The weakly bound states and resonances of the BeHe2 triatomic system

In the present study, we carry out a full search of the bound states and resonances of the He(2)Be triatomic system, with its isotopic variants (3)He(2)(9)Be, (3)He(4)He(9)Be, and (4)He(2)(9)Be using the hyperspherical method. Three-body long-range effects are also included in the computation by adding to the additive potential the Axilrod-Teller triple-dipole term. In addition, the possibility of the occurrence of Efimov-type states in these systems is discussed. We have found a bound state for each of the (3)He(2)(9)Be and (3)He(4)He(9)Be trimers, while one weakly bound excited state is also found to exist for the (4)He(2)(9)Be system.     

Loosely bound states of three particles

Loosely bound states of three particles for particles interacting via short range forces were predicted by Efimov. They are shown to arise naturally in the adiabatic hyperspherical approximation presently used to compute reactions along hyperspherical coordinates. Because of the loosely bound nature of such states, the asymptotic form of the effective hyper-radial potential is critical. A variational wave function is suggested and is shown to reproduce the asymptotic potenial obtained by Efimov. Implications and extensions of the theory are suggested.     

Non-Born-Oppenheimer approximation for very weakly bound states of molecular anions

The influence of nuclear rotation on weak electron binding in the long range field of a linear polar molecule is treated in a way that leads ultimately, with suitable approximation, to the familiar equations for close coupling of electron-nuclear-rotational motions. Subsequently, a conventional pseudopotential approximation is invoked to examine the rotational spectra of HCN and DCN anions. It is shown that the number of rotationally excited anion states cannot be reliably predicted by assuming that zero binding occurs when the rotational energy equals the electron affinity obtained in the Born-Oppenheimer approximation. A method is suggested for combining accurate molecular orbital and parameterized pseudopotential methods to provide accurate electron affinities for very weakly bound anionic states.     

Rotational spectrum, potential energy surface, and bound states of the weakly bound complex He-N2O

Pure rotational transitions of the weakly bound complex He-N(2)O and three minor isotopomers (He-(14)N(15)NO, He-(15)N(14)NO, and He-(15)N(15)NO) were measured in the frequency region from 6 to 20 GHz. Predictions for the microwave transition frequencies were based on the infrared work by Tang and McKellar [J. Chem. Phys. 117, 2586 (2002)]. In the case of (14)N containing isotopomers, nuclear quadrupole hyperfine structure of the rotational transitions was observed and analyzed. The resulting spectroscopic parameters were used to determine geometrical and dynamical information about the complex. An ab initio potential energy surface was calculated at the coupled cluster level of theory with single and double excitations and perturbative inclusion of triple excitations. This surface was constructed using the augmented correlation consistent polarized valence triple zeta basis set for all atoms with the inclusion of bond functions for the van der Waals bond. Bound state calculations were done to determine the energies of low-lying rovibrational levels that are supported by the potential energy surface. The resulting transition energies agree with the experimental values to 1% or better.     

Excited vibrational states near dissociation in weakly bound triatomic systems

The vibrational motion of a weakly bound M₃ complex is investigated using hyperspherical coordinates and an adiabatic separation of the radial and angular motions within the molecule. It is shown that the vibrational energy levels near dissociation correlate to an M atom orbiting about a rotating M₂ core. Application is made to the neon trimer assuming an approximate pairwise additive potential.     

Anharmonicity of weakly bound M(+)-H2 complexes

The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M(+)-H(2)) complexes. The H-H stretching frequency shifts of M(+)-H(2) (M(+) = Li(+), Na(+), B(+), and Al(+)) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H-H stretching frequency of Li(+)-H(2), B(+)-H(2), Na(+)-H(2), and Al(+)-H(2) is red-shifted by 121, 202, 74, and 62 cm(-1), respectively, relative to that of unbound H(2). The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).     

Fusion of weakly bound nuclei

We discuss the effect of the break-up of weakly bound nuclei on the fusion cross section of these projectiles with light, medium and heavy mass targets, at energies close to the Coulomb barrier. The discussion is based mostly on data obtained by our group in collaborative experiments. We also show that the break-up process at near and sub-barrier energies is responsible for the vanishing of the usual threshold anomaly of the optical potential.     

Adiabatically bound valence anions of guanine

It is believed that guanine, a basic component of DNA and RNA, has the smallest affinity to an excess electron among all nucleic acid bases. Our experimental and computational findings indicate, however, that many so far neglected tautomers of guanine support adiabatically bound anionic states in the gas phase. The computed values of electron vertical detachment energy for the most stable anionic tautomers are within a broad range of the dominant feature of the photoelectron spectrum. We suggest that guanine might be the strongest excess electron acceptor among nucleic acid bases. Thus it might be critical to radiobiological damage of DNA and it might contribute to those chemical transformations of DNA that proceed through bound anionic states.     

Electron dynamics of image potential states in weakly bound adsorbate layers: A short review

Image potential states on physisorbate-covered metal surfaces are fascinating model systems for the transition from the quasi-free behavior of electrons at bare metal surfaces towards the molecular electronics in more complex organic overlayers. This article discusses the fundamental properties of these systems and the experimental highlights in the study of rare gas and simple molecular overlayers on noble metal surfaces since the early 90s.     

Mixed valence—Rydberg states

A survey of recent calculations involving the mixing of Rydberg and valence states in the spectra of molecular systems is undertaken. It is pointed out that when such states undergo curve crossings with one another, minimal energy splittings of 1.0–2.0 eV can occur at the respective 50-50 composition points. The significance of such large interactions between valence and low-lying Rydberg states is considered in terms of the properties of the resulting mixed CI states, particularly with reference to the oscillator strengths for the pertinent ground state transitions and also the spatial extension of the corresponding upper orbitals. It is thereupon argued that such mixings have important consequences in the spectra of a wide variety of systems, including those of O₂, ethylene and ethane.     

Spectroscopic and theoretical study of the weakly bound H2-HCCCN dimer

Rotational spectra of the H(2)-HCCCN complex were studied using a pulsed-nozzle Fourier transform microwave spectrometer. Complexes containing the main and several minor isotopologues of cyanoacetylene (HCCC(15)N, DCCCN, and various (13)C containing isotopologues) and the two spin isomers of the H(2) molecule (paraH(2) and orthoH(2)) were investigated. Transitions of complexes with (14)N and D containing isotopologues have nuclear quadrupole hyperfine structures, which were measured and analyzed. Transitions of orthoH(2) molecule containing complexes show additional hyperfine structures due to nuclear magnetic proton spin-proton spin coupling of the hydrogen nuclei in the H(2) molecule. For orthoH(2)-HCCCN, both strong a- and weaker b-type transitions were measured and analyzed using a semirigid asymmetric rotor model. For the paraH(2)-HCCCN complex, only a-type transitions could be observed. The dimer complexes are floppy and have near T-shaped structures. Intermolecular interaction potential energy surfaces were calculated for H(2)-HCCCN using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations [CCSD(T)]. Three orientations of the hydrogen molecule within the complex were considered. Equal weighting of the surfaces corresponding to the three hydrogen orientations provided an averaged potential energy surface. Bound-state rotational energy levels supported by the surfaces were determined for the different hydrogen orientations, as well as for the averaged surface. Simple scaling of the surfaces improved the agreement with the experimental results and produced surfaces with near spectroscopic accuracy.     

A weakly bound,quantum-mechanical buridan's ass

The spherical limit of a single particle in the field of three nuclei is shown to exhibit a variety of phenomena associated with the stability of the various extrema involved. Some consequences with respect to the general problem of detecting broken symmetry solutions of variational problems are pointed out.     

Yb valence states in YbC2: a HERFD-XANES spectroscopic investigation

The valence state of Yb in YbC(2) was analyzed using high-energy-resolution fluorescence detection (HERFD) X-ray absorption near-edge structure (XANES) spectroscopy and time-of-flight neutron powder diffraction to clarify a controversy in the literature. The unit cell volume of YbC(2) suggests a mixed Yb valence, which was formerly determined to be 2.8 by magnetization measurements and paramagnetic neutron scattering techniques. However, the nature of the intermediate valence was not clearly established. Both homogeneous and heterogeneous mixed valences were assumed in different publications. The temperature-dependent behavior of the valence state was only predicted, albeit not explicitly studied. In this work, the valence state of Yb in YbC(2) is, therefore, investigated thoroughly by HERFD-XANES spectroscopy at low and high temperatures. Our measurements result in an average Yb valence of 2.81 that is temperature-independent from 15 to 1123 K. These findings are confirmed by neutron powder diffraction experiments, which reveal a constant C-C distance of 128.7(9) pm in a temperature range from 5 to 100 K. A significant temperature dependence of the Yb valence state in YbC(2) can, therefore, be excluded by our experimental results.     

Water induced weakly bound electrons in DNA

We have studied the effect of humidity on the electronic properties of DNA base pairs. We found that the hydrogen links of the nucleobases with water molecules lead to a shift of the pi electron density from carbon atoms to nitrogen atoms and can change the symmetry of the wave function for some nucleobases. As a result, the orbital energies are shifted which leads to a decrease in the potential barrier for the hole transfer between the G-C and A-T pairs from 0.7 eV for the dehydrated case to 0.123 eV for the hydrated. More importantly, the pi electron density redistribution activated by hydration is enhanced by the intrastrand interactions. This leads to a modification of the nucleobase chemical structures from the covalent type to a resonance structure with separated charges, where some pi electrons are not locked up into the covalent bonds. Within the (G-C)(2) sequences, there is overlapping of the electronic clouds of such unlocked electrons belonging to the stacked guanines, that significantly increases the electron coupling between them to V(DA)=0.095 eV against the V(DA)=0.025 eV for the dehydrated case. Consequently, the charge transfer between two guanines within the (G-C)(2) sequences is increased by 250 times due to hydration. The presence of nonbonded electrons suppress the band gap up to approximately 3.0 eV, that allows us to consider DNA as a narrow band gap semiconductor.     

Theoretical and experimental study of weakly bound CO2-(pH2)2 trimers

The infrared spectrum of CO(2)-(pH(2))(2) trimers is predicted by performing exact basis-set calculations on a global potential energy surface defined as the sum of accurately known two-body pH(2)-CO(2) (J. Chem. Phys. 2010, 132, 214309) and pH(2)-pH(2) potentials (J. Chem. Phys. 2008, 129, 094304). These results are compared with new spectroscopic measurements for this species, for which 13 transitions are now assigned. A reduced-dimension treatment of the pH(2) rotation has been employed by applying the hindered-rotor averaging technique of Li, Roy, and Le Roy (J. Chem. Phys. 2010, 133, 104305). Three-body effects and the quality of the potential are discussed. A new technique for displaying the three-dimensional pH(2) density in the body-fixed frame is used, and shows that in the ground state the two pH(2) molecules are localized much more closely together than is the case for the two He atoms in the analogous CO(2)-(He)(2) species. A clear tunneling splitting is evident for the torsional motion of the two pH(2) molecules on a ring about the CO(2) molecular axis, in contrast to the case of CO(2)-(He)(2) where a more regular progression of vibrational levels reflects the much lower torsional barrier.     

Laser induced amplified spontaneous emission from the B2Pi, L2Pi, and I2Sigma+ valence states of NO

Amplified spontaneous emission (ASE) from single rovibrational levels of valence (non-Rydberg) states of NO molecules has been investigated. The B2Pi (v=24 and 25), L2Pi (v=5 and 6), and I2Sigma+ (v=6) levels have been populated through laser optical-optical double resonance excitation via the Rydberg A2Sigma+ state. Term values for the 2Pi states have been determined with an accuracy of +/-0.03 cm(-1). Analyses of rotationally resolved dispersed ASE spectra in the near infrared region have shown that all the lower states belonged to the Rydberg states. The valence approximately Rydberg coupling in the upper manifolds has driven ASE systems from the valence to the Rydberg levels where they benefit from the strong intensities of inter-Rydberg transitions with Deltav=0. The experimentally predicted valence approximately Rydberg interactions have been compared with theoretical treatments.     

Atomic valence states in molecular orbital theory

Atomic valence states in simple valence bond and molecular orbital theories of electronic structure have been compared. A basic difference emerges which can be characterised by the presence of one-centre Coulomb terms in the molecular orbital valence state energy. The recognition of this difference is important when performing generalised Hückel calculations: the Coulomb integrals are now given by charge dependent orbital electronegativities, and not the negative of the appropriate ionisation potentials of the kind available in the existing tabulations by Hinze and Jaffé and other authors. The analysis is given in detail for the special case of tetrahedrally coordinated boron and nitrogen, as found in cubic boron nitride.

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Zusammenfassung Es werden atomare Valenzzustände in einfachen Valenzbindungen und MO Theorien der Elektronenstruktur verglichen. Ein wesentlicher Unterschied ergibt sich durch die Anwesenheit von Einzentren-Coulomb-Termen in den MO-Valenzzustandsenergien. Das Erkennen dieser Unterschiede spielt eine entscheidende Rolle bei der Durchführung von Berechnungen nach der allgemeinen Hückel-Theorie: die Coulomb-Integrale sind nun durch Z-abhängige Orbitalelektronennegativitäten und nicht durch geeignete Ionisationspotentiale, wie sie in den Tabellen von Hinze und Jaffé und anderen Autoren verfügbar sind, gegeben. An Hand des tetraedisch gebundenen Bors und Stickstoffs, wie sie in kubischen Bornitriden vorkommen, wird eine genaue Analyse der Methode an einem speziellen Beispiel durchgeführt.

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Résumé Comparaison des états de valence atomiques dans les méthodes de la mésomérie et des orbitales moléculaires. Il apparait une différence fondamentale, que l'on peut caractériser par la présence de termes coulombiens monocentriques dans l'énergie de l'état de valence en orbitales moléculaires. Il importe d'être conscient de cette différence lorsqu'on effectue des calculs Hückel-étendu: les intégrales coulombiennes sont alors données par des électronégativités orbitales dépendant de la charge et non par l'opposé des potentiels d'ionisation du type de ceux que l'on obtient dans les tables de Hinze et Jaffé ou d'autres auteurs. On donne une analyse détaillée pour le cas particulier de l'azote et du bore à coordination tétraédrique, tel qu'il apparait dans le nitrure de bore cubique.

     

The valence and Rydberg excited states of CH2: A theoretical exploration

Using the completed active space second‐order perturbation (CASPT2) method, valence and Rydberg excited states of CH₂ molecule are probed with the large atomic natural orbital (ANO‐L) basis set. Five states are optimized and the geometric parameters are in good agreement with the available data in literatures, furthermore, the state of 2¹B₁ is obtained for the first time. Valence and Rydberg excited states of CH₂ are also calculated for the vertical transitions with the ANO‐L+ basis set that is constructed by adding a set of 1s1p1d Rydberg orbitals into the ANO‐L basis set. Two Rydberg states of the p?³A₂ and r?³B₁ at 9.88 and 10.50 eV are obtained for the first time, and the 3a₁ → 3d_yz nature of the state p?³A₂ and the 3a₁ → d_x2?y2 nature of the state r?³B₁ are confirmed. © 2012 Wiley Periodicals, Inc.