Showing papers on "Excited state published in 1987"

Wave Function of the Universe

Abstract: The quantum state of a spatially closed universe can be described by a wave function which is a functional on the geometries of compact three-manifolds and on the values of the matter fields on these manifolds. The wave function obeys the Wheeler-DeWitt second-order functional differential equation. We put forward a proposal for the wave function of the "ground state" or state of minimum excitation: the ground-state amplitude for a three-geometry is given by a path integral over all compact positive-definite four-geometries which have the three-geometry as a boundary. The requirement that the Hamiltonian be Hermitian then defines the boundary conditions for the Wheeler-DeWitt equation and the spectrum of possible excited states. To illustrate the above, we calculate the ground and excited states in a simple minisuperspace model in which the scale factor is the only gravitational degree of freedom, a conformally invariant scalar field is the only matter degree of freedom and $\ensuremath{\Lambda}g0$. The ground state corresponds to de Sitter space in the classical limit. There are excited states which represent universes which expand from zero volume, reach a maximum size, and then recollapse but which have a finite (though very small) probability of tunneling through a potential barrier to a de Sitter-type state of continual expansion. The path-integral approach allows us to handle situations in which the topology of the three-manifold changes. We estimate the probability that the ground state in our minisuperspace model contains more than one connected component of the spacelike surface.

PbS in polymers. From molecules to bulk solids

Abstract: The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron‐hole‐in‐a‐box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 A.

Transition probability, B(E2)up-arrow, from the ground to the first-excited 2/sup +/ state of even-even nuclides

Abstract: Adopted values for the reduced electric quadrupole transition probability, B(E2)up-arrow, from the ground state to the first-excited 2/sup +/ state of even-even nuclides are given in Table I. Values of tau, the mean life of the 2/sup +/ state, E, the energy, and ..beta../sub 2/, the quadrupole deformation parameter, are also listed there. The ratio of ..beta../sub 2/ to the value expected from the single-particle model is presented. The intrinsic quadrupole moment, Q/sub O/, is deduced from the B(E2)up-arrow value. The product E x B(E2)up-arrow is expressed as a precentage of the energy-weighted total and isoscalar E2 sum-rule strengths.

Fluorescent excitation of interstellar H2

Abstract: The infrared emission spectrum of H2 excited by ultraviolet absorption, followed by fluorescence, was investigated using comprehensive models of interstellar clouds for computing the spectrum and to assess the effects on the intensity to various cloud properties, such as density, size, temperature, and the intensity of the UV radiation field. It is shown that the absolute H2 IR line intensities depend primarily on the density of the cloud and the strength of the incident UV radiation, and to a much lesser exent on the temperature of the gas, the total thickness of the cloud, and the optical properties of the grains. A variety of recent observational results are discussed with reference to theoretical models. It is shown that the rich H2 emission spectrum of the reflection nebula NGC 2023 can be reproduced by a model with density of about 10,000/cu cm, temperature of about 80 K, and UV flux approximately 300 times that of the Galactic background starlight.

New 10 μm infrared detector using intersubband absorption in resonant tunneling GaAlAs superlattices

Abstract: We demonstrate a novel 10.8 μm superlattice infrared detector based on doped quantum wells of GaAs/AlGaAs. Intersubband resonance radiation excites an electron from the ground state into the first excited state, where it rapidly tunnels out producing a photocurrent. We achieve a narrow bandwidth (10%) photosensitivity with a responsivity of 0.52 A/W and an estimated speed of 30 ps.

Photochemistry and Photophysics of Coordination Compounds

Abstract: Topic 1: Metal-Centered Excited States.- Polarized Luminescence of [Pt(CN)2bipy] Single Crystals - Magnetic Field and Temperature Effects.- Light-Induced Excited Spin State Trapping in Iron(II) Complexes.- Infrared Luminescence Spectroscopy of V3+ Doped Cs2NaYX6 (X=Cl,Br).- Recent Progress in Uranyl Photo-Physics.- Effects of Macrocyclic and Cryptand Ligands on Photophysics of Eu3+ Ions.- Topic 2: Photophysics and Photochemistry of Cr(III) Complexes.- Ligand Field Analysis of the Doublet Excited States in Chromium(III) Trischelated Complexes.- Quenching of Three Photoaquation Modes of a Chromium(III) Acidoammine.- Stereochemical Constraints on the Excited State Behavior of Chromium (III).- Excited State Behavior as a Probe of Ground-State Ion-Pair Interactions in Chromium(III)-Polypyridyl Complexes.- Counterion Effects on Doublet Splittings of Chromium(III) Complexes.- Spectrum-Structure Correlations in Hexacoordinated Transition Metal Complexes.- Multiphoton-Induced Picosecond Photophysics of Chromium(III)- Polypyridyl Complexes.- Topic 3: Excited State Properties of Tris-2,2'-Bipyridine Ruthenium(II) and Related Complexes.- On the Orbital Nature of the Luminescent Excited State of Orthometalated Transition Metal Complexes.- Correlations Between Optical and Electrochemical Properties of Ru(II)- Polypyridine Complexes: Influence of the Ligand Structure.- Towards a Dynamic Model for the Ru(bpy)32+ System.- Broad-Band Emission and Zero-Phonon Lines of Single-Crystal [Ru(bpy)3] (PF6)2 - A Comparison.- Magnetic-Field Effects and Highly Resolved Vibronic Structure of [Ru(bpy)3]2+.- Highly Resolved Optical Spectra of [Os(bpy)3]2+ Doped into [Ru(bpy)3]X2.- Excited State Behaviors of Ruthenium(II) Complexes as Studied by Time Resolved and Temperature and Solvent Dependent Emission Spectra.- The Lowest Excited States of [Ru(2,2'-Biyridine)2]2+.- Quenching of Excited Ru(bpy)32+ with Methylviologen at Low Temperatures.- Kinetics of the Chemiluminescent Oxidation of Aqueous Br- by Ru(bipyr)33+.- Synthesis and Photophysical Studies of Ortho-Metalated Pd(II) Complexes Including Two Novel Pd(II)/Rh(III) Dimers.- Photoproperties of Ortho-Metalated Ir(III) and Rh(III) Complexes.- Ground and Excited State Interactions in Multimetal Systems.- Photophysical and Photochemical Properties of Ruthenium(II) Mixed- Ligand Complexes: Precursors to Homonuclear and Heteronuclear Multi- metal Complexes Containing Ruthenium(II), Platinum(II), Rhenium(I) and Rhodium(III).- Ground- and Excited-State Acid-Base Equilibria of (2,2'-Bipyridine)- Tetracyanoruthenate(II).- Topic 4: Photoredox Processes.- Kinetics and Mechanism of Photochemical Formation of a Pyrazine Bridged Fe(II) Protoporphyrin IX Polymeric Compound.- Charge-Transfer States and Two-Photon Photochemistry of Cu(NN)2+ Systems.- Photoinduced Multielectron Redox Reactions in the [PtCl6]2-/Alcohol System: Visible Light Reduction of Platinum Centers.- Dynamic and Static Outer-Sphere and Inner-Sphere Quenching Processes.- Photochemistry and Spectroscopy of Ion Pair Charge Transfer Compounds.- Backward Electron Transfer within Geminate Radical Pair Formed in the Electron Transfer Quenching.- Photochemistry of Copper Complexes and its Catalytic Aspects.- Intramolecular Excited State Electron Transfer from Naphthalene to Cobalt(III).- Photochemistry of Coordination Compounds of Main Group Metals. Reductive Elimination of Thallium (III) Complexes.- Topic 5: Organometallic Photochemistry.- Photophysics and Photochemistry of Tungsten Carbyne Complexes.- The Photoisomerization and Photosubstitution Reactions of the Ruthenium Cluster HRu3 (CO)10 (?-COCH3).- Multiple Emission from (?5-C5H5)Re(CO)2L (L = a Substituted Pyridine) Complexes in Room-Temperature Solution.- Photochemically Induced C-C-Bond Formation in the Coordination Sphere of Transition Metals.- Triplet Quenching by Metal Carbonyls.- Photoexcitation of W(CO)6 Solutions Containing ?-Diimine Ligands. Kinetics and Mechanism of Chelation for a Series of Photoproduced W (CO)5 (?-Diimine) Intermediates.- Kinetics and Mechanism of C-H Activation Following Photoexcitation of (?5-C5H5)Ir(CO)2 in Hydrocarbon Solutions.- Identification of H2-, D2-, N2-Bonded Intermediates in the Cr(CO)6 Photocatalyzed Hydrogenation Reactions.- Spectroscopy and Photochemistry of Ni (CO)2 (?-Diimine) Complexes.- Fe(CO)3(R-DAB), a Complex with Two Close-Lying Reactive Excited States.- The Photocatalytic Metathesis Reaction of Olefins.- Photochemical Generation of Nineteen-Electron Organometallic Complexes and Their Use as Reducing Agents in Micellar Systems.- Probing Organometallic Photochemical Mechanisms with Quinones: Photolysis of Mn2 (CO)10.- Laser Flash Photolysis of Phosphine-Substituted Dimanganese Carbonyl Compounds.- Topic 6: Methods, Applications, and Other Aspects.- Electron Trapping in Colloidal TiO2 Photocatalysts: 20 ps to 10 ns Kinetics.- The Radiation Sensitivity of Select Metal Chelate Polymers: Mechanistic Changes at Higher Energies.- Temperature Dependent Emission of Copper Porphyrins in Liquid Solution.- Pressure Effects on Nonradiative Deactivation from Metal Complex Excited States in Solution.- Heterogeneous Photocatalysis by Metal Sulfide Semiconductors.- Inorganic Photoinitiators for Photolithographic Applications.- Photochemical Behaviour of Luminescent Dyes in Sol-Gel and Boric Acid Glasses.- Industrial Applications of Organometallic Photochemistry.- Synthesis and Characterization of a ?-?X?-diruthenium Complex as a Precursor to an Efficient Water Oxidation Catalyst.- The Application of Diffuse Reflectance Laser Flash Photolysis to Metal Phthalocyanines in an Opaque Environment.- Quenching of Singlet Oxygen by Cobalt Complexes.- Recent Advances in Inorganic and Organometallic Photolithography.- Author Index.

Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

Abstract: A knowledge of the geometries at which excited molecules return to the electronic ground state (S0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S1 (lowest excited singlet) and T1(lowest triplet) surfaces, as well as S0–S1 conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S0 and S1 surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two-electron two-orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S0 and S1 surfaces touch. The results obtained for the simple model are supported by ab initio large-scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π-donor-to-π-acceptor single bonds, and by similar calculations for “push-pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S0, S1, and T1 states. The likely consequences of these results for the detailed description of the mechanisms of cis-trans isomerization, the formation of twisted internal charge-transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.

Presumption for a Quantum Energy Gap in the Quasi-One-Dimensional S?=?1 Heisenberg Antiferromagnet Ni(C2H8N2)2NO2(ClO4)

Abstract: Magnetic susceptibility and inelastic neutron scattering experiments have been performed in the nearly ideal one-dimensional Heisenberg antiferromagnet with spin one, Ni(C2H8N2)2NO2ClO4. The experimental results are consistent with the recent theoretical predictions for a quantum energy gap between the ground state and the first excited states.

Time-Resolved Fluorescence of Lanthanide Probes and Applications in Biotechnology

Abstract: The relaxation of excited molecular states is often followed by the emission of light. Molecular excited states are related to the orbital, vibrational, and rotational levels of the orbital electrons. In addition, the electron spin determines the singlet and triplet states. In the singlet state the spin of an electron is paired, whereas in the triplet state the spin is not paired. When a molecule is excited to an upper singlet energy level, such as S2 (Figure 1), it can rapidly (10−10 sec) go to the lowest excited state S1 without emission of light. From S1 the molecule may go to any of the rotational and vibrational levels of the ground electronic state S0. This happens either by fluorescence emission or by nonradiative internal processes, referred to as intersystem crossing. From T1, the molecule can return to S0 by nonradiative processes or by a radiative process called phosphorescence. Under suitable conditions S1 and T1 can also transfer their excitation energy to other molecules.

Photo-excitation in conjugated polymers

Abstract: Conjugated polymers such as polyacetylene form a group of model semiconductors in which the 'one-dimensional' character of the polymer strongly modifies the behaviour of charges added to the chains from that usually observed in three-dimensionally bonded semiconductors. First, the polymer chain can undergo local reorganisation of the pi -electron bonding in the vicinity of a charge added to it. This has the effect of localising the charge in a state that can be described as a soliton for the particular case of trans-polyacetylene, and more generally as a polaron. Secondly, the motion of charged excitations is highly anisotropic, with strong confinement to the chain through the 'polaronic' structural relaxation of the chain. This anisotropy strongly affects charge transport and charge separation following photo-excitation. The authors review some of the recent work in this area and present polarisation-dependent measurements of photoluminescence (PL) and photo-induced absorption (PA) on highly oriented films of polyacetylene and poly(phenylenevinylene). They find that charge separation, detected through PA from photogenerated solitons or bipolarons, is the result of inter-chain electron transfer, whereas intra-chain excitation leads to charge confinement, and decay by PL (for poly(phenylenevinylene)) and by non-radiative channels. They find furthermore that these non-radiative channels become more efficient as the length of the conjugated polymer chain increases, and consider that this occurs through rapid motion of the excited state to recombination centres. They have identified both monomolecular and bimolecular kinetics for these processes.

Antiferromagnetic correlations in almost-localized Fermi liquids.

Abstract: A Monte Carlo method is used to calculate various properties of one-band Gutzwiller wave functions which are formed by restricting the charge fluctuations in noninteracting wave functions. Gutzwiller's approximate formula for the kinetic energy is tested both for the ground state and excited states. The ground state is found to have strong antiferromagnetic short-range spin-spin correlations for nearly-half-filled bands, thus extending previous work on the half-filled case. These correlations are very sensitive to the choice of occupied Bloch states and when the occupation is distributed uniformly over the band they disappear. From this fact we conclude that correlations are present only at temperatures low compared to the coherence temperature. In the almost-localized limit it is advantageous to describe the system by an effective Hamiltonian which separates into a term due to the kinetic energy of the charge carriers and one due to the Heisenberg spin-spin coupling. We show that the almost-localized Fermi liquid can gain energy from both terms in the effective Hamiltonian. In other words the restrictions on charge fluctuations can cause spin correlations which in turn can stabilize the Fermi-liquid ground state.

Kinetics of Nonequilibrium Low-Temperature Plasmas

Abstract: One Low-Temperature Plasma. General Information.- 1.1. Quasineutrality. Debye Screening.- 1.2. Ideal Plasma.- 1.3. Equilibrium Plasma.- 1.4. Local Thermodynamic Equilibrium. Elementary Processes.- 1.5. Features of the Transport Phenomena.- 1.6. Nonequilibrium Low-Temperature and High-Temperature Plasmas.- Two Elementary Processes in a Low-Temperature Plasma.- 2.1. Elastic Collisions.- 2.1.1. Coulomb Collisions.- 2.1.2. Elastic Scattering of Electrons by Atoms and Molecules.- 2.2. Inelastic Collisions of Electrons with Atoms, Ions, and Molecules.- 2.2.1. Excitation and Quenching by Electron Impact.- 2.2.2. Electron Impact Ionization and Ternary Recombination (Electron-Ion-Electron).- 2.2.3. Excitation of Vibrational and Rotational States of Molecules by Electron Impact.- 2.3. Inelastic Collisions with Heavy Particles.- 2.3.1. Excitation and Quenching.- 2.3.2. Ionization and Three-Particle Recombination.- 2.3.3. Associative Ionization and Dissociative Recombination.- 2.3.4. Processes Forming Molecular Ions.- 2.3.5. Charge Transfer.- 2.3.6. Formation of Negative Ions.- 2.4. Elementary Radiative Processes.- 2.4.1. Bound-Bound Transitions.- 2.4.2. Photoionization and Radiative Recombination.- 2.5. Average Energy Transferred to an Atom in Collisions.- 2.5.1. Energy Transfer in Elastic Collisions of Atoms with Electrons.- 2.5.2. Energy Transfer in Inelastic Collisions of Electrons with Atoms.- 2.5.3. Diffusion Coefficients for a Weakly Bound Electron in the Energy Space of the Atom.- Three Radiative Transport of Excitation.- 3.1. Basic Characteristics of Radiative Excitation Transport.- 3.2. Radiative Excitation Transport Equation.- 3.3. Effective-Lifetime Approximation.- 3.4. Radiative Excitation Transport in an Inhomogeneous Medium.- 3.5. Domain of Application of the Theory.- Four Criteria for the Onset of Nonequilibrium States.- 4.1. Criterion for Detachment of the Electron Temperature.- 4.2. Criterion for Equilibrium Ionization and for an Equilibrium Distribution of the Atoms over Levels.- 4.3. Criterion for the Breakdown of the Maxwellian Distribution.- Five Population Kinetics of Excited States.- 5.1. Qualitative Picture of the Population Distribution in a Nonequilibrium Plasma.- 5.2. System of Kinetic Balance Equations for the Populations of Excited States.- 5.3. Numerical Methods of Solving the System of Kinetic Equations for the Populations.- 5.4. Diffusion Approximation.- 5.5. Discrete Methods and the Modified Diffusion Approximation.- 5.5.1. Single-Quantum Approximation.- 5.5.2. Single-Quantum Approximation with Allowance for Radiative Transitions, the Predominant-Sink Model.- 5.5.3. Modified Diffusion Approximation.- 5.6. Comparison of the Populations Found Analytically with the Results of Computer Calculations and Experiments.- 5.7. Influence of Atom-Atom Collisions on the Population Distribution.- 5.8. Allowance for Sources of Excited Atoms in the System of Balance Equations.- 5.8.1. Processes of External Population of an Excited Level.- 5.8.2. Processes Which Destroy Excited States.- 5.9. Features of the Impact-Radiation Kinetics in a Rarefied Plasma.- 5.10. Some Applications of the Theory.- 5.10.1. Determining the Electron Temperature and Density from the Measured Populations of Excited States.- 5.10.2. Population Inversion in the Recombinational Decay of a Plasma.- Six Kinetics of Ionization and Recombination.- 6.1. Elementary Kinetics of Ionization and Recombination.- 6.1.1. Electron-Impact Ionization and Ternary Recombination.- 6.1.2. Ionization and Recombination in Collision with Heavy Particles.- 6.1.3. Penning Ionization.- 6.1.4. Associative Ionization and Dissociative Recombination.- 6.1.5. Radiative Recombination.- 6.1.6. Dielectronic Recombination.- 6.1.7. Recombination of Positive and Negative Ions.- 6.2. Basic Kinetic Equations for Ionization and Recombination and the Results of Their Numerical Solution.- 6.2.1. Definition of the Ionization and Recombination Coefficients.- 6.2.2. Results of Numerical Determination of the Ionization and Recombination Coefficients.- 6.3. Coefficients of Impact-Radiative Recombination in the Diffusion and Modified Diffusion Approximations.- 6.3.1. The Diffusion Approach in the Kinetics of Recombination and Ionization.- 6.3.2. Coefficients of Impact-Radiative Ionization and Recombination in the Modified Diffusion Approximation.- 6.3.3. Comparison with Experimental Data and Numerical Results.- 6.3.4. Influence of Collisions with Heavy Structureless Particles on the Ionization and Recombination Coefficients.- 6.3.5. Impact-Dissociative Recombination and Impact-Associative Ionization.- 6.4. Electron Density under Steady Nonequilibrium Conditions.- 6.4.1. Equation for the Nonequilibrium Electron Density.- 6.4.2. Discussion of the Experimental Data and Comparison with Theoretical Results.- Seven Electron Energy Distribution and Energy Balance.- 7.1. Kinetic Equation and the Electron Energy Balance.- 7.1.1. Weakly Ionized Plasma.- 7.1.2. Highly Ionized Plasma.- 7.2. Inelastic Collisions. Their Influence on the Electron Energy Balance, Excitation Rate, and Ionization.- 7.2.1. Inelastic Collision Integral. Energy Loss in Inelastic Collisions.- 7.2.2. The Distribution Function and the Rate of Excitation and Ionization in a Highly Ionized Plasma.- 7.2.3. Rate of Excitation and Ionization and the Distribution Function in a Weakly Ionized Plasma.- 7.2.4. Distribution Function in the Presence of a Source of Fast Electrons.- 7.3. Self-Consistent Distributions for Electrons over Energies and Atoms over Excited States.- 7.3.1. Self-Consistent Distributions of Atoms over Levels and Electrons over Energies.- 7.3.2. Influence of a Non-Maxwellian Distribution on the Coefficient of Impact-Radiative Ionization.- 7.4. Electron Energy Distribution in a Strong Electric Field.- 7.4.1. Electron Energy Distribution in a Weakly Ionized Plasma.- 7.4.2. Townsend Ionization Coefficient.- Eight Transient Nonequilibrium Plasmas.- 8.1. Criteria of Quasisteadiness.- 8.1.1. Quasisteadiness in the Excited States.- 8.1.2. Quasisteadiness of the Electron Temperature.- 8.2. Ionizational Relaxation.- 8.2.1. Ionization Growth during Optical Heating and behind Shock Fronts.- 8.2.2. Recombinational Decay of a Plasma.- 8.2.3. Recombination of Electrons in an Expanding Plasma.- 8.3. Radiation from Transient Plasmas.- 8.4. Relaxation of the Distribution Function.- 8.5. Instabilities of a Nonequilibrium Plasma in an External Electric Field.- 8.5.1. Ionizational Instability of a Highly Nonequilibrium Plasma.- 8.5.2. Overheating Instability of the Electron Gas.- Nine Some Topics in the Kinetics of Molecular Plasmas.- 9.1. Electron Energy Balance.- 9.2. Electron Energy Distribution Function.- 9.3. Distribution of Molecules over Vibrational Levels.- 9.3.1. Quasiequilibrium. The Structure of the Vibrational Distribution.- 9.3.2. Steady-State Distribution in the Case of Strong Excitation.- 9.3.3. Vibrational Energy Balance.- 9.4. Electron-Ion Recombination in Molecular Gases.- 9.4.1. Recombination Accompanied by the Excitation of Molecular Rotations.- 9.4.2. Recombination Accompanied by Excitation of Molecular Vibrations.- 9.4.3. Limits of Applicability. Comparison with Experiment.- 9.5. Some Topics in the Kinetics of Atomic-Molecular Plasmas.- Appendixes.- Appendix 1. Values of the Level-Population Coefficients of Atomic Hydrogen.- Appendix 2. Differential and Finite-Difference Fokker-Planck Equations.- Appendix 3. Energy Level Schemes of Atoms.- Appendix 4. Sample Calculation of the Populations of Excited Atoms in the Modified Diffusion Approximation.- References.

Mini-soliton stars

Abstract: Nontopological soliton solutions of a complex scalar field in general relativity are derived. The theory is renormalizable (except for graviton loops), and these solutions are valid both classically and quantum mechanically. We study the ground states (which are stable) as well as the excited states, but restrict ourselves to spherically symmetric ones. Their physical characteristics can be rather remarkable. For example, if the mass of the scalar field is about 30 GeV, then a mini-soliton star could have a radius \ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ cm, a mass \ensuremath{\sim}${10}^{10}$ kg, and a density \ensuremath{\sim}${10}^{41}$ times that of a neutron star.

Quantum simulation of hydrogen in metals.

Abstract: The path-integral method of quantum simulation is applied to an empirical model for hydrogen in niobium. Results for the density distribution of D, H, and the positive muon over the unit cell show the dramatic increase of quantum effects along this series. Calculations on the activation energy for diffusion confirm the importance of excited states at high temperature pointed out by Emin, Baskes, and Wilson and suggest that hydrogen diffusion is approximately classical in this regime.

Purely rotational coherence effect and time‐resolved sub‐Doppler spectroscopy of large molecules. II. Experimental

Abstract: In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases.

The electronic state‐selective photodissociation of CH2BrI at 248, 210, and 193 nm

Abstract: The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser‐molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin‐orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin‐orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes.

State-to-state chemistry with fast hydrogen atoms. Reaction and collisional excitation in H + CO2

Abstract: We present a detailed theoretical study of vibrational–rotational excitation and reaction in collisions of CO2 with 1.9–2.6 eV hydrogen atoms. Minima and saddle points on the potential surface have been characterized using ab initio configuration interaction calculations, and a global surface has been developed by a combination of many-body expansion and surface-fitting methods. The collision dynamics have been studied using quasiclassical trajectories, with the CO2 vibrational states characterized by a Fourier-transform action calculation. For non-reactive scattering there is reasonable correlation between the vibrational modes that are excited and the regions of the potential surface sampled during the collisions. Most of the lower states of CO2 are excited by direct collisions that do not sample potential wells. Collisions which do sample wells lead to short-lived HOCO and HCO2 complexes, in which either the H dissociates to produce highly excited overtone and combination states of CO2, or a CO bond breaks to give OH + CO having close to statistical vibrational–rotational distributions. Comparison of cross-sections and final-state distributions with experiment is excellent for the reactive collisions, and is good on a relative but not absolute basis for the non-reactive collisions.

Theory of the Rydberg-atom two-photon micromaser

Abstract: A continuous-wave maser operating on a two-photon transition between Rydberg levels is expected to oscillate with about one atom and a few tens of microwave photons at any time in its superconducting cavity. We analyze in detail the characteristics of this new microscopic quantum electronics device presently under construction in our laboratory.

A study of nonrigid aromatic molecules by supersonic molecular jet spectroscopy. I. Toluene and the xylenes

Abstract: Dispersed emission and time of flight mass spectra are presented for jet‐cooled toluene, and o‐, m‐, and p‐xylene. The spectra exhibit features, typically within 100 cm−1 of the S1⇄S0 origins, which are assigned to transitions associated with the internal rotation of the ring methyl groups. A model is developed which treats this methyl motion as that of a one‐dimensional rigid rotor. The spacings of the peaks in the spectra are used to solve for the rotational constant B of the methyl rotor, and for the size and shape of the n‐fold barrier to rotation (i.e., V3, V6, etc.) within this model. For toluene and p‐xylene, the barrier is found to be small in both the ground (S0) (V6∼10 cm−1) and excited (S1) (V6∼25 cm−1) electronic states. For m‐xylene, the ground state is again found to have a low barrier (V6∼25 cm−1), but the excited state has a potential barrier of V3=81 cm−1, V6=−30 cm−1. The barrier to rotation of the ring methyl groups is observed to be the highest for o‐xylene. In this case the ground sta...

Structure/Reactivity and Thermochemistry of Ions

Abstract: Capture Collision Theory.- Collision Theory: Summary of the Panel Discussion.- Optical Studies of Product State Distributions in Thermal Energy Ion-Molecule Reactions.- Crossed-Molecular Beam Studies of Charge Transfer Reactions at Low and Intermediate Energy.- Production, Quenching and Reaction of Vibrationally Excited Ions in Collisions with Neutrals in Drift Tubes.- Kinetic Energy Dependence of Ion-Molecule Reactions: From Triatomics to Transition Metals.- Reactions of Transition Metal Ions with Cycloalkanes and Metal Carbonyls.- Gas Phase Metal Ion Chemistry: Summary of the Panel Discussion.- Dynamics of Dissociation and Reactions of Cluster Ions.- Growing Molecules with Ion/Molecule Reactions.- AB Initio Studies of Interstellar Molecular Ions.- Structures and Spectroscopic Properties of Small Negative Molecular Ions - Theory and Experiment.- AB Initio Calculations on Organic Ion Structures.- Formation of Anions in the Gas Phase.- Proton Transfer Reactions of Anions.- Assignment of Absolute Gas Phase Basicities of Small Molecules.- Kinetics and Equilibria of Electron Transfer Reactions: A? + B = A + B?. Determinations of Electron Affinities of A and B and Stabilities of.Adducts A2? and (A * B)?.- Ion Thermochemistry: Summary of the Panel Discussion.- Entropy-Driven Reactions: Summary of the Panel Discussion.- Organic Ion/Molecule Reactions: Summary of the Panel Discussion.- Experimental and Theoretical Studies of Small Organic Dications, Molecules with Highly Remarkable Properties.- Structure and Reactivity of Gaseous Ions Studied by a Combination of Mass Spectrometric, Nuclear Decay and Radiolytic Techniques.

Finite-size corrections and numerical calculations for long spin 1/2 Heisenberg chains in the critical region

Abstract: Leading and next-to-leading-order finite-size corrections to the ground and first excited states are calculated for the spin-1/2 anisotropic Heisenberg model in the critical region. The analytic results are compared to numerical data obtained for chains up to a length of N=1024. It is found that, near the isotropic point, the asymptotic region where the results obtained for N to infinity are applicable sets in at very large N values, and for obtaining good accuracy in fitting the numerical data one has to take into account several correction terms, even at large (N>100) chain lengths.

The à 2Π–X̃ 2Σ+ transition of HC2 isolated in solid argon

Abstract: Fourier transform absorption spectra have been obtained between 700 and 7900 cm−1 at a resolution of 0.2 cm−1 for Ar:C2H2 samples codeposited at 12 K with a beam of argon atoms that had been excited in a microwave discharge. Detailed isotopic substitution studies have confirmed that the predominant product species is HC2, which contributes not only the absorptions previously assigned to its two stretching fundamentals but also several weaker absorptions in the 2000–3600 cm−1 spectral region and a prominent, complicated pattern of absorptions between 3600 and 7800 cm−1. The previous assignment of the 3611 cm−1 HC2 absorption as the CH‐stretching fundamental is reviewed, and the assignment of an absorption at 2104 cm−1 as ν2+ν3 of ground‐state HC2 is discussed. The near infrared absorption band system has been assigned to the A 2Π–X 2Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state. The position of the transition origin could not be definitively e...

Spectroscopic and electrochemical properties of dimeric ruthenium(II) diimine complexes and determination of their excited state redox properties

Abstract: The complexes Ru(bpy)/sub 2/(ppz)/sup 2 +/ and ((bpy)/sub 2/Ru(ppz)Ru(bpy)/sub 2/)/sup 4 +/, where ppz is the planar ligand 4',7'-phenanthrolino-5',6':5,6-pyrazine, have been prepared and characterized. Resonance Raman spectra establish that the visible spectra of Ru(bpy)/sub 2/L/sup 2 +/ and ((bpy)/sub 2/Ru-L-Ru(bpy)/sub 2/)/sup 4 +/ complexes, where L is a bis-diimine, in general, are composed of MLCT transitions which terminate in the ..pi..* orbitals localized on the different ligands. The luminescence, which is detectable at room temperature in fluid solutions of both the mono- and bimetallic complexes, can be assigned as a L(..pi..*) ..-->.. Ru(II) t/sub 2/ transition. An approximate but general correlation between the lower energy MLCT absorption maximum and the emission maximum suggests that in many other bimetallic complexes of Ru(II) the emission energy is shifted beyond usual detection limits. Analysis of the emission and electrochemical data indicates that the MLCT states of bridged 2,3-dipyridylpyrazine (dpp) and ppz dimeric complexes are weak reductants but very strong oxidants. The implications of this general pattern of excited state redox potentials are discussed.

Theoretical study of a Cu + ion impurity in a NaF host

Abstract: The Cu+ ion impurity in a NaF host has been modeled using a finite cluster of ions to represent the crystal lattice. Several approximations to the lattice potential in the region of the cluster were compared to the exact Madelung potential. The error in the calculated nearest‐neighbor distance for the pure host was found to be proportional to the error in the lattice potential. Hartree–Fock calculations were carried out for the ground 1A1g and excited 1,3Eg and 1,3T2g states of the NaF:Cu+ system. The resulting energy level structure was compared to the experimental spectra. The symmetric‐stretch potential energy curve, vibrational frequencies, and Franck–Condon factors were calculated for the 1A1g and 1,3T2g states. Using a single configuration coordinate model and a semiempirical spin–orbit coupling scheme, the relative intensities and bandwidths were calculated for absorption to the 1,3T2g states and compared to experiment.

State to state photodissociation of H2O in the first absorption band

Abstract: The photodissociation of H2O in the first absorption band is studied from single rotational states of vibrationally excited water. A tunable IR laser is used to prepare single rotational states in the asymmetric stretch mode. The subsequent photodissociation at 193 nm favors product formation from these single prepared states. The formation of the OH product in different rotational, Λ‐doublet, and spin states is analyzed for a series of initial rotational states of H2O. This is the first direct photodissociation studied on a state to state level. The product state distributions depend sensitively upon the prepared state in the parent molecule H2O and exhibit pronounced quantum structure. The experimental results are understood almost quantitatively in terms of theory. The photodissociation of water turns out to be a limiting case of a dissociation which is governed by transfer of parent motion to products. The experiment leads to a highly improved understanding for the selective population of Λ‐doublet states.

Structure and vibrational spectroscopy of the water dimer using quantum simulation

Abstract: A method is described for calculating the vibrational frequencies of a molecular cluster using a combination of quantum simulation and local-mode variational theory. The method is applied to the water dimer and results are compared with normal-mode theory, frozen field local-mode theory, and with experiment. It is shown that the new approach is more rapidly convergent than the other techniques. Also, providing a suitable potential surface is available, the method is readily applicable to other molecular clusters.