Showing papers on "Excited state published in 1980"

Kondo effect in real metals

Abstract: Starting from the most general form of the Anderson hamiltonian, the behaviour of magnetic impurities in real metals is considered, taking into account the orbital structure of the local impurity electrons, crystal field and spin orbit splittings. The analysis is carried out in an atomic limit, in which the impurity has a well defined integer valency (a Schrieffer Wolff transformation is then valid). The main steps of a scaling procedure are described in detail. As the temperature goes down, the excited states of the ground state configuration decouple one after the other. The hierarchy of these decouplings, and their interplay with Kondo singularities are analyzed. When a Fermi liquid picture applies as T → 0, the number of independent parameters may be reduced considerably using symmetry and universality arguments which bypass the numerical description of the crossover region. That first part sets a language in which to describe specific problems. We apply that language to the case where the atomic ground state is an orbital singlet. In the absence of anisotropies, the only parameters are the impurity spin S and the number of orbital channels n. We show that an anomalous fixed point occurs at finite coupling when n > 2 S. That fixed point is unstable with respect to anisotropies. The scaling trajectories are discussed for a cubic crystal field for several choices of valencies. The universality of the low temperature behaviour is clarified. A similar analysis is carried out when the atomic ground state only has one electron (or hole). The influence of crystal field and spin orbit interactions is analyzed — and their relevance to the Kondo crossover and to universality is ascertained.

The theory of electron-molecule collisions

Abstract: The current state of the theory and its application to low-energy electron-molecule collisions is reviewed. The emphasis is on elastic scattering and vibrational and rotational excitation of small diatomic and polyatomic molecules. New and traditional theoretical approaches are described, and the results of calculations are compared with existing experimental measurements.

Observation of the Infrared Spectrum of H 3

Abstract: The infrared ${\ensuremath{ u}}_{2}$ band of ${\mathrm{H}}_{3}^{+}$ has been observed. A direct infrared absorption method combining a liquid-nitrogen-cooled multiple-reflection discharge cell and a difference-frequency laser system has been used for the detection. Fifteen absorption lines have been measured in the region of 2950-2450 ${\mathrm{cm}}^{\ensuremath{-}1}$ and assigned. This is the first spectroscopic detection of this fundamental molecular ion in any spectral range.

Cross-section and rate formulas for electron-impact ionization, excitation, deexcitation, and total depopulation of excited atoms

Abstract: For electron-induced ionization, excitation, and de-excitation, mainly from excited atomic states, a detailed analysis is presented of the dependence of the cross sections and rate coefficients on electron energy and temperature, and on atomic parameters. A wide energy range is covered, including sudden as well as adiabatic collisions. By combining the available experimental and theoretical information, a set of simple analytical formulas is constructed for the cross sections and rate coefficients of the processes mentioned, for the total depopulation, and for three-body recombination. The formulas account for large deviations from classical and semiclassical scaling, as found for excitation. They agree with experimental data and with the theories in their respective ranges of validity, but have a wider range of validity than the separate theories. The simple analytical form further facilitates the application in plasma modeling.

Porphyrins. 40. Electronic spectra and four-orbital energies of free-base, zinc, copper, and palladium tetrakis(perfluorophenyl)porphyrins

Abstract: Absorption and emission spectra and emission quantum yields are given for free-base (H/sub 2/), Zn, Cu, and Pd derivatives of tetrakis(perfluorophenyl)porphyrin (TFPP). The four-orbital model is used to rationalize differences in the optical properties among the derivatives of TFPP and the same derivatives of porphine (P), octaethylporphyrin (OEP), and tetraphenylporphyrin (TPP). It is concluded from absorption data that the energy difference between the two excited singlets, i.e., /sup 1/E(a/sub 2u/,e/sug g/) - /sup 1/E(a/sub 1u/,e/sub g/), decreases along the series Pd > Cu > Zn > H/sub 2/ and also along the series OEP > P > TFPP > TPP. The theory and date are reviewed, and it is concluded that the energy difference between the two excited triplets, i.e., /sup 3/E(a/sub 2u/,e/sub g/) - /sup 3/E(a/sub 1u/,e/sub g/), also decreases along these series. However the condition of degeneracy, e.g., /sup 1/E(a/sub 2u/,e/sub g/) - /sup 1/E(a/sub 1u/,e/sub g/) = 0, occurs for different molecules in the singlet and triplet cases. Theory also suggests that if /sup 3/E(a/sub 2u/,e/sub g/) > /sup 3/E(a/sub 1u/,e/sub g), the molecule should form a /sup 2/A/sub 1u/ cation radical and if /sup 3/E(a/sub 2u/,e/sub g/) < /sup 3/E(a/sub 1u/,e/sub g/) the molecule should formmore » a /sup 2/A/sub 2u/ radical. 4 figures, 2 tables.« less

Dynamics of dense laser-induced plasmas

Abstract: Calculations have been made to determine the influence of a dense plasma of hot electrons and holes on the primary channels of energy relaxation and redistribution of photoexcited carriers in Si, particularly collisions between carriers, plasmon emission, impact ionization, phonon emission, and carrier diffusion. At high carrier densities, Auger recombination is sufficiently fast to ensure that the electrons and holes rapidly reach quasiequilibrium with a common quasi-Fermi level at a temperature which is lowered by the partitioning of energy into thermally excited plasmons. The appropriate dielectric function has been calculated. At sufficiently high temperatures and carrier densities, energy can diffuse at a rate that is comparable to (and, in some cases, faster than) the rate at which the energy is transferred to the lattice. The steady-state carrier density and temperature, and consequently the ultimate extent to which the lattice is heated, depend critically on the parameters of the exciting laser.

Light induced electron transfer reactions of metal complexes

Abstract: Properties of the excited states of tris(2,2'—bipyridine) and tris(l,lO—phenanthroline) complexes of chromium(III), iron(II), ruthenium(II), osmium(II), rhodium(III), and iridium(III) are described. The electron transfer reactions of the ground and excited states are discussed and interpreted in terms of the driving force for the reaction and the distortions of the excited states relative to the corresponding ground states. General considerations relevant to the conversion of light into chemical energy are presented and progress in the use of polypyridine complexes to effect the light induced decomposition of water into hydrogen and oxygen is reviewed.

A multichannel quantum defect approach to dissociative recombination

Abstract: The molecular multichannel quantum defect theory, initially developed to describe the escape of an electron from the molecular core region, is extended to include dissociation of the molecule into two atoms. The mixing between the two types of channels results from electronic interaction between a Rydberg series and dissociative valence states of the neutral molecule. This interaction is introduced as an additional phaseshift into the continuum parts of the wavefunctions. The resulting unified treatment of (pre)ionisation and (pre)dissociation is applied to the process of dissociative recombination, following the approach of C.M. Lee (1973). The 'indirect' process proceeding via electron capture in vibrationally excited Rydberg states is taken into account fully by the inclusion of closed electron-ion channels in the calculation. Model calculations are presented and the influence of various physical parameters on the dissociative recombination profile is tested.

Luminescence of porphyrins and metalloporphyrins. Part 1.—Zinc(II), nickel(II) and manganese(II) porphyrins

Abstract: Metal-free and ZnTPP complexes exhibit both fluorescence and low temperature phosphorescence, MnIITPP shows only low temperature phosphorescence while NiIITPP is non-luminescent. The absorption spectra show that there is little interaction between the metal and the porphyrin π-system so that the excited states can be regarded as (ππ*) in nature. Where phosphorescence occurs, the Stokes shift is 3900 ± 100 cm–1 and decreases with increased metal–porphyrin interaction. The rate constants for intersystem crossing and for non-radiative decay from the lowest (ππ*) triplet state depend upon spin–orbital coupling properties. ZnII and NiI exhibit fairly weak spin–orbital coupling but the effectiveness of MnII is enhanced by the paramagnetic character and by favourable geometry. NiIITPP possesses a low energy (dd) excited singlet state which facilitates rapid internal conversion within the singlet manifold.

Photofragmentation infrared emission studies of vibrationally excited free radicals CH3 and CH2I

Abstract: Time and wavelength‐resolved infrared fluorescence techniques are used to study the photofragmentation dynamics of CH2I2 and CH3I at the excimer laser wavelengths of 248 and 308 nm. Emission is detected from vibrationally excited CH2I and CH3 radicals as well as from the excited iodine atoms [I*(2P1/2→2P3/2)] produced in the photolysis. A complete infrared fluorescence spectrum of the highly excited CH2I radical is obtained as a function of time after the 248 nm dissociating laser pulse, providing both spectroscopic and vibrational deactivation data for the radical. Significant CH2I emission is observed at all wavelengths, indicating that excitation occurs into a very high density of states, nearing the vibrational quasicontinuum. Stronger emission features are observed in the region of the C–H stretching vibrations, the CH2 bending motion, and a combination band of these two modes. Deactivation rates for various spectral features of the highly excited CH2I radical with CH2I2 and argon are presented, alon...

Molecular multiphoton ionization spectroscopy

Abstract: Multiphoton ionization (MPI) spectroscopy is rapidly evolving into a valuable adjunct to VUV spectroscopy for the study of the highly excited states of molecules. The technique is described, and several examples are given of its applications. Studies of the 3pz Rydberg states of the methylbenzenes and of the 1B2u state of benzene show that state assignments can be made and new multiphoton structure seen that is not available in one-photon spectra. MPI of benzene in a supersonic jet demonstrates dramatic resolution improvement and shows that natural linewidth information can be gained for large molecules. Collisional effects in MPI spectra are briefly discussed with reference to Xe and to laser enhanced collisional broadening of nitric oxide in the supersonic beam.

Vibrational energy distribution of the CH3 radical photodissociated from CH3I

Abstract: The n→σ* transition in CH3I at 257 nm produces mainly excited I(2P1/2) atoms and the available energy is mainly released as translation. A recent experiment by Y. Lee and co‐workers measured the distribution of the methyl radicals over the bending umbrella vibration, the only mode excited. This dissociation process is attractively simple because it can be expressed in terms of two linear coordinates rC–I, the distance between the carbon and iodine nuclei and rC–H3, the distance between the carbon nucleus and the plane of the three protons. The potential surface chosen for the excited surface dissociating to I(2P1/2) atoms was (atomic units) An essentially exact close coupling calculation using this potential energy together with an appropriate ground state potential fitted the absorption curve very well and the vibrational distribution approximately. Both the experimental and theoretical distributions for λ=266 nm peak at v=2. Calculations for other wavelengths show that the umbrella mode distribution wil...

Calculation of rotational–vibrational preionization in H2 by multichannel quantum defect theory

Abstract: Multichannel quantum defect theory is adapted to treat simultaneous rotational and vibrational preionization in H2. The strongly preionized spectrum between the N+=0 and N+=2 rotational thresholds of photoionization of H2X1Σg+(J″=0, v″=0) to produce H2+X2Σg+(N+, v+=0) is computed as example and good agreement is obtained with the photoionization data of Dehmer and Chupka.

Excited stretching vibrations of water: the quantum mechanical picture

Abstract: Quantum mechanical eigenvalues derived from a Morse oscillator basis are reported for stretching vibrational states of H2O with total quantum numbers ν = ν1 + ν3 = 0–8, on the Sorbie-Murrell potential. Spectroscopic absorption intensities for all interstate transitions were calculated in a bond dipole model. The dominant feature of the eigenvalue spectrum is a pattern of local mode doublets, such that linear combinations of the component wavefunctions for doublets with splittings less than 10 cm-1 are localized in the bond directions. The timescales for intramolecular energy transfer derived from these splittings increase along the most strongly localized progression from 0·5 ps at ν = 1 to 30 ns at ν = 8. This most localized progression is substantially the most strongly coupled in absorption to the ground state. The strongest transitions in general are subject to a bond excitation rather than a normal coordinate excitation selection rule Δn = ± 1. The relative intensities of particular pairs of transiti...

Optical dephasing and vibronic relaxation in molecular mixed crystals: A picosecond photon echo and optical study of pentacene in naphthalene and p‐terphenyl

Abstract: Picosecond photon echo and spectroscopic measurements have been used to establish that optical dephasing in the pure electronic and several vibronic transitions of pentacene in naphthalene is induced by pseudolocal phonon scattering in the ground and excited state. The frequency and low‐temperature lifetime of this local phonon are 18 cm−1 and 3.5 ps in the groundstate and 13.8 cm−1 and 11 ps in the excited state (vibron independent). The deuterium isotope effect on the local phonon frequency demonstrates its molecular character and suggests its assignment as an in‐plane librational mode. For the system pentacene in p‐terphenyl photon echo measurements indicate the existence of a librational mode of ?30 cm−1 with a ?1.5 ps lifetime. Line shift measurements on both mixed crystal systems further show that the observed temperature induced shift is due to a difference in quadratic electron–bandphonon coupling of the ground and excited state and crystal anharmonicity (thermal expansion). We further report for ...

Intramolecular vibrational energy redistribution and the time evolution of molecular fluorescence

Abstract: We note the presence of contradictory estimates of intramolecular vibrational relaxation rates in the literature where large molecules in high energy states, corresponding to huge densities of vibrational levels, have been ascribed relaxation rates orders of magnitude smaller than those assigned to smaller molecules with much smaller densities of vibrational levels. This unphysical disparity is explained as arising from vague (or undefined) definitions of intramolecular vibrational relaxation and/or from a consideration of quantities which are not directly measured or measurable. A resolution of a portion of the problem is already well known for electronic relaxation, but the application of those results to a description of the time evolution of the molecular fluorescence, produced during the intramolecular vibrational relaxation of the electronically excited molecules, requires a generalization of the electronic relaxation theory to separate and describe the ’’unrelaxed’’ and ’’relaxed’’ emission spectra. We provide this general theory of the time variation of the emission spectrum for molecules conforming to both the intermediate and statistical limits of intramolecular vibrational relaxation with emphasis placed upon the distinguishability between these two cases. The intermediate case analysis utilizes egalitarian and random coupling type models with essentially identical conclusions from both. The time evolution and relative yields associated with the emission spectra are described for both continuous and short pulse excitation, and reasons are provided for the absence of observation of time varying emission spectra in the experiments of Smalley and co‐workers. Quantum beats are possible in principle in the sparse intermediate case. Their observability depends, however, on the detection method. When the emission spectrum can be resolved, beats are expected only in the frequency integrated intensity.

Vibrational predissociation in hydrogen bonded complexes

Abstract: A vibrationally excited hydrogen bonded complex, A–H*⋅⋅B, is metastable since the vibrational energy of the A–H* chemical bond exceeds that needed to sever the hydrogen bond. After a time the excited complex spontaneously breaks up and fragments A–H and B are produced. We show that this vibrational predissociation process is inefficient when there is unfavorable Franck–Condon overlap between the vibrational wave functions which characterize the bound complex A–H*⋅⋅⋅B and the wave functions which describe the fragments A–H+B. We also show that the vibrational predissociation process is most efficient when the fragments are produced in rotationally or vibrationally excited states. Analytical expressions are presented which allow numbers for the lifetime of A–H*⋅⋅⋅B to be easily obtained with a hand electronic calculator. These lifetimes are exceedingly sensitive to the parameters which characterize the multidimensional potential surface of the hydrogen bonded complex. Since the surface is poorly understood, the resulting lifetimes have only a qualitative significance. Sample calculations of vibrational predissociation of HF*⋅⋅⋅HF and its isotopes together with figures of potential surfaces, wave functions, and coupling terms serve to illustrate the ideas of this paper.

Vibrational relaxation in jet‐cooled alkyl benzenes. II. Fluorescence spectra

Abstract: Dispersed fluorescence spectra of a series of laser‐excited n‐alkylbenzenes cooled in a supersonic free jet have been obtained under effectively collision‐free conditions. The series includes all members of the n‐alkylbenzene family from methyl‐ through n‐hexylbenzene. Pulsed laser excitation was cleanly made into the 000, 6b01, 1210, and 18a10 vibronic bands of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. Resonance fluorescence from the O00 excitation experiments permitted clear assignment of the six ring modes (6a, 6b, 1, 12, 18a, 9a) which in combinations and short progressions dominate the fluorescence spectra of these molecules. Except for 6a and 1, the frequency of these ground state vibrations is found to be constant throughout the alkylbenzene series to better than 1 part in 100. Fluorescence from the 6b10, 1210, and 18a10 excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) oc...

Time Correlations between the Two Sidebands of the Resonance Fluorescence Triplet

Abstract: A new type of time correlation analysis of resonance fluorescence is presented. A strontium atomic beam is excited by a 28-\AA{}A-off-resonance laser. The photons of the two sidebands of the fluorescence triplet are shown to be emitted in a well-defined time order. A simple interpretation of this effect is given which implies a quantum jump of the atom from the lower to the upper state through a multiphoton process.

Dynamics of the primary events in bacterial photosynthesis

Abstract: This paper considers the primary light-induced electron-transfer (ET) processes in the reaction center of photosynthetic bacteria which involved ET from the electronically excited state of the bacteriochlorophyll a dimer (BChL)/sub 2/ to bacteriopheophytin (BPh) ((I.1)) and ET from BPh/sup -/ to ubiquinone (Q) ((I.2)). Ultrafast reactions I.1 and I.2, which are practically temperature independent over the range 4 to 300 K, cannot be accounted for in terms of low-temperature nuclear tunneling through a nuclear barrier. Two mechanisms for ultrashort, temperature-independent processes I.1 and I.2 were examined. The rate of the ET reaction (I.2) is considerably longer than characteristic medium-induced vibrational relaxation rates, so that process I.2 has to occur from a thermally equilibrated nuclear configuration of BPh/sup -/Q. Reaction I.2 is assigned to an activationless nonadiabatic ET process, the short lifetimes for this reaction stemming from a large value of the electronic coupling V approximately equal to 4 cm/sup -1/. We propose that the ultrafast reaction (I.1) occurs from a nonequilibrium nuclear configuration of the (BChl)/sub 2/*BPh initially excited state which is located above the crossing point of the nuclear potential surfaces for (BChl)/sub 2/*BPh and for (BChl)/sub 2//sup +/BPh/sup -/. Such a novel ET mechanism involves competition between ETmore » and vibrational relaxation. A theory has been developed to handle this problem and applied to reaction I.1. A microscopic molecular scheme for the primary events of charge separation in bacterial photosynthesis is proposed, which rests on the optimization of the intramolecular distortions of the equilibrium nuclear configurations and the intermolecular spatial organization of the donor and the acceptor. The molecular scheme is successful in accounting for the qualitative and the quantitative features of the primary ET rates.« less