Showing papers on "Excited state published in 1970"

Superlattice and negative differential conductivity in semiconductors

Abstract: We consider a one-dimensional periodic potential, or "superlattice," in monocrystalline semiconductors formbeyd a periodic variation of alloy composition or of impurity density introduced during epitaxial growth. If the period of a superlattice, of the order of 100A, is shorter than the electron mean free path, a series of narrow allowed and forbidden bands is expected duet o the subdivision of the Brillouin zone into a series of minizones. If the scattering time of electrons meets a threshold condition, the combined effect of the narrow energy band and the narrow wave-vector zone makes it possible for electrons to be excited with moderate electric fields to an energy and momentum beyond an inflection point in the E-k relation; this results ina negative differential conductance in the direction of the superlattice. The study of superlattices and observations of quantum mechanical effects on a new physical scale may provide a valuable area of investigation in the fieId of semiconductors.

Phonon Sidebands, Multiphonon Relaxation of Excited States, and Phonon-Assisted Energy Transfer between Ions in Solids

Abstract: A unified treatment is given in the adiabatic approximation for phonon sideband intensities, multiphonon relaxation transition probabilities, and phonon-assisted energy-transfer probabilities. The intensity distribution of phonon sidebands is determined by coupling constants of the vibrational modes with electrons or holes and a criterion for the appearance of discrete sidebands is given. Transition probabilities of multiphonon relaxation processes among various excited levels of an ion are shown to depend exponentially on the energy gap between these levels, in agreement with recent experimental results. A similar dependence is derived for the energy-transfer probabilities between two ions on the energy mismatch between excitation energies of these ions.

Classical Aspects of Energy Transfer in Molecular Systems

Abstract: The decay time of the luminescence of a molecule S in front of a metal mirror depends markedly on its distance from the mirror. This phenomenon is quantitatively explained by considering the radiation field of this dipole, given by Hertz classical equation. This field arrives at the molecule, after being reflected at the mirror, with a retardation of the order of 10−15 sec. The decay time of the luminescence depends on the phase shift produced by this retardation, and thus on the ratio of the distance of the oscillator from the mirror, and the wavelength of the emitted light. By measuring the distance dependence of the decay time of the luminescence this retardation effect can be studied. In quantum‐mechanical terms the phenomenon can be described as being due to a stimulation or inhibition of the emission of the light quantum. In contrast to the known cases of stimulated emission, the stimulating field is the radiation field of the emitter quantum itself. The energy transfer from an excited molecule S to an acceptor A can be treated in a similar manner by considering the phenomenon as a retardation effect. In classical terms the field of S induces A to oscillate, and the induced field of A arriving at S slows down this oscillator. Simple equations are given for the energy transfer from an excited dipole or quadrupole, and for a row of many dipoles, oscillating in phase, to a weakly absorbing acceptor layer. The latter case is considered as a model for a J‐aggregating dye and by comparison with experimental data conclusions concerning the size of a J aggregate are drawn.

Polarization Dependence of the Two-Photon Absorption of Tumbling Molecules with Application to Liquid 1-Chloronaphthalene and Benzene

Abstract: The two‐photon absorption cross section δ for photons of any polarization (linear, circular, or elliptical) is averaged over all orientations of the absorbing molecule. The result is given by 〈δ〉 = δFF + δGG + δHH, where δF, δG and δH are molecular parameters and F, G, and H are simple functions of the polarization vectors. It is shown how the δ's may be calculated from theory and also how they may be measured by experiment. Experiments using only linearly polarized light are insufficient to determine all three δ's; hence, circularly polarized light will play an essential role in this spectroscopy. For absorption of two linearly polarized photons with angle θ between their polarization vectors, the angular dependence is 〈δ〉 = A + Bcos2θ, where A and B are simple combinations of the δ's. We obtain two exact symmetry rules which permit allowed two‐photon transitions of different symmetries to be distinguished. For transitions from totally symmetric ground states the rules are: (1) If the excited state transforms like xy, yz, or zx, then δF = 0. (2) If the excited state transforms like x2, y2, or z2, then δG = δH. In cases of near resonance, when a single intermediate state dominates the formula for the cross section, we show that δF = δH, and that linearly polarized light suffices for a complete investigation. These results are applied to liquid 1‐chloronaphthalene. We find two allowed two‐photon transitions which are assigned 1B1g (perpendicular nodes) at 37 700 cm−1 and 1 Ag (total symmetry) at 42 600 cm−1. This is in reasonable agreement with theoretical predictions of other authors. We have also examined the region of the second excited singlet of benzene, near 6.2 eV. We were not able to detect any two‐photon absorption, setting an upper limit of about 10−51 cm4·sec/photon·molecule on its 〈δ〉. This leads to an unequivocal assignment of 1B1u for this state according to calculations of Jortner. In an Appendix we examine the effect of “hot spots” in the laser beam on the observed cross sections. We show that the elimination of the hot spots is of some importance, contrary to a statement of other authors.

Potential Curves of Diatomic Rare‐Gas Molecules and Their Ions, with Particular Reference to Xe2

Abstract: The excited and ionized states of the heavier rare‐gas molecules are discussed, existing evidence on the dissociation energies of the ions are reviewed, and estimates of these dissociation energies are made. Estimated potential curves for the Xe2+ ion and for the lower excited states of the Xe2 molecule are given. Applications to the interpretation of the observed spectra of the heavier rare gases, especially Xe2, are discussed.

Electronic Energies and Electronic Structures of the Fluoromethanes

Abstract: The high‐resolution He i and He ii photoelectron spectra of all fluoromethanes in the series CH4 to CF4 and their deuterated analogs have been recorded and are compared with the Koopmans' theorem results of near‐Hartree–Fock calculations performed in a Gaussian basis. The agreement is very good in general and offers an unambiguous assignment of almost all of the bands observed. In particular, repeated correlations are demonstrated between the compositions of the orbitals from which the electrons are ejected and the characters of the resulting photoelectron bands. Identifiable trends throughout the series are stressed and an anomalous feature in the CF4 spectrum is noted. Jahn–Teller effects in CH4 and CH3F are clearly evident, but as expected, they are not observed in CHF3 and CF4. Comparison of the photoelectron spectra excited with He i and He ii radiation shows wide variations in the relative intensities of various bands in certain of the more symmetric molecules, suggesting that relative intensities c...

Electron Energy Distributions and Collision Rates in Electrically Excited N 2 , CO, and C O 2

Abstract: Electron energy distributions have been obtained for electrically excited ${\mathrm{N}}_{2}$, CO, C${\mathrm{O}}_{2}$, and their mixtures by numerically solving the Boltzmann equation for conditions typical of electric discharges. Reported electron cross-section data have been used in the calculation. The calculated distribution functions were found to be markedly non-Maxwellian, having energy variations which reflect the important electron-molecule energy exchange processes. Solution of the electron energy conservation equation using these distribution functions revealed that vibrational and electronic excitation of ${\mathrm{N}}_{2}$, CO, and C${\mathrm{O}}_{2}$ dominates electron-molecule energy exchange processes for average electron energy in the 1-3-eV range typical of electric discharges. Electron-molecule vibrational excitation rates were also evaluated for a variety of gas mixtures and discharge conditions. The importance of these results to molecular gas-discharge lasers is discussed.

Transversely excited atmospheric pressure co2 lasers

Abstract: Laser action at 10. 6 μm has been achieved in CO2–N2–He gas mixtures at all pressures up to atmospheric by means of fast electrical discharges transverse to the laser axis. The atmospheric pressure of operation is very promising as the laser energy per pulse increases with pressure and the average power per unit volume is proportional to the square of the operating pressure. A brief description of a simple laser of this type, operating at atmospheric pressure is given along with some of the major achievements to date using this technique.

A possible role for singlet oxygen in the initiation of fatty acid autoxidation

Abstract: A mechanism for the initiation of autoxidation in fatty acids is proposed which involves singlet state oxygen, formed through a photosensitization reaction, as the reactive intermediate. Both singlet oxygen generated in a radio-frequency gasdischarge, and photosensitization by natural pigments, were shown to catalyze the oxidation of methyl linoleate. The involvement of singlet oxygen was shown by the identification of nonconjugated hydroperoxides as products common to both photooxidation and singlet O2 oxidation. Nonconjugated hydroperoxides could not be detected among the free radical autoxidation products. Further proof for the above mechanism was gained by showing that compounds known to react strongly with singlet oxygen, inhibited the photooxidation. With the exception of chlorophyll, all sensitizers could be completely inhibited. Although singlet oxygen formation can account for approximately 80% of the observed chlorophyll photooxidation, at least one other mechanism must be involved. It is postulated that proton abstraction by the photoactivated carbonyl group of chorophyll could account for the remaining 20% of the observed photooxidation. The conclusion is drawn that oxygen, excited to its singlet state by a photosensitization process, plays the important role of forming the original hydroperoxides whose presence is necessary before the normal free radical autoxidation process can begin.

Infrared Absorption in Some II-VI Compounds Doped with Cr

Abstract: The optical absorption spectra of substitutional ${\mathrm{Cr}}^{2+}$ ($3{d}^{4}$) in single crystals of ZnS (cubic and mixed cubic-and-hexagonal), ZnSe, ZnTe, CdS, and CdTe have been measured for Cr concentrations of ${10}^{18}$-${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and at 2, 15, and 300\ifmmode^\circ\else\textdegree\fi{}K in the wave-number range $5\ensuremath{\le}\overline{\ensuremath{ u}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The room-temperature spectrum in the range $500\ensuremath{\le}\overline{\ensuremath{ u}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$ is dominated by a broad absorption band at 5500 ${\mathrm{cm}}^{\ensuremath{-}1}$ arising from the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E$ transition. These two levels originate from the crystal-field splitting of the $^{5}D$ free-ion ground state. At helium temperatures distinct lines appear on the low-energy side of the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E_{2}$ absorption band in all samples except for CdTe. These lines are associated with zero-phonon transitions and phonon-assisted transitions. As the temperature is increased, the relative intensities of some of the lines change due to the thermal population of low-lying levels belonging to the orbital ${T}_{2}$ state. In the case of ZnSe these levels have been studied in the far infrared in the range $5\ensuremath{\le}\overline{\ensuremath{ u}}\ensuremath{\le}200$ ${\mathrm{cm}}^{\ensuremath{-}1}$, both with and without a magnetic field. An interpretation of the results is made based on crystal-field theory with a strong, static, Jahn-Teller distortion of $E$ symmetry in the $^{5}T_{2}$ orbital ground state. There is little or no Jahn-Teller effect in the $^{5}E$ orbital excited state. The optical transitions exhibit a strong coupling to $E$-mode phonons of average energy \ensuremath{\sim}70 ${\mathrm{cm}}^{\ensuremath{-}1}$.

cw OPERATION OF AN ORGANIC DYE SOLUTION LASER

Abstract: A cw laser using a 2.5×10−4 M solution of rhodamine 6G in water containing a deaggregating agent (1.5% Triton‐X100) is described. The laser consists of a 4.5‐mm transverse‐flow hemispherical cavity excited longitudinally by the 5145‐A output of a 1‐W argon ion laser. With mirror reflectances of 97.5% and 99.5%, a 200‐mW threshold excitation was measured. Power output was approximately 30 mW at 597 nm with 960 mW excitation power.

Cooperative Phenomena in Resonant Electromagnetic Propagation

Abstract: Phenomena of coherent resonant propagation can be considered as resulting from the cooperative interaction of a certain number of excited two-level systems. It is shown that these phenomena can be characterized by a specific "maximum cooperation number" and by the associated "cooperation time." These are defined for the superradiant state, but their meaning and usefulness can be extended to other situations. The alternative description of of superradiance as a spontaneous or as a stimulated effect is also discussed and it is shown that with the help of the new concepts, the Dicke quantum perturbative treatment can be reconciled with the semiclassical theories.

Excess Electrons in Polar Solvents

Abstract: In this paper we consider a structural model for localized excess electron states in polar solvents with particular reference to dilute metal ammonia solutions and to the hydrated electron. The over‐all energetic stability of these species was assessed by considering simultaneously the electronic energy and the medium rearrangement energy. The present model consists of a finite number of loosely packed molecules on the surface of the cavity which are subjected to thermal fluctuations and a polarizable continuum beyond. The electronic energy was computed utilizing an electrostatic microscopic short‐range attraction potential, a Landau‐type potential for long‐range interactions, and a Wigner‐Seitz potential for short‐range repulsive interactions. The medium rearrangement energy includes the surface tension work, the dipole–dipole repulsion in the first solvation layer, and most importantly the short‐range repulsive interactions between the hydrogen atoms of the molecules oriented by the enclosed charge. The gross features of localized electron states in different solvents can be rationalized in terms of different contributions to the medium rearrangement terms. The energetic stability of the localized state of excess electrons in polar solvents was established and the cavity size in the ground state of the solvated electron could be uniquely determined. Experimental energetic and structural data such as volume expansion, coordination numbers, heats of solution, and spectroscopic properties are in qualitative agreement with the predictions of the present model. Optical line shape data calculated from the theoretical model do not agree with experiment; this discrepancy suggests that more data are required in regard to the excited states of the solvated electron.

Antiferromagnetic model with known ground state

Abstract: The ground state of the one-dimensional antiferromagnetic model, with the Hamiltonian H=1/2 j Sigma i=1n sigma i. sigma i+1+1/4j Sigma i=1n sigma i. sigma 1+2 n sigma i. sigma 1+2 (j>O, N even, N+1 identical to 1, N+2 identical to 2) can be given explicitly in terms of spin eigen-functions. Various properties of the ground state are studied. An approximate determination of the low-lying excited states shows the usual linear frequency-wave-vector relationship.

Quantum Theory of the Intensities of Molecular Vibrational Spectra

Abstract: Formulas for the transition probabilities and hence the absolute intensities of molecular vibrational spectra are obtained from a unified quantum field treatment. The theory of infrared, Raman, and hyper‐Raman spectroscopy of molecular vibrations is developed by assuming these processes occur as time‐ordered steps involving the creation or destruction of one quantum of vibrational energy and changes in the occupation number of one, two, or three photons, respectively. The formulas obtained by this method for ir transitions become equivalent to the earlier treatment of Jones and Simpson if the energy difference of the ground and first excited electronic energy levels are very large relative to that of the vibrational quantum. The formulas obtained for Raman transitions are very similar to those obtained by the method originated by Albrecht and developed further by Savin; we get not only the original terms of Albrecht but also the trace terms obtained by Savin. Furthermore by using third‐order time‐dependent equations from the start we avoid many of the difficulties of the earlier treatments; our equations predict different conditions for the resonant Raman effect than do the earlier equations, and experiments are suggested for testing the new equations. The formula which we give for the absolute intensity of the hyper‐Raman effect appears to be the first ever given.

Combined SCF and CI Method for the Calculation of Electronically Excited States of Molecules: Potential Curves for the Low‐Lying States of Formaldehyde

Abstract: The success of various ab initio methods for the calculation of electronic spectra for polyatomic molecules is discussed with reference to the formaldehyde system. It is found that a limited CI calculation based on the SCF MO's of the ground‐state wavefunction yields inaccurate potential curves for the excited states of H2CO and overestimates their transition energies from the ground state. It is concluded that the major cause for these deficiencies lies in the fact that the ground‐state MO's do not represent a good starting point for the description of the excited states, and thus a second series of calculations is carried out using open‐shell SCF techniques in an attempt to improve upon this situation. The resulting excited‐state potential curves do show the proper angular behavior expected from experimental studies, but the calculated transition energies are too low because the correlation energy error in the SCF treatment is greater for the closed‐shell ground state than for the open‐shell excited species. To correct this shortcoming a limited CI calculation is carried out for each excited state based on the SCF MO's of its parent or main configuration. This combined SCF and CI treatment leads to excellent results for both transition energies and potential curves of ground and excited states and also produces relatively good dipole moments for both types of states.

Some Propensity Rules in Collision‐Induced Rotational Quantum Jumps

Abstract: When a mixture of lithium vapor and argon is irradiated with the light from an argon ion laser and the resulting fluorescence of the Li2 B 1Πu − X 1Σg+ band system is examined under high resolution, a pattern of collision‐induced satellite lines is observed to accompany the parent resonance fluorescence series. Under conditions of low pressure these satellite lines originate from single inelastic events which alter the rotational state of the excited Li2 molecule. The relative intensities of the satellite lines are found to be markedly different depending on whether the collision‐induced transition originates from the upper or lower component of the Lgr; doublet of the Π state, referred to as c or d, respectively. An increase in J(+ ΔJ jump) is favored over a decrease (− ΔJ) for d → c jumps whereas − ΔJ is favored over + ΔJ for c → d jumps. On the other hand ± ΔJ changes that preserve the character of the Λ component, i.e., c → c and d → d, occur with nearly equal probability for the same value of ΔJ. Thi...

Temperature Dependence of Surface Acoustic Wave Velocity on α Quartz

Abstract: Measurements of the temperature dependence of surface acoustic wave velocity were performed for propagation on X, Y, AC, AT, and along the x axis of several other rotated Y cuts of quartz over the temperature range −25° to +75°C. A pulsed rf technique was employed. To facilitate the measurement of angular dependence of the temperature coefficient of velocity, surface wave delay lines were constructed in which the waves were excited on the quartz by means of an interdigital electrode structure on a glass substrate which was brought into contact with the quartz. Calculations of the temperature coefficients of velocity and delay time were performed using an iterative computer program based on the work of Coquin and Tiersten. The measurements and calculations are in good agreement in most cases, the largest discrepancy being approximately five percent. It is concluded that the orientation which best combines low‐temperature dependence of delay time, high coupling constant, and a minimum of deleterious side effects is 42½° rotated Y‐cut quartz with propagation along the x axis.

Photoreactivity of n,π* Excited States of Alkyl Ketones

Abstract: ion of a hydrogen from the solvent are often observed (vide infra). Having discussed the energies and primary photophysical processes of the $1 and T1 states of alkyl ketones, we can now construct an energy diagram as shown for acetone in Figure 3. We shall assume that acetone is a good model for the photophysical behavior of other aliphatic ketones. The important features of this diagram are: (a) the $1 (n,~r*) state of acetone relatively low in energy compared to other simple chromophores; (b) the Tx (n,~r*) state of acetone is relatively high in energy compared to most chromophores (only simple benzene derivatives possess a higher triplet energy); (c) the lifetime of 31 is long enough to allow participation in bimolecular processes of low activation energy. Because of their high triplet energy, aliphatic ketones are commonly used as photosensitizers for unsaturated compounds possessing low absorption above 2800 QVANTUM YIELDS AND PtEAiCTIVITIE$ The quantum yield (~b) for formation of a photoproduct, a measure of the efficiency with which the photoreaction proceeds, is by definition equal to the number of molecules of photoproduct formed per photon of light absorbed. It can also be viewed as the probability that the initially generated electronically excited state will yield the given photoproduct. Take, for example, a unimolecular photochemical isomerization of A to B occurring www.annualreviews.org/aronline Annual Reviews A nn u. R ev . P hy s. C he m . 1 97 0. 21 :4 99 -5 60 . D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by C ol um bi a U ni ve rs ity o n 03 /0 6/ 09 . F or p er so na l u se o nl y. PHOTOREACTIVITY OF n,,r* EXCITED STATES 505 exclusively from the Tt state of an alkyl ketone (sA). If every photon absorbed is assumed to result in the formation of the St state of A (1A), the quantum yield for formation of B ($B°) will be the product of the probability of 1A leading to 3A times the probability of 3A proceeding on to B. ~B° will then be given by Equation 5 where kr is the unimolecular rate constant for the photolsomerizatlon of 3A to B. ~B° = = dpsTkrrT 5. ~ + kS T + k~ kr + ka + k The reactivity of an electronically excited state toward a primary photochemical process is defined as the rate constant for that process. Thus for the photoisomerizafion of A to B, kr is a measure of the reactivity of aA toward the photoisomerization (or the primary photoprocess which proceeds the isomerization). The quantum yield for a photoreaction, which depends upon the relative rates of the various competing processes occurring from the electronically excited states (Equation 5), is not in general a good measure of either the absolute or the relative photoreactivity of excited states (27-29). The use of energy transfer quenching (30) has become an extremely popular technique for estimating the reactivity of excited states toward various primary photochemical processes. In this method the yield of product formation is decreased by the addition of a compound which is ~nown to quench a specific excited state by electronic energy transfer. For example, the quantum yield for the triplet state isomerization of A to B will be lowered by the introduction of the additional pathway for deactivation of *A shown in Equation 6, resulting in the expression for $~ given in Equation 7. Division of $~0 (Equation 5) by $~ (Equation 7) yields

The Method of Optimized Valence Configurations: A Reasonable Application of the Multiconfiguration Self-Consistent-Field Technique to the Quantitative Description of Chemical Bonding*

Abstract: Publisher Summary This chapter discusses the method of optimized valence configurations, which is reasonable application of the multi-configuration self-consistent field technique to the quantitative description of chemical bonding. It is the purpose of this chapter to review and bring up to date the conceptual features, the analysis, and results obtained by the application of the method of Optimized Valence Configurations to diatomic molecules. Several significant conclusions can be drawn from the experience obtained in developing this method, and in assessing its relationship to other schemes. (1) It is possible to quantitatively separate the “molecular” aspects of the changing correlation energy of two approaching atoms from the remaining correlation. (2) The number of significant configurations representing extra molecular correlation is small and readily obtainable by a sequence of limited multi-configurational self-consistent-field computations, followed by a single configuration interaction involving all new orbitals thus obtained while those configurations representing atomic correlation are indeed numerous, but are easily accounted for by a suitable perturbation technique. (3) An excellent initial guess for excited starting orbitals can be obtained by maximization of the exchange integral between the orbital of the Hartree-Fock configuration being correlated and the excited orbital.

Electron Spectroscopic Investigation of Auger Processes in Bromine Substituted Methanes and Some Hydrocarbons

Abstract: Auger spectra in gaseous CH4, CH3Br, CH2Br2, CHBr3, CBr4, C2H6, and C6H6 are excited by electron impact at about 5 keV energy and observed by means of a spherical electrostatic spectrometer at 90° to the primary electron beam. Besides the energies and the intensities of the Auger lines and the chemical shifts, the analysis of the spectra gives the energies and the lifetimes of the doubly ionized molecules. The line widths together with energy considerations indicate that some of these doubly ionized molecules rapidly dissociate. Comparisons are also made between the bromine and the carbon spectra.

Measured Lifetimes of Rotational and Vibrational Levels of Electronic States of N2

Abstract: The lifetimes of vibrational levels of the B 3πg and C 3πu electronic states of molecular nitrogen and of the B 2Σu+ electronic states of the positive molecular nitrogen ion were measured by photometrically monitoring the excited level number densities in a pulsed invertron. Heavy‐particle pressure damping cross sections were obtained from experimental studies of the pressure dependence of the reciprocal lifetimes. Lifetimes of individual rotational levels from K = 14 to K = 35 of the B 2Σu+(υ = 0) levels were measured. The lifetimes of all rotational levels were identical. However, our results suggest a dependence of the line strength on the rotational level different from that predicted by the Born–Oppenheimer approximation with zero rotational–vibrational interaction and zero electron spin–rotational interactions.