Excited States Of He Research Articles

He<sub>2</sub>分子激发态的势能函数和离解极限研究

He<sub>2</sub>分子激发态的势能函数和离解极限研究

Thermometry in noble gas dielectric barrier discharges at atmospheric pressure using optical emission spectroscopy

We measure the temperature, T, of dielectric barrier discharges (DBD), in noble gases using optical emission spectroscopy (OES), by analysing rotational bands in the emission spectra of the first negative system (FNS) of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> . This has the advantage that rotational structure can be fully resolved even with a spectrograph of average performance, and that the rotational temperature, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> (~ T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gas</sub> ) can then be determined from a conventional Boltzmann plot. Ionization of N2 occurs mainly via Penning transfer from metastable excited states of He (ca. 20 eV) or Ne (ca. 16.6 eV). Using two glass-walled DBD chambers of very different volumes (0.1 and 20 liters), we have studied atmospheric-pressure discharges in flowing helium (He) or neon (Ne) containing traces of nitrogen. Discharges were excited by audio-frequency (10 kHz) high voltage (HV) using a needle as the HV electrode and a dielectric (alumina)-covered planar grounded counter-electrode. OE spectra were acquired with a 0.5 m focal length spectrograph, coupled to an intensified charge coupled device (ICCD) detector. Using the (0-0) R-branch of the FNS N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> (B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> - X <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) bands near a wavelength of 391.4 nm, we have measured axial (inter-electrode) distributions of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> for the two different reactor volumes in both He and Ne. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> values were found to be highest at the needle electrode, of about 450 K and 740 K for He and Ne, respectively; in He, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> dropped to a minimum of about 405 K at the mid-gap position in the small chamber, and ~ 360 K near the planar electrode in the large chamber. We conclude that temperatures in noble gas discharges depend critically on thermal conductivities of the particular gases (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">He</sub> = 1.9; K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ne</sub> = 0.6, both in mW.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and on other experimental factors that influence heat transfer.

Sturmian expansions for two-electron atomic systems: Singly and doubly excited states

We present a configuration interaction (CI) method based on the Sturmian expansion for bound states of a two-electron atomic system. These Sturmian functions are solutions of one-electron quantum mechanical problems, where the eigenvalue is the magnitude of a short-range potential. Also, they fulfill the long-range boundary conditions of Coulomb potentials. We choose to expand the Sturmians of the CI basis using ${L}^{2}$ Laguerre-type functions. We compute ground and single-excited states energies for He and ${\mathrm{H}}^{\ensuremath{-}}$. Moreover, we are able to obtain energies and widths of double excited states of He, using a Sturmian basis with outgoing boundary conditions. In all cases, our ansatz outperforms other CI calculations, for similar basis size.

Electron‐pair extracule densities for low‐lying excited states of He and Li+

AbstractThe electron‐pair extracule density is the probability density function for the position vector of the center‐of‐mass of an electron pair. Accurate calculations of spherically averaged extracule densities d(R) and their moments are reported for the 23S, 23P, and 21P states of the helium atom and lithium cation. The computations are based on explicitly correlated wave functions with energy errors of only 2–3 nanohartrees. The role of electron correlation is probed by comparison with Hartree‐Fock calculations, and Fermi correlation is probed by the differences between the singlet and triplet P states. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Static polarizabilities of helium and alkali atoms, and their isoelectronic ions

A rather simple method of calculation of the scalar and tensor components of static polarizability was developed for a wide range of atoms and ions. The general formulas for polarizabilities of SL terms, J levels, and F components of hyperfine structure are given. The results of calculation for ground and some excited states of He and alkali atoms (Li, Na, K, Rb, Cs) are presented. The comparison of the results with available theoretical and experimental data shows that, in most cases, the differences do not exceed 10%. The polarizabilities for the Li+, Be+, B2+, C3+, Ne7+, Mg+, Al2+, Si3+, Ar7+, Ca+, Sc2+, Ti3+, Sr+, and Ba+ ions are also given.

Ab initio interaction potentials for X and B excited states of He–I2 for studying dynamics

Ab initio CCSD(T) and MRCI approaches were employed to construct potential energy surfaces of the ground and the B electronic excited states of He–I2 complex, while full quantum mechanical methods were applied to study its spectroscopy and dynamics. A description of the approach adopted, together with the results obtained and their comparison with recent experimental data, as well as further improvements are presented.

Angular effects in autoionization of 3P doubly excited states in He

The first members of dipole allowed 3Po doubly excited series in helium have been observed in resonant photoexcitation of 1s2s 3Se metastable atoms. A good agreement measured relative photoionization cross sections is achieved when theory includes the radiation damping and, also important, the effects of spin-orbit multiplet splitting on electron angular distribution.

Primordial helium recombination. I. Feedback, line transfer, and continuum opacity

Precision measurements of the cosmic microwave background temperature anisotropy on scales l>500 will be available in the near future. Successful interpretation of these data is dependent on a detailed understanding of the damping tail and cosmological recombination of both hydrogen and helium. This paper and two companion papers are devoted to a precise calculation of helium recombination. We discuss several aspects of the standard recombination picture, and then include feedback, radiative transfer in He i lines with partial redistribution, and continuum opacity from H i photoionization. In agreement with past calculations, we find that He ii recombination proceeds in Saha equilibrium, whereas He i recombination is delayed relative to Saha due to the low rates connecting excited states of He i to the ground state. However, we find that at z<2200 the continuum absorption by the rapidly increasing H i population becomes effective at destroying photons in the He i 21Po-11S line, causing He i recombination to finish around z≃1800, much earlier than previously estimated.

Radiative decay spectra of selected doubly excited states in helium

Radiative decay spectra of doubly excited states in He have been measured after selective excitation with monochromatized synchrotron radiation. The decay spectra are in excellent agreement with predictions based on calculations.

No-core shell model in an effective-field-theory framework

We present a new approach to the construction of effective interactions suitable for many-body calculations by means of the no-core shell model (NCSM). We consider an effective field theory (EFT) with only nucleon fields directly in the NCSM model spaces. In leading order, we obtain the strengths of the three contact interactions from the condition that in each model space the experimental ground-state energies of 2H, 3H and 4He be exactly reproduced. The first ( 0 + ; 0 ) excited state of 4He and the ground state of 6Li are then obtained by means of NCSM calculations in several spaces and frequencies. After we remove the harmonic-oscillator frequency dependence, we predict for 4He an energy level for the first ( 0 + ; 0 ) excited state in remarkable agreement with the experimental value. The corresponding 6Li binding energy is about 70% of the experimental value, consistent with the expansion parameter of the EFT.

Modeling study on the influence of the pressure on a dielectric barrier discharge microplasma

In this paper we present a one-dimensional fluid model to study the properties of very small dielectric barrier discharges (DBD), which can be used as microchip plasma sources for analytical spectrometry. The influence of the pressure on this kind of discharge was investigated to obtain more insight in the variations of discharge current and the differences in particle densities that arise when operating at different pressures. The studied discharge gas is helium and is described with passive He background atoms, He+ and He2+ ions, He metastable atoms, higher excited states of He, He2* excimers and electrons. We show that the plasma activity, expressed in discharge current, is remarkably higher for pressures ranging from 50 to 140 mbar. For most pressures the calculated maximum current density is about 0.1 A cm−2, whereas in the range from 50 to 140 mbar the maximum current density is about 0.3 A cm−2. We also report how the plasma densities alter at different operating pressures, with spatially averaged densities at maximum current ranging from about 1016 m−3 to 1018 m−3, depending on the plasma species, and identify the main underlying processes, which are responsible for these evolutions.

Double-charge-transfer cross sections in inelastic collisions of bare ions with helium atoms

Double-charge-transfer cross sections into singly and doubly excited states for collisions of $\ensuremath{\alpha}$ particles with helium atoms have been studied in the energy range of 50 to $500\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$. We have also studied the double electron capture cross sections into ground states for collisions of $^{7}\mathrm{Li}^{3+}$ and $^{10}\mathrm{B}^{5+}$ with helium atoms. In our study we have applied the four-body boundary corrected continuum intermediate state approximation. The intermediate continuum states of each electron and static correlations of the electrons have been taken into account in this formalism. Present calculated results for total charge transfer cross sections for the reaction $\ensuremath{\alpha}+\mathrm{He}$ and ${\mathrm{Li}}^{3+}+\mathrm{He}$ compare favorably well with the existing experimental and other theoretical predictions. Due to the nonavailability of any theoretical and experimental finding for the reaction ${\mathrm{B}}^{5+}+\mathrm{He}$, the energy variation of capture cross sections into the ground state of ${\mathrm{B}}^{5+}$ ion has been shown within the same energy range. However, charge transfer cross sections into singly and doubly excited states of He are compared with the available theoretical observations only due to nonavailability of any experimental result. In this case as well, agreement is very encouraging.

Electron-Impact Ionization and Excitation of Helium to then=1–4Ionic States

We present high-precision (e,2e) measurements and calculations for the e-He four-body Coulomb breakup problem. Cross-section ratios for ionization and excitation of the first three excited states of He+ relative to the ground state have been measured for incident energies between 112 and 319 eV. Comparing the data with predictions from a state-of-the-art hybrid distorted-wave+convergent R matrix with pseudostates (close coupling) approach shows that treating the projectile-target interaction at least to second order is crucial to obtain reasonable agreement between theory and experiment. Nevertheless, our benchmark studies reveal significant theoretical problems for the symmetric energy-sharing cases, thus indicating the need for further improvement.

High-lying doubly excited states of He and H−: electron correlations and classification schemes

Abstract High-lying doubly excited states of He and H− are studied and energies and intrinsic characteristics of their wave-functions are reported. Results for energies of 3Po and 1D doubly excited states associated with the hydrogenic thresholds up to N = 20 are presented and compared to available data from the literature. The classification of these doubly excited states by approximate quantum numbers is reexamined.

Electron Correlations and Classification Schemes for Intershell Doubly Excited States

High-lying intershell doubly excited states of He and H− are studied and energies and intrinsic characteristics of their wave functions are reported. Results for energies of 3S and 1Pe doubly excited states associated with the hydrogenic manifold up to N = 20 are presented and compared to available data from the literature. The classification of these doubly excited states by approximate quantum numbers is reexamined.

Double photoexcitation of He atoms by attosecond xuv pulses in the presence of intense few-cycle infrared lasers

We studied the photoelectron spectra of He atoms resulting from double photoexcitation to autoionizing states by an attosecond xuv pulse in the presence of intense few-cycle Ti:sapphire lasers using the time-dependent hyperspherical close-coupling method. In its first application we show that the combination of a weak xuv pulse and an intense infrared laser offers an efficient means for probing states that cannot be reached by single photoabsorption experiments alone. The method provides an alternative approach of studying Stark induced states at field strength much higher than that available for a dc electric field. Using parameters from the presently available attosecond pulses and infrared lasers we showed that the four singlet 2l2l{sup '} doubly excited states of He are prominently excited in such experiments. The dependence of the shape of the autoionizing states thus generated has been studied with respect to the intensity and the carrier-envelope phase of the few-cycle laser pulses.

A study of population inversion of He I and Ne I in regenerative sooting discharges

The mechanisms for population inversion among the excited states of He I and Ne I in regenerative sooting discharges have been investigated as a function of the geometry and the physical parameters of the graphite hollow cathode sources. The effect of the state of sooting of the hollow cathode, the gas pressure and the role of the cycling of idis on the population inversion of the excited He I and Ne I levels have been investigated.

Energies and Auger widths of the triply excited states of He -

Some triply excited states of He- ion are studied using the saddle-point complex-rotation method. The energies and Auger widths of these resonances are calculated with the relativistic corrections and mass polarization taken into account. The total Auger width is obtained by coupling the important open channels and summing over the other channels. These results are in good agreement with other theoretical and experimental data in the literature.

Time-resolved measurements of the gas temperature in a H2/CH4 medium pressure microwave 915 MHz pulsed plasma

Experimental measurements of the time variation of gas temperature in a non-equilibrium hydrogen-methane microwave plasma operating at medium pressure and under pulsed mode are presented. The hydrogen plasma is created using a 915 MHz microwave generator at an input microwave power up to 17 kW and the gas temperature is determined from the rotational distribution of the G 1Σg+ excited state of H2(T(G)). In continuous discharge, the rotational temperature T(G) of H2(G 1Σg+) is found to be nearly equal to the gas temperature determined from rotational distribution of the a3Πu metastable state of C2 molecule obtained from absorption spectroscopy. The time evolution of H2(G 1Σg+) rotational distribution in pulsed discharges reaches a Boltzmann equilibrium only 100 µs after the beginning of the pulse. Variations of the T(G) in-pulse value as a function of pressure, volumetric power density and gas velocity is studied.

Density-functional calculations on singly and doubly excited Rydberg states of many-electron atoms

Nearly 100 low-, moderately high-, and high-lying singly and doubly excited states of He, Li, and Be have been calculated using a nonvariational, work-function-based exchange potential within the nonrelativistic Hohenberg-Kohn-Sham density-functional theory ~DFT!. The nonlinear gradient included in the Lee-Yang-Parr correlation functional is used to incorporate the correlation potential. The generalized pseudospectralmethod is used for nonuniform and optimal spatial grid discretization and solution of the Kohn-Sham equation to obtain accurate eigenvalues, expectation values, and radial densities for both ground and excited states. The results are compared with the available theoretical and experimental data. The discrepancy in the calculated singly excited state energies is within about 0.01% for He ~for other atoms, it is less than 0.2%!, while that for the doubly excited states of He is well within 3.6%. The deviations in the calculated single- and doubleexcitation energies for 31 selected states are in the error ranges 0.009‐0.632 % and 0.085‐1.600 %, respectively. The overall agreement of the present results is quite gratifying, especially in the light of DFT’s difficulties in dealing with excited states. The exchange-only results are practically of Hartree-Fock quality, and the correlation-included results are usually slightly overestimated. The present method offers a simple, computationally efficient and general scheme for accurate calculation of multiply excited Rydberg states within DFT.