Showing papers on "Excited state published in 1969"

Modes of propagating light waves in thin deposited semiconductor films

Abstract: We report theory and experiment on modes of propagating light waves in deposited semiconductor films. The modes are excited by a novel prism‐film coupler which is also used for the measurement of their phase velocities. Up to 50% of the incident laser energy has been fed into a single mode of propagation. The positions and linewidths of the modes, the wave intensity inside the film, and a dramatic view of the mode spectrum displayed by the scattered light are discussed in detail.

Ground State of a One‐Dimensional N‐Body System

Abstract: The problem of N quantum‐mechanical equal particles interacting pairwise by inverse‐cube forces (``centrifugal potential'') in addition to linear forces (``harmonical potential'') is considered in a onedimensional space. An explicit expression for the ground‐state energy and for the corresponding wavefunction is exhibited. A class of excited states is similarly displayed.

Semiempirical, Pseudopotential Calculation of Alkali–Noble-Gas Interatomic Potentials

Abstract: A relatively simple semiempirical method is suggested for calculating interatomic potentials of ground and low‐lying excited states of diatomic systems composed of an alkali atom and a ground‐state noble‐gas atom, and its applicability to other systems is discussed. Two types of interaction are included: an electrostatic interaction which yields the asymptotic R−6 behavior characteristic of van der Waals potentials, and a pseudointeraction which represents the effect of the Pauli exclusion principle on overlapping electron states. For the electrostatic interaction, the noble‐gas atom is treated as a polarizable dipole with two parameters, namely a polarizability α and a “radius” r0. The value of r0 is chosen by adjusting the depth of the ground‐state well to agree with recent scattering data. To determine the interatomic potentials, the total Hamiltonian including spin–orbit coupling is diagonalized in a limited basis set of atomic orbitals. Results are presented and compared with previous calculations an...

Excited-state two-proton tautomerism in hydrogen-bonded n-heterocyclic base pairs

Abstract: The optical absorption and luminescence spectra of 7-azaindole and its doubly hydrogen-bonded dimer were investigated as a model for the study of electronic interactions in DNA base pairs. It is demonstrated that a biprotonic phototautomerism occurs in the dimer and in suitable ethanol solvates in fluid solvents but that the phenomenon is not observed in a rigid solvent matrix. The normal violet structured fluorescence of 7-azaindole monomer becomes a broad green fluorescence in the tautomer. It is shown that spectral band interchanges, excimer formation, excited-state single-proton transfer, and proton tunneling cannot account for the luminescence change, but that the molecular exciton effect facilitates the cooperative two-proton reversible transfer. It is proposed that biprotonic phototautomerism with molecular environmental sensitivity could provide a mechanism for the initial step in ultraviolet mutagenic effects in DNA.

Quality factor of general ideal antennas

Abstract: An expression for the quality factor of an ideal antenna with both TE and TM modes excited is derived and applied to several special cases.

Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules. I. The CI Formulation and Studies of Formaldehyde

Abstract: A configuration interaction (CI) procedure which is designed to produce energies and wavefunctions from relatively large scale computations on ground and excited molecular states is presented based on an orthonormal set of molecular orbitals. The method of generating configurations is referenced to the particular state of interest and involves a two‐pass procedure: First, the generation of configurations by single and double excitations from selected important configurations (parents), subject to a threshold criterion, followed by diagonalization of the energy to obtain an initial approximation to the wavefunctions of interest; second, the use of these wavefunctions as parents and a repeat of the generation and diagonalization steps to produce the final CI wavefunction. CI studies of the 1,3A2 (n→π*), 1,3A1 (π→π*, n→3p), 1B1 (σ→π*, π→3s), and 1B2 (n→3s) states of formaldehyde are reported for several large Gaussian basis sets of near atomic Hartree–Fock quality based on molecular orbitals determined from ...

Radiative Recombination in Highly Excited CdS

Abstract: The processes involved in the laser effect in CdS excited by electron bombardment have been studied with the aid of an experimental technique allowing the spectral dependence of the optical gain to be obtained. It is shown that at least three different processes can lead to a laser effect in CdS. The first, which corresponds to a low gain, is due to the annihilation of a free exciton with the emission of a photon and an LO phonon. The second, yielding a medium gain, is due to an exciton-exciton interaction; and the third, which results in a high gain, involves an exciton-electron interaction. These last two processes have been studied theoretically, and the results are compared with the experimental data.

De‐excitation of Electronically Excited Sodium by Nitrogen

Abstract: A semiquantitative calculation is made of the cross section for the quenching of Na(32P) by molecular nitrogen, as a function of initial kinetic energy and of final vibrational quantum number, υf, of the nitrogen molecule. The large observed cross section, which is of gas‐kinetic order, can be explained in terms of an intermediate ionic state, involving Na+ and N2− (υ = υ−). This state is unstable at infinite separation of Na and N2, but because of the Coulomb attraction it becomes stable at collision distances below about 3 A. As a result of the vibrational structure of both the intermediate and final states, we treat the reaction in terms of a diffusion of the probability flux through a two‐dimensional network of potential‐energy curves parametrized by both the electronic state and also the vibrational quantum numbers υ− and υf. At each potential‐energy curve crossing we compute the transition matrix element for insertion into a Landau–Zener type of transition probability. The transition matrix element ...

Perturbed Hartree–Fock Theory. I. Diagrammatic Double‐Perturbation Analysis

Abstract: The coupled Hartree–Fock scheme, for calculating atomic and molecular properties which are second order in some perturbing field, is studied with the help of a double‐perturbation expansion and diagrammatic techniques. The double‐perturbation expansion, with the external field as one perturbation (H(1)) and the difference between the true electron‐repulsion potential and the Hartree–Fock potential as the other (V), is compared with an iterative solution of the set of coupled Hartree–Fock equations for the first‐order perturbed orbitals. The term in the double‐perturbation expansion that is second order in H(1) and zeroth order in V is shown to be the Dalgarno “uncoupled” second‐order energy. The coupled Hartree–Fock second‐order (in H(1)) energy expression includes all terms first order in V and some of the terms to all orders in V, up to infinite order. It is shown how to separate the diagrams contributing to the CPHF energy from those that do not; in particular, the CPHF equations are composed of the so‐called “bubble diagrams” and their corresponding exchange diagrams. It is shown that each particle–hole pair, or “bubble,” is independent of all the other particle–hole pairs, in the sense that the total energy contribution from the diagram can be factored into contributions from the individual particle–hole pairs. It is shown that the first‐order perturbed wave‐function obtained from the coupled Hartree–Fock formalism includes effects due to all singly‐excited determinants and a certain class of linear combinations of doubly‐excited determinants, namely those singlet spin eigenfunctions which correspond to two electrons being excited without spin flips from Hartree–Fock unexcited orbitals to virtual orbitals; there exists a second, linearly independent, doubly excited singlet function which is not included in the CPHF wavefunction and does not contribute to the perturbation energy. The contribution to the second‐order energy from the doubly excited determinants amounts to about 10% of the total CPHF second‐order energy in the case of some electric and magnetic properties of H2 and 25% of the total second‐order energy corresponding to the electric dipole polarizability of the Be atom. The larger contribution of the terms involving doubly excited determinants for Be has its origin in the fact that a double excitation from the 2s to the 2p orbital gives a large fraction of the correlation energy between the 2s electrons and that the single excitation from the 2s to the 2p orbital is the largest single contribution to the polarizability. An alternate iterative scheme for the solution of the coupled Hartree–Fock equations is derived. This alternate iterative scheme, which includes certain terms summed to infinite order, is expected to be a considerable improvement over the simple scheme which starts with the “uncoupled” approximation as the zeroth iteration. Comparisons of the different schemes for the properties considered in this paper are made to confirm this conclusion. The CPHF energy (second order in H(1)) is partitioned approximately into a part due to correcting the unperturbed Hartree–Fock wavefunction for correlation and a part due to correcting the uncoupled Hartree–Fock perturbed wavefunction for correlation; the contribution from the former was shown to be about 10%–20% of that from the latter.

Reactions of excited atoms and molecules with atoms and molecules

Abstract: Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH+, RH 2 + , and H 2 + by collisions of excited rare gas atoms R* with H2. With HD as the target molecule the isotope effect for the production of RD+ and RH+ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.

Estimates of the Efficiencies of Production and Detection of Electron-Excited Auger Emission

Abstract: The efficiency of production of electrons emitted by the Auger process from the surface of a solid irradiated with a beam of electrons in the energy region below 2 keV is estimated. It is shown that the probability of photon production is negligible and that to a first approximation the optimum primary energy required for the ionization of an inner atomic level is between 3 and 3.5 times the critical ionization potential of that level. Calculation of the Auger current is made for a chosen system, that of oxygen adsorbed on a copper surface, and is found to be about 2×10−11 A in the group of oxygen peaks around 500 eV, for a complete monolayer, detected on a spherical screen of the size typically found in LEED display systems. Consideration of the two methods, the retarding field and the electrostatic deflection, that have been used for detection of Auger currents, leads to the conclusions that, in the retarding field analyzer, the current in the second derivative increases as the square of the amplitude o...

Fluorescence Quenching and Isotope Effect of Tryptophan

Abstract: The fluorescence spectra and quantum yields of trytophan (TRP) and several tryptophan derivatives in water and a polar glass have been measured over wide temperature ranges. For TRP, the wavelength of maximum emission shifts from 310 nm at 80°K to 355 nm at room temperature with almost all of the red shift occurring between 170° and 230°K, which is the temperature range where the glass softens. The successively more red‐shifted spectra have no isoemissive wavelength, which supports the view that reorientation of several solvent molecules in the solvent shell of the excited TRP molecule and not a 1:1 exciplex is responsible for the red shift. The quantum yield remains constant until a temperature is reached at which solvent reorientation is virtually complete. Above that temperature, the quenching of the 355‐nm emission can be fitted with a nonradiative de‐excitation having an activation energy of 7 kcal/mole. A model for these spectral changes and quenching mechanism will be offered. In deuterated solvents, a large isotope effect of fluorescence yield of TRP has been reported. This effect is clearly not caused by proton transfer in the excited state since it is found to be virtually the same for TRP and 1‐Me‐TRP. At temperatures below those at which solvent reorientation occurs, the isotope effect vanishes, and above them it approaches an asymptotic value, the quenching activation energy being independent of the isotopic constitution of the solvent. The fluorescence of most proteins and hormones originates in their tryptophan residues. The quantum yields, isotope effects, and emission spectra of such polypeptides are compared with the corresponding parameters for solvated TRP.

Deactivation of Vibrationally Excited Carbon Dioxide (ν3) by Collisions with Carbon Dioxide or with Nitrogen

Abstract: The rate constants associated with the deactivation of vibrationally excited CO2*(ν3) by collision with either CO2 or N2 have been experimentally determined from 300°–1000°K by a laser fluorescence method. The reactions investigated were CO2*(ν3)+N2⇆ lim k−5k5CO2+N2*, CO2*(ν3)+N2→ lim k5CO2*(ν1, ν2)+N2, CO2(ν3)+CO2→ lim k1CO2*(ν1, ν2)+CO2, N2*+CO2→ lim k7CO2*(ν1, ν2)+N2, where the asterisk denotes a vibrationally excited molecule, and the quantities in parentheses represent the specific excited modes of CO2. The rate constant k5 varies with temperature T (°K) as 8.60 × 107 / T3/2torr−1·sec−1. From 400°–1000°K the rate constant k1 (per torr−1 ·second−1) varies with temperature as log10k1 = A − BT−1 / 3 with A = 6.79 and B = 30.8. From 300°–400°K the measured values of k1 are greater than those corresponding to the above relation. The rate constant k6 increases from a value of 110 torr−1·sec−1 at room temperature to a value of 2700 torr−1·sec−1 at 1000°K, but the variation of k6 with temperature cannot be s...

Use of the CNDO Method in Spectroscopy. IV. Small Molecules: Spectra and Ground‐State Properties

Abstract: The modified CNDO method has been applied to the study of the electronic‐absroption spectra of several small molecules; namely, formaldehyde, formic acid, formamide, allene, ketene, and diazomethane, with quite satisfactory results. The lowest energy excited state of the last three compounds has been assigned as the state which arises from a π → π′* transition. A study has also been made of the calculation of ground‐state properties from the spectroscopic parameterization. It has been concluded that although this method can give reasonable estimates of these properties, it is most reliable when used to evaluate ground‐state properties which depend upon calculated eigenvalues.

Self-Coupling of a Two-Level System by a Mirror

Abstract: We discuss, from a classical as well as a quantum-mechanical point of view, the effect of the coupling of an excited two-level system with itself due to emission and absorption of electric-dipole radiation, the emitted radiation being reflected by a nearby mirror. Expressions familiar from resonance-scattering theory are found for the shift of the transition frequency and linewidth from their values in the absence of the mirror. This renormalization effect leads to spatial modulation in the transition frequency and width of the excited state due to its resonant interaction with itself via the radiation field reflected from the mirror. This formalism is applied to some recent experiments with monomolecular dye layers. The experimental values of the fluorescence lifetime are in good agreement with the theory for distances from 500 to 6000 \AA{}.

Matrix‐Isolation Study of the Infrared and Ultraviolet Spectra of the Free Radical HCO. The Hydrocarbon Flame Bands

Abstract: The photoproduction of H or D atoms from a variety of sources in a carbon monoxide matrix or in an argon matrix to which a small concentration of carbon monoxide has been added leads to the appearance of prominent ultraviolet absorptions between 2100 and 2600 A, all of which may be assigned to HCO or DCO. Both the CO‐stretching and the bending vibrational modes are appreciably excited in the transition. Evidence is presented indicating that the transition observed in the matrix experiments is the same one responsible for the hydrocarbon flame bands. Using the frequencies observed in the matrix experiments, a tentative assignment for the hydrocarbon flame bands has been proposed which is in reasonable agreement with the observed band structure of the emission system. In the upper state, the carbon and oxygen atoms of HCO are approximately singly bonded. Observation of the infrared absorption frequencies of isotopically substituted HCO in an argon matrix has prompted reconsideration of the valence force field appropriate to ground‐state HCO. Interaction between the CH‐stretching and the CO‐stretching modes has been found to play an important role. Factors leading to the stabilization of HCO in an argon matrix in the present experiments, in contrast to the results of previous studies, are discussed.

Low‐Energy Electron‐Impact Study of the First, Second, and Third Triplet States of Benzene

Abstract: The six lowest excited states of benzene have been investigated in the gas‐phase free molecule at low pressure by electron‐impact spectroscopy. Incident electron energies of 13.6 and 20.0 eV and scattering angles from 9° to 80° were used. Three singlet–singlet transitions at 5.0, 6.2, and 6.9 eV were identified. These transitions agree with the results of optical absorption and higher‐energy electron‐impact experiments. In addition, three triplet states were observed at 3.9, 4.7, and 5.6 eV. The positions of the first two triplet states agreed with optical data on solid benzene and threshold electron‐impact experiments. The third triplet state at 5.6 eV was assigned on the basis of the relative intensity of the transition at various scattering angles. The ratio of the intensity of this transition to the allowed singlet–singlet transition at 6.9 eV was in a constant proportion to the corresponding ratio for the first triplet state (3.9 eV) at all scattering angles. The spacings of the first and second and second and third triplet states in benzene were determined to be 0.80 and 0.85 eV. The difference in the spacing is not significant with respect to experimental error.

Energy Migration in Lanthanide Chelates

Abstract: Intramolecular energy transfer in lanthanide chelates can occur through different paths depending on the nature of the chelate. Contrary to prevailing notions, it is shown that energy transfer does not require the participation of the lowest triplet level of the chelate. The experimental results from a study of solutions of more than 600 chelate systems are consistent with a transfer mechanism via the ligand‐excited singlet state, with rate constants ket of about 1011 sec−1 and higher. Transfer via the lowest triplet state may predominate, however, when the rate of energy transfer from the singlet excited level is smaller than the rate of intersystem crossing to the triplet level. Whatever the mechanism, intramolecular energy transfer is generally a very efficient process, regardless of the fluorescence quantum yield of the chelated lanthanide ion. Low quantum yields of sensitized fluorescence are usually the result of radiationless decay processes following intramolecular energy transfer. This quenching ...

Theoretical Investigation of the Lowest Double‐Minimum State E, F 1Σg+ of the Hydrogen Molecule

Abstract: Accurate potential‐energy curve for the first excited 1Σg+ state of the hydrogen molecule has been calculated for 1 ≤ R ≤ 12 a.u. The curve is known to have a double minimum, and for the outer minimum a significantly lower energy has been obtained than all previously reported results. The diagonal corrections for nuclear motion have also been computed and found to have a very sharp peak in the vicinity of the potential maximum. Vibrational and rotational levels have been calculated for all isotopes of the hydrogen molecule, and some results for H2, D2and T2 are shown and compared with experimental data. For the outer minimum, two, three, and four vibrational levels for H2, D2, and T2, respectively, are reported which lie below the lowest levels observed experimentally. For the observed levels good agreement with experimental data is obtained; however, the agreement is significantly worse when only the clamped nuclei potential without the diagonal corrections is used in the calculation of the vibrational e...

Theory of atomic structure including electron correlation. i. three kinds of correlation in ground and excited configurations.

Abstract: There are novel correlation effects in excited states and configurations unlike those in closed shells. A theory for general nonclosed shells, and a method for calculation, are developed by separating the correlations into three mathematically and physically distinct types: (1) "internal," (2) "polarization plus semi-internal," and (3) "all-external" correlations. The first two of these are unique to open shells and strongly dependent on number of electrons, symmetry, and $Z$. They are however shown to be calculable by a finite configuration interaction method, and their energy contributions and wave functions are computed using a fully automatic program for 113 states of $1{s}^{2}2{s}^{n}2{p}^{m}(n=0, 1, 2; m=0, 1,\dots{}, 6)$ configurations for $Z=5 \mathrm{through} 11$. Both effects are found to be important in magnitude. The detailed wave functions obtained, which include those of positive and negative ions and of highly excited states containing inner $2s$ holes, are useful for obtaining atomic properties such as transition probabilities. The remaining all-external correlation energy is found to be, as predicted by the present theory, just like the correlation in closed shells, i.e., mainly made up of transferable pair correlations (evaluated in Paper II) and approximately transferable through $Z$ in a given isoelectronic sequence.

Coherent Resonance Fluorescence Excited by Short Light Pulses

Abstract: Fluorescence produced by resonant coherent light pulse passing through medium assuming infinite relaxation times and negligible pulse attenuation

Ring Puckering in Five‐Membered Rings. II. The Microwave Spectrum, Dipole Moment, and Barrier to Pseudorotation in Tetrahydrofuran

Abstract: The microwave spectrum of tetrahydrofuran has been studied. Nine complete rotational spectra have been observed. These arise from the ground and eight excited states. All of these states are less than 200 cm−1 from the ground state. The rotational constants and dipole moments exhibit a strong nonlinear dependence on the quantum number of the excited state. Vibration–rotation interaction is strong and the spectra of the first four states deviate from that of rigid rotor spectra. These deviations permit the determination of two energy separations: Δ01 = 0.67 cm−1 and Δ23 = 1.5 cm−1. All of the results are interpreted in terms of a model of restricted pseudorotation with a potential function of [30(1‐cos2φ) / 2] + [40(1‐cos4φ) / 2] cm−1, where φ is the angle of pseudorotation. The dipole moment varies from 1.52 to 1.76 D depending upon the pseudorotation state. The details of this variation indicate that the twisted configuration is at lower energy than the bent configuration.

Validity conditions for local thermodynamic equilibrium.

Abstract: Validity criteria for partial and complete local thermodynamic equilibrium (L.T.E.) are re-examinated on the basis of generalized cross section formulas which are in agreement with experiments and with quantum mechanical results. The validity conditions for L.T.E. obtained deviate both in the absolute values and in the temperature dependence from those given by other authors. The influence of resonance absorption on the establishment of L.T.E. is considered and quantitatively taken into account. The influence of atom-atom collisions on the population densities of highly excited levels is also discussed. A general formula is derived permitting an estimate wether the excited levels are populated due to electronic or due to atomic collisions. The condition obtained is quite severe for plasmas in which the electron temperature is different from the gas temperature. For example: In a helium non-L.T.E. plasma of 1% ionization degree (forT e=2×104 °K) all levels of principal quantum numbersn>12 will essentially be populated due to atomic collisions at a gas temperature ofT a=1.6×104 °K. The influence of diffusion on the L.T.E. conditions is discussed for the various kinds of particle interactions. It is found that deviations from L.T.E. due to particle diffusion may be more severe than is generally assumed.

The collisional deactivation of metastable atoms and molecules in the upper atmosphere

Abstract: It is now possible to produce comparatively large concentrations of highly reactive metastable species in laboratory sources and to study metastable atoms and molecules with very long radiative lifetimes by photoionization mass spectroscopy, by vacuum ultraviolet absorption techniques, or by direct observation of the feeble radiation emitted by these excited species. These methods are being used widely to study the collisional deactivation of aeronomically important metastable species by quenching or by reaction. New information on radiative lifetimes, diffusion coefficients, and reaction rate coefficients is now available. These results are tabulated and reviewed in the light of atmospheric measurements and current theories of the terrestrial airglow and aurora.

High‐Resolution Photoionization Study of the H2 Molecule near Threshold

Abstract: Relative photoionization and absorption cross sections of H2 (ordinary and para) have been measured from 745–810 A at 300 and 78°K, with a resolution width of 0.04 A. The photoionization data show the presence of extensive structure due to autoionization of vibrationally excited Rybderg states. Analysis of the data on the D–X(6, 0) band in the region of the ionization threshold leads to the conclusion that the ionization potential of H2 lies between 124 418.2 and 124 393.5 cm−1 and is probably very near the former value. Analysis of the data on the B″–X(4, 0) band requires either that the ionization potential lies below 124 407.2 cm−1 (which is rather unlikely) or that autoionization of the R(2) and R(3) lines occurs in violation of one of the autoionization selection rules proposed by Beutler and Junger. Some Rybderg states that lie just below their ionization limits and do not autoionize spontaneously can be made to ionize in weak electric fields and by collision with unexcited H2 molecules. Direct ioni...

Properties of the photoactive chlorophyll-aII in photosynthesis.

Abstract: 1. The complete difference spectrum of the reaction of the photoactive chlorophyll-an is presented.2. The reaction of excited chlorophyll-aII is of the type of a sensitizer. It is not engaged directly in the electron transfers. This is in contrast to the photoactive chlorophyll-aI which is an electron donor in its excited state.3. The chlorophyll-aII-reaction can be separated from the overall reaction by heating chloroplasts 5 min at 50°C.4. Chlorophyll-an is the reaction center of the well-known poison DCMU.5. Properties of chlorophyll-aII are depicted in Tab. 1. They are compared with those of chlorophyll-aI and the O₂-evolution system.

Interference effects in the radiative decay of closely spaced levels

Abstract: In this paper we present a study of the radiative decay of coherently excited, closely spaced, molecular (or atomic) levels. Interference effects in the decay process cannot be treated in general by displaying the damping matrix in a diagonal form, so that each decay channel is assumed to be characterized by its own lifetime. The problem of the radiative decay of coherently excited indistinguishable levels (characterized by the same symmetry) is treated by a self-consistent extension of the Wigner-Weisskopf method, or alternatively, by the ‘unitarity relations’ method, developed in elementary particles physics. Finally, the Fano configuration interaction method is applied for the decay of a manifold of states characterized by the same polarization. General expressions for photon counting rates are derived and the nature of a general quantum beat experiment is discussed.