Showing papers on "Excited state published in 1989"

Interstellar polycyclic aromatic hydrocarbons: the infrared emission bands, the excitation/emission mechanism, and the astrophysical implications.

Abstract: A comprehensive study of the PAH hypothesis is presented, including the interstellar, IR spectral features which have been attributed to emission from highly vibrationally excited PAHs Spectroscopic and IR emission features are discussed in detail A method for calculating the IR fluorescence spectrum from a vibrationally excited molecule is described Analysis of interstellar spectrum suggests that the PAHs which dominate the IR spectra contain between 20 and 40 C atoms The results are compared with results from a thermal approximation It is found that, for high levels of vibrational excitation and emission from low-frequency modes, the two methods produce similar results Also, consideration is given to the relationship between PAH molecules and amorphous C particles, the most likely interstellar PAH molecular structures, the spectroscopic structure produced by PAHs and PAH-related materials in the UV portion of the interstellar extinction curve, and the influence of PAH charge on the UV, visible, and IR regions

Molecule Formation and Infrared Emission in Fast Interstellar Shocks. III. Results for J Shocks in Molecular Clouds

Abstract: The structure and emission spectrum of J shocks in molecular gas are studied over a broad range of conditions. It is found that at high densities chemistry has a profound effect on the emission spectrum: the density behind the shock is sufficiently high that some of the internal energy of the newly formed H2 molecules is transformed to the gas as heat by collisional deexcitations, producing the H2 formation plateau. In this temperature plateau, endothermal reactions and neutral-neutral chemical reactions with activation energies can proceed efficiently, producing significant quantities of warm H2, CO, OH, and H2O and enhanced columns of warm atoms and ions. The heat generated by the H2 formation is radiated in collisionally excited atomic fine-structure lines.

Degenerate quantum-beat laser: Lasing without inversion and inversion without lasing.

Abstract: A single lasing mode driven by a three-level ``quantum-beat'' atomic configuration can show gain without population inversion or optical absorption into an excited state without spontaneous or stimulated emission.

Cavity Quantum Electrodynamics

Abstract: Ever since Einstein demonstrated that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have generally believed that excited atoms inevitably radiate. Spontaneous emission is so fundamental that it is usually regarded as an inherent property of matter. This view, however, overlooks the fact that spontaneous emission is not a property of an isolated atom but of an atom‐vacuum system. The most distinctive feature of such emission, irreversibility, comes about because an infinity of vacuum states is available to the radiated photon. If these states are modified—for instance, by placing the excited atom between mirrors or in a cavity—spontaneous emission can be greatly inhibited or enhanced.

Electronic spectra and photophysics of platinum(II) complexes with alpha-diimine ligands - Solid-state effects. I - Monomers and ligand pi dimers

Abstract: Two types of emission behavior for Pt(II) complexes containing alpha-diimine ligands have been observed in dilute solution. If the complex also has weak field ligands such as chloride, ligand field (d-d) excited states become the lowest energy excited states. If only strong field ligands are present, a diimine 3(pi-pi/asterisk/) state becomes the lowest. In none of the cases studied did metal-to-ligand charge transfer excited state lie lowest.

New mechanism for hydrogen desorption from covalent surfaces: The monohydride phase on Si(100).

Abstract: A new mechanism for the thermal desorption of molecular hydrogen from the monohydride phase on Si(100) has been identified. The unusual first-order desorption kinetics that are observed are due to the irreversible excitation of a hydrogen adatom into a delocalized, two-dimensional band state on the surface with an activation energy of 47 kcal/mol. The desorption reaction occurs between this excited hydrogen adatom and a second, localized hydrogen adatom.

High‐resolution near‐infrared spectroscopy of water dimer

Abstract: High-resolution near-infrared spectra are reported for all of the O-H stretch vibrational bands of the water dimer. The four O-H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that, for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.

Electron–vibration coupling in semiconductor clusters studied by resonance Raman spectroscopy

Abstract: The resonance Raman spectrum of 45(+−3) A diameter CdSe clusters was measured. The incident photons were resonant with the HOMO–LUMO transition in the clusters. At low temperature, one mode at 205 cm−1 is observed, as well as two overtones, with the integrated areas under these peaks in the ratio of 9:3:1. This mode is assigned as the longest wavelength longitudinal optical vibration of the cluster. The strength of the coupling between the lowest electronic excited state and the LO vibration is found to be 20 times weaker in these clusters than in the bulk solid. The CdSe cluster resonance Raman spectrum is shown to be consistent with the recently measured homogeneous cluster absorption spectrum.

A collisional-radiative model applicable to argon discharges over a wide range of conditions. I. Formulation and basic data

Abstract: A collisional-radiative model with an extended region of applicability is developed for an argon atom plasma. Atom-atom inelastic collisions and diffusion losses of the metastable states along with the electron-atom inelastic collisions and radiative processes are considered in this model, taking into account 65 effective levels. Among the analytical expressions used for the corresponding cross sections, special attention is paid to those determining the set of cross sections for excitation by electrons from the ground state, owing to the possibility of utilising the formulae recommended in kinetic modelling studies of discharges in argon or in mixtures including argon atoms. The numerical method developed makes it possible to investigate the mechanisms by which the excited levels are populated in a non-equilibrium argon plasma characterised (even in the case of a non-Maxwellian electron distribution) by a set of parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground state atom population n1, the discharge tube (or the plasma column) radius R and the optical escape factors Lambda mn and Lambda m, which are dependent only on the quantities Ta, n1 and R in many cases of practical interest.

Coupled translational-vibrational activation in dissociative hydrogen adsorption on Cu(110).

Abstract: The initial dissociative sticking probability of ${\mathrm{H}}_{2}$ on Cu(110) has been measured using vibrationally hot, translationally cold-seeded beams. The results indicate a dissociative barrier which is accessed through energy partition in both translational and vibrational degrees of freedom in the im- pinging hydrogen molecule. The experiment has enabled a determination of the translational requirement in dissociating hydrogen in the first vibrational excited state, and the results place the barrier at \ensuremath{\sim}1 eV, in good agreement with theoretical predictions.

A theoretical study of multidimensional nuclear tunneling in malonaldehyde

Abstract: Various aspects of the intramolecular proton transfer in malonaldehyde have been investigated theoretically within the reaction surface Hamiltonian framework, which was recently applied with a two‐dimensional surface to this molecule by Carrington and Miller. The present calculation, which involves a three‐dimensional reaction surface and a high level of ab initio accuracy, gives a tunneling splitting which is ∼50% smaller than experiment and a hydrogen/deuterium isotope effect that is within 40% of experiment with no adjustable parameter. The vibrational wave function has been analyzed to extract an effective curvilinear tunneling path on the hypersurface. The path calculations, and other analysis, clearly demonstrate the limitations of one‐dimensional models for polyatomic tunneling systems like malonaldehyde. In addition, tunneling splittings have been calculated for excited vibrational states of malonaldehyde, leading to new insight into the multidimensional character of proton transfer.

Intersubband emission from semiconductor superlattices excited by sequential resonant tunneling

Abstract: We present the first observation of infrared light emission from a semiconductor superlattice in a resonant-tunneling experiment. Radiation from the three lowest intersubband transitions is observed, proving resonant tunneling an effective means of populating high-lying states of the superlattice. The relative strength of the emission lines enables us to estimate the electron temperature below the optical-phonon energy to reach about 140 K, whereas transitions originating above the optical-phonon energy are strongly quenched.

Supercomputer algorithms for reactivity, dynamics and kinetics of small molecules

Abstract: Recent Advances in Electronic Structure Theory and their Influence on the Accuracy of Ab initio Potential Energy Surfaces.- Modern Electronic Structure Calculations: The Accurate Prediction of Spectroscopic Band Origins.- Potential Energy Surfaces of Several Elementary Chemical Reactions.- Calculation and Characterization of Reaction Valleys for Chemical Reactions.- Computed Potential Energy Surfaces for Chemical Reactions.- An Ab initio Study on the Coordination of Formaldehyde, Carbon Dioxide, Dinitrogen and Related Molecules to Iron(0) and Nickel(0) Fragments.- Kinetic Paths from the Hyperspherical Perspective: Ab initio Potential Energy Surface for the O(3P)+H2 Reaction.- Exact Quantum Results for Reactive Scattering Using Hyperspherical (APH) Coordinates.- Computational Strategies and Improvements in the Linear Algebraic Variational Approach to Rearrangement Scattering.- How Variational Methods in Scattering Theory Work.- Quantum Dynamics of Small Systems Using Discrete Variable Representations.- Finite Element Calculations of Scattering Matrices for Atom-Diatom Reactive Collisions. Experiences on an Alliant FX/8.- Investigations with the Finite Element Method. The Collinear H + H2, F + H2 and Ne + H2+ Reactions.- Calculation of Multichannel Eigenvalues and Resonances.- Accurate Determination of Polyatomic Infrared Spectra.- The Calculation of Ro-Vibrational Spectra Using Supercomputers.- Approximate Quantum Techniques for Atom Diatom Reactions.- Approximate Quantum Mechanical Calculations on Molecular Energy Transfer and Predissociation.- Temperature-Dependent Rate Constants for Ion-Dipole Reactions: C+(2P) + HCl(X1?+).- Classical Path Approach to Inelastic and Reactive Scattering.- Intramolecular Energy Transfer in HC and HO Overtone Excited Molecules.- Classical Trajectory Studies of Gas Phase Reaction Dynamics and Kinetics Using Ab initio Potential Energy Surfaces.- Quasiclassical Calculations for Alkali and Alkaline Earth + Hydrogen Halide Chemical Reactions Using Supercomputers.- Dynamics of the Light Atom Transfer Reaction: Cl + HCl ? ClH + Cl.- The Modeling of Complex Gas Phase Reactions: From Expert Systems to Supercomputers.

State‐resolved evidence for hot carrier driven surface reactions: Laser‐induced desorption of NO from Pt(111)

Abstract: State‐specific diagnostics are used to characterize the laser‐induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non‐Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance.

Valence bond approach to exact nonlinear optical properties of conjugated systems

Abstract: Diagrammatic valence‐bond (DVB) theory is extended to dynamic nonlinear susceptibilities of interacting π electrons in Pariser–Parr–Pople (PPP) or other quantum cell models whose correlated ground state ‖G〉 is known. Corrections φ(1)(ω) and φ(2)(ω2,ω1) to ‖G〉 due to oscillating electric fields are found directly as linear combinations of VB diagrams. Any nonlinear optical coefficient is reduced to matrix elements that implicitly include all excited states, as verified for shorter polyenes. Static and dynamic χ(3) coefficients for cis and trans polyenes to N=12 carbons illustrate the importance of retaining the full spectrum. The coefficients βijk(ω,ω) for second harmonic generation are found for polar molecules like aniline and nitroaniline. Divergent responses are treated by lifetimes Γ for the resonant states, as shown for third harmonic generation in hexatriene with Γ=0 and in octatetraene with Γ>0. Electron–electron interactions reverse the sign of γijkl(ω,ω,ω) in linear polyenes, except for the large...

Dual-wavelength ratiometric fluorescence measurements of membrane potential.

Abstract: This work shows that the voltage across membranes in two very different preparations, lipid vesicles in suspension and individual HeLa cells under a microscope, is linearly related to the ratio of fluorescence excited from the two wings of the absorption spectrum of a voltage-sensitive dye. The dye di-4-ANEPPS [1-(3-sulfonatopropyl)-4-[beta-[2-(di-n-butylamino)-6-naphthyl] vin yl]pyridinium betaine] is well characterized from earlier investigations and responds via a rapid (less than millisecond) spectral shift to membrane potential changes. The resultant small change in fluorescence intensity monitored at a single wavelength is useful for measurements of temporally well-defined voltage transients such as action potentials. The dual-wavelength approach described in this work extends the usefulness of this fast potentiometric dye by filtering out complex or artifactual changes in fluorescence intensity and providing a voltage-dependent signal that is internally standardized. Thus, rapid measurements of membrane potential are made possible in nonexcitable cells.

Statistics of the excitations of the resonating-valence-bond state

Abstract: By extending recent proposals for choosing the phases in resonating-valence-bond ground states to the case of excited states, we show for these wave functions that the hole excitations are charged, spinless fermions and the spin excitations are neutral, spin-(1/2 bosons. We also show that for a system with periodic boundary conditions, all states are at least fourfold degenerate.

A simple model for correcting the zero point energy problem in classical trajectory simulations of polyatomic molecules

Abstract: A simple model is proposed for correcting problems with zero point energy in classical trajectory simulations of dynamical processes in polyatomic molecules. The ‘‘problems’’ referred to are that classical mechanics allows the vibrational energy in a mode to decrease below its quantum zero point value, and since the total energy is conserved classically this can allow too much energy to pool in other modes. The proposed model introduces hard sphere‐like terms in action–angle variables that prevent the vibrational energy in any mode from falling below its zero point value. The algorithm which results is quite simple in terms of the cartesian normal modes of the system: if the energy in a mode k, say, decreases below its zero point value at time t, then at this time the momentum Pk for that mode has its sign changed, and the trajectory continues. This is essentially a time reversal for mode k (only!), and it conserves the total energy of the system. One can think of the model as supplying impulsive ‘‘quantu...

Time dependent calculations of the absorption spectrum of a photodissociating system with two interacting excited electronic states

Abstract: We use a time dependent method for solving the Schrodinger equation to calculate the photon absorption cross section for the photodissociation of a model H+3 system. The coupling V between the excited states is found to alter the absorption cross section if the time scale ℏ/V is less than the dissociation time. The influence of the relative orientation of the transition dipoles, on the absorption spectrum, is also investigated.

Thermal double donors in silicon

Abstract: A family of double donors with only slightly differing binding energies can be generated in silicon containing oxygen. In the 30 years since they were discovered the microscopic structure of these defects has not been unravelled in spite of being investigated with all the tools of solid state physics.

Subnatural linewidth averaging for coupled atomic and cavity-mode oscillators

Abstract: We calculate the spontaneous-emission spectrum and the spectrum of weakly driven fluorescence for a two-level atom coupled to a resonant-cavity mode. For strong atom-cavity coupling the spectra split into two peaks that can have subnatural linewidths. If the cavity linewidth is negligible, the spontaneous-emission spectrum has half the radiative linewidth of the atom; the spectrum of weakly driven fluorescence shows an additional 36% squeezing-induced narrowing. These effects can be observed using coupled-field and collective-polarization oscillators excited in a cavity containing N two-level atoms.

Collisional losses from a light-force atom trap

Abstract: We have studied the collisional loss rates for very cold cesium atoms held in a spontaneous-force optical trap. In contrast with previous work, we find that collisions involving excitation by the trapping light fields are the dominant loss mechanism. We also find that hyperfine-changing collisions between atoms in the ground state can be significant under some circumstances. PACS numbers: 32.80.Pj, 34.50.Rk Spontaneous-force light traps' have provided a way to obtain relatively deep static traps for neutral atoms. These allow one to produce samples containing large numbers of very cold atoms. In this paper we present an experimental study of the collisions which eject atoms from such a trap. These collisions are of considerable interest because the temperatures of the trapped atoms (10 K) are far lower than in usual atomic collision experiments. The theory of such low-energy collisions and their novel features have been discussed by several authors. Perhaps the most notable feature is that the collision times are very long, and the collision dynamics are dominated by long-range interactions and spontaneous emission. These collisions also have important implications with regard to potential uses of optically trapped atoms. For many applications the maximum density that can be obtained is a critical parameter, and these collisions limit the attainable density. There have been two experimental studies of collisions in optical traps. Gould et al. measured the cross section for associative ionization of sodium. However, there is no evidence that this process is significant in limiting trapped-atom densities, and for some atoms, including cesium, it is energetically forbidden. Prentiss et al. studied the collisional losses which limited the density of sodium atoms which were held in a spontaneous-force trap. Their surprising and unexplained results were a direct stimulus for our work. In particular, they observed no dependence of the loss rate on the intensity of the trapping light. This was quite surprising, because a ground and an excited atom interact at long range via the strong 1/r resonant dipole interaction, and a portion of the excited-state energy can be converted into sufficient kinetic energy to allow the atoms to escape from the trap. By comparison, two atoms in their ground states interact only through a much weaker short-range 1/r Van der Waals attraction, and even when such collisions occur, they may not produce significant kinetic energy to cause trap loss. This implies that the dominant collisional loss mechanism would involve the excited atomic states, and thus depend on the intensity of the light which causes such excitations. In this paper we present measurements which show that, in contrast with Ref. 9, the collisional loss rate has a marked dependence on the trap laser intensity. We will present strong circumstantial evidence that the dependence at very low intensities is due to hyperfinechanging collisions between ground-state atoms. We believe that the loss rates at higher intensities are associated with collisions involving excited states, and are the type discussed by Gallagher and Pritchard. As discussed in Ref. 7, these collisions are very different from normal ground-excited-state atomic collisions in which two initially distant atoms, A and A approach, collide, and separate in a time much less than the radiative lifetime of the excited state. In contrast, for these very-low-temperature collisions the absorption and emission of radiation in the midst of the collision drastically alter the motion. In particular, if the excitation takes place when the two atoms are far apart (R) 1000 A) they will reradiate before being pulled into the small-R region where energy transfer occurs. However, if they are sufficiently close when excited, they can be pulled close enough together for substantial potential energy to be transferred into kinetic energy before decaying. The two dominant transfer processes are excited-state fine-structure changes and radiative redistribution. In the first, A changes its fine-structure state in the collision and the pair acquire a fine-structureinterval worth of kinetic energy. The second process, radiative redistribution, refers to A Areemitting a -photon which, because of the A-A* attractive potential, has substantially less energy than that of the photon which was initially absorbed. This energy difference is transferred to the subsequent kinetic energy of the ground-state atoms. The trap loss rate depends on the probability of exciting such "close" A-A pairs, and this probability is determined by the frequency and intensity of the exciting radiation. Light which is tuned to the red of the atomic resonance frequency, vo, excites pairs which are closer together (and shifted in energy) and thus is more eN'ective at causing trap loss than light which is at vo. We tested this hypothesis by examining how the loss