Respective Ground States Research Articles

Ab initio study of geometrically metastable multiprotonated species: MHk+n

The geometries and stabilities relative to fragmentation of H4O2+, H4S2+, H3F2+, H3Cl2+, H2Ne2+, H2Ar2+; H4F3+, H4Cl3+, H3Ne3+, H3Ar3+; H4Ne4+, and H4Ar4+ have been studied using the quadratic configuration interaction (with single and double excitations plus approximate triple excitations included) QCISD(T)/6–311G(2df,2p) method at second-order Mo/ller–Plesset optimized geometries MP2(full)/TZP+ZPE/6–31G**. All of the triply charged H4F3+, H4Cl3+, H3Ne3+, and H3Ar3+ and more highly charged H4Ne4+ and H4Ar4+ ions, as well as doubly charged H2Ne2+ were not found to possess local minima on their zero-point corrected ground state surfaces, although H4Cl3+ is only slightly unstable. Tetrahedral (Td) structures for H4O2+ and H4S2+, planar triangular (D3h) H3F2+, triangular pyramidal (C3v) H3Cl2+ and bent (C2v) H2Ar2+ were found to be local minima on the respective ground state surfaces. The latter five species lie above their respective ion-plus-ion dissociation products by 61, 91, 111, 67, and 116 kcal/mol, and have barriers to dissociation of 38, 20, 12, 34, and 5 kcal/mol (all energies being zero-point corrected). Such multiply charged cations store a great deal of energy, which may be released by the addition of a single extra electron to form the corresponding cation of one less charge.

A quasiclassical trajectory study of OH rotational excitation in OH+CO collisions using a b i n i t i o potential surfaces

We have performed large basis set configuration interaction calculations to characterize the two potential surfaces (2A′ and 2A″ ) which correlate to the ground state of OH+CO. Only planar geometries of the four atoms are considered, and the calculations restrict the OH and CO bond distances to their isolated diatomic values. Global representations of these potential surfaces have been developed and used in quasiclassical trajectory studies of rotational excitation in low energy (1–6 kcal/mol) collisions of OH and CO in their respective rovibrational ground states. We find that the collisional excitation cross sections are about equal for the two surfaces, and there is a monotonic increase in each cross section with translational energy. For OH rotational quantum numbers N between 2 and 6 there is approximately a factor of 2–3 decrease in the cross section for each unit increase in N. The energy and N dependence of these cross sections are generally in excellent agreement with recent experiments. We have also explored the sensitivity of these cross sections to the nature of the potential energy surface, and we have used a surface that describes the formation of the intermediate complex HOCO to determine sensitivity of the rotationally inelastic cross sections to complex formation. In agreement with the experiments, we find that the low energy, high N cross sections are appreciably perturbed by complex formation.

Molecular beam epitaxy growth and characterization of CdxHg1−xTe (0.4<x<1) based quantum wells

CdxHg1−xTe epilayers and CdxHg1−xTe/CdyHg1−yTe quantum well heterostructures have been grown by molecular beam epitaxy. The various parameters of the growth technique have been carefully optimized: control of the thickness by the observation of the oscillations of the reflection-high-energy electron diffraction, the alloy composition by measuring the Cd sticking coefficient in our growth procedure, and the quality of the interfaces by performing transmission electron microscopy. The photoluminescence spectra at 5 K of thick CdxHg1−xTe epilayers are dominated by bound exciton recombination below the fundamental band gap Eg. By contrast, the main emission in the quantum wells photoluminescence spectra is due to an intrinsic recombination of the confined carriers in their respective ground state. The influence of CdxHg1−xTe well thickness and the CdyHg1−yTe barrier composition of this quantum well transition energy are presented.

Contribution to the systematics of r0-derived molecular structure determinations from rotational parameters

This work investigates and enumerates the types of molecular structure that can, in principle, be obtained when moments of inertia, planar moments, or rotational constants or different forms of their isotopic differences are least-squares fitted to the respective experimental ground state values. Ther0-structure, ther0-derived “pseudo-Kraitchman” structuresrΔI andrΔB, and an inequality relation between them are discussed. A least-squares treatment that tries to determine not only the set of structural parameters but also constant, i.e., isotope-independent, rovibrational contributions to the moments of inertia or rotational constants is possible and merits preference. It is shown that the resulting structures are, however, identical with the formerrΔ⌈ orrΔB-structures, respectively.

Tip model of cold fission

Cold fission is defined to be the limiting case of nuclear fission where virtually all of the available energy is converted into the total kinetic energy of the fragments. The fragments have, therefore, to be born in or at least close to their respective ground states. Starting from the viewpoint that cold fission corresponds to most compact scission configurations, energy constraints have been exploited to calculate minimum tip distances between the two nascent fragments in binary fission. Crucial input parameters to this tip model of cold fission are the ground-state deformations of fragment nuclei. It is shown that the minimum tip distances being compatible with energy conservation vary strongly with both the mass and charge fragmentation of the fission prone nucleus. The tip distances refer to nuclei with equivalent sharp surfaces. In keeping with the size of the surface width of leptodermous nuclei, only configurations where the tip distances are smaller than a few fm may be considered as valid scission configurations. From a comparison with experimental data on cold fission this critical tip distance appears to be 3.0 fm for the model parameters chosen. Whenever the model calculation yields tip distances being smaller than the critical value, a necessary condition for attaining cold fission is considered to be fulfilled. It is shown that this criterion allows to understand in fair agreement with experiment which mass fragmentations are susceptible to lead to cold fission.and which fragment-charge divisions are the most favored in each isobaric mass chain. Being based merely on energy arguments, the model cannot aim at predicting fragment yields in cold fission. However, the tip model proposed appears well suited to delineate the phase space where cold fission phenomena may come into sight.

Carrier freezeout in silicon

Carrier freezeout calculations must take into account the occupancy of all bound (ground and excited) impurity states. The calculations can be simplified if a single effective state energy which is a function of impurity concentration and temperature is used. Effective donor and acceptor states at energies E D ∗ and E A ∗ , respectively, are determined for the common shallow impurities (P, As, Sb and B) in Si. These effective states are assumed twofold and fourfold degenerate, respectively, so that the common formulae for partial ionization, which account for only the ground state, can still be used. A simple hydrogenic model is used to predict the dependence of the overlapping of the impurity state wave functions on doping concentration. For low temperatures E D ∗ and E A ∗ converge to their respective ground state energies and are only weakly dependent on doping concentration. With increasing temperature and with decreasing impurity concentration, the separation between the effective impurity states and the respective band edges increases. An empirical analytical expression is developed for the dependence of E D ∗ and E A ∗ on temperature and impurity concentration.

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba2Cu3O7−δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mossbauer spectroscopy using the resonances of155Gd,161Dy,151Eu, and57Eu. In GdBa2Cu3O7−δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa2Cu3O7−δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa2Cu3O7−δ and EuBa2Cu3O7−δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa2 (Cu0.995Fe0.005)3O7−δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.

Study of the stability of the ground states and K-isomeric states of 250Fm and 254102 against spontaneous fission

By employing the 249Cf(4He, 3n) and 208Pb(48Ca, 2n) reactions, experiments to study the stability against spontaneous fission of the nuclides 250Fm and 254102 as well as of the two-quasi-particle (2q-p) K isomers 250mFm (T1/2 = 1.8 ± 0.1 s) and 254m102 (T1/2 = 0.28 ± 0.04s) have been performed. The ground-state spontaneous fission of the two nuclides has been discovered and the corresponding branching ratios bsf and partial half-lives Tsf, respectively, have been determined to be: (6.9 ± 1.0) × 10−5, 0.83 ± 0.15 yr for 250Fm; (1.7 ± 0.5) × 10−3, (3.2 ± 0.9) × 104s for 254102. As a by-product of these studies, new data about cross sections of the 206,208Pb(48Ca, xn) reactions have been obtained. Experiments designed to search for the spontaneous fission decay of the 2q-p K-isomeric states in 250Fm and 254102 have not revealed the effect in question. The lower limits of the ratios of the partial spontaneous fission half-lives for the 2q-p K-isomeric states to those for the respective ground states, Tsf*/Tsf, have been established to be ⩾ 10−1 for 250mFm/250Fm and ⩾ 5 × 10−3 for 254m102/254102. This means that the stability of the 2q-p K-isomeric states in 250Fm and 254102 against spontaneous fission is practically not inferior to that of the ground states of these nuclei. In accord with the experimental findings, the theoretical estimates of Tsf*/Tsf made in the present paper show that, due to the influence of the specialization and blocking effects on the potential energy and the effective mass associated with fission, spontaneous fission from 2q-p K-isomeric states cannot be facilitated but, on the contrary, should be essentially hindered compared with ground-state spontaneous fission.

Combined bond polarization function basis sets for accurate ab initio calculation of the dissociation energies of AHn molecules (A=LI to F)

AbstractAn alternative route toward developing basis sets for post‐Hartree‐Fock calculations, the hybrid bond polarization function method, is investigated. Two new basis sets, denoted 6‐31G(d, p)+ B and 6‐31 + G(d,p)+B, are defined for the first‐row hydrides. The dissociation energies of the first‐row hydride species in their respective ground states are computed using full fourth‐order Møller‐Plesset theory, and compared with results obtained with large polarized basis sets containing no bond functions. It is shown that results are competitive even with basis sets as large as 6‐311++G(3df,3pd), while computation times are reduced by a factor of 4 to 20. On empirical grounds, the basis set superposition error should be neglected entirely.

Ab initio SCF calculations of 3P Mg and Be atom activation of methane

In studying the insertion of metal atoms into CH bonds, ab initio molecular orbital calculations were used to obtain the energies and geometries of the reaction products methylmagnesium hydride (CH 3MgH) and methylberyllium hydride (CH 3BeH). The two species were successfully geometry-optimized in their respective ground states and in their lowest-lying excited triplet states: 7 a 1 13e 1 for the Mg-system (3-21G) and 5 a 1 12 e 1 for the Be-species (3-21G, 6-31G*). In both systems it was revealed that the excited states behaved like two weakly interacting species, the metal hydride and methyl radical moieties. This was in contrast to the ground state molecules which exhibited much shorter metal—carbon bonds. Investigations of possible reaction pathways involving different approaches of the 3 P Mg and Be atoms with respect to a methane molecule were carried out. No transition states or meta-stable species were found for any one of four specific approaches examined at the 3-21G level of sophistication but energy minima were obtained for each geometry in the context of restricted symmetry.

State-to-state collisional excitation of NH 3 by He and H 2 studied in a laser crossed molecular beam experiment

The astrophysically important inelastic collisions of ammonia with He and H 2, are studied in a crossed beam experiment. State-to-state cross sections for the excitation of both ortho and para NH 3 starting from their respective ground states 0 0 and 1 1 are presented. In the experiment a pulsed nozzle beam of NH 3, seeded in Ar, is crossed with a secondary beam of He or H 2. Both the initial cold beam and the collisionally excited NH 3 are detected with complete state selectivity by REMPI (resonance-enhanced multiphoton ionisation) via the B̃ state of ammonia. Initial state preparation is achieved by rotational cooling that yields a primary NH 3 beam that consists almost exclusively of the lowest quantum states of ortho and para NH 3. There are considerable differences between the experimental results and recent theoretical calculations on the collisional excitation of NH 3 by He and H 2, which demonstrates that direct state-to-state experiments are essential for the description of such complicated collision systems.

Detection of nucleon correlations via pion double-charge-exchange reactions.

Double-charge-exchange reactions induced by pions furnish a direct means of detecting nucleon-nucleon correlations in nuclei. For the particular case of double isobaric analog transitions the effect of correlations is easily isolated and quite strong. We show that the magnitudes of the double-charge-exchange cross sections for the isotopes $^{42}\mathrm{Ca}$, $^{44}\mathrm{Ca}$, and $^{43}\mathrm{Ca}$, recently measured at low energies, can be explained well by our taking account of the position correlations of the valence neutrons in the respective ground states.

Electric dipole collectivity in light nuclear systems.

\ensuremath{\alpha}${\mathrm{\ensuremath{-}}}^{18}$O and \ensuremath{\alpha}-Ca cluster configurations of $^{22}\mathrm{Ne}$ and the $^{46}$,48,52Ti isotopes, respectively, have been studied in a microscopically founded model, treating antisymmetrization between the clusters correctly. Depending on the respective ground state structure, these systems show rotational bands with weak (Ti isotopes) and strong ${(}^{22}$Ne) parity splitting. Irrespective of the degree of parity splitting, both cases exhibit E1 transitions between these cluster states, which are strong on the appropriate molecular scale. Though the influence of inelastic channels is discussed in qualitative terms only, both $^{22}\mathrm{Ne}$ and the Ti isotopes appear to be promising candidates for the molecular-dipole degree of freedom to be seen in light nuclei.

Magnetic moments of Gd nuclei

Low temperature nuclear orientation experiments down to 2 mK on147GdFe,149GdFe,149GdGd and153GdGd have yielded the magnetic hyperfine interaction strength. Bhf as 31.0(1.6) μNT, 28.3(2.0) μNT, 33.8(4.7) μNT and 13.3(2.1) μNT respectively. From these values the respective ground state magnetic moments ¦μ¦ of147Gd,149Gd and159Gd were deduced as 1.12(20) μN, 1.01(16) μN and 0.40(8) μN.

Rotational spectra of mono-18O-substituted ozones in the ν2 excited vibrational state

Abstract Millimeter wave rotational spectra of the mono-18O-substituted ozones, 16O18O16O, 18O16O16O, were studied for the ν2 excited vibrational state at selected frequencies between 59 and 367 GHz. The spectra were fitted using both Watson A and S reductions to parameter sets which were complete through the sextic terms and included two octic terms. The parameters were compared with those of the respective ground states fitted in the same way. The changes in the inertial defects with vibrational state agree very well with those calculated from the quadratic force constants of the normal species.

CEPA calculations of potential energy surfaces for open-shell systems.

Abstract Quantum-chemical ab initio calculations have been performed for the van der Waals interaction between helium and oxygen atoms in their respective ground states: He( 1 S)+ O( 3 P). As long as fine-structure effects are neglected, there are two low-lying electronic states, 3 Σ − and 3 Π resulting from the degeneracy of the O( 3 P) ground state. Both states are purely repulsive at the SCF level, after inclusion of electronic correlation by the CEPA method they exhibit shallow van der Waals (dispersion) minima at large interatomic separation: R ϵ = 3.61 A, ϵ = 1.0 meV ( 3 Σ − ) and R ϵ = 3.05 A, ϵ = 2.3 meV ( 3 Π). The analysis of the results shows the very slow convergence of the dispersion interaction with increasing basis size, while SCF repulsion and the repulsion due to the change of the intra-atomic correlation are obtained reasonably accurately with moderate basis stes. Van der Waals coefficients C 6 , C 8 , C 10 , potential curves of the type HFD (i.e. Hartree-Fock plus damped dispersion) and the influence of fine-structure effects (mainly spin-orbit coupling) on the shape of the adiabatic potential curves are discussed as well.

Scattering of metastable rare gas atoms from solid surfaces

This work reports on measurements of the survival probabilities (SP) and angular distributions of metastable He atoms backscattered from clean and adsorbate covered metal surfaces, as well as on data for He ∗, Ne ∗ and Ar ∗ reflected from LiF and NaCl surfaces. The SP for He ∗ are always below ∼ 10 −3, they increase for the clean metal surfaces in the order Cu<Pd<W, as well as upon adsorption of CO or oxidation. Equally small values (∼10−4) were found for He ∗ and Ar ∗ reflected from the ionic crystals, while a considerably higher value (∼ 10 −2) was determined for Ne ∗ in this case. The angular distributions vary considerably from those of the respective ground state atoms which has to be attributed to the marked change of the interaction potentials.

Excitation processes in an r.f.-boosted, pulsed hollow cathode lamp

Time and space resolved emission spectroscopy was used to examine the excitation processes in an r.f.-boosted, pulsed hollow cathode lamp. Sealed commercial hollow cathode lamps with copper cathodes and neon or argon buffer gases were driven with temporally spaced unidirectional current pulses and radio frequency bursts. The neon filled lamp was studied most extensively; the argon lamp was used for comparison. Three excitation periods were considered: the current pulse, the r.f. burst, and the afterglow. Each of these periods was marked by a unique set of emission characteristics that suggested different combinations of excitation processes. Excitation during the current pulse appears to be a combination electron impact and charge exchange. Charge exchange excites the 3 d 95 s levels of Cull while electron impact excites the other ion levels and the neutral levels. Charge exchange continues to excite the ion spectrum during the afterglow. Afterglow emission from the neutral spectrum apparently results from two types of recombination processes. The exact recombination mechanisms could not be unambiguously assigned. Emission results from the r.f. burst suggest that electron impact is the dominant excitation process. Excitation of the neutral and ion spectra are from their respective ground states. There appears to be little ionization by electron impact. To evaluate the lamp's suitability as a line source for analytical atomic spectroscopy the profile of the 324.8 nm neutral resonance line was examined during the current pulse and the r.f. burst. The profile during the current pulse is strongly dependent on radial position in the cathode bore. Near the edges of the cathode the line is severely self-reversed. The profile during the r.f. burst is uniform across the cathode bore and is affected only slightly by self-absorption.

Selective excitations in actinide nuclei via alpha pickup

The $\ensuremath{\alpha}$-cluster pickup reaction ($d,^{6}\mathrm{Li}$) has been studied at ${E}_{d}=55$ MeV on targets of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$. Members of the ground-state rotational bands in $^{228}\mathrm{Ra}$ and $^{234}\mathrm{Th}$ are excited and absolute reduced $\ensuremath{\alpha}$ widths obtained from finite-range distorted-wave analysis are in good agreement with values deduced from $\ensuremath{\alpha}$ decay. In addition three excited groups of states are very strongly populated in both nuclei with spectroscopic strength per group comparable with those of the respective ground state bands. These groups are apparently excited rotational bands with band heads at ${E}_{x}=700\ifmmode\pm\else\textpm\fi{}40, 1070\ifmmode\pm\else\textpm\fi{}60, \mathrm{and} 1390\ifmmode\pm\else\textpm\fi{}60$ keV in $^{228}\mathrm{Ra}$ and ${E}_{x}=810\ifmmode\pm\else\textpm\fi{}30, 1150\ifmmode\pm\else\textpm\fi{}40, \mathrm{and} 1470\ifmmode\pm\else\textpm\fi{}40$ keV in $^{234}\mathrm{Th}$. The selective and strong excitation in this particular multi-nucleon transfer reaction of several excited bands is not predicted by existing theoretical models. An attempt has been made to describe the systematics of excited ${0}^{+}$ states in the actinide region with the interacting boson model. Excitation energies are reasonably well described but intruder states are present and transfer strengths are not reproduced properly. The observation of strong $\ensuremath{\alpha}$-cluster pickup to excited rotational bands suggests coherent contributions from both neutron and proton pair excitations which can lead to strong four-body correlations and/or to new types of collective excitations which favor quartet structure. It is found that about 25% of the nuclear charge (matter) at $r\ensuremath{\simeq}10.6$ fm must be associated with $\ensuremath{\alpha}$ particles. This high $\ensuremath{\alpha}$-clustering probability indicates $\ensuremath{\alpha}$-particle condensation in low-density nuclear matter.NUCLEAR REACTIONS $^{232}\mathrm{Th}$, $^{238}\mathrm{U}(d,^{6}\mathrm{Li})$, $E=54.8$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$; DWBA analysis; $^{228}\mathrm{Ra}$, $^{234}\mathrm{Th}$ deduced levels, ${S}_{\ensuremath{\alpha}}$, $\ensuremath{\gamma}_{\ensuremath{\alpha}}^{}{}_{}{}^{2}$ (10.5 fm), $\ensuremath{\alpha}$-clustering probabilities.

Tests of orbit–lattice interaction models using spin–lattice relaxation data of Er3+, Dy3+, and Yb3+ in Cs2NaYCl6

Measurements of the electron spin–lattice relaxation rate are reported for Er3+, Dy3+, and Yb3+ in the cubic host Cs2NaYCl6. Expressions for the anisotropic direct relaxation rates of the respective Γ8 and Γ6 ground states are given in terms of the eleven coupling parameters of the dynamic orbit–lattice interaction for octahedrally coordinated rare earths. Experimental direct process relaxation data are used to test the predictions of three models: (1) the electrostatic model of Buisson and Borg, (2) a nearest neighbor, effective point charge model, and (3) the nearest neighbor superposition model of Newman and co-workers. None of these proves satisfactory in providing a self-consistent explanation of the data from all three ions.