Showing papers on "Excited state published in 1985"

Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

Abstract: The unidentified infrared emission features (UIR bands) are attributed to a collection of partially hydrogenated, positively charged polycyclic aromatic hydrocarbons (PAHs). This assignment is based on a spectroscopic analysis of the UIR bands. Comparison of the observed interstellar 6.2 and 7.7-micron bands with the laboratory measured Raman spectrum of a collection of carbon-based particulates (auto exhaust) shows a very good agreement, supporting this identification. The infrared emission is due to relaxation from highly vibrationally and electronically excited states. The excitation is probably caused by UV photon absorption. The infrared fluorescence of one particular, highly vibrationally excited PAH (chrysene) is modeled. In this analysis the species is treated as a molecule rather than bulk material and the non-thermodynamic equilibrium nature of the emission is fully taken into account. From a comparison of the observed ratio of the 3.3 to 11.3-micron UIR bands with the model calculations, the average number of carbon atoms per molecule is estimated to be about 20. The abundance of interstellar PAHs is calculated to be about 2 x 10 to the -7th with respect to hydrogen.

Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism

Abstract: Photophysical parameters have been determined for coumarin laser dyes in a variety of organic solvents, water, and mixed media. The response of fluorescence emission yield and lifetime to changes in solvent polarity was a sensitive function of coumarin substitution pattern. Most important were substituent influences which resulted in enlarged excited-state dipole moments for the fluorescent state. For dyes displaying sharp reductions in emission yield and lifetime with increased solvent polarity, protic media and particularly water were most effective in inhibiting fluorescence. The temperature dependence of emission yield and lifetime was measured for two solvent-sensitive dyes in acetonitrile and in a highly viscous solvent, glycerol. The quenching of coumarin fluorescence by oxygen for dyes with lifetimes > 2 ns was also observed. The dominant photophysical features for coumarin dyes are discussed in terms of emission from an intramolecular charge-transfer (ICT) excited state and an important nonradiative decay path involving rotation of the amine functionality (7-position) leading to a twisted intramolecular CT state (TICT). The role of excited-state bond orders involving the rotating group in determining the importance of interconversions of the type ICT ..-->.. TICT is discussed. 73 references, 1 figure, 3 tables.

Excited-state properties of a triply ortho-metalated iridium(III) complex

Abstract: The characterization of the ground and luminescent excited states of a triply ortho-metalated complex of ppy, fac-Ir(ppy)/sub 3/ (ppy = 2-phenylpyridine) is effected. This complex, which is the first triply ortho-metalated ppy species to be characterized, is one of the strongest transition-metal photoreductants thus far reported. 20 references, 2 figures.

Inhibited spontaneous emission by a Rydberg atom.

Abstract: Spontaneous radiation by an atom in a Rydberg state has been inhibited by use of parallel conducting planes to eliminate the vacuum modes at the transition frequency. Spontaneous emission is observed to "turn off" abruptly at the cutoff frequency of the waveguidelike structure and the natural lifetime is measured to increase by a factor of at least 20.

Light-induced excited-spin-state trapping in iron(II) spin-crossover systems. Optical spectroscopic and magnetic susceptibility study

Abstract: Single-crystal optical absorption spectra of the iron(I1) sph-crossover compounds [Fe(pt~)~]@F~), (ptz = 1-propyltetrazole) and [Fe(2-pi~)~]Cl,.EtOH (2-pic = 2-(aminomethy1)pyridine) have been.measured between room temperature and 8 K. The wellestablished thermally induced spin transition high spin (?,& + low spin (lAIg) as well as the recently discovered phenomenon of 'light-induced excited-spin-state trapping (LIWT)" is followed and analyzed. A kinetic study by optical spectroscopy of the relaxation from the light-induced high-spin (HS) state to the low-spin (LS) state has been carried out for [Fe(ptz),](BF,), in the temperature range 53-61 K. The magnetic susceptibility of the light-induced HS state of this compound has also been measured.

Resonances in the K shell excitation spectra of benzene and pyridine: Gas phase, solid, and chemisorbed states

Abstract: K shell excitation spectra of the aromatic molecules benzene and pyridine in the gas phase are compared to those for the solids (ices) and for monolayers chemisorbed on Pt(111). The gas phase and solid spectra are essentially identical and even the spectra for the chemisorbed molecules exhibit the same resonances. Because of the orientation of the molecules upon chemisorption the latter spectra show a strong polarization dependence as a function of x‐ray incidence. This polarization dependence in conjunction with a multiple scattering Xα calculation for the benzene molecule allows us to assign the origin of all K shell resonances. The resonances are found to arise from transitions to π* antibonding orbitals and to σ* shape resonances in the continuum. The shape resonances are characterized by potential barriers in high (l=5 and 6) angular momentum states of the excited photoelectron. The polarization dependence and energy position of the resonances allow the molecular orientation on the surface to be determined and show that the change in the carbon–carbon bond length is less than 0.02 A.

Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules

Abstract: Rate constants for both radical ion pair formation and recombination in porphyrin - quinone donor-acceptor molecules are reported. The ion pair formation and recombination depend on the exothermicity of the respective electron-transfer reaction in a manner previously reported. The question as to what fraction of energy change is due to relaxation processes involving the solvent vs. the porphyrin - quinone molecules has not been answered by these studies. However, the measured reorganization energy was found to be approximately solvent independent. It was observed that in these molecules the donor-acceptor distance is restricted, and the highly exothermic charge recombination reactions do not produce electronically excited states to the donor or acceptor. 11 references, 2 figures.

Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low‐pressure argon positive column

Abstract: The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low‐pressure argon positive column is investigated theoretically and experimentally. A simple yet self‐consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady‐state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05

Femtosecond dynamics of resonantly excited excitons in room-temperature GaAs quantum wells.

Abstract: We investigate the effect of excess excitonic populations created by resonant ultrashort excitation on the optical-absorption properties of GaAs quantum wells. We find that under these conditions at room temperature excitons produce more absorption bleaching than equal densities of free-carrier pairs. This bleaching partly recovers as the excitons ionize to give free carriers. Hence, we directly measure the thermal ionization time of excitons at room temperature for the first time, and find that $\ensuremath{\tau}\ensuremath{\sim}300$ fs.

Near-Infrared Spectroscopy of Organic Substances

Abstract: The near-infrared spectral region is generally defined as the wavelength range from 700 nm to about 2500 nm, although there is considerable variation in wavelength ranges of the different instrument types. The absorption bands in this region are due to overtones and combinations of the fundamental mid-IR molecular vibration bands. The energy transitions are between the ground state and the second or third excited vibrational states. Because higher energy transitions are successively less likely to occur, each overtone is successively weaker in intensity. Since the energy required to reach a second o r third level excited.

Photodissociation of acetylene at 193.3 nm

Abstract: The authors have measured the translational energy release for processes C2H2 193 nm/ C2H + H (I) and C2H 193 nm/ C2 + H (II) using the molecular time-of-flight method. By measuring the maximum release of translational energy for process I it has been determined that the C-H bond energy in acetylene, D0(C2H-H), is 132 +/- 2 kcal/mol. Since the translational energy distribution peaks well away from zero, it is quite unlikely that internal conversion of the electronically excited C2H2 is an important channel for dissociation in channel I. Because of the relatively small amount of energy appearing as product rotation, it is possible to get information on the vibrational structure of C2H radical from the translational energy distribution and it is found that the bending frequency in C2H is 550 +/- 100 cm . This experiment is consistent with earlier work which observed C2( /sub u/) in fluorescence. 21 references, 20 figures, 3 tables.

Ground- and excited-state properties of Li2 and Li2+ from ab initio calculations with effective core polarization potentials

Abstract: Potential curves, vibrational term values, and spectroscopic constants for eighteen low-lying electronic states of Li2 and eight electronic states of Li2+ are obtained from all-electron SCF/valence CI calculations including core polarization effects by an effective potential. Previous theoretical results for experimentally known states appear to be significantly improved. Deviations from experimental data amount to ΔDc ⩽ 50 cm−1, Δωc ⩽ 0.5 cm−1, ΔRc ⩽ 0.005 A, and ΔBc ⩽ 0.002 cm−1, respectively. For unobserved or uncertain data predictions are made whose accuracy is believed to be given by these bounds. In several cases, the purely theoretical data have been combined with experimental information to yield even more precise predictions, e.g. for the hump height of the B-state potential, the ground-state ionization energy and spectroscopic constants of the I3Πu state. Transition moment functions and radiative lifetimes are obtained for all molecular states under consideration. Our calculated A-state lifetimes for unperturbed rovibronic levels are within the experimental uncertainty of 1–2%.

Possibility of direct observation of quantum jumps

Abstract: In a single-atom double-resonance experiment involving a strong and a weak transition, quantum jumps on the weak transition cause the fluorescence of the strong transition to turn on and off abruptly. The fluorescence is off when the weak transition is excited and on when it is not. Thus quantum jumps on the weak transition can be directly monitored by observation of the random telegraph signal radiated by the strong transition. We present here a simple theory of this effect for the case of incoherent excitation.

Semiconductor cluster beams: One and two color ionization studies of Six and Gex

Abstract: Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.

Correlated spontaneous-emission lasers: Quenching of quantum fluctuations in the relative phase angle.

Abstract: In quantum-beat and Hanle-effect experiments, spontaneous-emission events from two coherently excited states are strongly correlated. A doubly resonant laser cavity driven by such atomic configurations can have vanishing diffusion coefficient for the relative phase angle.

Spectroscopic Properties of the Quasi One-Dimensional Tetracyanoplatinate(II) Compounds

Abstract: Within the last ten years significant advances have been made in the knowledge and interpretation of the physical and chemical properties of organic and inorganic compounds with quasi one-dimensional crystal structures. Among these the Mx[Pt(CN)4] · n H2O compounds (with M = Na, K, Ba, Mg, Eu...) show a series of very unusual properties. These are mainly displayed by the lowest excited states which for most compounds exhibit a strong polarized luminescence. By the choice of the cation M it is possible to position the lowest transition energies within the large range from the near ultraviolet to the red. Further, high pressure application allows a continuous tuning, even down to the near infrared. By temperature variation and the application of high magnetic fields an assignment of the low-energy excited states can be achieved. Two distinctly different types of states are present, delocalized states and localized states (self-trapped excitons). By the occurrence of these different types of states the tetracyanoplatinates(II) represent interesting model systems.

Vibrational energy levels of formaldehyde

Abstract: Vibrational energies for nonrotating H2CO and D2CO are calculated using unadjusted and adjusted ab initio quartic force fields in normal mass‐weighted coordinates. Converged energies are obtained using uncoupled anharmonic oscillator and vibrational self‐consistent field basis sets and are compared with experiment. Strong ‘‘Fermi‐like’’ resonances are found involving the CH symmetric and antisymmetric stretches.

Photophysics and Photochemistry in the Vacuum Ultraviolet

Abstract: Photophysics of Highly-Excited States.- 1. Introduction.- 2. Quantum Defect Theory.- 3. Core Effects.- 4. Intermolecular Rydberg Correlations.- 5. Field Effects.- 6. Rydberg States in Condensed Media.- 7. Concluding Remarks.- Methods for Studying Higher Excited States in Molecules and Molecular Crystals by Means of Synchrotron Radiation.- 1. Introduction.- 2. Properties of Synchrotron Radiation Sources: A Short Overview.- 3. Some General Aspects of Synchrotron Radiation Instrumentation.- 4. Absorption and Reflection Spectroscopy.- 5. Time-Resolved and Energy-Resolved VUV Luminescence Spectroscopy.- 6. Photoemission.- 7. Perspectives.- 8. Appendix.- Nonlinear Optics and Laser Spectroscopy in the Vacuum Ultraviolet.- 1. Introduction.- 2. Vacuum Ultraviolet Lasers.- 3. Nonlinear Optical Methods for Producing Tunable VUV Radiation.- 4. Applications to Chemical Physics.- 5. Future Goals.- Multiphoton Ionization and Third-Harmonic Generation in Atoms and Molecules.- 1. Introduction.- 2. Resonantly-Enhanced Multiphoton Ionization.- 3. Multiphoton Ionization and Third-Harmonic Generation in Rare Gases.- Electron-Impact Spectroscopy of Molecules.- 1. Introduction.- 2. General Theory.- 3. Experimental.- 4. Experimental Results.- Elements of Quantum Defect Theory. I. Introduction and Formalism.- 1. Introduction to Quantum Defect Theory.- 2. Multichannel Quantum Defect Theory in Atoms.- Elements of Quantum Defect Theory. II. A Unified Theory of Rydberg and Autoionizing States.- 1. Introduction.- 2. Eigenchannel Quantum Defect Theory.- 3. Ab initio Calculations of Quantum Defect Parameters.- 4. Applications.- Elements of Quantum Defect Theory. III. Diatomic Molecules.- 1. Introduction to Molecular QDT.- 2. Comparison with Experiment.- 3. Inclusion of Molecular Dissociation.- Negative-Ion States.- 1. Introduction.- 2. Negative-Ion Properties.- 3. Unimolecular Electron Attachment.- 4. Multiply-Charged Negative Ions.- 5. Photoabsorption by Negative Ions.- 6. Recent Studies of Long-Lived He Negative Ions.- 7. Production of Negative Ions by Photon Reactions.- 8. Collision of High-Rydberg Atoms with Electron-Attaching Molecules and the Possible Influence of Dipole Bound States.- 9. Negative Ions of Molecular Clusters.- Superexcited nd Ionic State Relaxation Processes in Vacuum Ultraviolet Excited Polyatomic Molecules.- 1. Introduction.- 2. Fundamental Processes.- 3. Competing Decay Channels of Superexcited States.- 4. Methods for Studying Superexcited and Ionic States and Their Relaxation Processes.- 5. Further Remarks on Molecular Autoionization.- 6. Decay of Ion States.- Photoionization Dynamics of Small Molecules.- 1. Introduction.- 2. Shape Resonances in Molecular Photoionization.- 3. Molecular Autoionization.- 4. Concluding Remarks.- Photodissociation Dynamics of Gas-Phase Small Molecules.- 1. Introduction.- 2. ICN Photodissociation in the Near Ultraviolet.- 3. VUV Photodissociation of HCN.- 4. VUV Photodissociation of the Cyanogen Halides.- 5. Conclusions.- VUV Spectroscopy of Rare-Gas Van Der Waals Dimers.- 1. Introduction.- 2. General Background.- 3. Experimental Apparatus.- 4. Dependence of the Cluster Ion Signals on Nozzle Stagnation Pressure.- 5. Appearance Potentials of the Dimer Ions and Dissociation Energies of the Dimer-Ion Ground States.- 6. Molecular Rydberg Structure Between the Atomic Fine-Structure Thresholds: Dissociation Energies of X+2(C 2?1/2u) and XY+(B 2?1/2 and D 2?1/2+).- 7. Molecular Rydberg Structure in the Spectra of NeY Near the Ne 3s Atomic Resonance.- 8. Molecular Rydberg Structure in the Spectra of ArKr and ArXe Near the Ar 5s and 3d Atomic Resonances.- 9. Molecular Rydberg Structure in the Spectra of X2 and XY Near the High-Lying Atomic Rydberg States.- 10. Summary and Conclusions.- Excitons and Energy Transfer in Insulators.- 1. Prologue.- 2. Frenkel Excitons.- 3. Wannier Excitons.- 4. Deep Wannier Atomic Impurity States.- 5. Excitons and Impurity States in Rare-Gas Fluids.- 6. Molecular Wannier Impurity States in Rare-Gas Solids.- 7. Excited-State Dynamics in Rare-Gas Solids.- 8. Self-Trapping of Excitons in Rare-Gas Solids.- 9. Lattice Relaxation Around Impurity States.- 10. Electronic Relaxation of Impurity States.- 11. Electronic Energy Transfer in Rare-Gas Solids.- 12. Perspectives.- Regular and Irregular Motion in Classical and Quantum Systems.- 1. Introduction: Order and Chaos.- 2. Integrable Systems.- 3. Perturbation of Integrable Systems and the KAM-Theorem.- 4. Area-Preserving Mappings.- 5. Chaotic Systems.- 6. A Generic System is Neither Integrable Nor Ergodic, But Shows a Stochastic Transition.- 7. The Birkhoff-Gustavson Normal Form and Invariant Tori Below the Critical Energy.- 8. Do Quantum Systems Exhibit Chaotic Behavior?.- 9. Integrable Systems and Tori Quantization.- 10. Quantization of Chaotic Systems.- 11. Statistics of Energy Levels.- 12. Quantizing a Generic System.- 13. Conclusions and Discussion.- New Trends in Atomic Diamagnetism.- 1. Introduction.- 2. The Coulomb Spectrum.- 3. The Landau Spectrum.- 4. The Atom in a Magnetic Field.- 5. Classical Mechanics of Diamagnetism.- 6. Quantum Mechanical Approach.- 7. Semi-Classical Methods.- 8. Experimental Studies in Atomic Physics.- 9. Prospects and Conclusions.- Evidence for Motional Electric Field Effects in the Absorption Spectrum of Lithium Vapor in a Magnetic Field.- 1. Introduction.- 2. Results and Discussion.- On the Spectrum Of V(?) $$V\left( \rho \right) = - \frac{{qt}}{\rho } + \frac{{rt}}{{{\rho ^2}}} + stp + t{p^2}.$$. Physical Implications for a Variety of Problems.- 1. Introduction.- 2. Solution of the Eigenvalue Problem.- 3. Applications.- 4. Summary.- Perturbation Spectroscopy.- 1. Introduction.- 2. Perturbation Theory and MCD Spectroscopy.- 3. Electric Field-Effect Spectroscopy.- 4. Pressure-Effect Spectroscopy.- Circular Dichroism and Magnetic Circular Dichroism Studies in the Vacuum Ultraviolet.- 1. Introduction.- 2. Instrumentation.- 3. Results.- Electric Field Studies in the Vacuum Ultraviolet.- 1. Introduction.- 2. Experimental.- 3. General Results.- 4. Excited-State Parameters.- 5. Arc-Suppressor "Solvent" Effects.- 6. Conclusions.- Valence-Shell and Rydberg Transitions in Large Molecules.- 1. Introduction.- 2. The Rydberg Concept.- 3. From Spectra to Photochemistry.- 4. Rydberg States of Transition Metal Complexes.- Multiphoton Spectroscopy and Photochemistry.- 1. Introduction.- 2. Nonlinear Photochemistry.- 3. Two Examples.- 4. Other Systems.- Excess Energy Dependence of Vibrational Relaxation and Photophysical Branching Ratios in Isolated Aromatic Molecules: Relevance to Vacuum Ultraviolet Photochemistry.- 1. Introduction.- 2. Spectral Characteristics of Gas-Phase Fluorescence.- 3. Excess-Energy Dependence of Radiationless Transitions: Theory.- 4. Excess-Energy Dependence of Radiationless Transitions: Experiment.- 5. Intramolecular Vibrational Relaxation.- 6. Photochemical Implications of the Energy Dependence of Electronic Relaxation Processes.- Theoretical Studies of the Electronic Structure and Spectra of NH3+.- 1. Introduction.- 2. Franck-Condon Analysis of the 2A1(NH3+) ? 1A1 (NH3) Spectrum.- 3. Ab initio Calculations on Low-Lying States of NH3+.- Theoretical Correlations of Organic Photochemical Reactions in the VUV.- 1. Introduction.- 2. 1,4-Cyclohexadiene.- 3. Methylene Cyclopropane.- Photochemistry of Saturated Alcohols and Open-Chain Ethers at 185 nm in the Liquid Phase.- 1. Introduction.- 2. Discussion.- Photolysis of Cyclic Ethers and Acetals at 185 nm in the Liquid Phase.- 1. Discussion.- List of Participants.

Laser-induced freezing.

Abstract: Laser beams are crossed in a two-dimensional colloidal liquid of strongly interacting spherical particles to produce an external periodic potential and stimulate a density modulation. If the modulation wave vector is at the peak in the static structure factor, a sufficiently large potential induces a phase transition to a structure which exhibits solidlike order having density modes other than those directly excited. A Landau theory is presented and discussed in terms of our results.

Relativistic plasma-wave excitation by collinear optical mixing.

Abstract: The relativistic plasma wave excited when the frequency difference between two copropagating C${\mathrm{O}}_{2}$ laser beams equals the plasma frequency is detected for the first time. The plasma-wave frequency, wave number, spatial extent, and saturation time are directly measured by use of 7-mrad, collective, ruby Thomson scattering and the forward-scattered ir spectrum. The wave amplitude $\frac{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}{{n}_{0}}$ is inferred to be (1-3)% which gives a longitudinal electric field of 0.3 to 1 GV/m at a laser intensity of 1.7\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$, in reasonable agreement with theory.

Radiative Properties of Rydberg States in Resonant Cavities

Abstract: Publisher Summary Rydberg atoms are excited atomic systems in which an electron has been promoted to a level with a very large principal quantum number n . These atoms offer remarkable radiative properties owing to a number of reasons: first, the size of these atoms and hence the electric dipole matrix elements between neighbor levels—proportional to n 2 —are typically ( n ∼ 30) three orders of magnitude larger than the corresponding quantities for ground or low excited atomic or molecular states. As a result, the intrinsic coupling of these atoms to the radiation field is unusually strong. Second, transitions between Rydberg levels fall in the millimeter-wave range. Quantum effects involving ultimately the interaction of a single photon with a single atom are observable, which is quite novel in this frequency domain. Third, these atoms have relatively long spontaneous emission lifetimes, which means that in spite of their very strong coupling to microwaves, they can live for a very long time in excited states and basically behave as metastable species on which very precise spectroscopic investigations can be made. Rydberg atom collective absorption properties can also be used to reduce the cavity temperature and thus to decrease the detection noise of solid state detectors.

Symmetry breaking in molecular calculations and the reliable prediction of equilibrium geometries. The formyloxyl radical as an example

Abstract: A systematic approach to symmetry breaking in molecular calculations, based on MCSCF and multireference CI (MRCI) wave functions, is presented. A series of MCSCF expansions is generated by successively incorporating resonance effects and size effects into the wave functions. The character of the potential surface obtained at each level is analyzed. As an example, the potential energy curves of the ground state (σ) and the first excited state (π) of the formyloxyl radical (HCO2) are characterized. The σ and π equilibrium structures are shown to be symmetric, with an adiabatic σ−π excitation energy of 9.2 kcal/mol. Unlike earlier theoretical studies, our MCSCF model produces a qualitatively correct potential surface. Therefore, we are able to extract reliable vibrational frequencies from the MRCI potential surface.