Helium Spectra Research Articles

Evidence of the2s2p(1P) doubly excited state in the harmonic generation spectrum of helium

By solving the two-active-electron time-dependent Schr\odinger equation in an intense, ultrashort laser field, we investigate evidence of electron correlations in the high-order harmonic generation spectrum of helium. As the frequency of the driving laser pulse varies from 4.6 to 6.6 eV, the 13th, 11th, and 9th harmonics sequentially become resonant with the transition between the ground state and the isolated $2s2p({}^{1}P)$ autoionizing state of helium, which dramatically enhances these harmonics and changes their profiles. When each of the 9th and 13th harmonics are in resonance with this autoionizing state, there is also a low-order multiphoton resonance with a Rydberg state, resulting in a particularly large enhancement of these harmonics relative to neighboring harmonics. When the 11th harmonic is in resonance with the $2s2p({}^{1}P)$ autoionizing state, the 13th harmonic is simultaneously in resonance with numerous higher-energy autoionizing states, resulting in a competition between these two harmonics for intensity. These results demonstrate that even electron correlations occurring over a narrow energy interval can have a significant effect on strong-field processes such as harmonic generation.

Atomic processes in planetary nebulae

AbstractProgress in the study of the atomic processes in planetary nebulae (PNe) is reviewed, focusing on the literature published since the last IAU symposium on PNe five years ago. High quality photoionization cross sections and recombination coefficients are now available for the first six ions of the trans-iron elements selenium and krypton, enabling robust modeling of their ionization structure and consequently converting the measured ionic abundances to elemental abundances. Major progress has been achieved in utilizing the recombination spectra of helium and heavy element ions to probe the nebular physical conditions. New ab initio, density-dependent effective recombination coefficients have been calculated for the recombination spectra of O ii and N ii, down to very low temperatures (~ 100–300 K). Plasma diagnostics based entirely on those heavy element recombination lines are developed and applied to the observations. It is shown that these heavy element recombination lines originate predominately from cold plasmas of temperatures ~ 1,000 K, in agreement with the predictions of the bi-abundance model that has been proposed to explain the dichotomy of nebular plasma diagnostics and abundance determinations using collisional excited lines (CELs) on the one hand and optical recombination lines (ORLs) on the other.

The Proton and Helium cosmic ray spectra from 50 GeV to 15 TeV

To date, very little has been done in the field of research of the nuclei cosmic ray spectra in the energy range between 1 and 10 TeV per nucleon and especially there were even fewer direct measurements in space. The PAMELA experiment (Casolini et al., 2008) has a possibility to make progress in this range. The method of the measurement of the helium and proton cosmic ray spectra with the energy higher than 50 GeV/nucleon was developed mainly with the use of the PAMELA calorimeter. This analysis method and the results obtained based on 3 years of measurements are presented.

Precision Velocity Measurements of Pulsed Supersonic Jets

We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t ≤ 10(-4), typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/<v> < 3 × 10(-4) has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.

Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots

Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature.

COSMIC-RAY HELIUM HARDENING

Recent observations by the ATIC-2, CREAM and PAMELA experiments suggest that the spectrum of cosmic- ray (CR) helium is harder than that of CR protons below the knee energy and all CR spectra become hard at 100 GeV/nucleon. According to DSA theory, the spectrum of accelerated particles at a shock does not depend on CR el- ements, but depends only on the velocity profile of the shock. Thus, naively, recent CR observations seem to show that the acceleration site of CR helium is different from that of CR protons. We propose a new idea, that higher energy CRs are generated in a more helium-rich region, to explain the hardening without introducing different sources for CR helium. We argue that CRs are produced in a chemically enriched region, such as a superbubble, and the outward-decreasing abundance naturally leads to the hard spectrum of CR helium if CRs escape from the supernova remnant shock in an energy-dependent way. Our idea uses the fact that CRs escaping from SNRs generally have a different spectrum than those of the acceleration site. The runaway CR spectrum depends not only on the acceleration spectrum at shocks but also on the evolution of the maximum energy and the number of accelerated CRs. We also suggest that the spectral hardening of CRs is caused by the decreasing Mach number in the high-temperature medium. Both the inhomogeneous abundance and the high temperature can be realized in the hot superbubbles with multiple supernovae.

COSMIC-RAY PROTON AND HELIUM SPECTRA FROM THE FIRST CREAM FLIGHT

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004-2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of ~38.5 km with an average atmospheric overburden of ~3.9 g cm$^{-2}$. Individual elements are clearly separated with a charge resolution of ~0.15 e (in charge units) and ~0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of -2.66 $\pm$ 0.02 for protons from 2.5 TeV to 250 TeV and -2.58 $\pm$ 0.02 for helium nuclei from 630 GeV/nucleon to 63 TeV/nucleon. They are harder than previous measurements at a few tens of GeV/nucleon. The helium flux is higher than that expected from the extrapolation of the power law fitted to the lower-energy data. The relative abundance of protons to helium nuclei is 9.1 $\pm$ 0.5 for the range from 2.5 TeV/nucleon to 63 TeV/nucleon. This ratio is considerably smaller than the previous measurements at a few tens of GeV/nucleon.

Cosmic-ray energy spectrum around the knee obtained by the Tibet experiment and future prospects

The observation of the extensive air showers has been carried out since 1990 by Tibet air-shower array to study the origin of the cosmic-rays. In order to investigate the chemical composition around the knee, air-shower core detector was used in two experimental phases aiming to separate light primary elements (proton and helium). In a first phase experiment made in 1996–1999, primary proton and helium spectra were measured using a hybrid detector consisted of Tibet II air-shower array, emulsion chambers, and burst detectors. The result indicated that the fraction of light elements among the all particle spectrum is less than 30%. We performed the second phase experiment in 2002–2005 to measure proton + helium together to confirm the previous result with higher statistics by improving the detection efficiency for the air-shower cores. The P + He spectrum was obtained in a good agreement with previous result by several times higher statistics. The flux of the light elements, together with the all-particle energy spectrum, indicates the dominance of nuclei heavier than helium in the energy range around the knee. We also present the outline of the next phase experiment, which is to measure the heavy elements around the knee, using low threshold air-shower core detector named YAC.

Electron-impact ionization excitation of helium in the quasiphoton regime

The triply differential cross section of ionization excitation of helium, leaving the residual ion in the n = 2 excited states, is evaluated for the kinematics considered experimentally by Dupre et al. [J. Phys. B 25, 259 (1992)]. The interaction of the incident electron with the target is described at the first order, while the interaction of the ejected electron with the residual ion is treated very accurately within the formalism of the Jacobi matrix method. In the quasiphoton limit and for low ejected electron energies, the presence of series of doubly excited states, mainly below the n = 3 single ionization threshold in helium, makes the triply differential cross sections extremely sensitive to both the energy and the emission angle of the ejected electron. We show that the convolution of our results with a Gaussian energy profile, in which the full width at half-maximum corresponds to the energy resolution in the experiment, has a significant effect. Our results suggest that it is also important to account for the finite resolution on the measurement of the scattering angle when the experimental data are compared to the theoretical predictions. Comparison of our theoretical results convoluted both in energy and in angle with the experimental data demonstrates the importance of an accurate description of the helium spectrum. A possible two-step mechanism involving single ionization of the target followed by excitation of the core electron is proposed to explain the remaining discrepancies.

Role of metastable atoms in the propagation of atmospheric pressure dielectric barrier discharge jets

In the experiment of plasma jets generated in a tube dielectric barrier discharge configuration, three distinguishable modes, namely, laminar, transition, and turbulent jet modes, have been identified. Flows of helium, neon, and argon gases shared the hydrodynamic law when their plasma jets spraying into ambient air of atmospheric pressure and room temperature. Aiming to reveal the basic processes, we propose that plasma jet length is mainly determined by reactions involving metastable atoms. These processes are responsible for the variation in plasma jet length versus gas flow rate and working gas species. To investigate this proposal in detail, we have obtained three significant experimental results, i.e., (1) the plasma jet lengths of helium, neon, and argon are different; (2) the plasma jet length of krypton slightly changes with gas flow rate, with three modes indistinguishable; and (3) there are large differences between optical emission spectra of helium, neon, argon, and krypton flow gases. These observations are in good agreement with our proposal.

About possibility of isotope dating of native gold by the (U-Th)/He method

For investigation of helium in native gold, a new measuring complex was created and used: the high sensitivity mass spectrometer MSU-G (ZAO SKB “SPECTRON”). The sensitivity of measuring 4He was 5.3 × 10−13 cm3/g per impulse. Experiments in stepwise heating of samples have been carried out, and the kinetics of radiogenic 4He emanation from native gold was investigated. Migration parameters (activation energy and frequency factor) were determined. Model calculations of stability (closure temperature) of radiogenic 4He in the native gold structure with a given time and temperature of thermal influences were made using the data received. The concentration of 4He in native gold from the original deposit Nesterovskoe is (4.7 ± 0.1) × 10−5 cm3/g in the sample from the placer; from Chudnoe deposit, it is (3.8 ± 0.1) × 10−5 cm3/g; from sulfide deposits of Kitoiskii knot of Eastern Sayani, it is (1.9 ± 0.1) × 10−5 cm3/g; and from the South Muiskii ore region it is (8.7 ± 0.5) × 10−7 cm3/g. The received curve lines of kinetics of 4He emanation from native gold show that radiogenic helium is well bonded in the native gold structure: in all the examined samples, most 4He emanates only by reaching the temperature of 950–1000°C. A specific feature of the kinetics of radiogenic 4He emanation in all examined samples is an outburstlike emanation in the form of a peak of large amplitude in the area of temperatures near the melting temperature point of gold. This is stipulated by the existence of helium bubbles released by metals only while they melt. The spectrum of helium thermal desorption from native gold has a complicated form and is a result of superposition of several peaks. This proves the migration of groups of atoms located in the gold structure in different energy states. Very large values of the activation energy of helium migration from native gold were received: up to 161–176 kcal/mol. Extremely large values of the frequency factor, from 2 × 1018 to 3 × 1032, correspond to such values of activation energies. This is caused probably by helium migration in the form of gas bubbles. The received data indicate the very high stability of the (U-Th)/He isotope system in native gold. Using the (U-Th)/He method of isotope geochronology seems to be very promising for isotope dating of these strategic raw materials.

Helium lines in RR Lyrae spectra

Context. During a comparative study of atmospheric phenomena that occur during the pulsation cycles of 10 RRab stars I unexpectedly found helium lines in emission and/or absorption in all 10 stars during rising light. The progression of events in the time-evolution of the helium spectrum differs in detail from star to star, but 9 of the 10 stars share a number of general characteristics, illustrated approximately by the behavior of RV Oct described in this Note. Aims. My long-term aim is to provide a comprehensive empirical description of atmospheric phenomena that take place during the pulsation cycles of RRab stars. The short-term goal of this paper is to inform readers that measurements of helium lines place new constraints on shock-wave models for RRab stars. Methods. Results presented here are based on measurements and analysis of observations made with the echelle spectrograph of the du Pont 2.5-m telescope at Las Campanas Observatory. Results. A general pattern of behavior emerges albeit with significant star-to-star variation. He I λ5376 (D3) appears first as a pure emission feature, unlike the Balmer lines for which the emission profile is always overlain initially by strong redward-displaced absorption. The phase of maximum D3 emission coincides approximately with that of Hα, but the duration of conspicuous D3 emission (in excess of the local continuum) is less than 15 mn, approximately half the duration of similarly defined Hα emission. The emission phase is followed by the appearance, first, of redward-displaced absorption, and shortly thereafter by a second violet-displaced absorption component that strengthens with advancing phase. Both absorption components gradually weaken and disappear after radial-velocity minimum, which lags luminosity maximum by no more than 0.01P as discussed in Sect. 3. However, bona fide He I λ5876 emission flux in excess of the local continuum flux reappears near maximum light for some stars in the sample and persists for about 45 mn. The radial velocities of the initial and final He I λ5876 emission features differ little from the time-average value of the metallic-line radial velocities for RV Octantis. However, the absorption features appear to distort the location of the emission centroid, when they are strong.

STEREO Observations of Energetic Ions in Corotating Interaction Regions During the May 2007 Solar Events

Elemental composition and energy spectra of ∼ 0.1 – 1.0 MeV/n heavy ions were analyzed in two corotating interaction region (CIR) events starting on 18 May (CIR1), and 24 May (CIR2), 2007, that occurred during two weak solar energetic particle (SEP) events. The data were taken by the Suprathermal Ion Telescope (SIT) instruments onboard STEREO-A and B. The ion intensity increases related to the CIR2 were much more pronounced compared to those associated with the CIR1, although the total pressure, the magnetic field magnitude and the speed difference between high and slow solar wind were less enhanced. We found that the ratio of CIR2 to CIR1 helium fluence decreases with energy as a power – law below ∼ 1 MeV/n. The energy spectra of hydrogen and helium inside the CIR2 region are much softer compared to the spectra observed in the CIR1 region. Based on these findings, we argue that the larger intensities in the CIR2 event could be due to an additional source population. The elemental abundances in CIR2 and the high-speed stream of the CIR1 region have a SEP-like composition. The energy spectra of hydrogen and helium inside the CIR2 were close to the preceding SEP event spectra. The similarity in the elemental abundances and in the spectral slopes suggests that the seed population of CIR2 are ions from the preceding SEP event. For the CIR1 event the ions seem to be contaminated by SEPs.

Time-dependent hartree-fock theory for atoms

The coupled and uncoupled versions of time-dependent Hartree–Fock theory are presented. The theory provides a way of calculating the linear response of an atomic system to an external time-dependent perturbation, and thence the Green's function. It is shown that the coupled version is equivalent to the Random Phase Approximation and that it contains correlation to at least first order, while the uncoupled version contains no correlation. The effect of the correlation included in the coupled version on the dipole spectrum of two electron systems is studied. This study is based on numerical solution of the equations for the dipole response of helium and lithium II. The coupled and uncoupled dipole spectra of helium and parts of these spectra of lithium II are given. Phase shifts for electron-helium ion scattering are also given. Calculations for helium of long range forces, average excitation energies involved in the theory of range and straggling of fast charged particles and the Lamb shift, and the two photon decay spectrum and radiative lifetimes for the decay (1s2s)1S (1s2)1S, are described. It is concluded that, for two electron systems, the correlation included in the coupled theory displaces rather too much dipole oscillator strength into the continuum (while maintaining the Thomas-Kuhn sum rule) and overestimates exchange in the energy levels and low energy phase shifts, and that the coupled dipole spectrum is more accurate than the uncoupled one.

Neutron scattering study of the excitation spectrum of solid helium at ultra-low temperatures

There has been a resurgence of interest in the properties of solid helium due to the recent discovery of non-classical rotational inertia (NCRI) in solid 4He by Chan and coworkers below 200 mK which they have interpreted as a transition to a ‘supersolid’ phase. We have carried out a series of elastic and inelastic neutron scattering measurements on single crystals of hcp 4He at temperatures down to 60 mK. While we have found no direct evidence of any change in the excitation spectrum at low temperatures, we have found that the excitation spectrum of solid 4He shows several interesting features, including extra branches in addition to the phonon branches. We interpret these extra branches as single particle excitations due to propagating vacancy waves, which map on to the famous ‘roton minimum’ long known in the excitation spectrum of superfluid liquid 4He. The results show that in fact solid 4He shares several features in common with the superfluid.

TDDFT–Generalized kick perturbations and monitoring observables for calculation of excitation energies

This article introduces a generalized way to calculate excitation energies of many-body Hamilton operators by propagating kicked orbitals within time-dependent density functional theory (TDDFT). We use generalized δ(t) perturbations and monitor certain observables of the reduced one-body density to calculate the excitation energies of a static many-particle problem. This generalizes the usual procedure of a linear momentum kick and monitoring the induced t-dependent dipole moment or higher moments. In particular we propose to monitor the time-dependent local charges (e.g. Voronoi, …) in a molecule. As a simple application the S = 0 singlet excitation spectrum of helium is calculated with this formalism included in a standard TDDFT-software package for atoms, QPROP.

Helium Line Formation and Abundance during a C‐Class Flare

During a coordinated campaign which took place in May 2001, a C-class flare was observed both with SOHO instruments and with the Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak. In two previous papers we have described the observations and discussed some dynamical aspects of the earlier phases of the flare, as well as the helium line formation in the active region prior to the event. Here we extend the analysis of the helium line formation to the later phases of the flare in two different locations of the flaring area. We have devised a new technique, exploiting all available information from various SOHO instruments, to determine the spectral distribution of the photoionizing EUV radiation produced by the corona overlying the two target regions. In order to find semiempirical models matching all of our observables, we analyzed the effect on the calculated helium spectrum both of A(He) (the He abundance) and of the uncertainties in the incident EUV radiation (level and spectral distribution). We found that the abundance has in most cases (but not in all) a larger effect than the coronal back-radiation. The result of our analysis is that, considering the error of the measured lines, and adopting our best estimate for the coronal EUV illumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300 K) and transition region yields reasonably good matches for all the observed lines. This value is marginally consistent with the most commonly accepted photospheric value: A(He)=0.085.

Simulation of the ARGO-YBJ experiment operated in Scaler Mode

The ARGO-YBJ experiment is a large area full coverage detector operating at high mountain altitude (4300 m a.s.l., Tibet, China, ∼6700 m 2 active surface). In scaler mode, the particle rate is recorded at fixed time intervals (0.5 s), allowing the achievement of an energy threshold as low as E t h ∼ 1 GeV for the photons. This result, together with the estimate of the effective area as a function of the primary energy, has been obtained using the simulation code CORSIKA6201 for the shower development in the atmosphere and a dedicated and detailed detector simulation, ARGOG, based on GEANT3.21. With this simulation chain, and using the known primary proton and helium spectra, we obtained counting rates consistent with the measured ones, making us confident in the expectation of the detector performance as far as the photon shower detection is concerned, which is of crucial importance to determine the capability of the ARGO-YBJ experiment to detect gamma-ray bursts (GRBs).

Cosmic ray data and their interpretation: about the Tibet Shower Array: Chemical Composition of Cosmic Rays at the Knee Viewed from the Tibet Air Shower Experiment

A brief summary is given about present status of composition measurement of cosmic rays by direct observations and indirect ones which provides key information on the origin of the cosmic rays. The result of Tibet hybrid experiment measuring proton and helium spectra at the knee is presented, in which the decreasing fractions of both of the proton and helium component to the all particle spectrum with increasing energy are observed suggesting that the knee is dominated by heavy nuclei. It is planed to explicitly measure the iron spectrum at the knee by detecting the core structure of the air showers in the next phase of Tibet experiment ‘YAC’ ( Y angbajing A ir shower C ore array).

The Gamma‐Ray Albedo of the Moon

We use the GEANT4 Monte Carlo framework to calculate the γ-ray albedo of the Moon due to interactions of cosmic-ray (CR) nuclei with moon rock. Our calculation of the albedo spectrum agrees with the EGRET data. We show that the spectrum of γ-rays from the Moon is very steep with an effective cutoff around 3-4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a narrow pion-decay line at 67.5 MeV, perhaps unique in astrophysics. Apart from other astrophysical sources, the albedo spectrum of the Moon is well understood, including its absolute normalization; this makes it a useful standard candle for γ-ray telescopes. The steep albedo spectrum also provides a unique opportunity for energy calibration of γ-ray telescopes, such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). Since the albedo flux depends on the incident CR spectrum, which changes over the solar cycle, it is possible to monitor the CR spectrum using the albedo γ-ray flux. Simultaneous measurements of CR proton and helium spectra by the Payload for Antimatter-Matter Exploration and Light-Nuclei Astrophysics (PAMELA), and observations of the albedo γ-rays by the GLAST Large Area Telescope (LAT), can be used to test the model predictions and will enable the LAT to monitor the CR spectrum near the Earth beyond the lifetime of the PAMELA.