calculations.

Molecular and electronic structure in the metal‐to‐ligand charge‐transfer excited states of d6 transition‐metal complexes in solution

Abstract: The ground and metal-to-ligand (MLCT) excited electronic states of the complexes Os(bpy)/sub n/(P/sub 2/)/sub 3-n//sup 2 +/ (bpy = 2,2'-bipyridine, P/sub 2/ = cis-(C/sub 6/H/sub 5/)/sub 2/PCH=CHP(C/sub 6/H/sub 5/)/sub 2/; n = 1-3) have been studied by resonance Raman spectroscopy, time-resolved resonance Raman spectroscopy, and emission spectroscopy. The time-resolved resonance Raman evidence confirms that the charge-transfer electron density is localized in the lowest ..pi..* orbital of one bpy ligand rather than delocalized over the ..pi..* orbitals of all of the available bpy ligands on the vibrational time scale. The amount of charge transferred from the metal to bpy ..pi..* in the MLCT state has been determined. Application of Badger's rule to the Raman data and Franck-Condon analysis of the emission data lead to two independent determinations of average bond length displacements in the MLCT state. The two approaches yield displacement values that agree within 0.001-0.003 A for the three complexes studied, suggesting that it may be possible to determine excited-state structures in solution with a precision silimar to that of a good X-ray crystal structure. Analysis of time-resolved resonance Raman and emission data for Ru(pby)/sub 3//sup 2 +/, Ru(bpy)/sub 2/(en)/sup 2 +/, and fac-Re(bpy)(CO)/sub 3/Cl by these methods yields resultsmore » in agreement with the osmium data. The results suggest a general prescription for the determination of the molecular and electronic structures of electronically excited states in solution. 40 references, 6 figures, 4 tables.« less

Jet spectroscopy of anthracene and deuterated anthracenes

Abstract: Fluorescence excitation and SVL fluorescence spectra of jet-cooled h10-, 9d1-, 9,10d2-, and d10-anthracene are reported. Ground state vibrational assignments are presented for all these species and are compared with literature values. In addition, assignments for the first excited singlet state of h10-anthracene are made using SVL spectra and rotational band contours as guides. The work presented herein serves as an essential reference for other work from this research group concerning the dynamics of excited anthracene (see accompanying papers), and completes the spectroscopy of the polyacene series.