2s2p 1P Research Articles

Charge distribution of He states, excited by electron capture in He+ (26 keV)-He collisions

Using anticrossing spectroscopy we investigated one-electron capture in He+ (26 keV) – He collisions. The intensity of the triplet line λ (1s4l 3D → 1s2p 3P) ≈ 447.2 nm emitted by the fast atoms was measured as a function of an external electric field in the collision volume varied from −30 kV/cm to +30 kV/cm. The radiative decay of excited He atoms in an almost field-free region outside the collision volume was observed. The highly asymmetric intensity function could be reproduced theoretically using the density-matrix formalism. It indicates that the excited He atoms are in a state with a highly forward-backward asymmetric charge distribution, immediately after the charge-exchange collision.

Enhancement of edge impurity transport with ECRH in the HL-2A tokamak

Edge impurity transport is studied in electron cyclotron resonance heating (ECRH) L-mode plasmas of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy with which radial profiles of impurity line emissions are measured from the core region inside the last closed flux surface (LCFS) and the edge region in the scrape-off layer, simultaneously. The radial profile of carbon emissions of C V (2271 Å: 1s2s 3S–1s2p 3P) reconstructed into the local emissivity profile is analysed with a one-dimensional impurity transport code, and the diffusion coefficient and convective velocity of impurity ions are determined in the core region of the HL-2A tokamak. The impurity source is also determined with the measured absolute emissivity profiles of C IV (1548 Å: 1s22s 2S–1s22p 2P) located at the LCFS. The ratio of C V to C IV can therefore be used as an index to characterize the core impurity transport between the LCFS and the radial region of the C V emission at a normalized radius of about ρ = 0.6. The ratio measured from ohmic discharges shows a gradual decrease with electron density. However, the ratio suddenly decreases by a factor of three when the ECRH focused in the plasma centre is switched on, suggesting a strong enhancement of the impurity transport. The analysis with the transport code indicates a change in the convective term. The convective velocity of C4+ ions changes from inward to outward direction during the ECRH phase, while an inward velocity usually exists in the ohmic phase. Possible mechanisms for the reversal of the convective velocity are discussed.

Optimization of bremsstrahlung and characteristic line emission from aluminum plasma

Intense XUV and soft X-ray emission from laser-produced plasma sources are currently of great interest for a variety of research applications including inertial confinement fusion, and X-ray backlighting. Optimization of X-ray lines such as the Heα line (resonance line 1S2P 1P1−1S21S0), the inter-combination line (1S2P 3P1−1S21S0), the di-electronic satellite (1S2P22D−1S22P 2P) lines and the Kα lines along with bremsstrahlung radiation in the spectral range 0.8–8.5keV has been carried out using a 20J/500ps laser system. Optimization has been performed by changing the laser energy at fixed laser focal position and with focal position variation while keeping the laser energy constant.

Stimulated photon echo at the transition 0–1 in ytterbium vapour: circular polarizations

For the first time, stimulated photon echoes (SPE) generated in a gas at the transition 0↔1 by circularly polarized resonant radiation pulses were analysed both theoretically and experimentally. Experiments were performed for the inter-combination transition (6s2) 1S0↔ (6s6p) 3P1 of 174Yb (type 0↔1). The SPE generated by three radiation pulses of identical circular polarizations has the same circular polarization. When the second or the third exciting radiation pulse has a circular polarization opposite to that of the other two pulses, the SPE polarization coincides with that of the oppositely polarized pulse. Finally, when the first exciting radiation pulse has a circular polarization opposite to the circular polarizations of the second pulse and third pulse, the SPE does not appear either in pure ytterbium vapour or in its mixture with a heavy atomic buffer.

Negative ion resonance measurements in the autoionizing region of helium measured across the complete angular scattering range (0°–180°)

Excitation function measurements for the decay of the 2s22p 2P and 2s2p2 2D triply excited negative ion resonances in helium to singly excited n = 2 states have been measured. These excitation functions have been determined across the complete angular range (0–180°) using a magnetic angle changer with a soft-iron core. The convergent close-coupling method has been used to calculate the cross sections, with the underlying complexity of the problem not yet being able to be fully resolved. Agreement between the present experimental data and previous experimental data is good, with these excitation functions confirming the presence of an unusual (2s22p)2P resonance behaviour in the 21S channel at 90°, where this would not usually be expected. Resonance energy and width values have been obtained, with a mean energy for the (2s22p)2P resonance of 57.20 ± 0.08 eV and a mean width of 73 ± 20 meV, and a mean energy of the (2s2p2)2D resonance of 58.30 ± 0.08 eV and a mean width of 59 ± 27 meV. Resonant cross section and ρ2 values have been calculated across the angular range for the first time, providing angular distribution data on decay propensities for both resonances.

Absorption and emission of single attosecond light pulses in an autoionizing gaseous medium dressed by a time-delayed control field

An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the 2s2p(1P) autoionizing state, which is coupled to the 2s2(1S) autoionizing state by a femtosecond infrared laser with the intensity in the order of 10^{12} W/cm2. The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for delta-function pulses reveals the strong modification of the Fano lineshape in the spectra, where these features are quite universal and remain valid for realistic pulse conditions. We further account for the propagation of pulses in the medium and show that the EUV signal at the atomic resonance can be enhanced in the gaseous medium by more than 50% for specifically adjusted laser parameters, and that this enhancement persists as the EUV propagates in the gaseous medium. Our result demonstrates the high-level control of nonlinear optical effects that are achievable with attosecond pulses.

The stimulated photon echo generated by linearly polarized exciting pulses at the 0–1 transition in ytterbium vapour

The polarization of the stimulated photon echo (SPE) generated in a gas at the 0↔1 transition was analysed for the first time for all combinations of three linearly polarized resonant radiation exciting pulses. Experimental results were obtained for the SPE generated at the inter-combination transition (6s2) 1S0↔(6s6p) 3P1 of 174Yb (type 0↔1). In the case where one of the exciting pulses had linear polarization crossed with that of the other two exciting pulses, the SPE polarization proved to be linear and parallel to the crossed pulse polarization. When the second and third exciting pulses had linear polarizations crossed with that of the first one, no SPE in pure ytterbium appeared. For high dilution of ytterbium with the Xe atomic buffer, a collision-induced SPE appeared and showed a non-monotonic amplitude dependence on the buffer pressure and a polarization coinciding with the polarization of the first pulse.

Depolarization of the Resonance Line of C4+ due to the Electron Beam Density Effect

We investigate theoretically the linear polarization of the Helium-like resonance line 1s2p 1P1 1s2 1S0 emitted from C4+ ions using a steady-state collisional-radiative model, taking into account both the excitation from the ground level 1s2 1S0 and the metastable level 1s2s 1S0 in the population of the 1s2p 1P1 upper level of the resonance line. We analyze the behavior of the linear polarization of the resonance line with respect to the electron density in a wide range varying from 1011 to 1017 cm-3 and for three specific energies of the electron beam. Our results show that when the electron density is sufficiently high, the excitation 1s2s 1S0  1s2p 1P1 provide a significant contribution to the population rate of 1s2p 1P1 level which lead to an important depolarization of the resonance line.

Electronic spectroscopy of ytterbium in a neon matrix

The low-lying electronic states of Yb isolated in a solid Ne matrix are characterized through absorption and emission spectroscopy. The absorption spectra of matrix isolated Yb while pumped into its triplet states have been recorded for the first time and the 6s6p (3)P(J) → 5d6s (3)D(1, 2) transition frequencies obtained. Under matrix conditions, the structure of these states is found to be qualitatively the same as in the free atom, but the intersystem crossing rate is observed to be several orders of magnitude greater. A proposed explanation for this is curve crossings between the bound potential energy surface correlated to the 6s6p (1)P(1) state and the potential energy surfaces correlated to the 5d6s (3)D(1, 2) states in isolation. The potentials of the Yb·Ne dimer in its lowest electronic states are computed ab initio and used in a pairwise cluster model to explicitly demonstrate these curve crossings.

Towards a measurement of the 2s2p 3P0 → 2s2 1S0 E1M1 two photon transition rate in Be-like xenon ions

Dielectronic recombination (DR) of Be-like 136Xe50+ was measured by employing the electron-ion merged-beam technique at the storage ring ESR of GSI in Darmstadt, Germany. We present results covering the center-of-mass energy range 0 − 4 eV. In addition to Rydberg series of resonances associated with ground-state intra L-shell excitations, resonances were found which are formed during DR of initially metastable 136Xe50+(2s2p 3P0) parent ions. This state is the first excited state and is long lived because the J=0 → J=0 transition and the absence of nuclear spin make the rare E1M1 two-photon transition the lowest-order decay channel. Future measurements which will monitor the strength of the resonances as function of storage time may be exploited for a measurement of the E1M1 two photon transition rate.

Electron impact double-excitation of helium 2ℓ2ℓ' autoionizing states using the (e, 2e) technique

Coplanar (e, 2e) triple differential cross sections (TDCS) measurements are reported for the helium autoionizing doubly excited states, (2s2)1 S, (2p2)1D, and (2s2p)1P, for an incident electron energy of 250 eV and a scattering angle of −13°, corresponding to a momentum transfer of 1.06 a.u. The presence of autoionization results in a clearly visible recoil peak in the TDCS structure with a shape that is strongly dependent on the orbital angular momentum L of the resonance. The resonance contributions together with the strong background of direct ionization processes lead to a complicated asymmetric structure of the resonance profile in the TDCS.

Towards a storage-ring measurement of the hyperfine induced 2s2p 3P0 → 2s2 1S0 transition rate in berylliumlike sulfur

We report on progress towards an accurate measurement of the hyperfine induced 2s2p 3P0 → 2s2 1S0 transition rate in Be-like 33S12+ using isotopically enriched ion beams in a heavy-ion storage ring.

Geometric phases generated by the non-trivial spatial topology of static vector fields linearly coupled to a neutral spin-endowed particle: application to 171Yb atoms trapped in a 2D optical lattice

We have constructed the geometric phases emerging from the non-trivial topology of a space-dependent magnetic field B(r), interacting with the spin magnetic moment of a neutral particle. Our basic tool, adapted from a previous work on Berry’s phases, is the space-dependent unitary transformation , which leads to the identity, , at each point r. In the ‘rotated’ Hamiltonian , is replaced by the non-Abelian covariant derivative where can be written as A1(r)Sx + A2(r)Sy + A3(r)Sz. The Abelian differentials Ak(r)·dr are given in terms of the Euler angles defining the orientation of B(r). The non-Abelian field transforms as a Yang–Mills field; however, its vanishing ‘curvature’ reveals its purely geometric character. We have defined a perturbation scheme based upon the assumption that in the longitudinal field A3(r) dominates the transverse field A1, 2(r) contributions, evaluated to second order. The geometry embedded in both the vector field A3(r) and the geometric magnetic field is described by their associated Aharonov–Bohm phase. As an illustration we study the physics of cold 171Yb atoms dressed by overlaying two circularly polarized stationary waves with orthogonal directions, which form a 2D square optical lattice. The frequency is tuned midway between the two hyperfine levels of the (6s6p)3P1 states to protect the optical B(r) field generated by the lattice from the dressed atom instability. The geometric field B3(r) is computed analytically in terms of the Euler angles. The magnitude of the second-order corrections due to the transverse fields can be reduced to the per cent level by a choice of light intensity that keeps the dressed atom loss rate ⩽5 s−1. A second optical lattice can be designed to confine the atoms inside 2D domains where . We extend our analysis to the case of a triangular lattice.

Resonant enhancement of a single attosecond pulse in a gas medium by a time-delayed control field

An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p(1P) and 2s2(1S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photon wave packet by an ultrashort dressing field for the first time. We analyse the photon and electron dynamics and determine the dressing condition that maximizes this enhancement. The result demonstrates new possibilities of attosecond optical control.

Photon echo generated at the transition 0–1 in ytterbium vapor

Photon echo generated at the inter-combination transition (6s2) 1S0 − (6s6p) 3P1 of 174Yb was investigated for pure ytterbium vapor and for its mixtures with atomic buffers. In pure ytterbium vapor, the polarization of photon echoes at this 0–1 transition coincides with the polarization of the second exciting pulse for all combinations of linear and circular polarizations of exciting radiation pulses. Photon echo does not appear either for linear orthogonal or for opposite circular polarizations of exciting pulses in pure ytterbium. In mixtures of ytterbium with atomic buffers (Kr, Xe), collision induced photon echo arises only for exciting pulses of linear orthogonal polarizations, its power is essentially less than that of the ordinary echo generated by pulses with parallel polarizations in the same mixture. Polarization of collision induced echo is linear, and it coincides with polarization of the first exciting pulse. Experimental results agree with calculations, and they confirm that the collision induced photon echo at this transition arises exclusively due to anisotropy of depolarizing collisions.

Experimental constraints on the polarizabilities of the6s2 1S0and6s6p 3P0ostates of Yb

We utilize accurate experimental data available in the literature to yield bounds on the polarizabilities of the ground and first excited states of atomic Yb. For the 6s^2 1S0 ground state, we find the polarizability alpha to be constrained to 134.4<alpha<144.2 in atomic units, while for the 6s6p 3P0 excited state we find 280.1<alpha<289.9. The uncertainty in each of these values is 1.0. These constraints provide a valuable check for ab initio and semi-empirical methods used to compute polarizabilities and other related properties in Yb.

Excitation of He atoms by 10–30 keV He+ impact

The anticrossing spectra of the helium line λ(1s4l–1s2p 3P) = 447.2 nm were measured for 10–30 keV He+–He collisions. The theoretical intensity functions were calculated taking into account cascade processes, the inhomogeneity of the axial electric field in the collision volume and the density distribution of the target He atoms. Comparing the theoretical intensities with the measured ones, the post-collisional states of He atoms were determined. The results indicate that for this projectile-energy range, the electronic charge distributions of the excited-atom states change from almost symmetric to strongly asymmetric ones with large electric dipole moments. This result suggests that for projectile energies of 20–30 keV, Paul-trap promotion becomes the main excitation mechanism.

Differential cross sections and electron impact coherence parameters for elastic electron scattering from laser-excited 138Ba

We have measured and calculated differential cross sections and the P3 collision Stokes parameter for elastic electron scattering from the laser-excited (…6s6p 1P1) and cascade-populated (…6s5d 3D1, 2) levels of barium at 5, 10, and 20 eV impact energy. Measurements were performed using a crossed-beam apparatus, where excited-state atomic targets were prepared with resonant laser radiation incident on the electron–atom interaction region. Calculations were performed in the convergent close-coupling (CCC), relativistic CCC, and optical-model potential approximations. We present measured and calculated partial differential cross sections, which relate to differential cross sections and electron impact coherence parameters through the target state anisotropy introduced by the laser geometry.

A complex scaled multiconfigurational time-dependent Hartree–Fock method for studying resonant states

A new complex scaled multiconfigurational time-dependent Hartree–Fock (CMCTDHF) method (also called the complex scaled multiconfigurational linear response (CMCLR)) has been developed. CMCTDHF uses complex multiconfigurational Hartree–Fock states as initial states, and in real space has been successfully used to study electronic excitation energies and linear response properties. With CMCTDHF, shape and Feshbach resonances of an N-electron system can be obtained by studying the excited states of an (N + 1)-electron system with the trajectory method. Our first application of CMCTDHF has been for the beryllium doubly excited state 1Po (1s2 2p 3s) with a 14s 14p 5d Gaussian basis and a 2s 2p 3s 3p 3d complete active space (CAS). We find the resonance position and width are (11.01, 0.434) eV, which agrees very well with the experimental result (10.89, 0.531) eV. This is the first time that the complex scaling method has been successfully used to study beryllium doubly excited states. Our calculations show that CMCTDHF is a practical and important tool for studying resonant states.

Spectroscopy and dynamics of barium-doped helium nanodroplets

Excitation spectra up to the ionization threshold are reported for barium atoms located on the surface of helium nanodroplets. For states with low principal quantum number, the resonances are substantially broadened and shifted towards higher energy with respect to the gas phase. This has been attributed to the repulsive interaction of the excited atom with the helium at the Franck-Condon region. In contrast, for states with high principal quantum number the resonances are narrower and shifted towards lower energies. Photoelectron and ZEKE spectroscopy reveal that the redshift results from a lowering of the ionization threshold due to polarization of the helium by the barium ionic core. As a result of the repulsive interaction with the helium, excited barium atoms desorb from the surface of the droplets. Only when excited to the 6s6p (1)P(1) state, which reveals an attractive interaction with the helium, the atoms remain attached to the droplets.