Excitation transfer from He(235,21,S) into bound and continuum states of CS2 studied by electron and optical spectroscopy and by coincidence measurements

For the reaction of He(2³ S, 2¹ S) with CS₂ we present a large body of experimental data: electron spectra, fluorescence spectra, electron-ion and electron-photon coincidence spectra. The combination of all these data allows us to characterize the reaction in great detail in a way which is rather direct and free of speculations. As for other molecules with positive electron affinity the charge exchanged channel He⁺ + CS ₂ ^? plays an important role as intermediate state. It is possible to isolate experimentally the electron energy spectra which correspond to formation of the individual electronic CS ₂ ⁺ states out of the intermediate He⁺ + CS ₂ ^? state. This has not been achieved so far for CS₂ or any other molecule by straightforward evaluation of experimental data.     

Experimental observation of optical bound states in the continuum

We present the experimental observation of bound states in the continuum. Our experiments are carried out in an optical waveguide array structure, where the bound state (guided mode) is decoupled from the continuum by virtue of symmetry only. We demonstrate that breaking the symmetry of the system couples this special bound state to continuum states, leading to radiative losses. These experiments demonstrate ideas initially proposed by von Neumann and Wigner in 1929 and offer new possibilities for integrated optical elements and analogous realizations with cold atoms and optical trapping of particles.     

Simultaneous measurement of beta- decay to bound and continuum electron states

We report the first measurement of a ratio lambda(beta(b))/lambda(beta(c)) of bound-state ((lambda(beta(b))) and continuum-state (lambda(beta(c))) beta(-)-decay rates for the case of bare 207Tl81+ ions. These ions were produced at the GSI fragment separator FRS by projectile fragmentation of a 208Pb beam. After in-flight separation with the Brho-deltaE-Brho method, they were injected into the experimental storage-ring ESR at an energy of 400.5A MeV, stored, and electron cooled. The number of both the 207Tl81+ ions and their bound-state beta(-)-decay daughters, hydrogen-like 207Pb81+ ions, were measured as a function of storage time by recording their Schottky-noise intensities. The experimental result, lambda(beta(b))/lambda(beta(c)) = 0.188(18), is in very good agreement with the value of 0.171(1) obtained from theory employing spectra of allowed transitions.     

Three-particle bound states for partly nonlocal interactions with continuum bound states

We determine the binding energies of the model triton for two partly nonlocal interactions consisting of a local potential with a strong repulsive core and a nonlocal separable interaction acting only inside the core region, which is responsible for the occurrence of a continuum bound state at very high energies in the total interaction. The ground state of the three-particle system does not collapse in this case as for purely nonlocal interactions. The occurrence of a continuum bound state is therefore only a necessary but not a sufficient condition for such an unphysical collapse in few particle systems.     

Surface bound states in the continuum

We introduce a novel concept of surface bound states in the continuum, i.e., surface modes embedded into the linear spectral band of a discrete lattice. We suggest an efficient method for creating such surface modes and the local bounded potential necessary to support the embedded modes. We demonstrate that the surface embedded modes are structurally stable, and the position of their eigenvalues inside the spectral band can be tuned continuously by adding weak nonlinearity.     

Electronic surface states investigated by means of photoemission spectroscopy

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.     

Guiding light through optical bound states in the continuum for ultrahigh‐Q microresonators

Light is usually confined in photonic structures with a band gap or relatively high refractive index for broad scientific and technical applications. Here, a light confinement mechanism is proposed based on the photonic bound state in the continuum (BIC). In a low‐refractive‐index waveguide on a high‐refractive‐index thin membrane, optical dissipation is forbidden because of the destructive interference of various leakage channels. The BIC‐based low‐mode‐area waveguide and high‐Q microresonator can be used to enhance light–matter interaction for laser, nonlinear optical and quantum optical applications. For example, a polymer structure on a diamond membrane shows excellent optical performance that can be achieved with large fabrication tolerance. It can induce strong coupling between photons and the nitrogen–vacancy center in diamond for scalable quantum information processors and networks.

     

Conical intersections and bound molecular states embedded in the continuum

Nuclear dynamics on coupled potential surfaces can lead to bound states embedded in the continuum. For one type of conical intersection situation, an explicit proof is presented that such states exist. Non-Born-Oppenheimer effects are responsible for the binding of these states. Once the Born-Oppenheimer approximation is introduced, these states at best become resonances which decay via potential tunneling. The tunneling is completely suppressed by the coupling between the electronic states. A numerical example is given.     

Strong-coupling expansions for multiparticle excitations: continuum and bound states

We present a new linked cluster expansion for calculating properties of multiparticle excitation spectra to high orders. We use it to obtain the two-particle spectra for systems of coupled spin-half dimers. We find that even for weakly coupled dimers the spectrum is very rich, consisting of many bound states. The number of bound states depends on both geometry of coupling and frustration. Many of the bound states can only be seen by going to sufficiently high orders in the perturbation theory, showing the extended character of the pair attraction.     

Three-particle system for nonlocal interactions with continuum bound states

The energy spectrum of the model triton is studied for a series of rank-two separable twobody interactions, each of which has a fixed deuteron wave function and a continuum bound state fixed at high energy. In each case the three particle system collapses, having an unphysical deeply bound ground state. With increasing two body repulsion, the collapsed state becomes less bound. If the continuum bound state moves to higher energies, the collapsed state becomes more bound. Our results show that for non-local two-body interactions which support a continuum bound state, or a resonance pole sufficiently close to the real axis, the three particle system is not a low energy system.     

Study of theS-matrix near bound states in the continuum

The behaviour ofS-matrix for potentials generating bound states in continuum in the neighbourhood of the positive bound state energies is studied. It is shown that unlike the case of usual negative energy bound states, theS-matrix does not have a pole at the positive bound state energy but becomes unity at the energy corresponding to bound states in continuum. Calculations ofS-waveS-matrix for a local potential constructed by Stillinger and Herrick and a separable nonlocal potential constructed by the present authors verify these results. Our results indicate that the bound states embedded in continuum constructedvia the von Neumann and Wigner procedure cannot be interpreted as resonances with zero width.     

Two-electron bound states in a continuum in quantum dots

A bound state in a continuum (BIC) might appear in open quantum dots for the variation in the dot’s shape. By means of the equations of motion of the Green’s functions, we investigate the effect of strong intradot Coulomb interactions on that phenomenon within the framework of the impurity Anderson model. The equation that the imaginary part of the poles of the Green’s function equals zero yields the condition for BICs. As a result, we show that the Coulomb interactions replicate the single-electron BICs into two-electron ones. The text was submitted by the authors in English.     

Vibrational combination states of polyatomic molecules investigated by ultrashort two-pulse spectroscopy

A first infrared pulse at frequency ν₁ generates transitions to intermediate states in S₀ and a simultaneous visible pulse at ν₂ raises the excited molecules to a state in S₁. Tuning the frequencies ν₁ and ν₂ over several hundred wave numbers and holding the sum ν₁ + ν₂ constant allows the observation of various combination vibrations. The technique is demonstrated on polyatomic molecules consisting of 42 atoms.     

Quantum bound states embedded in a continuum and their classical analogs

We show that both a rigid and a nonrigid dipole can be trapped by an external uniform magnetic field in classical mechanics. The trapped states of a dipole present a nontrivial example of classical bound states embedded in a continuum (BSEC) that can be treated as analogs of quantum BSECs. For example, the classical motion of a dipole is confined to a finite region in space, though there are no classical turning points. We also examine the quantum motion of a dipole in a magnetic field and show that, for the most natural choices of the parameters (the rigid rotating dipole or the one bound by oscillator potential, uniform time-independent magnetic field, etc.), there are no quantum BSEC solutions.     

Rydberg states of HCN observed by electron impact spectroscopy

The electron energy loss spectrum of HCN has been determined in the energy region 8–13.6 eV at impact energies of 100, 50 and 30 eV. It is shown that energy loss spectra of HCN at intermediate impact energies can be satisfactorily analysed unlike the diffuse unassigned optical absorption spectra that have previously been reported. Rydberg series have been assigned using term values and quantum defects together with ionization potentials obtained by photoelectron spectroscopy.     

Vanadium valency and hybridization in V-doped hafnia investigated by electron energy loss spectroscopy

The valency of vanadium, and thus indirectly the oxygen stoichiometry, of V-doped hafnia synthesized under different atmospheres have been investigated on a nanometer scale by means of electron energy loss spectroscopy (EELS). The EELS V L_2,3 spectra are compared with the results of crystal field multiplet calculations and experiments on reference vanadium oxides. The EELS spectra indicate that V-doped hafnia prepared under reducing (H₂) and neutral (Ar) atmosphere are unambiguously substituted with trivalent vanadium atoms leading to the creation of oxygen vacancies in the structure. On the contrary, stoichiometric (Hf, V)O₂ compound (i.e. V⁴⁺) is more likely to be stabilized under oxidative (air) atmospheres. We also show that the amount of hybridization alters for the different compounds studied but may in part be analyzed by high spatially resolved EELS. The crystal field multiplet calculations particularly indicate that a simple reduction of the Slater integrals gives a good account of the spectral modification induced by hybridization for the case of tetravalent vanadium atoms. Received 17 November 2000 and Received in final form 17 April 2001