Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

The connection between photon mass andB (3) : The poincaré group

The longitudinal vacuum fieldB ⁽³⁾ is an experimental observable which produces by magnetization a well-defined square-root beam power density dependence. Its longitudinal polarization implies that the helicities of the photon are +1, 0, and –1, and that the little group of the Poincaré group is the rotation group 0(3) of a massive boson. The mass of the photon (m) is therefore related directly toB ⁽³⁾ through the Proca equation, and it is concluded that experimental evidence forB ⁽³⁾ is also evidence for finitem.     

Microscopic three-cluster study of the low-energy 9 Be photodisintegration

The ⁹Be and ⁹B nuclei are investigated in a microscopic three-cluster model involving α + α + n (or α + α + p) configurations. The ⁸Be (0 ⁺ , 2 ⁺ ) + n and ⁵He (3/2 ^- , 1/2 ^- ) + α (or mirror) channels are included by taking account of the unstable nature of ⁸Be and ⁵He. Spectroscopic properties of ⁹Be and ⁹B are analyzed. We show that the ⁵He + α configurations cannot be neglected to derive accurate results. The ⁹Be(γ,αα)n photodisintegration cross-section is shown to be mainly determined by ⁸Be + n channels at low energies, but ⁵He + α channels become important beyond E _γ≈ 4 MeV. Received: 7 September 2001 / Accepted: 19 November 2001     

Large copper isotope effect on the pseudogap in the high-temperature superconductor HoBa 2 Cu 4 O 8

The copper isotope effect (⁶³Cu vs. ⁶⁵Cu) on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa₂Cu₄O₈ has been investigated by inelastic neutron scattering. For the ⁶³Cu compound there is clear evidence for the opening of an electronic gap in the normal state at T ^* ≈ 160 K far above T _c = 79.0 K. Upon substitution of ⁶³Cu by ⁶⁵Cu, T _c decreases marginally to 78.6 K, whereas T ^* is increased to about 185 K. This large copper isotope shift Δ T ^* ( Cu ) = T ^* ( ⁶⁵ Cu ) - T ^* ( ⁶³ Cu ) ≈ 25 K - together with the corresponding oxygen isotope shift Δ T ^* ( O ) = T ^* ( ¹⁸ O ) - T ^* ( ¹⁶ O ) ≈ 50 K found in an earlier investigation - suggests that phonons or lattice fluctuations involving both the copper and the oxygen ions are important for the pairing mechanism in high-T _c materials. Received 13 October 2000     

Isotope dependent predissociation in the C 1 Σ + , v =0 and v =1 states of CO

Rotationally resolved spectral lines in the C - X (1, 0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm^-1 in the vacuum ultraviolet. Improved molecular constants for the excited state are derived and no effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the C ¹ Σ ⁺ , v = 1 state for five natural isotopomers of carbon monoxide: τ( ¹² C ¹⁷ O ) = 280 ps, τ( ¹² C ¹⁸ O ) = 210 ps, τ( ¹³ C ¹⁶ O ) = 295 ps, τ( ¹³ C ¹⁷ O ) = 160 ps, and τ( ¹³ C ¹⁸ O ) = 150 ps. Within the accuracy of the present measurements no effects of J-dependent lifetimes were observed, for neither of the isotopomers. In addition direct time domain measurements of the lifetime of the C ¹ Σ ⁺ , v = 0 and v = 1 states of the main isotopomer are performed in a pump-probe experiment using a picosecond VUV-laser, yielding τ( ¹² C ¹⁶ O ) = 1780 ps for v = 0 and τ( ¹² C ¹⁶ O ) = 625 ps for v = 1. For C ¹ Σ ⁺ , v = 0 in ¹²C¹⁶O and ¹³C¹⁶O the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay in this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C ¹ Σ ⁺ , v = 0 state of the ¹³C¹⁸O isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for ¹²C¹⁶O.) From the combined data predissociation yields upon excitation of the C ¹ Σ ⁺ , v = 1 state are derived, lying in the range 0.84-0.91 for the five less abundant isotopomers; for the main ¹²C¹⁶O isotopomer a strongly deviating predissociation yield of 0.65 is deduced. Received 21 December 2000 and Received in final form 26 March 2001     

Interaction between aromatic amine cations and nonpolar solvents: Infrared spectra of isomeric aniline -Ar n ( ) complexes

Infrared (IR) photodissociation spectra of the aniline⁺-Ar_n cations, An ⁺ - Ar _n (n = 1, 2), are analyzed in the vicinity of the N-H stretch fundamentals. The complexes are produced in an electron impact (EI) ion source which produces predominantly the most stable cluster isomers. Two isomers of An⁺-Ar are identified by their characteristic N-H stretch frequencies: the planar proton-bound global minimum, in which the Ar ligand forms a nearly linear H-bond to the amino group, and the less stable π-bound local minimum, in which the Ar atom is attached to the π-electron system of the aromatic ring. This result is the first unambiguous detection of the most stable H-bound An⁺-Ar dimer. All previous spectroscopic studies of An⁺-Ar employed resonance enhanced multiphoton ionization (REMPI) of neutral An-Ar and identified only the less stable π-bound cation due to restrictions arising from the Franck-Condon principle. The EI-IR spectrum of An⁺-Ar₂ shows that the most stable structure of this trimer features two equivalent H-bonds (C_2v symmetry). The interpretation of the experimental data is supported by quantum chemical calculations. The ab initio potential of An⁺-Ar calculated at the UMP2/6-311G(2df, 2pd) level features H-bound global minima ( D _e = 513 cm^-1) and π-bound local minima ( D _e = 454 cm^-1), with a barrier of V _b ≈ 140 cm^-1 for isomerization from the π-bound toward the H-bound minimum. Received 4 February 2002 Published online 13 September 2002     

Electron-pair densities of group 2 atoms in their and terms

Electron-pair intracule (relative motion) and extracule (center-of-mass motion) densities are studied in both position and momentum spaces for the ¹ P and ³ P terms of the group 2 atoms Be (atomic number Z =4), Mg (Z =12), Ca (Z =20), Sr (Z =38), Ba (Z =56), and Ra (Z =88). In position space, the ¹ P - ³ P difference in the intracule densities shows that the probability of a small interelectronic distance is larger in the triplet for all the six atoms, as reported for the lightest Be atom in the literature. The position-space extracule density clarifies that the triplet electrons are more likely to be at opposite positions with respect to the nucleus than the singlet electrons for all the atoms. In momentum space, the singlet generally has a larger probability of a small relative momentum between two electrons as a na?ve manifestation of the Fermi hole in the triplet. The extracule density in momentum space shows that the ¹ P term has a distribution larger in a large center-of-mass momentum region than the ³ P term. Received: 26 August 1998 / Received in final form: 1 February 1999     

Magnetization of an electron plasma by microwave pulses: Faraday induction

It has been argued theoretically that the recently proposed vacuum fieldB ⁽³⁾ is not accompanied by a real electric fieldE ⁽³⁾ . Experimental evidence for this interence is available in the data reported by Deschampset al. [10], using microwave magnetization of an electron plasma set up in helium gas. Faraday induction due toB ⁽³⁾ does not occur in the inert gas and is not observed experimentally in the absence of free electrons. WheneverB ⁽³⁾ interacts with free electrons, however, Faraday induction occurs through a pulse of induced magnetization (i.e., induced orbital electronic angular momentum).     

Merged beam study of the associative ionisation ( C + , N + ? and ? O + )+ O -

Total cross-sections have been measured for the associative ionisation of C ⁺ + O ^- , N ⁺ + O ^- and O ⁺ + O ^- by means of a merged-beam set-up operating with keV beams. These original measurements might be relevant to the understanding of some astrophysical objects or laboratory-made plasmas (flames and etching plasmas). The magnitude of these cross-sections is particularly large whatever the associating system, as these are in the range of 1×10 ^-14 cm² at thermal energies. Their behaviour as a function of energy significantly differs from one system to another, and is characterised by the Wigner law at low energy, and a rapid fall-off at higher energy due to competition with non-associative ionisation processes. Received 10 December 2001 and Received in final form 12 March 2002     

Noether's theorem and the fieldB (3)

It is shown that the longitudinal, magnetic flux density,B ⁽³⁾ , of vacuum electromagnetic radiation can be accommodated rigorously within Noether's theorem, which relates fundamental spacetime symmetries to fundamental conservation laws. This demonstration linksB ⁽³⁾ to the canonical energy-momentum tensorT _µv that appears in Einstein's field equations of general relativity. Thus,B ⁽³⁾ provides a link between electromagnetism and gravitation which might eventually lead to an unified understanding of field theory.     

The role of OH ions in the stabilization of F colour centres in LiF

Thermally Stimulated Depolarisation Current (TSDC) and optical methods are applied to a range of alkali-fluoride crystals in order to establish a model for the stable F ₂ ⁺ - like colour centres in LiF:OH^-. The experimental results for LiF:OH^- suggest that the OH^- defects are partially destroyed under ionising irradiation or during crystal growth. The low-temperature dielectric relaxation signals in LiF:OH^- and LiF:Mg²⁺,OH^- are attributed to highly interacting hydroxide ions and products of their destruction located in extended lattice defects. In LiF:OH^-, in contrast to other alkali halides, the results advocate for a defect-structure model, which considers a neutral defect (ND, probably O₂ or H₂) sited at the anion vacancy of the O^2--V _a ⁺ dipole and which possibly is the “nucleus” for the F ₂ ⁺ centre. The proposed F ₂ ⁺ (ND, O^-) model seems to better explain the dielectric results, compared to the older F ₂ ⁺ (O^2-) and F ₂ ⁺ (O^-) models. The estimate for the electric dipole moment derived from the experimental TSDC bands, gives a value for the F ₂ ⁺ - like centre in LiF:OH^- between those of the F ₂ ⁺ (O^-) and F ₂ ⁺ (O^2-) defects, in good agreement with the proposed F ₂ ⁺ (ND,O^-) model. The reduction of the activation energy barrier of the (re)orientation process of the Mg²⁺V _c ^- (OH^-) complexes in LiF:Mg²⁺,OH^-, and the low-temperature shift of their TSDC band, compared to the single Mg ₂ ⁺ V _c ^- peak in LiF:Mg²⁺, are tentatively ascribed to an increase in the crystal-lattice parameters owing to the presence of OH^- and/or products of its destruction. Received 31 August 2001 / Received in final form 30 March 2002 Published online 9 July 2002     

On the nature of the Aharonov-Bohm effect

It is shown that zero-field potentials may be responsible for the Aharonov-Bohm effect. A magnetic field B=curlA ^f has a physical (gauge-invariant) meaning for field potentials A ^f, whereas a circulation ∮_C A ⁰ d r has a physical meaning for zero-field potentials A ⁰.     

Ising models on hyperbolic graphs

We consider Ising models on a hyperbolic graph which, loosely speaking, is a discretization of the hyperbolic planeH ² in the same sense asZ ^d is a discretization ofR ^d . We prove that the models exhibit multiple phase transitions. Analogous results for Potts models can be obtained in the same way.     

Theoretical transition probabilities and lifetimes in nickel-like Se,Y and Sn ions

The 17 levels of the 3 d ¹⁰ , 3 d ⁹ 4 s and 3 d ⁹ 4 p configurations, and the electric-dipole transitions among these levels are calculated for the three nickel-like ions Se⁶⁺, Y¹¹⁺ and Sn²²⁺ by using large-scale multiconfiguration Dirac-Fock wavefunctions. From these computations, the excitation energies and transition probabilities as well as the lifetimes of 3 d ⁹ 4 p levels are derived, including all dominant effects of relativity, correlation and of the rearrangement of the electron density within the same framework. Comparison is made with the scarce number of experimental data and previous semi-empirical computations.     

Luminescence characteristics of ultraviolet upconversion from Er 3 + :YAG crystal by Ar + laser (488 nm) excitation

Ultraviolet and violet upconversion signals at 271 nm, 317 nm, 381 nm and 407 nm were observed when an erbium-doped YAG crystal was pumped by an Ar⁺ laser (488 nm). The dependence of intensity of luminescence emitting from the ⁴S _3/2 state and the ²P _3/2 state on pump power (I) was experimentally investigated. Changes from I¹ down to I ^1/2 for the ⁴S _3/2 state and from I² down to I¹ for the ²P _3/2 state were observed. The upconversion mechanism was discussed by means of the rate equations. It appears that energy-transfer upconversion (ETU) is a dominant process for the Er³⁺:YAG crystal used in our experiment. Received 20 March 2001 and Received in final form 11 July 2001     

Formation of Cs molecules via Feshbach resonances stabilized by spontaneous emission: theoretical treatment with the Fourier grid method

This paper proposes a general method to investigate Feshbach resonances in atomic collisions similar to Cs(6 s ) + Cs(6 p ) in the thermal or cold regime. In order to compute the predissociation widths of the C ¹ Π _u (6 s + 5 d ) bound vibrational levels of Cs₂, coupled both with the (2) ³ Σ ⁺ _u (6 s + 6 p ) continuum and with the (2) ³ Π _u (6 s + 5 d ) vibrational series, a Fourier grid method is implemented, with an optical potential. A convenient way of optimizing the latter is proposed. A large number of resonances are found and calculations of their cross-sections for stabilization into ground state molecules show that the rate may be important. This confirms the interpretation of Lintz and Bouchiat [Phys. Rev. Lett. 80, 2570 (1998)] who observed dimer formation in cell experiments. Possible generalization to the cold regime relies on the possibility to tune the position of a resonance to coincide with the maximum of the collisional energy distribution. Received 14 February 2002 Published online 28 June 2002     

The shapes of Auger decay lines in photoelectron satellite spectra

The theory of the shapes of Auger decay lines of satellite two-hole-one particle states accompanying photoionization based on the Green's function method is developed. The lineshapes of Auger decay of satellite states [2 s 2 p ]( ^1,3 P )3 s ( ² P ), [2 s 2 p ]( ¹ P )4 s ( ² P ) and [3 s 3 p ]( ³ P )4 s ( ² P ) in valence p-photoelectron spectra of Ne and Ar atoms are calculated (hole states are indicated by square brackets throughout). It is shown that in some cases the Auger lineshapes reproduce the shape of the photoelectron satellite line, but in other cases Auger line may be narrower then the photoelectron line and may have opposite direction of asymmetry. The theoretical results are in agreement with experimental low-energy Auger spectra. Received: 25 May 1998 / Accepted: 2 October 1998     

Theoretical design of azaacene-based non-fullerene electron transport material used in inverted perovskite solar cells

ABSTRACT

Inverted perovskite solar cells (PSCs) have attracted much attention due to their low-temperature and solution-based process. Electron transport layers are important components in inverted PSCs. Non-fullerene n-type organic small molecules seem to be more attractive as electron transport layers, because their structures are easy to be synthesised and modified. In this paper, density functional theory and semi-classical Marcus electron transfer theory were used to explore the electron transport properties in three azaacene derivatives, including one experimentally reported molecule, 1,4,9,16-tetrakis((triisopropylsilyl)ethynyl)quinoxalino[2^?,3^?:4^″,5^″]cyclopenta[1^″,2^″,3^″:5^′,6^′]acenaphtho[1^′,2^′:5,6]pyrazino[2,3-b]phenazine (1), and two theoretically designed molecules (2 and 3). Compound 2 is formed by substituting i-Pr groups in compound 1 with H atoms, which is designed to evaluate the effect of i-Pr groups on the electron transport properties. Compound 3 is designed by adding one more benzopyrazine group to the conjugation structure of compound 1. It shows that i-Pr group can increase HOMO and LUMO energy levels and improve solubility in organic solvent and hydrophobicity. Enlarging conjugation can not only decrease LUMO energy level and electron reorganisation energy, but also can increase solubility and electron mobility. So our designed compound 3 is expected to be a potential electron transport material in inverted PSCs.     

Cold rubidium molecule formation through photoassociation: A spectroscopic study of the 0 g - long-range state of 87 Rb 2

We report the detailed analysis of translationally cold rubidium molecule formation through photoassociation. Cold molecules are formed after spontaneous decay of photoexcited molecules from a laser cooled atomic sample, and are detected by selective mass spectroscopy after two-photon ionization into Rb ₂ ⁺ ions. A spectroscopic study of the 0 _g ^- (5 S + 5 P _3/2 ) pure long-range state of ⁸⁷Rb₂ is performed by detecting the ion yield as a function of the photoassociation laser frequency; the spectral data are theoretically analyzed within the semiclassical RKR approach. Molecular ionization is resonantly enhanced through either the 2 ³ Π _g or the 2 ³ Σ ⁺ _g intermediate molecular states. Some vibrational levels of the latter electronic state are observed and assigned here for the first time. Finally, cold molecules formation rates are calculated and compared to the experimentally measured ones, and the vibrational distribution of the formed molecules in the a ³ Σ ⁺ _u ground triplet state is discussed. Received 18 January 2001 and Received in final form 10 April 2001     

Analytical Evaluation of Cu<Superscript>2+</Superscript> Selective Behavior of calix[4]arene Derivative

The present article describes the solvatochromic effect including solvent system selection, time study and a detailed complexation study along with exploration of extraction properties of 5,11,17,23-tetrakis[(diethylamino)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (4) that bears nitrogen atom as a donor group available for chelating metal ions. Complexation properties of 4 toward selected transition metal ions have been investigated by UV-visible and fluorescence spectroscopies. The% efficiency of 4 toward selected transition metal ions was found in order Cu²⁺> Ni²⁺> Hg²⁺> Zn²⁺> Co²⁺> Cd²⁺> Pb²⁺. It has been noticed that 4 is not only proved to be an efficient Cu²⁺ selective chromoionophore but also possesses an effective extraction property for transferring Cu²⁺ ions from an aqueous to dichloromethane layer. The FT-IR spectroscopic method has also been applied for further confirmation of the complexation phenomenon of 4 with Cu²⁺ ion and found adequate.