1s Ground State Research Articles

Self-consistent solution of Dyson’s equation up to second order for atomic systems

In this paper, the single-particle Green’s function approach is applied to the atomic many-body problem. We present the self-consistent solution of the Dyson equation up to second order in the self-energy for nonrelativistic spin-compensated atoms. This Dyson second-order scheme requires the solution of the Hartree–Fock integro-differential equations as a preliminary step, which is performed in coordinate space (i.e., without an expansion in a basis set). To cope with the huge amount of poles generated in the iterative approach to tackle Dyson’s equation in second order, the BAGEL (BAsis GEnerated by Lanczos) algorithm is employed. The self-consistent scheme is tested on the atomic systems He, Be, Ne, Mg, and Ar with spin-saturated ground state S01. Predictions of the total binding energy, ionization energy, and single-particle levels are compared with those of other computational schemes [density functional theory, Hartree–Fock (HF), post-HF, and configuration interaction] and with experiment. The correlations included in the Dyson second-order algorithm produce a shift of the Hartree–Fock single-particle energies that allow for a close agreement with experiment.

Atomic diamagnetism within a dense plasma.

In this paper we have studied the influence of plasma electron polarization around a charged atomic impurity on the diamagnetic response behavior of the impurity ion, when subject to large-scale magnetic fields within high-density plasmas. As a typical example, we consider the two-electron ion C4+ (Z=6) in its ground state 1s(2):(1)S. Calculation performed within the Hartree-Fock approximation under the framework of the ion sphere model for the plasma-embedded impurity ion suggests that, in a high-density regime, the diamagnetic shift of the ground state is a bivariate function of the magnetic field and the plasma electron density. Also, it is shown that the magnitude of the diamagnetic susceptibility of the impurity ion increases with increasing plasma electron density, implying that the ion becomes more diamagnetic as a direct consequence of the increased orbital radii of its bound charges under enhanced density-induced screening.

Vibrational relaxation of highly excited NCNO in collisions with He, Ar and N2

The relaxation of vibrationally highly excited NCNO has been studied using the technique of time-resolved infrared fluorescence (TR-IRF). A pulsed dye laser operating at 600 nm, or in some experiments at 640 nm, excited NCNO to its S1 state whence radiationless transitions transfer the molecules to high vibrational levels of the S0 ground state below the dissociation limit. IRF was observed at ca. 4.6 μm, corresponding to fundamentals in the CN stretching vibration of NCNO. Values of ΔE , the mean energy removed per collision with the diluent gas (Ar, N2 or He) were derived as a function of E , the mean internal energy content of the excited molecules. The variation of ΔE with E was approximately linear and, at E = 10000 cm−1, ΔE = 0.44 ± 0.03 cm−1 for Ar, ΔE = 0.62 ± 0.04 cm−1 for N2 and ΔE = 0.57 ± 0.04 cm−1 for He.

Laser-induced continuum structures and double poles ofthe S-matrix

Double poles of the S-matrix and avoided level crossings arerelated to branch points in the complex plane. Although thenumber of these branch points is very small, they determinethe dynamics of the system in a decisive manner. We study, inthe adiabatic regime, the effects induced by two strong laserfields in the continuum of the H atom. In the photoabsorptionspectra of the probe field from the ground 1s state, thecoupling to the 2s, 5s, 5d and 5g states is considered. Thequasi-level avoided crossing in the laser-induced continuumstructures is traced theoretically as a function of theintensities of the strong laser fields. Under certainconditions, the quasi-levels cross and the S-matrix has adouble pole. The relation of the laser-induced continuumstructures to branch points in the complex plane is discussed.

S- and P-wave meson spectroscopy in the nonrelativistic quark model

The mass spectrum of the ground state 1S 0 , 3S 1 and orbitally excited 3P 0 , 1P 1 , 3P 1 , 3P 2 mesons is considered in a nonrelativistic constituent quark model. A good description of the mass spectrum of these mesons is obtained with the same constituent quark masses m u = m d =305±2 and m s =483±3 MeV/ c 2. It is suggested that the nonrelativistic quark model describes better the masses of “bare” q q ̄ states, such as found in the K-matrix analysis, than the measured ones, at least for such broad states as the K ∗ 0(1430) . The a 0(980) is a quarkonium in this scheme and belongs to the lightest scalar nonet together with the K ∗ 0(1430) , with the “bare” q q ̄ mass 1191±8 MeV/ c 2, f 0(980) and f 0(1370). The extracted value of the K 1( 3P 1)−K 1( 1P 1) mixing angle ( θ K ) is (31±4)°. A constant in the mass formulae for P-wave mesons might be interpreted as the effective gluon “mass”.

Quantum model simulation of complete S→S1 population transfer by means of intense laser pulses with opposite chirp

The effects of short (0.5–2 ps) chirped laser pulses on the vibrational population transfer from the electronic ground state S0 to the excited state S1 are investigated via numerical simulations of the wave packet dynamics. It is demonstrated for a model system that both positively and negatively chirped laser pulses with high intensities can achieve almost complete population transfer to the vibrational ground or low excited states in the electronically excited state. The underlying mechanisms of the transfer are, however, different, i.e., suppression and compensation of intrapulse pump–dump processes for the negatively and positively chirped pulses, respectively. The process induced by the negatively chirped laser pulse is applied to design complete S0→S1 population transfer to the lowest vibrational states of the first electronically excited state of 9-(carbazolyl)-anthracene (C9A) which could not be observed by means of traditional, i.e., continuous wave or ns spectroscopy.

Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters

X − ⋅(H 2 O) n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies.

Photoluminescence of concentration series of CaF2: Mn phosphors excited by VUV radiation

Fluorescent characteristics of a series of powder CaF2: Mn phosphors (from 0.01 to 2.47 wt. % of Mn in the mixture) excited by VUV radiation with quantum energies up to 14 eV at 293 K and up to 12 eV at 85 K are measured. Narrow excitation bands of Mn2+ centers found at 7.9 and 8.6 eV (at 293 K) are assigned to partially forbidden transitions of electrons from the ground state 6S split by the crystalline field (10 Dq=0.71 eV from the literature) in two sublevels to the excited level corresponding to the 6D term of a free Mn2+ ion (3d5 → 3d44s transitions). A wide nonelementary excitation band in the region of 9.1–10.3 eV is interpreted as photogeneration of near-activator D-excitations: allowed transitions of electrons from levels that are split from the top of the valence band under the influence of an impurity ion to the free 4s-orbital of a Mn2+ ion. Channels of energy transport in the CaF2: Mn system are briefly analyzed.

Resonant multiphoton ionization of the 1s state of a hydrogen atom in a strong laser field

The process of resonant multiphoton ionization of a hydrogen atom in the ground 1s state is studied by direct numerical integration of the nonstationary Schrodinger equation for a quantum system in an electromagnetic field. The dependence of photoionization probability on the radiation intensity is found to be nonmonotonic. It is established that the minima of ionization correspond to multiphoton resonances between the ground state and one of the excited (Rydberg) atomic states perturbed by the laser field. It is shown that ionization is suppressed due to rearrangement of Rydberg states in a strong electromagnetic field and is accompanied by efficient Raman Λ transitions, which connect a set of closely lying Rydberg states via the continuum.

Ab initio calculations, IR, Raman and UV supersonic jet studies of vibrations in 2,6-dicyano-3,5-dimethylaniline

The infrared and Raman spectra for 2,6-dicyano-3,5-dimethylaniline (DCDMA(NHxD2−x), x=2,1,0) in a solid state and the laser induced fluorescence (LIF) excitation spectra of vacuum isolated jet cooled molecules have been investigated. The ground state S0 and the S1 excited state geometries of DCDMA(NHxD2−x) molecules as well as vibrational frequencies have been evaluated by ab initio calculation using GAUSSIAN94 programs. The DCDMA nonplanar in its ground state S0 (RHF/6-31G*) assumes planar geometry, C2v, in the S1 state (CIS/6-31G*). The correlation between the normal coordinates in different species and in different electronic states has been performed using Dushinsky matrices. The electronic origins were found at 29861cm−1 for DCDMA(NH2), 29852cm−1 for DCDMA(NHD) and 29843cm−1 for DCDMA(ND2). The fundamental frequencies derived from the IR, Raman and LIF excitation spectra have been assigned using the results of ab initio calculations. The scaling factors were found to be 0.890 and 0.846 for the ground state and the singlet excited state frequencies, respectively. The dominant feature of the LIF excitation spectra of DCDMA(NHxD2−x), x=0,1,2, is the presence of a progression in the low frequency mode 5 (112cm−1) involving a symmetrical in-plane motion of the cyano group forming the hydrogen bonds with the amino group.

Microscopic solvent structure of subcritical and supercritical methanol from ultraviolet/visible absorption and fluorescence spectroscopies

Ultraviolet/visible absorption and fluorescence spectroscopies at different temperatures and pressures were applied to investigate the microscopic solvent structures of subcritical and supercritical methanol using 4-nitroanisole, ethyl-(4-dimethylamino)benzoate, Reichardt’s dye, and anthracene as the probe molecules. It was found that at temperatures higher than 150 °C the long winding chains of sequentially hydrogen-bonded methanol molecules were probably broken, but the small hydrogen-bonded aggregates possibly existed in methanol even at higher temperature. It was also found that the solvation process of the anthracene molecule in the S0-ground state obeyed the Langmuir adsorption model. However, in the case of fluorescence measurements in supercritical methanol, we detected deviations from the simple Langmuir adsorption model. These deviations were explained in terms of preferential solvation of the solvent molecules around photoexcited anthracene. Judging from the experimental results, it was concluded that the local density augmentation of the supercritical methanol around the nonpolar solute was a short-ranged effect, which did not correspond directly to the large isothermal compressibility of fluid near the critical point.

Total and partial recombination cross sections forF6+

Total and partial recombination cross sections for F{sup 6+} are calculated using close-coupling and distorted-wave theory. For total cross sections, close-coupling and distorted-wave results, which include interference between the radiative and dielectronic pathways, are found to be in good agreement with distorted-wave results based on a sum of independent processes. Total cross sections near zero energy are dominated by contributions from low-energy dielectronic recombination resonances. For partial cross sections, the close-coupling and distorted-wave theories predict strong interference for recombination into the final recombined ground state 1s{sup 2}2s{sup 2}{sup 1}S{sub 0} of F{sup 5+}, but only weak interference for recombination into the levels of the 1s{sup 2}2s2p configuration. {copyright} {ital 1999} {ital The American Physical Society}

Internal Conversion in 1-Aminonaphthalenes. Influence of Amino Twist Angle

With the 1-aminonaphthalenes 1N5 and 1DMAN a fast radiationless process occurs in n-hexane, diethyl ether, and acetonitrile, which is shown to be internal conversion (IC). The IC reaction is slower with 1N4 and much less efficient with 1MAN and 1AN. This IC process is thermally activated and slows down with increasing solvent polarity, due to a larger IC activation energy. In the ground state S0, the amino twist angle θ relative to the naphthalene plane increases in the order 1MAN, 1AN, 1N4, 1N5, 1DMAN, as derived from absorption and fluorescence spectra, 1H NMR spectra, ground-state dipole moments, and ab initio calculations. For the five 1-aminonaphthalenes in the equilibrated S1 state, the twist angle and the radiative rate constant have similar values. The different IC efficiences of these molecules are therefore determined by the structural differences (amino twist angle) between S1 and S0. A correlation is found between the IC efficiency in these molecules and the twist angle θ. The IC process to S0 starts from the equilibrated S1 state, which is vibronically coupled with S2 due to a small energy gap ΔE(S1,S2). It is therefore concluded that the extent of vibronic coupling and the magnitude of the twist angle θ are the determining factors in the IC process.

Electron Scattering from the Ground and Excited States of Barium

We have used the nonrelativistic convergent close-coupling (CCC) method to investigate electron scattering from the ground (6s2)1S state and excited (6s6p)1 Po1 and (6s5d)1,3De2 states of barium. For the scattering from the barium ground state, we have found very good agreement with measurements of (6s6p)1 Po1 apparent cross sections at all energies. Similarly, good agreement is found for differential cross sections for elastic scattering and (6s6p)1 Po and (6s5d)1 De2 excitations and with the (6s6p)1 Po1 state electron{photon angular correlations. For the scattering from excited states of barium we have found good agreement with elastic (6s6p)1 Po1 scattering and the (6s5d)1De2 → (6s6p)1 Po1 transition for both differential cross sections and electron–photon angular correlations.

Nanosecond time-resolved resonance Raman investigation of the radical cation of 4,4′-dibromobiphenyl and the T1→Tn transition of 4,4′-dibromobiphenyl

We have taken nanosecond time-resolved resonance Raman spectra of the T1→Tn transition of 4,4′-dibromobiphenyl in cyclohexane solution and of the radical cation of 4,4′-dibromobiphenyl in acetonitrile solution. The resonance Raman spectrum of T1 4,4′-dibromobiphenyl is consistent with a planar quinoidal structure. The C–Br bond appears stronger in the T1 state than in the S0 ground state and this is in agreement with photochemistry studies done for 4-bromobiphenyl that indicate that the T1 state must be further excited by excimer formation and/or electron transfer with an appropriate donor prior to a debromination reaction.

Destabilization of Atomic Anions: The case of F-and O2-

Destabilization of atomic anions is studied for the ground1S and first excited state 3P of theten-electron system. Using large and diffuse basis sets,configuration-interaction calculations show that for decreasingnuclear charge Z the ground state 1S is absorbedinto the continuous spectrum at the critical parameter valueZc(1S) ≈ 8.74 e0, and thetriplet state at Zc(3P) = 9e0. This implies instability of the di-anion O2-and excludes existence of stable excited bound states ofF-. The numerical results give strong evidence of differentabsorption mechanisms into the continuum for 1S and3P, involving zero-energy threshold bound or virtualstates, respectively. Arguments from the rigorous theory ofSchrödinger operators lead to the expectation that the observedphenomena are manifestations of more universal properties common toall anionic systems.

Vibrational Predissociation of p-Difluorobenzene·Ar Studied by Mass-Analyzed Threshold Ionization Spectroscopy

Mass-analyzed threshold ionization (MATI) spectroscopy has already proven to be a powerful method for the study of the fragmentation energetics and dynamics of molecular clusters in the cationic state. In this study, its application was extended to the investigation of the vibrationally induced predissociation (VP) of van der Waals (vdW) clusters in vibronically excited states (S1). To verify the feasibility of MATI spectroscopy for studies of this type, the p-difluorobenzene·Ar1 (p-DFB·Ar1) complex has been chosen as a model system. The state and mass selectivity of MATI spectroscopy promise to give useful supplementary information about the VP process, which would be difficult or even impossible to obtain by conventional methods such as fluorescence spectroscopy and time-resolved resonant two-photon ionization spectroscopy. (Butz, K. W.; et al. J. Phys. Chem. 1986, 90, 3533. Jacobson, B. A.; et al. J. Chem. Phys. 1988, 89, 5624.) In accordance with the pioneering studies of Parmenter et al., who investigated the predissociation of the vibronically excited p-DFB·Ar1 complex very extensively by means of UV fluorescence spectroscopy, (Butz et al., 1986), the MATI spectra give evidence for the strong mode selectivity of the VP process. However, from the MATI results, evidence is given that additional fragmentation channels appear, which have not been observed in the fluorescence spectra. On the basis of the fragmentation thresholds observed in the MATI spectra, we also deduced upper and lower limits for the dissociation energies of the complex in the S0, S1, and ionic ground state, which differ significantly from those determined by Parmenter et al.

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions.

The benzene–argon complex: A ground and excited state ab initio study

Equilibrium dissociation energies De of the benzene–argon van der Waals complex are calculated in the ground state S0 and in the excited state S1 using integral-direct coupled cluster methods. The results confirm previous investigations of S0, showing that high quality correlation consistent basis sets and connected triple excitations are imperative for a good description of the van der Waals complex. We estimate the CCSD(T) dissociation energy De=389±2 cm−1 for the ground state S0. Using the CCSD linear response approach the frequency shift (redshift) δνe=19 cm−1 is obtained. Accurate spectroscopic structural data and frequency shifts δν0 for the 601 band of the S1←S0 transition are available for most of the benzene–rare gas atom complexes. However, the experimental determination of absolute dissociation energies of these complexes is connected with much larger uncertainties. The theoretical result agrees very well with the experimentally available redshift, showing that integral-direct coupled cluster methods will become an important tool in the study of van der Waals complexes in the future.

Third-Order Nonlinear Optical Properties in the Excited State of Well-Defined Thiophene−Dimethylsilyl Co-oligomers

A new series of oligomers for nonlinear optics (NLO), the thiophene−dimethylsilyl co-oligomers, (TSi)n−T with n = 1−3, have been synthesized in order to combine third-order NLO response and transparency in the visible region. Stationary-state absorption spectra, fluorescence spectra, and time-resolved absorption (picosecond) have been carried out. In sharp contrast with oligothiophenes, the absorption spectra of (TSi)n−T are not red-shifted as n increases, but as for short oligothiophenes their quantum yield of fluorescence is low because of the competitive triplet-state formation and their emission wavelength range is remarkably solvent-independent. These data show slight increase of the dipole moment between the S0 ground state and the S1 excited state, in agreement with theoretical calculations (ZINDO method). Fast intersystem crossing is observed between singlet and triplet states and S1 → Sn and T1 → Tn spectra have been measured. Optical nonlinearity (γ) of the excited state has been measured and co...