Showing papers on "Excited state published in 1979"

Resonance ionization spectroscopy and one-atom detection

Abstract: Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of Hes(2/sup 1/S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and timemore » resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states.« less

Estimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processes

Abstract: Rate constants for electron-transfer quenching of Ru(bpy)32+* (bpy is 2,2'-bipyridine) by a series of organic quenchers have been determined in acetonitrile (μ = 0.1 M) at 22 ± 2 °C. The reactions studied were based on three different series of structurally related quenchers having varying redox potentials. They include oxidative quenching both by a series of nitroaromatics (ArNCh) and by a series of bipyridinium ions (P2+) and reductive quenching by a series of aromatic amines (R^NAr). After corrections for diffusional effects, the quenching rate constant (kq') data fall into two classes both of which can be treated successfully using Marcus-Hush theory. For case 1, which includes the data for oxidative quenching by P2+ and reductive quenching by RaNAr, RT In kq varies as AG21/2 where | AGasI « /2. AG23 is the free energy change for electrontransfer quenching within an association complex between the quencher and excited state and is the vibrational contribution to the activation barrier to electron transfer. The experimental data are also consistent with Marcus-Hush theory over a more extended range in AG22 where the free energy dependence includes a quadratic term. For case II, which includes quenching by several of the nitroaromatics, RT In kq varies as AG23 and evidence is obtained from the remainder of the data for a transition in behavior from case 11 to case I. The microscopic distinction between the two cases lies in competitive electron transfer to give either groundor excited-state products following the electron-transfer quenching step. For case II, back-electron transfer (A32) to give the excited state, e.g., Ru(bpy)33+,ArNC>2— Ru(bpy)32+*,ArN02, is more rapid than electron transfer to give the ground state (A30), e.g., Ru(bpy)33+,ArN02~ Ru(bpy)32+,ArN02· For case I, electron transfer to give the ground state is more rapid. The different behaviors are understandable using electron-transfer theory when account is taken of the fact that k30 is a radiationless decay rate constant, and the electron-transfer process involved occurs in the abnormal free-energy region where — AG22 > . An appropriate kinetic treatment of the quenching rate data allows estimates to be made of redox potentials for couples involving the excited state. Formal reduction potentials in CH3CN (μ = 0.1 M) at 22 ± 2 °C are £(RuB33+-/2+*) = —0.81 ± 0.07 V and £(RuB32+‘/+) = +0.77 ± 0.07 V. Comparisons between groundand excited-state potentials show that the oxidizing and reducing properties of the Ru(bpyb2+ system are enhanced in the excited state by the excited-state energy, that the excited state is unstable with respect to disproportionation into Ru(bpy)3+ and Ru(bpy)33+, and that the excited state is thermodynamically capable of both oxidizing and reducing water at pH 7. A comparison between the estimated 0-0 energy of the excited state and the energy of emission suggests that there may be only slight differences in vibrational structure between the ground and excited states.

Multiphoton Dissociation of Polyatomic Molecules

Abstract: The dynamics of infrared multiphoton excitation and dissociation of SF/sub 6/ was investigated under collision free conditions by a crossed laser-molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments were carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF/sub 6/, the pulse duration of the CO/sub 2/ laser and the frequency in both one and two laser experiments. Translational energy distributions of the SF/sub 5/ dissociation product measured by time of flight and angular distributions and the dissociation lifetime of excited SF/sub 6/ as inferred from the observation of secondary dissociation of SF/sub 5/ into SF/sub 4/ and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer on a nanosecond timescale, and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. The model shows that the energy fluencemore » of the laser determines the excitation of molecules in the quasi-continuum and the excess energy with which molecules dissociate after the laser pulse. The role played by the laser intensity in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. 63 references.« less

Origin of the nonlinear second-order optical susceptibilities of organic systems

Abstract: An all-valence-electron self-consistent-field linear-combination-of-atomic-orbitals molecular-orbital procedure including configuration interactions for calculating the magnitude and sign of the nonlinear second-order molecular susceptibility components (hyperpolarizability) for substituted dipolar aromatic molecular systems is reported. Three fundamentally important examples, aniline, nitrobenzene, and $p$-nitroaniline, are considered. Analysis of the microscopic origin of their molecular second-order susceptibilities provides a direct means for understanding the macroscopic nonlinear optical response of organic molecular solids which have already been observed to possess exceptional nonlinear optical properties. The important excited states of aniline, nitrobenzene, and $p$-nitroaniline have been identified and examined in their relationship with the molecular second-order susceptibility-tensor components ${\ensuremath{\beta}}_{\mathrm{ijk}}$. The detailed nature of the charge separation accompanying these states has been discussed in terms of both the configurations composing the excited states, and also the one-electron molecular orbitals which determine those configurations. These results demonstrate how the bond-additivity approximation is inappropriate for $p$-nitroaniline. Finally, the frequency dependence of the ${\ensuremath{\beta}}_{\mathrm{ijk}}$ components in each case shows that the Kleinman relations are valid approximations only at relatively low frequencies.

Chemiluminescence of organic peroxides. Conversion of ground-state reactants to excited-state products by the chemically initiated electron-exchange luminescence mechanism

Abstract: : A mechanism for thermal production of excited states is discussed. Electron transfer from an easily oxidized donor to a suitable organic peroxide followed by oxygen-oxygen bond cleavage generates radical ion intermediates. Subsequent chemical reaction followed by charge annihilation generates electronically excitated state products. This sequence of reactions is called chemically initiated electron-exchange luminescence and is shown to be operating for diphenoyl peroxide, 1-phenylethylperoxy acetate, dimethyldioxeta-none, and 1,4-diphenyl-2,3-benzodioxin (0-xylylene peroxide). (Author)

Electronic structure of homoleptic transition metal hydrides: TiH4, VH4, CrH4, MnH4, FeH4, CoH4, and NiH4

Abstract: Ab initio molecular electronic structure theory has been applied to the family of transition metal tetrahydrides TiH4 through NiH4. For the TiH4 molecule a wide range of contracted Gaussian basis sets has been tested at the self‐consistent‐field (SCF) level of theory. The largest basis, labeled M(14s 11p 6d/10s 8p 3d), H(5s 1p/3s 1p), was used for all members of the series and should yield wave functions approaching true Hartree‐Fock quality. Predicted SCF dissociation energies (relative to M+4H) and M–H bond distances are TiH4 132 kcal, 1.70 A; VH4 86 kcal, 1.64 A; CrH4 65 kcal, 1.59 A; MnH4 – 36 kcal, 1.58 A; FeH4 0 kcal, 1.58 A; CoH4 27 kcal, 1.61 A; and NiH4 18 kcal, 1.75 A. It should be noted immediately that each of these SCF dissociation energies will be increased by electron correlation effects by perhaps as much as 90 kcal. For all of these molecules except TiH4 excited states have also been studied. One of the most interesting trends seen for these excited states is the shortening of the M–H bon...

Ionospheric modification and parametric instabilities

Abstract: Thresholds and linear growth rates for stimulated Brillouin and Raman scattering and for the parametric decay instability are derived by using arguments of energy transfer. For this purpose an expression for the ponderomotive force is derived. Conditions under which the partial pressure force due to differential dissipation exceeds the ponderomotive force are also discussed. Stimulated Brillouin and Raman scattering are weakly excited by existing incoherent backscatter radars. The parametric decay instability is strongly excited in ionospheric heating experiments. Saturation theories of the parametric decay instability are therefore described. After a brief discussion of the purely growing instability the effect of using several pumps is discussed as well as the effects of inhomogeneity. Turning to detailed theories of ionospheric heating, artificial spread F is discussed in terms of a purely growing instability where the nonlinearity is due to dissipation. Field-aligned short-scale striations are explained in terms of dissipation of the parametrically excited Langmuir waves (plasma oscillations); they might be further amplified by an explosive instability (except at the magnetic equator). Broadband absorption is probably due to scattering of the electromagnetic pump wave into Langmuir waves. This absorption is probably responsible for the ‘Overshoot’ effect: the initially observed level of parametrically excited Langmuir waves is much higher than the steady state level.

Local-density theory of multiplet structure

Abstract: In order to obtain multiplet energies and therefore energies of excited states of atoms and molecules, the local-density theory of Hohenberg, Kohn, and Sham has recently been extended to give the lowest energy of a specified angular momentum and spin symmetry. It is explained why this method does not work if the exchange correlation functional is taken to be symmetry independent. Instead it is shown how the local-density theory can be used to estimate the energies of states of mixed symmetry and how the multiplet splittings are obtained from these estimates. The new method is tested on light atoms and the local-density theory with exchange only reproduces the Hartree-Fock results within 0.1 eV. With correlation included, the error in the local-density approach is typically a factor of 3 less than in the Hartree-Fock approach.

The energy density functional formalism for excited states

Abstract: It is shown that the density can be used as the basic variable for calculating the properties of excited states. The correspondence is not between an eigenstate and its density, as is the case with the ground state, but between the subspace spanned by the number of lowest-energy eigenstates and the sum of their densities. An extension of the Hohenberg-Kohn-Sham theory (1964-5) for excited states has also been developed. The equations derived are similar in form to those for the ground-state density but the interpretation is different. The lowest-order approximation of the present theory coincides with Slater's 'transition-state' theory (1974).

Quantum limits on resonant mass gravitational radiation detectors

Abstract: The methods of quantum detection theory are applied to a resonant-mass gravitational-radiation antenna. Quantum sensitivity limits are found which depend strongly on the quantum state in which the antenna is prepared. Optimum decision strategies and their corresponding sensitivities are derived for some important initial states. The linear detection limit (${E}_{min}\ensuremath{\sim}\ensuremath{\hbar}\ensuremath{\omega}$) is shown to apply when the antenna is prepared in a coherent state. Preparation of the antenna in an excited energy eigenstate or in a state highly localized in position or momentum space leads to increased sensitivity. A set of minimum-uncertainty states for phase-sensitive detection is introduced.

Energy transfer from an excited dye molecule to the surface plasmons of an adjacent metal.

Abstract: The probability for direct energy transfer from a dye molecule to the surface plasmons (SP) on a nearby metal is calculated using the classical Sommerfeld model. For 600-nm emission from a dipole-oriented ⊥ to an Ag surface, the peak SP coupling probability is 93% at a distance of 120 nm from the metal. The coupling is demonstrated in experiments with an Ag film between a rhodamine–methanol solution and a high-index prism. The SP-coupled radiation produces an intense cone of radiation in the prism, each frequency component of which satisfies the SP dispersion relation.

Intramolecular rate processes in highly vibrationally excited benzene

Abstract: Intracavity cw dye laser techniques incorporating photoacoustic detection have been used to measure visible absorption spectra of gas phase benzene (C6H6 and deuterated analogues C6H5D, p‐C6H2D4, and C6HD5). The prominent spectral features are attributable to CH stretch overtones that correspond to highly localized (’’bond‐selective’’) excitations of individual CH oscillators. These features have broad (∼100 cm−1 FWHM) Lorentzian line shapes that narrow as a function of increasing vibrational energy for most isotopic species. A simple model is presented to interpret the broad line shapes and their energy dependence as a manifestation of intramolecular V→V′ (vibration‐to‐vibration) energy redistribution and/or dephasing processes that occur rapidly (on 5×10−14 sec time scales), but nonstatistically, in isolated benzene and other polyatomic molecules.

Experimental energy band dispersions and lifetimes for valence and conduction bands of copper using angle-resolved photoemission

Abstract: Energy-band dispersions and electron lifetimes have been determined for both valence and conduction-band states of copper using angle-resolved photoemission with polarized synchrotron radiation in the $5\ensuremath{\le}h\ensuremath{ u}\ensuremath{\le}35$ eV photon energy range. Dispersion relations for the occupied $s\ensuremath{-}p$ and $3d$ bands of Cu along the $\ensuremath{\Gamma}\ensuremath{-}X$ and $\ensuremath{\Gamma}\ensuremath{-}L$ symmetry lines (including critical points at $\ensuremath{\Gamma}$, $X$, and $L$) have been determined with an accuracy of 0.05-0.1 eV and \ensuremath{\lesssim}5% of the zone-boundary momentum. Band symmetries have been deduced using polarization selection rules. The dispersion relation has also been accurately determined for the unoccupied ${\ensuremath{\Delta}}_{1}$ conduction band along $\ensuremath{\Gamma}\ensuremath{-}X$ at \ensuremath{\sim}10-15 eV above the Fermi energy ${E}_{F}$; this band has a reduced effective mass (${m}^{*}\ensuremath{\simeq}0.90\ensuremath{-}0.94$) which is related to self-energy effects. Lifetimes have been directly measured for excited hole states (the lifetime broadening ${\ensuremath{\Gamma}}_{h}$ increases from \ensuremath{\sim}0.2 to 0.5 eV full width at half maximum for $d$-band energies from 2 to 5 eV below ${E}_{F}$) as well as for excited electron states in the ${\ensuremath{\Delta}}_{1}$ conduction band (${\ensuremath{\Gamma}}_{e}\ensuremath{\simeq}1.0\ensuremath{-}2.0$ eV for energies 10-15 eV above ${E}_{F}$). The energy dispersion and $h\ensuremath{ u}$-dependent photoionization cross section of the $s\ensuremath{-}p$ surface state on Cu(111) are reported. Previous theoretical and experimental studies of copper are compared with our accurate $E$ vs $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ dispersions.

Do highly excited reactive polyatomic molecules behave ergodically

Abstract: Controversy has been generated on the subject of intramolecular vibrational relaxation in reacting molecules. Some background information for unimolecular rate formulations and experimental studies is outlined. Models for unimolecular dissociation are discussed. There are essentially two methods for producing excited species: (1) Boltzmann thermal systems and (2) external sources, such as chemical activation, multiphoton excitation, molecular beams, single photon excitation, and photosensitized reaction. The hot molecules are subject to collisional intervention unless collision-free conditions are maintained. Photophysical experiments are another potential source of information on the internal energy distribution in excited electronic states of polyatomic molecules. The answer to the question appears to be, Yes. 113 references, 3 figures.

Ray interpretation of the material signature in the acoustic microscope

Abstract: The output voltage of the reflection acoustic microscope depends on the location on the object surface in a way that is characteristic of its elastic properties. We present a ray model showing that this dependence is due to interference between a narrow bundle of axial rays and rays associated with the leaky Rayleigh wave excited on the surface.

Carbon K-shell excitation in small molecules by high-resolution electron impact

Abstract: The excitation of 1s carbon electrons has been observed in CO, CH4, CF4, CO2, COS, C2H2 and C2H4 by means of the electron energy-loss technique with high resolution (70 meV in the 300 eV excitation energy range) and at an incident electron energy of 1.5 keV. The energies, widths and vibrational structures of excited states corresponding to the promotion of 1s carbon electrons to unoccupied valence and Rydberg orbitals have been obtained. The validity of the equivalent-core model, and the role of resonances caused by potential barriers, are discussed.

The origin of vibrational dephasing of polyatomic molecules in condensed phases

Abstract: The vibrational dephasing of polyatomic molecules in condensed phases by intermolecular vibrational energy exchange is treated theoretically In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramolecular anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral line shape Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be properly accounted for, and allows explicit expressions for the vibrational correlation function and corresponding spectral line shape functions to be derived and related to molecular parameters Application of the theory can have important consequences experimentally, since an analysis of the te

Soft-x-ray-induced secondary-electron emission from semiconductors and insulators: Models and measurements

Abstract: Secondary-electron energy distribution curves (EDC's) and the total secondary-electron yields relative to such for gold have been measured for seven semiconductors for which electron-electron scattering losses within the emitter were considered dominant and for nine insulators (alkali halides) for which electron-phonon scattering losses were expected to be dominant in the transport process. The secondary-electron spectra were excited by Al-$K\ensuremath{\alpha}$ (1487 eV) photons and were measured from evaporated dielectric films (of about 0.3 \ensuremath{\mu} thickness) on conducting substrates with an electrostatic hemispherical analyzer of about 0.03-eV resolution. Some of the dielectric photoemitters have appreciably narrower energy distributions and higher yields than has gold; CuI and CsI have EDC widths at half-maximum of about one-third of that for gold, and yield values of 11 and 30 times greater. The FWHM and secondary-electron yield for gold were measured to be about 4 eV and 0.50 electrons per normally incident photon, respectively. The shapes of the EDC's were found to be essentially unchanged for photon excitation in the 0.1-10-keV region. Strong structural features appear only in the alkali halide EDC's, and it is proposed that these are mainly the result of single-electron promotion of secondaries from the valence band by plasmon deexcitation. A relatively simple model for x-ray photoemission has been developed which assumes that direct excitation of secondaries by photoelectron and Auger-electron "primaries" is the dominant excitation mechanism, and accounts for both electron-electron and electron-phonon scattering in the transport process. Free-electron conduction-band descriptions are assumed. The theoretical and experimental curves are in satisfactory agreement.

Electrogenerated chemiluminescence. 35. Temperature dependence of the ECL efficiency of tris(2,2'-bipyridine)rubidium(2+) in acetonitrile and evidence for very high excited state yields from electron transfer reactions

Abstract: The temperature dependence of the ECL emission yield, @ECL, of Ru(bpy)?+ in acetonitrile has been determined and was found to correlate closely with the luminescence quantum yield, @e, of the lowest R~(bpy),~+ dn* metal-to-ligand charge transfer (MLCT) excited state. Evidence is presented that at low temperatures (< -30 "C) the ECL emission yield becomes equal to the luminescence quantum yield implying an efficiency for excited state formation in the electron transfer reaction of Ru(bpy)?+ with Ru(bpy),+ of near 100%. High efficiencies for excited state formation are rationalized on the basis of the excited state spectroscopic properties of Ru(bpy)gP+ and the resulting implications to electron transfer theory and the ECL mechanism are discussed.

Critically evaluated rate constants for gaseous reactions of several electronically excited species

Abstract: An extensive evaluation is presented of the available gas phase chemical kinetic rate constants for the interactions of the low lying electronic states of several atoms and molecules with numerous collision partners. These include the following excited states: C(21D2,21S0), N(22D3/2,5/2,22P1/2,3/2), P(32D3/2,5/2,32P1/2,3/2,), S(31D2,31S0). Se(43P0,41D2,41S0), Te(53P1,0,51D2,51S0), CO(a3Π,a′3Σ+,d3Δ,e3Σ−,A1Π), CS(a3Π,A1Π), OH(A2Σ+), OD(A2Σ+), O2(c1Σu−,C3Δu,A3Σu+,B3 Σu−), and S2(a1Δg,b1Σg+,A3Σu+, B3Σu−). Wherever possible, recommended values are suggested. Much of the data refers only to room temperature. To facilitate the evaluation, collision‐free radiative lifetimes often have been required. These also have been evaluated and are presented. The mechanisms of the interactions and the various potential kinetic channels are discussed. These include such processes as chemical reactions, electronic quenching to the ground electronic state, electronic cross relaxation to an adjacent excited state, and for molec...

Absolute cross sections from the "boomerang model" for resonant electron-molecule scattering

Abstract: The boomerang model is used to calculate absolute cross sections near the $^{2}\ensuremath{\Pi}_{g}$ shape resonance in $e$-${\mathrm{N}}_{2}$ scattering. The calculated cross sections are shown to satisfy detailed balancing. The exchange of electrons is taken into account. A parametrized complex-potential curve for the intermediate ${\mathrm{N}}_{2}^{\ensuremath{-}}$ ion is determined from a small part of the experimental data, and then used to calculate other properties. The calculations are in good agreement with the absolute cross sections for vibrational excitation from the ground state, the absolute cross section $v=1\ensuremath{\rightarrow}2$, and the absolute total cross section.

The Effects of Crystal Field and Temperature on the Photoluminescence Excitation Efficiency of Ce3+ in YAG

Abstract: Absorption and luminescence spectra of YAG:Ce3+ crystals have been measured at various temperatures between 4.2° and 300°K. The high resolution, low temperature spectra show resolved fine structure. Zero‐phonon transitions have been assigned and the phonon replicas correlated with lattice phonon energies. The characteristic absorption band of YAG:Ce3+ near 460 nm decreases in intensity, whereas that near 340 nm increases with increasing temperature. This effect is explained in terms of a tetragonal crystal field splitting of both the 4f1 ground state and 5d1 excited states of Ce3+ in YAG. When allowance is made for changes in the absorption of pump light with temperature the photoluminescence quantum efficiency of Ce3+ in YAG is constant up to 300°K. The significance of these crystal field effects for the use of YAG:Ce as a high temperature lamp phosphor is discussed.