Showing papers on "Excited state published in 1975"

Binding energies in atomic negative ions

Abstract: This article updates a ten‐year‐old review of this subject [J. Chem. Phys. Ref. Data 4, 539 (1975)]. A survey of the electron affinity determinations for the elements up to Z=85 is presented, and based upon these data, a set of recommended electron affinities is established. Recent calculations of atomic electron affinities and the major semiempirical methods are discussed and compared with experiment. The experimental methods which yield electron binding energy data are described and intercompared. Fine structure splittings of these ions and excited state term energies are given.

Bound states in the continuum

Abstract: Quantum-mechanical examples have been constructed of local potentials with bound eigenstates embedded in the dense continuum of scattering states. The method employed corrects and extends a procedure invented by von Neumann and Wigner. Cases are cited whereby deformation of the local potential causes the continuum bound state to move downward through the bottom of the continuum, and to connect analytically to a nodeless ground state. A doubly excited model atom is also displayed, with interactions between its two "electrons," having an infinite lifetime (in the Schr\"odinger equation regime). In the light of these examples, attention is focused on quantitative interpretation of real tunneling phenomena, and on the existence of continuum bound states in atoms and molecules.

Comparison of doubly-excited helium energy levels, isoelectronic series, autoionization lifetimes, and group-theoretical configuration-mixing predictions with large-configuration-interaction calculations and experimental spectra

Abstract: For comparison with our recent group-theoretical predictions of configuration-mixing coefficients, we report extensive configuration-interaction calculations for doubly excited states in the helium isoelectronic sequence below the $N=2$ ($S, P, D$ states), $N=3$ ($S, P, D, F, G$ states), $N=4$ ($S, P, D$ states), and $N=5$ ($^{1}P^{\mathrm{o}}$ states) ionization thresholds. Two new quantum numbers label the Rydberg series, and are used to predict three "selection rules" for dipole excitation and autoionization processes which agree with experiment. New autoionization widths are reported for the helium states and confirm our group-theoretically predicted selection rules. Our width of 0.151 eV for the $^{1}P^{\mathrm{o}}$ state at 62.92 eV is in agreement with the recent experimental value 0.132 \ifmmode\pm\else\textpm\fi{} 0.014 eV. Calculations of helium energies and autoionization widths are strongly affected by "avoided crossings" of the doubly excited states as $Z$ is varied continuously. The new quantum numbers project out states in ${\mathrm{H}}^{\ensuremath{-}}$ which correspond to observed $^{1}P^{\mathrm{o}}$ shape resonances above the $N=2 \mathrm{and} 3$ ionization thresholds.

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

Abstract: Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X 2A1, A 2B2, B 2B1, C 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the A 2B2, B 2B1, and C 2A2 states have all been found to be within 2 eV of the minimum of the X 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 A for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 A and 1.42 A and the inversion barrier is approximately 800 cm−1.

Emission properties of vinylcarbazole polymers

Abstract: The emission properties of poly(N‐vinylcarbazole) (PVK), poly(N‐ethyl‐2‐vinylcarbazole), and poly(N‐ethyl‐3‐vinylcarbazole) have been investigated. Dilute fluid solutions of all three vinylcarbazole polymers are observed to exhibit a structureless emission band at energies approximately 5400 cm−1 to the red of the (0,0) band of the normal molecular fluorescence of isolated monomer compounds. On the basis of earlier work appropriate model compounds, this emission is assigned as that from an intrachain excimer state in which two interacting pendant groups, one of which is electronically excited, achieve an overlapping, eclipsed, sandwichlike configuration. The emission spectrum of PVK is characterized by the appearance of a second structureless emission band which appears at higher energy than the intrachain excimer fluorescence common to all three vinylcarbazole polymers. This emission band is unique to PVK and under certain conditions to its model compound, 1,3‐bis(N‐carbazolyl)propane. This emission is assigned as that from a second intrachain excimer. Contrary to the lower energy intrachain excimer, the polymer conformation appropriate for formation of this state exists prior to the initial excitation step.

R-matrix theory of photoionization. Application to neon and argon

Abstract: The R-matrix method, which has been used recently to calculate electron-atom and ion collision cross sections and atomic polarizabilities is extended to enable atomic photoionization processes to be studied. Both the initial atomic bound state and the final atomic continuum state are expanded in terms of R-matrix bases. The method is programmed for a general atomic system and then used to calculate the photoionization cross sections of ground state neon and argon atoms leaving the residual ions in their ground or first excited states. Good agreement is obtained with recent experiments using synchrotron radiation both in resonant and non-resonant regions, showing that the method has a wide range of applicability.

Picosecond detection of an intermediate in the photochemical reaction of bacterial photosynthesis

Abstract: Preparations of photosynthetic reaction centers from Rhodopseudomonas sphaeroides were excited with flashes lasting approximately 8 psec. Immediately after the excitation, there appeared a transient state which was characterized by new absorption bands near 500 and 680 nm, by a bleaching of bands near 540, 600, 760, and 870 nm, and by a blue shift of a band near 800 nm. The transient state decayed with an exponential decay time,t, of 246 plus or minus 16 psec after the flash. As the transient state decayed, the radical cation of the reaction center bacteriochlorophyll complex appeared. This indicates that the transient state is an intermediate in the photooxidation of the bacteriochlorophyll. The absorpiton spectrum of the transient state shows the state to be identical with a state (P-F) which has been detected previously in reaction centers that are prevented from completing the photooxidation, because of chemical reduction of the electron acceptor. Analysis of the spectrum suggests that the formation of P-F involves electron transfer from one bacteriochlorophyll molecule to another within the reaction center, or possibly from bacteriochlorophyll to the bacteriopheophytin of the complex. The initial absorbance changes after flash excitation also include a bleaching of an absorption band at 800 nm. The bleaching decays with tau approximately equal to 30 pse. The bleaching appers not to be a secondary effect, but rather to revael another early step in the primary photochemical reaction.

Configuration interaction studies of O3 and O+3. Ground and excited states

Abstract: We report the results of extensive configuration interaction calculations (double ζ basis) on the ground and 15 excited states of ozone and the eight lowest states of the ozone positive ion. The studies on ozone are carried out as a function of both bond length and bond angle. We find that three excited states of ozone may be bound with respect to the ground states of O2 and O. Additional information is provided on the ring state (60 ° bond angle) previously reported. Two of the excited states of ozone are known spectroscopically and the ionization potentials of the first three states of O+3 are now known experimentally. These experimental results are in good agreement with the theoretical calculations.

Effect of surface plasmons on transitions in molecules

Abstract: A quantum theory of the coupling of excited molecules to the surface plasmon modes of a metal is described. For radiative decay, formulas are given for the separate decay into s photon, p photon and surface plasmon channels valid for all molecule−metal separations. Calculations of the quantum yield of fluorescence are described. The total decay rate is shown to equal that given by classical theory. Other processes considered briefly are the Raman effect and resonance energy transfer between identical molecules.

Spontaneous emission from a medium with a band spectrum

Abstract: A theoretical analysis is made of the spontaneous emission from an excited atom or a molecule in a one-dimensional periodic structure formed by a screened two-conductor line filled with periodically distributed dielectric disks. It is shown that when the transition frequency lies within or close to the forbidden band of the periodic structure, the decay of an excited state of an atom or a molecule produces a characteristic long-lived dynamic state. This dynamic state is a superposition of the excited and ground states with an admixture of a photon "cloud," which surrounds the atom or molecule. A study is made of the distribution of the electromagnetic energy of the dynamic state. The energy and the decay law of the excited state are obtained and the spectrum of the emission which accompanies this decay is obtained. It is shown that the spontaneous emission can be suppressed strongly in volumes much greater than the wavelength. The possibility of the suppression of spontaneous emission from lasers is discussed.

Flow system for the production of diatomic metal oxides and halides

Abstract: A flow system for production of diatomic metal oxides and halides is described. Molecules are produced by reacting metal vapor in a flowing, inert gas with a suitable oxidizer. Product molecules are frequently formed in electronically excited states, making this system particularly useful for optical emission spectroscopic studies. Specific advantages are: (a) large number of reactant products; (b) little thermal excitation in the reaction region; (c) spectra uncluttered by emission from undesirable species; and (d) in many cases, formation of molecules in electronically excited states not produced or detected in other sources. Also discussed is a variation of the design which is used to produce small metallic particles. These particles are formed by homogeneous nucleation of the metal vapor and range in size from 5 to 500 nm.

Calculation of vibronic structure of the π→π* transition of trans‐ and cis‐stilbene

Abstract: The calculations of the vibronic structure of the first singlet π→π* transition of cis− and trans−stilbene are presented as a step in the detailed study of photoisomerization reactions. In order to obtain the vibronic transition intensities we calculate the equilibrium geometries and vibrational modes of the ground and excited electronic surfaces. The calculated vibronic transition intensities of trans−stilbene are in good agreement with the observed ones. The calculated torsional contributions enable us to assign some of the lines in the spectrum of trans−stilbene and suggest the possibility of deviations from planarity in the ground state equilibrium conformation. The observed diffuse structure of the π→π* transition of cis−stilbene is reproduced and the contributions to photoisomerization of torsional and out−of−plane modes are considered.

Role of ab initio calculations in elucidating properties of hydrated and ammoniated electrons

Abstract: The properties of solvated electrons are analyzed in terms of a self- consistent modified continuum model based on the techniques of ab initio molecular quantum mechanics. The model is semiclassical in spirit, employing the quantum mechanical density for the excess charge and the first solvation shell in conjunction with classical electrostatics, and is developed in a general form which can be straightforwardly applied to special cases of interest, such as the solvated mono- and dielectron complexes. The advantages and disadvantages of the technique are discussed in relation to other more empirical approaches. Computational results are presented for excess electrons (mono- and dielectrons) in water and ammonia, and the role of long-range polarization of the medium in localizing the excess charge is analyzed. The variationally determined ground states are characterized in terms of equilibrium solvation shell geometry (appreciable cavities are implied for both water and ammonia), solvation energy, photoionization energy, and charge distribution. The finding of negative spin densities at the first solvent shell protons underscores the importance of a many electron theoretical treatment. Preliminary results for excited states are also reported. The calculated results are compared with experimental and other theoretical data, and the sensitivity of the results to variousmore » features of the model is discussed. Particular attention is paid to the number of solvent molecules required to trap the excess electron. (auth)« less

Selective laser excitation of charge compensated sites in CaF2:Er3+

Abstract: A technique of selective laser excitation has been developed and used to study the optical spectrum of the charge compensated sites in CaF2:Er3+. The technique allows the unambiguous interpretation and classification of the spectral lines observed from crystals with multiple sites. The absorption lines of specific sites have been excited with a narrow band dye laser. Nonresonant energy transfer between sites has not been observed and fluorescence from a single site can be obtained if no overlap occurs between absorption lines of different sites. The fluorescence transitions of CaF2:Er3 have been classified into two sites having an Er3+ ion and fluoride interstitial compensation and 16 sites having more than one Er3+ ion. The single‐pair sites correspond to those previously identified with C4v and C3v site symmetry. The identification of the remaining sites with clusters containing more than one Er3+ ion is confirmed by the observation of ion–ion interactions, including up‐conversion, cross relaxation, and...

Dissociation of methane by electron impact

Abstract: The total dissociation cross section for methane is reported for electron energies between 10 and 500 eV. A maximum cross section of 4×10−16 cm2 occurs at 85 eV. Dissociation into ionic and neutral fragments have about equal probability for energies between 50 and 500 eV. At lower energies most of the fragments are uncharged, ground state, or long lived excited state molecules. The experimental technique is discussed in detail and criteria established for determining gases for which similar methods can be applied.

Emission spectra of XeBr, XeCl, XeF, and KrF

Abstract: The emission spectra of XeBr, XeCl, XeF, and KrF at high pressure are reported and discussed. The spectra were obtained by observing spontaneous emission from electron beam excited mixtures of argon containing lesser amounts of xenon with the halogens or krypton with fluorine. The emitting state in these species is best described as an ionic species Xe+X− or Kr+F−. The wavelengths of these emission bands are in good agreement with a theoretical model in which the ionic binding energy of the noble gas halide ion pair is roughly equal to that of the nearest alkali halide. Our high pressure spectra imply that the lowest potential energy curve for XeF is bound.

Laser photoelectron spectrometry of the negative ions of silicon and its hydrides

Abstract: Fixed−frequency laser photoelectron spectrometry has been utilized to study the ions Si−, SiH−, and SiH−2. In each case, the photodetached electron energy spectrum consists primarily of sharp, easily assignable peaks. The electron affinities of Si, SiH, and SiH2 have been determined to be (1.385±0.005) eV, (1.277±0.009) eV, and (1.124±0.020) eV, respectively. Two bound excited states attributed to 2D and 2P terms have been observed for Si−, and their binding energies determined. Similarly, two bound excited states, assigned to 1Δ and 1Σ+ symmetry, have been discovered in SiH−. The internuclear distance re in the ground state of SiH− has been determined by a Franck−Condon factor analysis to be (1.474±0.004) A. In addition, spectroscopic constants of the excited states have been deduced. Transitions from the ground state of SiH− to both the 1A1 and 3B1 states of SiH2 have been observed; unlike the case of CH2, the 1A1 state is the lower in energy.

Stark Ionization of High-Lying States of Sodium

Abstract: By using stepwise excitation in an atomic beam, we have excited slow-moving atoms to pure high-lying quantum states at densities low enough to avoid loss by collision. The atoms were detected with high efficiency by Stark ionization. Results are presented of a study of the threshold field for ionization for $s$ states of sodium with principal quantum number $n$ from 26 to 37.

Photodissociation of molecular beams of methylene iodide and iodoform

Abstract: Molecular beams of CH2I2 and CHI3 have been dissociated by polarized ultraviolet light and the angular distribution of the fragments has been measured. The photofragment mass spectra show that the lower lying excited states of CH2I2 and CHI3 dissociate to give an iodine atom and the corresponding radical. The angular distribution fitted the form 1+βP2(ϑ), where ϑ is the angle between the detection and polarization directions. β was 0.90±0.16 for CH2I2 and 0.46±0.16 for CHI3. These values prove that the molecule dissociates in a time short compared to its period of rotation. Moreover, the first two absorption bands in CH2I2 are shown to be of A1→B1 type polarized along the I–I vector, and in CHI3 they are of A1→E type polarized perpendicular to the threefold axis. A crude exciton model amounts for the number and symmetries of the energy states.

Ab initio study of the π‐electron states of trans‐butadiene

Abstract: Extensive ab initio configuration interaction calculations were carried out on the π‐electron states of trans‐1,3‐butadiene. A double‐ζ contracted Gaussian basis set, augmented with two diffuse 2p π functions on each carbon atom, was used in the calculations, which were based on a frozen σ core obtained from a ground‐state SCF calculation. All excitations fro and to π orbitals were included in the CI treatment. Natural orbitals were obtained for many of the wavefunctions,and their spatial extenty was determined. Only five of the calculated excited states were found to have a valencelike charge distribution (computed vertical excitation energies in eV are given in parentheses): 1 3Bu (3.45), 13Ag(5.04), 21Ag(6.77), 33Bu (8.08), and 15Ag (9.61). These states all correlate with the valence N and T states of ethylene and can be readily described in terms of the ’’two‐vinyl model’’ as either N T (the first two) or TT (doubly excited, the last three). Except for the doubly excited 21Ag, all low‐lying singlet ex...

Isotopic and vibronic coupling effects in the valence electron spectra of H2 16O, H2 18O, and D2 16O

Abstract: Isotopic shift data for Hei induced high resolution valence electron spectra of H2 16O, H2 18O, and D2 16O are presented. From the differences in the vibrational subspectra we were able to deduce adiabatic ionization energies and geometries of the ions and to assign the excited vibrations to normal modes. The ν2 progression in the 2A1 band was found to be affected by strong vibronic coupling.