bluuebunny/crossref_metadata_2025 huggingface dataset:
C-Linker of Cyclic Nucleotide–gated Channels Controls Coupling of Ligand Binding to Channel Gating
Authors: Paoletti Pierre , Young Edgar C. , Siegelbaum Steven A.
Published: July 2002

Cyclic nucleotide–gated channels are composed of a core transmembrane domain, structurally homologous to the voltage-gated K+ channels, and a cytoplasmic ligand-binding domain. These two modules are joined by ∼90 conserved amino acids, the C-linker, whose precise role in the mechanism of channel activation by cyclic nucleotides is poorly understood. We examined cyclic nucleotide–gated channels from bovine photoreceptors and Caenorhabditis elegans sensory neurons that show marked differences in cyclic nucleotide efficacy and sensitivity. By constructing chimeras from these two channels, we identified a region of 30 amino acids in the C-linker (the L2 region) as an important determinant of activation properties. An increase in both the efficacy of gating and apparent affinity for cGMP and cAMP can be conferred onto the photoreceptor channel by the replacement of its L2 region with that of the C. elegans channel. Three residues within this region largely account for this effect. Despite the profound effect of the C-linker region on ligand gating, the identity of the C-linker does not affect the spontaneous, ligand-independent open probability. Based on a cyclic allosteric model of activation, we propose that the C-linker couples the opening reaction in the transmembrane core region to the enhancement of the affinity of the open channel for agonist, which underlies ligand gating.
A Marine Snail Neurotoxin Shares with Scorpion Toxins a Convergent Mechanism of Blockade on the Pore of Voltage-Gated K Channels
Authors: García Esperanza , Scanlon Martin , Naranjo David
Published: July 2002

κ-Conotoxin-PVIIA (κ-PVIIA) belongs to a family of peptides derived from a hunting marine snail that targets to a wide variety of ion channels and receptors. κ-PVIIA is a small, structurally constrained, 27-residue peptide that inhibits voltage-gated K channels. Three disulfide bonds shape a characteristic four-loop folding. The spatial localization of positively charged residues in κ-PVIIA exhibits strong structural mimicry to that of charybdotoxin, a scorpion toxin that occludes the pore of K channels. We studied the mechanism by which this peptide inhibits Shaker K channels expressed in Xenopus oocytes with the N-type inactivation removed. Chronically applied to whole oocytes or outside-out patches, κ-PVIIA inhibition appears as a voltage-dependent relaxation in response to the depolarizing pulse used to activate the channels. At any applied voltage, the relaxation rate depended linearly on the toxin concentration, indicating a bimolecular stoichiometry. Time constants and voltage dependence of the current relaxation produced by chronic applications agreed with that of rapid applications to open channels. Effective valence of the voltage dependence, zδ, is ∼0.55 and resides primarily in the rate of dissociation from the channel, while the association rate is voltage independent with a magnitude of 107–108 M−1 s−1, consistent with diffusion-limited binding. Compatible with a purely competitive interaction for a site in the external vestibule, tetraethylammonium, a well-known K-pore blocker, reduced κ-PVIIA's association rate only. Removal of internal K+ reduced, but did not eliminate, the effective valence of the toxin dissociation rate to a value <0.3. This trans-pore effect suggests that: (a) as in the α-KTx, a positively charged side chain, possibly a Lys, interacts electrostatically with ions residing inside the Shaker pore, and (b) a part of the toxin occupies an externally accessible K+ binding site, decreasing the degree of pore occupancy by permeant ions. We conclude that, although evolutionarily distant to scorpion toxins, κ-PVIIA shares with them a remarkably similar mechanism of inhibition of K channels.
Determinants of Voltage-Dependent Gating and Open-State Stability in the S5 Segment of Shaker Potassium Channels
Authors: Kanevsky Max , Aldrich Richard W.
Published: July 2002

The best-known Shaker allele of Drosophila with a novel gating phenotype, Sh5, differs from the wild-type potassium channel by a point mutation in the fifth membrane-spanning segment (S5) (Gautam, M., and M.A. Tanouye. 1990. Neuron. 5:67–73; Lichtinghagen, R., M. Stocker, R. Wittka, G. Boheim, W. Stühmer, A. Ferrus, and O. Pongs. 1990. EMBO [Eur. Mol. Biol. Organ.] J. 9:4399–4407) and causes a decrease in the apparent voltage dependence of opening. A kinetic study of Sh5 revealed that changes in the deactivation rate could account for the altered gating behavior (Zagotta, W.N., and R.W. Aldrich. 1990. J. Neurosci. 10:1799–1810), but the presence of intact fast inactivation precluded observation of the closing kinetics and steady state activation. We studied the Sh5 mutation (F401I) in ShB channels in which fast N-type inactivation was removed, directly confirming this conclusion. Replacement of other phenylalanines in S5 did not result in substantial alterations in voltage-dependent gating. At position 401, valine and alanine substitutions, like F401I, produce currents with decreased apparent voltage dependence of the open probability and of the deactivation rates, as well as accelerated kinetics of opening and closing. A leucine residue is the exception among aliphatic mutants, with the F401L channels having a steep voltage dependence of opening and slow closing kinetics. The analysis of sigmoidal delay in channel opening, and of gating current kinetics, indicates that wild-type and F401L mutant channels possess a form of cooperativity in the gating mechanism that the F401A channels lack. The wild-type and F401L channels' entering the open state gives rise to slow decay of the OFF gating current. In F401A, rapid gating charge return persists after channels open, confirming that this mutation disrupts stabilization of the open state. We present a kinetic model that can account for these properties by postulating that the four subunits independently undergo two sequential voltage-sensitive transitions each, followed by a final concerted opening step. These channels differ primarily in the final concerted transition, which is biased in favor of the open state in F401L and the wild type, and in the opposite direction in F401A. These results are consistent with an activation scheme whereby bulky aromatic or aliphatic side chains at position 401 in S5 cooperatively stabilize the open state, possibly by interacting with residues in other helices.
The Interaction of a Neutral Ryanoid with the Ryanodine Receptor Channel Provides Insights into the Mechanisms by Which Ryanoid Binding Is Modulated by Voltage
Authors: Tanna Bhavna , Welch William , Ruest Luc , Sutko John L. , Williams Alan J.
Published: July 2002

In an earlier investigation, we demonstrated that the likelihood of interaction of a positively charged ryanoid, 21-amino-9α-hydroxyryanodine, with the sarcoplasmic reticulum Ca2+-release channel (ryanodine receptor, RyR) is dependent on holding potential (Tanna, B., W. Welch, L. Ruest, J.L. Sutko, and A.J. Williams. 1998. J. Gen. Physiol. 112:55–69) and suggested that voltage dependence could result from either the translocation of the charged ligand to a site within the voltage drop across the channel or a voltage-driven alteration in receptor affinity. We now report experiments that allow us to assess the validity of these alternate mechanisms. Ryanodol is a neutral ryanoid that binds to RyR and induces modification of channel function. By determining the influence of transmembrane potential on the probability of channel modification by ryanodol and the rate constants of ryanodol association and dissociation, we demonstrate that the influence of voltage is qualitatively the same for both the neutral and positively charged ryanoids. These experiments establish that most, if not all, of the modification of ryanoid interaction with RyR by transmembrane holding potential results from a voltage-driven alteration in receptor affinity.
Severed Molecules Functionally Define the Boundaries of the Cystic Fibrosis Transmembrane Conductance Regulator's Nh2-Terminal Nucleotide Binding Domain
Authors: Chan Kim W. , Csanády László , Seto-Young Donna , Nairn Angus C. , Gadsby David C.
Published: July 2002

The cystic fibrosis transmembrane conductance regulator is a Cl− channel that belongs to the family of ATP-binding cassette proteins. The CFTR polypeptide comprises two transmembrane domains, two nucleotide binding domains (NBD1 and NBD2), and a regulatory (R) domain. Gating of the channel is controlled by kinase-mediated phosphorylation of the R domain and by ATP binding, and, likely, hydrolysis at the NBDs. Exon 13 of the CFTR gene encodes amino acids (aa's) 590–830, which were originally ascribed to the R domain. In this study, CFTR channels were severed near likely NH2- or COOH-terminal boundaries of NBD1. CFTR channel activity, assayed using two-microelectrode voltage clamp and excised patch recordings, provided a sensitive measure of successful assembly of each pair of channel segments as the sever point was systematically shifted along the primary sequence. Substantial channel activity was taken as an indication that NBD1 was functionally intact. This approach revealed that the COOH terminus of NBD1 extends beyond aa 590 and lies between aa's 622 and 634, while the NH2 terminus of NBD1 lies between aa's 432 and 449. To facilitate biochemical studies of the expressed proteins, a Flag epitope was added to the NH2 termini of full length CFTR, and of CFTR segments truncated before the normal COOH terminus (aa 1480). The functionally identified NBD1 boundaries are supported by Western blotting, coimmunoprecipitation, and deglycosylation studies, which showed that an NH2-terminal segment representing aa's 3–622 (Flag3-622) or 3–633 (Flag3-633) could physically associate with a COOH-terminal fragment representing aa's 634–1480 (634-1480); however, the latter fragment was glycosylated to the mature form only in the presence of Flag3-633. Similarly, 433-1480 could physically associate with Flag3-432 and was glycosylated to the mature form; however, 449-1480 protein seemed unstable and could hardly be detected even when expressed with Flag3-432. In excised-patch recordings, all functional severed CFTR channels displayed the hallmark characteristics of CFTR, including the requirement of phosphorylation and exposure to MgATP for gating, ability to be locked open by pyrophosphate or AMP-PNP, small single channel conductances, and high apparent affinity of channel opening by MgATP. Our definitions of the boundaries of the NBD1 domain in CFTR are supported by comparison with the solved NBD structures of HisP and RbsA.
Cysteine Mutagenesis Reveals Novel Structure–Function Features within the Predicted Third Extracellular Loop of the Type Iia Na+/Pi Cotransporter
Authors: Lambert Georg , Forster Ian C. , Stange Gerti , Köhler Katja , Biber Jürg , Murer Heini
Published: July 2002

The transport function of the rat type IIa Na+/Pi cotransporter is inhibited after binding the cysteine modifying reagent 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) to a cysteine residue substituted for a serine at position 460 (S460C) in the predicted third extracellular loop. This suggests that Ser-460 lies in a functionally important region of the protein. To establish a “structure–function” profile for the regions that flank Ser-460, the substituted cysteine accessibility method was employed. 18 mutants were constructed in which selected amino acids from Arg-437 through Leu-465 were substituted one by one for a cysteine. Mutants were expressed in Xenopus oocytes and transport function (cotransport and slippage) and kinetics were assayed by electrophysiology with or without prior treatment with cysteine modifying (methanethiosulfonate, MTS) reagents. Except for mutant I447C, mutants with cysteines at sites from Arg-437 through Thr-449, as well as Pro-461, were inactive. Cotransport function of mutants with Cys substitutions at sites Arg-462 through Leu-465 showed low sensitivity to MTS reagents. The preceding mutants (Cys substitution at Thr-451 to Ser-460) showed a periodic accessibility pattern that would be expected for an α-helix motif. Apart from loss of transport function, exposure of mutants A453C and A455C to MTSEA or 2-(triethylammonium)ethyl MTS bromide (MTSET) increased the uncoupled slippage current, which implicated the mutated sites in the leak pathway. Mutants from Ala-453 through Ala-459 showed less pH dependency, but generally stronger voltage dependency compared with the wild type, whereas those flanking this group were more sensitive to pH and showed weaker voltage dependence of cotransport mode kinetics. Our data indicate that parts of the third extracellular loop are involved in the translocation of the fully loaded carrier and show a membrane-associated α-helical structure.
Mutations within the P-Loop of Kir6.2 Modulate the Intraburst Kinetics of the Atp-Sensitive Potassium Channel
Authors: Proks Peter , Capener Charlotte E. , Jones Phillippa , Ashcroft Frances M.
Published: July 2002

The ATP-sensitive potassium (KATP) channel exhibits spontaneous bursts of rapid openings, which are separated by long closed intervals. Previous studies have shown that mutations at the internal mouth of the pore-forming (Kir6.2) subunit of this channel affect the burst duration and the long interburst closings, but do not alter the fast intraburst kinetics. In this study, we have investigated the nature of the intraburst kinetics by using recombinant Kir6.2/SUR1 KATP channels heterologously expressed in Xenopus oocytes. Single-channel currents were studied in inside-out membrane patches. Mutations within the pore loop of Kir6.2 (V127T, G135F, and M137C) dramatically affected the mean open time (τo) and the short closed time (τC1) within a burst, and the number of openings per burst, but did not alter the burst duration, the interburst closed time, or the channel open probability. Thus, the V127T and M137C mutations produced longer τo, shorter τC1, and fewer openings per burst, whereas the G135F mutation had the opposite effect. All three mutations also reduced the single-channel conductance: from 70 pS for the wild-type channel to 62 pS (G135F), 50 pS (M137C), and 38 pS (V127T). These results are consistent with the idea that the KATP channel possesses a gate that governs the intraburst kinetics, which lies close to the selectivity filter. This gate appears to be able to operate independently of that which regulates the long interburst closings.
Configuration interaction in the continuum states of molecules
Authors: J N Bardsley
Published: July 2002

The theory of configuration interaction is discussed in relation to molecular states which have sufficient energy to decay by electron emission or pre-dissociation. By considering the effects of electron emission a resonant scattering formalism is developed which is useful for electron-molecule collisions at very low energies. The application of the theory to pre-dissociation is demonstrated by an analysis of the recent study by Barrowet al. of the spectrum of selenium.
A spectroscopic study of the plasma generated by a laser from polyethylene
Authors: B C Boland  , F E Irons  , R W P McWhirter
Published: July 2002

This paper describes a spectroscopic study of the plasma produced when a giant pulse ruby laser of 5 J energy and 17 ns duration is focused into a 0.3 mm diameter spot on a polyethylene target in vacuum. Quantitative measurements of spectral intensities in the range from 20 Angstrom to 6000 Angstrom have yielded estimates of the electron and ion densities, the electron temperature and the streaming velocities of the ions. From these results it is estimated that the electron temperature, the ionization energy, the energy lost by radiation and the energy of recoil of the target account for only about 10% of the incident laser energy. The ion kinetic energy accounts for 70% of the incident energy with an experimental uncertainty of ± 70%. It is calculated that there are about 1016ions produced by the laser beam striking the target. It is shown that the electrons contained in the plasma expanding away from the target cool adiabatically. In the course of the work there have been identified several new lines hitherto unclassified.
Quantal treatment of one-electron excitations (n=2 levels) of helium in He+(1s) on He(1s2) collisions at low energy (150 eV)
Authors: B Stern  , J Baudon  , P Lefebure  , G Rahmat  , G Vassilev
Published: July 2002

A fully quantal close-coupling treatment of the excitation of helium on the levels 23S, 21S and 23P in the He+on He collision. In addition to the lower Sigmagstate, described at any distance by the configuration 1s sigmag(2p sigmau)2, eight simple configurations of symmetry Sigmagare introduced. After the 9*9 configuration interaction they combine into eight molecular states which dissociate into specific atomic states. The main interactions with the entrance channel consist of two localised couplings at two crossing points (R=1.34 au and 1.48 au). A two-state approximation is valid to interpret the 23S excitation. The results are in good agreement with the experimental data.
Many-body calculation of electron affinities: C2and a prediction for P2
Authors: L S Cederbaum  , W Domcke  , W von Niessen
Published: July 2002

The electron affinities of the molecules, C2and P2are calculated by the Green's function formalism which has been successful in determining accurate ionisation potentials of atoms and molecules. The adiabatic electron affinities obtained are AEA(C2)=3.60 eV and AEA(P2)=0.30 eV compared to the experimental values of 3.54 eV and 0.24+or-0.23 eV respectively. The vibrational structure in the radiative electron-attachment spectra of C2and P2is also computed. There is a close relationship between the vibrational structures in electron-attachment and photodetachment spectra.
Impact-parameter formulation of the impulse approximation for charge exchange
Authors: J S Briggs
Published: July 2002

An impulse approximation to the charge-transfer amplitude in an ion-atom collision is derived. When a straight-line trajectory is used for the nuclear motion the impact-parameter method provides the same charge-transfer cross section as the plane-wave treatment of nuclear motion. The impact-parameter formulation allows a simply physical interpretation to be given to various peaking approximations that have been used to simplify the impulse approximation. The full impulse approximation provides the same high-velocity limit as the second Born approximation but an additional peaking approximation only provides the correct limit when Z2/Z1to 0.
Feshbach projection-operator calculation of the resonant states of H2-
Authors: B D Buckley  , C Bottcher
Published: July 2002

A formulation of the Feshbach projection-operator technique for a three-electron system is presented which renders practicable the calculation of the H2-resonance positions, and ultimately their widths, by standard configurational interaction methods. Striking and hitherto unidentified resonances in the 0 to 12 eV region have been discovered and these may lead to a more complete interpretation of the existing experimental results.
Electron impact excitation of the resonance transitions of Li, Na and K
Authors: J V Kennedy  , V P Myerscough  , M R C McDowell
Published: July 2002

A unitarised distorted-wave polarised-orbital model is used to investigate electron impact excitation of the Li, Na and K resonance transitions at low and intermediate energies. Results are presented for integral (total) cross sections, differential cross sections, coincidence parameters, and, for K, spin-flip differential cross sections. They are compared where possible with experiment and other theoretical models. The evaluation of the adiabatic polarisation interaction is discussed. Core-exchange effects are investigated for the case of Na. Tables of T-matrix elements are available for all three cases to aid in interpretation of photon-electron coincidence experiments.
Differential cross sections and angular distributions of H-from dissociative electron attachment to H2between 3.75 eV and 13 eV
Authors: M Tronc  , C Schermann  , R I Hall  , F Fiquet-Fayard
Published: July 2002

An electron-impact spectrometer has been modified for the study of negative-ion fragments. This instrument has been used to observe differential cross sections and angular distributions of H-ions produced by dissociative attachment to H2through the2Sigmau+and2Sigmag+resonances. The measured angular distributions are analysed using the pure resonant scattering with potential scattering theoretical model which is discussed. Vibrational structure observed in the differential cross section between 11.30 eV and 13.30 eV is interpreted as resulting from predissociation of the 'a' resonance bands by the H2-2Sigmag+repulsive state.
Relativistic effects in K-shell ionisation by 7-15 MeV proton bombardment on heavy elements
Authors: A Berinde  , C Deberth  , I Neamu  , C Protop  , N Scintei  , V Zoran  , M Dost  , S Rohl
Published: July 2002

The K-shell ionisation induced by 7-15 MeV proton bombardment on thin targets of Ag, Gd, Yb, Ta, Au, Pb and Th has been measured. The data are corrected for the contribution of the 'nuclear satellites' produced by internal conversion in the residual nuclei from (p,xn) reactions. The plane-wave Born approximation (PWBA) explains the data only up to Yb. For heavier elements, only the binary-encounter approximation (BEA) and the semiclassical approximation (SCA) including relativistic effects are in overall agreement with the experiment. A comparison with the existing alpha-particle ionisation cross sections at equal velocities shows the presence of increased electron binding due to the finite projectile charge.
Energetic protons from autoionising states of H2
Authors: K Kollmann
Published: July 2002

The relative abundance of protons formed by electron impact on H2has been measured as a function of the primary electron energy, the proton kinetic energy and the angle between the electron and proton beams. Apart from the known thresholds for direct dissociative ionisation three (averaged) threshold energies 23.5+or-0.7, 26.1+or-0.5 and 27.7+or-0.6 eV have been found for groups of protons with kinetic energies ranging from 0.75 to 3, 0.75 to 3.9 and 3 to 4.85 eV, respectively. The experimental data are interpreted in terms of H2** autoionising states recently calculated by O'Malley (1969), Bottcher (1976) and Hazi (1975).
Stueckelberg phases: the three-parameter exponential model
Authors: D S F Crothers
Published: July 2002

A three-parameter semiclassical S-matrix, embracing Nikitin non-adiabatic parameters (1962) and Stueckelberg phases, is calculated within the two-state exponential model. Strong-coupling and weak-coupling asymptotic expansions of the Whittaker functions are employed. A simple phase-integral interpretation of the three parameters is given. The matrix elements reduce correctly in all known limiting cases, including the Rosen-Zener model, (1932), and the parabolic models for both curve crossings and perturbed symmetric resonance.
Calculation of transition probabilities for two-electron one-photon and hypersatellite transitions for ions with two vacancies in the K shell
Authors: M Gavrila  , J E Hansen
Published: July 2002

Transition probabilities have been calculated for two-electron one-photon Kalpha alphatransitions (2s2p to 1s2) and single-electron Kalphahhypersatellite transitions (2p to 1s) for ions with two K-shell vacancies in the initial state. The ions studied are N, Ne, Al, Cl, Ca, Fe and Ni. The Kalpha alphaand Kalphahrates were calculated within a single-particle atomic model, allowing for the relaxation of the single-particle orbitals when passing from the initial to the final state. The LS-coupling scheme and non-relativistic single-configuration Hartree-Fock wavefunctions were used to calculate oscillator strengths in the dipole-length approximation.
Production of 2s vacancies in collisions of 0.3-400 keV Ne0, Ne+and Ne2+with Ne
Authors: T Andersen  , E Boving  , P Hedegaard  , J Ostgaard Olsen
Published: July 2002

The production of 2s vacancies in collisions of 0.3-400 keV Ne0, Ne+and Ne2+with Ne has been studied by measuring the emission cross sections for the Ne II (2s2p6) and Ne III (2s2p5) states using VUV spectroscopy. The experimental data show the presence of different reaction mechanisms, the origins of which are discussed within the framework of the MO model.
Ab initio electron-molecule scattering theory including polarisation: elastic scattering and rotational excitation of H2
Authors: A Klonover  , U Kaldor
Published: July 2002

An ab initio L2basis-set theory for electron-molecule scattering is proposed. All terms of the potential (static, exchange and polarisation) are included. The T-matrix expansion technique of Rescigno et al. (1974) is extended to include polarisation via a one-electron optical potential, which is expanded in a diagrammatic perturbation series. Integral and differential cross sections for e-H2elastic scattering and rotational excitation at electron energy 0.6-10 eV are calculated with the lowest-order polarisation diagrams. Very good agreement with experiment is obtained for elastic scattering over the whole range and for rotational excitation above 4 eV.
Production of excited hydrogen atoms by photodissociation of the H2 molecule near threshold
Authors: I V Komarov  , V N Ostrovsky
Published: July 2002

The photodissociation of the hydrogen molecule near the dissociation threshold into H(1s)+H(2s,2p) states is considered. The experimentally observed formation of H(2p) can only be explained by the interaction of the covalent and ionic electronic states of the H2 molecule at large internuclear distances in the vicinity of the avoided crossing of the potential curves. The theory is based on the Landau-Zener model with a quantum description of nuclear motion. The calculated relative yields of H(2s) and H(2p) atoms are 59.8% and 40.2% compared with 57% and 43% obtained in a recent experiment (Mental and Guyon (1977)).
Ab initio electron-molecule scattering theory including polarisation: vibrational and vibrational-rotational excitation of H2
Authors: A Klonover  , U Kaldor
Published: July 2002

An ab initio L2-basis-set theory for low-energy electron-molecule scattering, recently proposed and tested for e-H2elastic scattering and rotational excitation, is applied to pure vibrational and vibrational-rotational excitation of H2by slow electrons (0.6-10 eV). An optical potential including direct, exchange and polarisation terms is expanded in a diagrammatic perturbation series and used in the framework of the T-matrix expansion method of Rescigno et al. (1974). Polarisation is shown to have a substantial effect at the lower end of the energy range, decreasing at higher energy and practically vanishing at 10 eV. Calculated integral and differential cross sections for both vibrational and vibrational-rotational excitation are in good agreement with experiment.
Plasma parameters in laser imploded targets from space-resolved X-ray spectroscopy
Authors: M H Key  , J G Lunney  , J M Ward  , R G Evans  , P T Rumsby
Published: July 2002

Plasma densities and temperatures are deduced from soft X-ray emission spectra of neon-filled glass microballoons imploded by a 0.2 terawatt laser. Spatial resolution allows separate characterisation of the ablation plasma, the imploded glass and the compressed gas. Comparison with computer simulations of the implosion shows sensitivity to models for energy transport by hot electrons.
Ab initio study of some configurations of Al2-, Al2and Al2+
Authors: M Leleyter  , P Joyes
Published: July 2002

The relative secondary emission intensity of the molecular ion Al2-sputtered from a pure aluminium target by ionic bombardment (primary ions: Ar+, 6.5 keV) is larger than that of Al-, whereas this is not the case for the positive ions Al2+and Al+. An ab initio calculation (LCAO-STO-MO-SCF-CI) is carried out on the configurations4Sigmag-and2Piuof Al2-,2Sigmag+and2Piuof Al2+and3Sigmag-and1Sigmag+of Al2within first a minimal basis and then an extended one. After configuration interaction, it is found that the ground states of Al2-, Al2+and Al2are respectively4Sigmag-(Re=5.0 Bohr, De-=2.42 eV),2Sigmag+(Re=6.50 Bohr, De+=1.24 eV) and3Sigmag-(Re=4.85 Bohr, De0=1.03 eV; experimental ground state of Al2:3Sigmag-, Re=4.85 Bohr, 1.2<or approximately=D0<or approximately=2 eV). The binding energy of Al2-(4Sigmag-), clearly larger than that of Al2+(2Sigmag+, allows us to explain the special experimental result given by secondary ionic emission with a stability of Al2-stronger that than of Al2+.
The 2p-4d and 2p-4s transition arrays in fluorine-like copper XXI to arsenic XXV spectra
Authors: R J Hutcheon  , G E Bromage  , R L Cooke  , M H Key  , C L S Lewis
Published: July 2002

Wavelengths and oscillator strengths for the 2s22p5-2s22p44d, 4s electric-dipole transitions in fluorine-like ions from copper to arsenic have been calculated using Cowan and Griffin's HXR method (1970) and Slater-Condon theory (1978). Configuration mixing effects have been considered in detail. The predictions are compared with experimental spectra of copper, zinc, gallium and arsenic, obtained by focusing 30 GW laser pulses on to planar targets. The spectra were recorded with a convex-crystal X-ray spectrograph, and lines measured with accuracies of +or-2 to +or-5 mAA are identified with the stronger 2p-4d and 2p-4s transitions.
An interferometric and spectroscopic study on a high-current argon arc
Authors: P Baessler  , M Kock
Published: July 2002

The plasmas under investigation have been generated in a 4 mm bore wall-stabilised arc operating at atmospheric pressure with currents in the range 20 to 100 A in argon as well as in a mixture of argon and hydrogen. The electron density was determined with a statistical error of less than 1% by means of 2 lambda interferometry at lambda 632.8 nm and lambda 1152.3 nm. Possible systematic uncertainty caused by excited atoms was estimated to be less than 3% for the electron density range 1.89*1016 cm-3<or=Ne<or=1.64*1017 cm-3. Under the assumption of local thermodynamical equilibrium, experimental transition probabilities were obtained for the Ar I 430.0 nm line (Anm=3.2*105 s-1+or-7%) and the Ar II 480.6 nm line (Anm=7*107 s-1+or-14%). The values are not line-wing corrected. For the considered electron density range, the experimental half and quarter widths (in the reduced wavelength scale) of the Hbeta line were found to be constant within the limits of the experimental accuracy. In addition, it is shown that the experimental data of the Ar I 1430.0 nm line transition probabilities given in the literature can be brought to a common value by considering the present reduced widths of the Hbeta line profile.
Ion-radiator dynamic effects in Stark broadening of He II lines
Authors: R L Greene
Published: July 2002

A recently proposed relaxation theory of Stark broadening by ion perturbers is shown to be valid for the case of charged radiators. In addition, the straight-line-path assumption for the radiator and for the ion perturbers is examined and shown to be a good approximation for the first few members of the hydrogen isoelectronic sequence. Results of sample calculations of the Stark profiles of the Lyman- alpha and Balmer- alpha lines of He II are presented which show significant dynamic effects at electron densities as high as 1018cm-3. Extrapolation of the results to the Paschen- alpha (4686 AA) line suggests that the discrepancy between experiment and the unified-theory calculations is primarily due to the static ion-radiator approximation of the earlier theory.
Some spectroscopic aspects of TEA CO2laser-hydrogen glow discharge interactions
Authors: P Pignolet  , B Dubreuil  , A Catherinot
Published: July 2002

TEA CO2laser induced perturbation of shape and intensity of the first Balmer lines emitted by a hydrogen low-pressure glow discharge is investigated for radiation intensity ILin the 10 MW cm-2range. Because of the AC Stark effect, shifts of levels na=4, 5, 6, 7 and 8 are measured (i.e. for IL=8 MW cm-2, Delta E5=-0.59 cm-1, Delta E6=1.81 cm-1, Delta E7=0.55 cm-1). In addition, splitting and optical satellites originating from high orbital quantum numbers are clearly observed on the Hdeltaline. The laser one- and two-photon ionisation processes almost completely depopulate the excited states na>or=8. Because of collisional radiative processes the states na=4, 5, 6 and 7 which do not undergo photoionisation are also strongly depopulated. From a kinetic model, an approximate calculation of the collisional excitation transfer rate coefficient shows that the infrared laser perturbation method can be a powerful tool for investigating collisional excitation transfers connecting excited state population in plasmas.
Multiple scattering theory of electron capture in intermediate- to high-velocity collisions
Authors: K Taulbjerg  , J S Briggs
Published: July 2002

A fully symmetric form of the transition matrix element for electron capture is derived from a distorted-wave approach. The method is close in spirit to the continuum distorted-wave method of Cheshire but improves upon that method in several important respects.
Intensity correlations in resonance fluorescence from coupled two-level atoms
Authors: M Wiegand
Published: July 2002

The normalised intensity correlation function of resonance fluorescence radiation from two-level atoms is studied in various respects. In the first part of the paper the author discusses only interference effects for several uncoupled, linearly arranged, atoms. It is shown that, depending on the distance between the atoms, the direction of observation and the strength of the driving field, an antibunching effect as well as a very big bunching effect may be observed. In the second part of the paper he studies for the case of two atoms and weak external fields the influence of the interaction between the atoms by means of a perturbation theory. It is shown that the coupling between the atoms strongly modifies the normalised intensity correlation function in comparison with the case of uncoupled atoms. He takes as the driving field a coherent external field as well as laser light and thermal light.
The rotating-wave approximation in bound-free transitions
Authors: J Javanainen
Published: July 2002

The author solves the Schrodinger equation in terms of continued fractions for a system exhibiting light-induced bound-free transitions over an energy gap, without making the rotating-wave approximation. He describes the evolution of the bound state and the photoelectron spectrum by investigating the analytic properties of the Fourier transforms of the probability amplitudes. The main consequence of non-RWA transitions is a Bloch-Siegert shift of the continuum threshold, which may be comparable to RWA dynamic level shifts. At high intensities the counter-rotating wave also influences the evolution of the bound state, and gives rise to multiphoton transitions competing with RWA multiphoton transitions with an odd number of photons, but these effects are probably hard to isolate experimentally.
Shift of the He II Pαline in high density plasmas
Authors: C Fleurier  , P Le Gall
Published: July 2002

The authors have measured the shift of the line He II 468.57 nm at electron densities ranging from 1 to 6*1023m-3at 4 eV. A value of 0.0155 nm at 1023m-3was measured. A linear variation with the density is observed. Three different effects supposed to be responsible for the shift are reviewed. Comparison of the authors' results with theory is precluded by the large uncertainty concerning the plasma polarisation shift. Good agreement is observed with previous measurements.
A double-resonance measurement of the hydrogen Balmer-beta lineshape
Authors: G Kolbe  , D D Burgess
Published: July 2002

The line profile of Balmer beta for neutral hydrogen was observed for Ne=1015cm-3, Te=1.5 eV and Ne=5*1014cm-3, Te=0.85 eV. A double-resonance technique was used to discriminate against any molecular or other impurities that may have been filling in the dip at the core of the line. No evidence of any such impurity was found. However, a satellite observed on the red wing of the line profile taken at Ne=5*1014cm-3was shown to be an impurity and not associated with the Balmer-beta line profile.
Stark broadening of the hydrogen Hαline at low densities: fine structure and spontaneous emission effects
Authors: C Stehle  , N Feautrier
Published: July 2002

Taking the Halphaline as a test case, the authors give a proper treatment of the broadening including energy splitting and spontaneous emission effects. At low perturber densities (from 1010to 1014cm-3) the profile is calculated in the line centre within the impact approximation. Each collision is well described by a semiclassical perturbational approach. The energy separation of the sublevels for n=2 and n=3 produces a splitting of the line into seven components and modifies the collisional cross sections. These components, well resolved at perturber densities lower than 1012cm-3, are gradually mixed for increasing densities.
Effect of resonance energy transfer on donor fluorescence decay: A study by the phase and modulation technique
Authors: S Georghiou
Published: July 2002

Expressions are derived which contain experimentally measurable parameters and allow the study by the phase and modulation technique of the process of resonance energy transfer in viscous media. Computations are made of the donor fluorescence phase angle and degree of modulation for the system phenanthrene-acridine in cyclohexanol, and the results are compared with those corresponding to a statistical mixing of the molecules during the lifetime of the excited donor.
Continuum processes involving atomic systems with configurations 1s2...2pq
Authors: M J Conneely  , K Smith  , L Lipsky
Published: July 2002

The close-coupling approximation has been used to compute cross sections for a wide range of processes (electron impact, photodetachment and photoionization) involving atomic systems with incomplete outer 3p subshells. The processes have been chosen for their astrophysical interest. The purpose of the investigation is to provide a unified theoretical model for the wide variety of processes investigated and to compare the predictions of this close-coupling approach with previous theoretical approximations and experiment.
Energy transfer in organic systems. IX. Effect of diffusion on transfer efficiency
Authors: J B Birks  , M Salete  , S C P Leite
Published: July 2002

The quantum efficiency of radiationless energy transfer from naphthalene to 9,10-diphenylanthracene (DPA) and from 2,5- diphenyloxazole to DPA has been observed as a function of acceptor concentration in deoxygenated solutions in benzene, cyclohexane, cyclohexanol and benzene-cyclohexanol mixtures. The results agree reasonably with the Yokota-Tamimoto relation for the effect of diffusion on Forster dipole-dipole transfer, but there is evidence that multipole-multipole interaction terms may increase in the critical transfer distance R0when the latter is small.
Charge exchange between protons and atomic hydrogen. Comparison between theory and experiment
Authors: A Salin
Published: July 2002

The continuum-distorted wave method is used to calculate cross sections for electron-capture by protons in atomic hydrogen in the energy range 50-400 keV. The theoretical results are compared with the experimental ones and the agreement is seen to be very satisfactory.
Variational method for elastic scattering involving long range forces
Authors: W R Garrett
Published: July 2002

A new variational method is suggested for elastic scattering problems which involve long range polarization forces. Through a more appropriate choice of trial function, the method requires fewer variational parameters than the usual Hulthen-Kohn procedure and provides a variational bound on the phase shift. Sample results are obtained for electron and positron scattering from hydrogen.
On the validity of the unified classical path treatment of Stark broadening
Authors: U Frisch  , A Brissaud
Published: July 2002

The unified classical path (UCP) theory is tested for a line model similar to the Anderson model, for which the true profile is known. The central intensity is found to be zero, unlike that of the true profile. It is also shown that the central intensity of Ly alpha predicted by the UCP theory is much lower than the true value (corrected for absorption effects) in a certain realistic range of densities and temperatures. An improvement of the UCP theory is proposed to avoid this difficulty.
The ionization of boron and the isoelectronic ions carbon 11, nitrogen III, and oxygen IV by electron impact
Authors: E Stingl
Published: July 2002

Electron impact ionization of boron, carbon II, Nitrogen III, and oxygen IV, in their ground state configuration 1s22s22p, is treated, where allowance is made for inner shell ionization. Partial wave cross sections were calculated using both the so called 'modified' coulomb Born and the coulomb Born exchange approximation. Comparison is made with the only existing results of Peach and Lotz.
Dissociative ionization by electron impact. II. N+and N++from N2
Authors: A Crowe  , J W McConkey
Published: July 2002

For pt. I see ibid., vol. 6, no. 10, 2088 (1973). The angular distributions of both N+and N++produced in the electron impact dissociation of N2are presented for incident electron energies up to 300 eV. The N+distributions show only a small anisotropy and are in excellent agreement with a previous measurement. Angular distributions for the N++ion are also found to show little anisotropy. Absolute ionization efficiency curves for N2+, N+and N++from their respective thresholds to 300 eV are presented and comparisons are made with the results of other workers where available.
New diagram lines in the β-region of the L spectrum of holmium 67
Authors: A N Nigam  , Q S Kapoor
Published: July 2002

Employing a 40 cm curved mica crystal spectrograph of the transmission type, the beta -region of the L emission spectrum of holmium 67 has been investigated. Two new quadrupole lines at lambda =1601.2 and 1593.7 (xu) corresponding respectively to the transitions L3N2and L3N3have been reported for the first time. Wavelength data of different workers on the already reported lines in this region have been given for the sake of comparison.
A quantum unified theory of Stark broadening in the wings of spectral lines
Authors: N T Mihn  , N Feautrier  , H Van Regemorter
Published: July 2002

A simple and tractable quantum expression of the line wings is given when the upper and lower levels of the spectral lines are both perturbed. This quantum profile converges towards the impact regime at small distances from the line centre and towards the quasistatic limit when the usual semiclassical approximations are made. The correspondence between quantum and semiclassical expressions is examined, and the passage from the former to the latter clarifies the implicit assumptions which are always made in any unified semiclassical approach.
A generalization of the Coulomb-projected Born approximation
Authors: A D Stauffer  , L A Morgan
Published: July 2002

The Coulomb-projected Born approximation proposed by Geltman (1971) has been generalized by allowing only part of the nuclear charge to act directly on the incident particle. Analytic expressions are derived for elastic scattering cross sections. Results are presented for the 1s-1s, 1s-2s and 1s-2p transitions in H and the 1s-2s transitions in He+. Various criteria for the energy at which the Born approximation becomes valid are discussed.
Vacuum ultraviolet fluorescence from photodissociation fragments of N2O
Authors: L C Lee  , R W Carlson  , D L Judge  , M Ogawa
Published: July 2002

The vacuum ultraviolet fluorescence from fragments of N2O has been studied and the fluorescence production cross sections have been measured. The fragments are produced by extreme ultraviolet excitation using synchrotron radiation in the wavelength region lambda lambda 175-750 AA. Structure appearing in the lambda lambda 620-680 AA region has been identified as the absorption Rydberg series VI and VII. Two broad fluorescence bands were observed with peak cross sections of 0.6 Mb at lambda 370 AA and 0.4 Mb at lambda 230 AA. The photodissociation and dissociative ionization processes that are probably responsible for the observed fluorescence are discussed.
Fine-structure intervals in the 33P and n3D states of4He
Authors: A C Tam
Published: July 2002

The 33P1-33P2interval in4He is measured by the step-wise excitation level-crossing technique at 658.76+or-0.13 MHz, in fair agreement with the previous measurement by Wieder and Lamb (1957). The n3D fine-structure intervals are calculated by extending the theory of Van den Eynde et al. (1971), and fairly good overall agreement is obtained with recent experimental intervals, notably those of Astner et al. (1976). The singlet-triplet mixing coefficients of Van den Eynde et al. are recalculated using experimental singlet-triplet splittings, giving results in excellent agreement with the experimental values of Derouard et al. (1976).
X-ray scattering by phonons in sodium fluoride
Authors: J D Pirie  , T Smith
Published: July 2002

Measurements of the absolute intensity of the x-ray thermal scattering from sodium fluoride crystals have been made at a representative set of positions in reciprocal space. The distribution of scattered intensity can be interpreted to give the ionic form factor dependence on neighbouring ionic positions during thermal motion. The simplest interpretation is a fractional change of the form factor of a negative ion with relative displacement of a nearest-neighbour positive ion, increasing with the magnitude of the scattering vectorK.

The measurements also give transverse and longitudinal phonon dispersion curves for the [1,0,0], [1,1,0] and [1,1,1] directions. The errors in the frequencies are small except for a few higher-frequency phonons. The slopes of the dispersion curves at low wave vectors suggest that the differences between the first- and zero-sound velocities are less than 3%.
Ab initio calculation of cohesion energy, compressibility and density of the LiF crystal
Authors: J Andzelm  , L Piela
Published: July 2002

Properties of the LiF crystal have been calculated ab initio by taking into account the electrostatic and valence repulsion as well as induction and dispersion effects. The calculations have been carried out with an accuracy up to S2 terms. Two- and three-ion forces have been considered and the valence repulsion of the nearest- and next-nearest neighbours have been computed. All terms have been obtained by using those wavefunctions for the ions which give the expectation values of the energy very close to the corresponding Hartree-Fock limits. The results show that the induction and dispersion effects are of great importance. The computed lattice parameter (4.04 AA), density (2.65 g cm-3), compressibility (1.44*10-12 cm2 dyn-1) and cohesion energy (-244.9 kcal mole-1) agree well with the experimental values of 4.01-4.03 AA, 2.646 g cm-3, 1.43*10-12 cm2 dyn-1 and -246.8+or-2.1 kcal mole-1, respectively.
Electron-phonon interaction in a shell-model crystal
Authors: J M Vail
Published: July 2002

Electrostatic interaction of an excess electron with a shell-model crystal is assumed. The electron-phonon Hamiltonian is derived in the harmonic approximation for the large polaron. The screening potential for the electron arising from shell motion is obtained and discussed for adiabatic, non-adiabatic and intermediate electron-shell interaction. The screening potential arising from core motion is illustrated by a perturbative treatment of electron-phonon interaction in the adiabatic approximation. The mathematical structure of the self-consistent field produced by these screening effects is compared to that of the ion-size correction. Static lattice distortion, associated with self-trapping or a lattice defect, is introduced and its effects on shell screening on the electron-phonon interaction are discussed. Finally, the F-centre is used to illustrate the effect of a lattice defect on shell screening.