Excited States Of Helium Atom Research Articles

Using a post-version continuum distorted wave formalism for calculation of the fully differential cross section of helium atom single ionization by proton impact

In the present work, the fully differential cross section of single ionization of helium atom is calculated by fast proton impact. The calculations are done as a four-body formalism and by using a post-version continuum distorted wave theory with considering symmetric wave functions in the both initial and final channels. The ionization is considered from the ground and $$2^{1}S$$ excited states of helium atom. In order to investigate the role of the static electronic correlations, the calculations are performed by single-parameter Hylleraas and two-parameter Silverman as the ground state of helium atom wave functions. The results of the present theory are compared with the experiment in 1MeV proton impact energy and various values of the ejected electron energy and momentum transfer in the scattering and azimuthal planes. The results show that the present theory is able to predict the binary and recoil peaks as well as the experiment for ejected electron energies and momentum transfers are discussed in this manuscript.

Modeling a helium atom from a collision of an electron with an ionized helium atom

A method which is based on quantum scattering theory and, particularly, on the Jost functions, is presented for solving the bound states problem of helium atom. Within this approach, the energies of bound states of helium atom were obtained as the sum of energies of a hydrogen atom and the energies that one electron needs to bind to an ionized helium atom in a collision process. We calculated the ground state energy and the energies of the first excited states of helium atom, and obtained remarkable results, This method could then be used as basis to study atoms with more electrons.

Structure of free complement wavefunction for the ground and the first excited state of helium atom

We review the fundamental ideas of free complement (FC) method through its application on both ground and first excited states of helium atom. We have found that lower energies can be obtained with fewer number of terms in the FC expansion of the ground state wavefunction. In this case, the optimization of orbital exponents was not necessary for achieving spectroscopic accuracy, especially at higher orders where the structure of the FC wavefunction converges to that of the exact one. We have discovered that permanents naturally appear in the FC expansion of the first triplet excited state wavefunction. Including permanents in the FC expansion is shown to be energetically important for the first triplet excited state of helium atom whereas it is not computationally favorable at higher orders. Finally, considering the group theoretical properties of the symmetric group [Formula: see text] and using immanants, a compact and more elegant form for the FC expansion of the first triplet excited state of the helium atom is achieved.

Anisotropic emission in quantum-beat spectroscopy of helium excited states

We present quantum-beat spectroscopy of excited states of helium atoms populated selectively with high-order-harmonic emission below the atomic ionization potential by means of low-pass filtering of the pump radiation. The created electron wave packet is ionized by few-cycle infrared (IR) pulses leading to characteristic peaks in the photoelectron yield, which beat with a frequency proportional to the energy gap between the states involved in the two-color photoionization process. Minimizing the direct ionization by the extreme ultraviolet (XUV) radiation, we can follow the evolution of the electron wave packet also in the region of temporal pump-probe overlap. A detailed time-frequency analysis of the quantum beats and direct comparison with the solution of the time-dependent Schr\odinger equation reveal the existence of quantum beats characterized by a final state of mixed parity. Finally, we show that by varying the carrier-envelope offset phase of the probe pulse, one can optically control the preferred direction of photoelectron emission and the contrast of such beats.

Attosecond XUV absorption spectroscopy of doubly excited states in helium atoms dressed by a time-delayed femtosecond infrared laser

In the present paper, we investigate the time-resolved transient absorption spectroscopy of doubly excited states of helium atoms by solving the time-dependent two-electron Schr\"odinger equation numerically based on a one-dimensional model. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser pulse. A superposition of doubly excited states populated by the XUV pulse is identified, which interferes with the direct ionization pathway leading to Fano resonance profiles in the photoabsorption spectrum. In the presence of an IR laser, however, the Fano line profiles are strongly modified: A shifting, splitting, and broadening of the original absorption lines is observed when the XUV attosecond pulse and infrared few-cycle laser pulse overlap in time, which is in good agreement with recent experimental results. At certain time delays, we observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections indicating that the XUV light can be amplified during the interaction with atoms. We further prove that the above pictures are general for different doubly excited states by suitably varying the frequency of the IR field to coherently couple the corresponding states.

Excited atoms in cavities of liquid He I: long-range interatomic repulsion and broadening of atomic lines

A theoretical analysis of the line broadening of localized atomic transitions in liquid helium is presented. It is shown that accurate information can be derived on the long-range part of the He*-He interaction as well as on the local structure near the He* emitters. The analysis confirms that in corona discharges in liquid helium the emitting He* atoms reside in cavities and that for known He*-He interaction the size of the cavities can be deduced from the line profile. The He*-He interaction was calculated using the full configuration interaction (CI) method as implemented in the Molpro package. The widths of atomic lines due to fluorescent transitions between different excited states of helium atoms were calculated as a function of external pressure in the range from 0.1 and 3.5 MPa using the static approximation method, and the input from the results of the CI calculation and cavity diameters calculated using the bubble model. The calculated widths showed excellent agreement with experimental data of liquid helium excited by corona discharges. A second, analytical analysis using a power function to represent the He*-He interaction showed qualitative agreement with the experimental data.

Evaluation of Coulomb and exchange integrals for higher excited states of helium atom by using spherical harmonics series

In this work we study the higher excited states of Helium Atom. The purpose is to evaluate Coulomb and exchange integral via spherical harmonics series. The Coulomb and exchange integrals energy shift is evaluated up to sixth order. This is the energy when the atom is perturbed by Coulomb potential between electrons. The energy levels obtained from both integrals are in agreement with the experimental data. For highly-excited states, the calculated energy approaches −54.416 eV, in agreement with the graphical results from the book by Powell and Crasemann [1].

Cross section for the single-electron capture by fast He + ions in inert gases

The cross section of the single-electron capture for collisions between dressed ions and He, Ne, and Kr atoms is calculated using the first Born approximation. The electron capture from all shells of target atom and into each excited state of helium atom up to 5 1,3D are take into account. The results of calculations compare with the other experimental and theoretical data.

Probing the molecule-like modes of doubly excited states of helium atoms

We use the time-dependent hypersperhical close-coupling method to study the double excitation of helium atoms by attosecond XUV pulses in the presence of few-cycle infrared lasers, aiming to probe the time-resolved correlated electron motion and to reveal the underlying ro-vibration modes directly in time domain.

Solving the electron and electron-nuclear Schrödinger equations for the excited states of helium atom with the free iterative-complement-interaction method

Very accurate variational calculations with the free iterative-complement-interaction (ICI) method for solving the Schrodinger equation were performed for the 1sNs singlet and triplet excited states of helium atom up to N=24. This is the first extensive applications of the free ICI method to the calculations of excited states to very high levels. We performed the calculations with the fixed-nucleus Hamiltonian and moving-nucleus Hamiltonian. The latter case is the Schrodinger equation for the electron-nuclear Hamiltonian and includes the quantum effect of nuclear motion. This solution corresponds to the nonrelativistic limit and reproduced the experimental values up to five decimal figures. The small differences from the experimental values are not at all the theoretical errors but represent the physical effects that are not included in the present calculations, such as relativistic effect, quantum electrodynamic effect, and even the experimental errors. The present calculations constitute a small step toward the accurately predictive quantum chemistry.

Some notes on the role of meta-stable excited state of helium atom in laser-induced helium gas breakdown spectroscopy

A study of the experimental results on the plasma emissions of water and ethanol vapor samples, induced by Nd:YAG laser in ambient helium and nitrogen gases at atmospheric pressure, is presented here. The result reveals distinct geometrical and spectral characteristics of the plasma emissions generated in the helium gas when compared to those observed from nitrogen gas plasma. Most remarkable is the narrow line width and low continuum background exhibited by emission lines of the analyte atoms from helium plasma, including the hydrogen emission line which is known to suffer from notorious broadening effects in conventional laser induced breakdown spectroscopy (LIBS). It is further shown on the basis of the measured spatial distributions and time profiles of the emission intensities, that the excellent spectral quality is attained by taking advantage of the meta-stable excited state of helium atoms for the delayed excitation of the hydrogen and other analyte atoms, this allows the detection of those atomic emissions to be performed under more favorable conditions. The result of this study has thus demonstrated the feasibility of achieving high-quality spectrochemical analysis, including hydrogen analysis with laser-induced helium gas breakdown spectroscopy.

An improved approach for hydrogen analysis in metal samples using single laser-induced gas plasma and target plasma at helium atmospheric pressure

We report in this paper the results of an experimental study on hydrogen analysis of solid samples in high pressure helium ambient gas employing the basic scheme of laser induced breakdown spectroscopy (LIBS). It is shown that the metastable excited state of helium atom can be utilized to induce delayed excitation of the ablated hydrogen atoms, and thereby avoid the Stark broadening effect as well as overcoming the undesirable mismatch effect, which are responsible for inefficient excitation respectively. It is further demonstrated that for samples of high boiling-point materials such as zircaloy, successful hydrogen analysis can be achieved by a newly introduced double excitation technique employing single laser realized in a modified configuration of the conventional LIBS method.

Atomic Compton profiles within different exchange-only theories

The Impulse Compton Profiles (CP's) J(q) and the <p^n> - expectation values for some inert gas atoms (He-Kr) are computed and compared within the Harbola-Sahni (HS), Hartree-Fock(HF) theories and a Self Interaction Corrected (SIC) density functional model. The Compton profiles for excited states of Helium atom are also calculated. While the calculated CP's are found to generally agree, they differ slightly from one another for small values of the Compton parameter q and are in good agreement for large q values. The <p^n> expectation values within the three theories are also found to be comparable. The HS formalism seem to mimic HF reasonably well in the momentum space, establishing the logical consistency of the former.

Population Mechanism of the Excited States of Helium Atoms at a Hollow Cathode in He–Ar Mixture Nanosecond Discharge

This paper reports results and analysis of a nanosecond pulsed discharge at a hollow cathode in the He–Ar Penning mixture as a step towards a possible interpretation of the effective mechanism of the recombination process. It has been concluded that the population of the excited states of atoms during subsequent time intervals has a recombination period. The limit of this phenomenon and future studies are discussed.

Effects of helium upon electron beam excitation of N+2 at 391.4 and 427.8 nm

Relativistic electron beam interactions with very small ratios of nitrogen to helium (10−1–10−4) have been found to produce extremely large N+2(B2Σ+u–X2Σ+g) intensities at 391.4 and 427.8 nm, compared to line intensities originating from helium. These results occurred in the total pressure regime of 0.1–500 Torr. The pressure scaling results presented here are inconsistent with previously proposed kinetic mechanisms for the N+2 laser pumped by helium. With a simple model of the chemical kinetics, we show that this effect is due to the collisional transfer of energy between excited states of helium atoms and the ground state of N+2.

Doubly excited states of helium atoms between the N=2 and N=3 He+thresholds

Some doubly excited autoionising states of helium atoms converging on the N=3 He+ threshold are calculated by use of a method of complex coordinates. Hylleraas-type wavefunctions are used for L=0 and L=1 resonances with products of Slater-orbital type wavefunctions with expansion lengths up to 319 terms for L>or=2 resonances. States of parities (-1)L and (-1)L+1 and with angular momenta of L<or=4 are calculated. Resonance energies and autoionisation widths are reported.

Spectroscopic Study on the Population Densities of Free Electrons and Excited States of Helium in a P.I.G. Plasma

A spectroscopic study is carried out on a helium plasma produced by the P.I.G. type discharge. The radial distributions of the emission coefficients of 2 3 P - n 3 D lines and the recombination continuum are obtained from the measured chord intensity-distributions by means of the Abel inversion. The population densities of n 3 D states of helium atoms and free electrons are calculated from the emission coefficients. From the Boltzmann plot which is the population density distribution of the excited states and free electrons, it is revealed that the highly excited states of helium atoms are in equilibrium with the free electrons, that is, the Saha equilibrium is established between them. The electron temperature and the electron density are found to be 0.23 eV and 5.5×10 13 cm -3 at the radial position R=5 mm, and 0.17 eV and 2.2×10 13 cm -3 at R=10 mm, respectively.

Population densities of excited states of helium atoms in a brush cathode discharge in a magnetic field

Spectral line intensities emitted by a brush cathode helium discharge in a magnetic field have been measured for the 2 1 S − n 1 P, 2 1 P − n 1 S, 2 1 P − n 1 D, 2 3 S − n 3 P, 2 3 P − n 3 S, and 2 3 P − n 3 D series up to a principal quantum number n = 10 and the population densities of the excited states have been obtained. For n ≳ 5, the states are in thermal equilibrium. The electron density and the electron temperature obtained from the Saha-Boltzmann equation are 1·7 × 10 13 cm -3 and 0·17 eV. These values are in good agreement with the values obtained from the measured recombination continuum. For the lower states, measured populations are in good agreement with those calculated by Drawin, Emard and Katsonis on the basis of a collisional-radiative model.

Ionizing Collisions of the Metastable and Long-lived Highly Excited States of Helium Atoms with Water Vapor

Long-lived excited states of He*(21S and 23S)and He**(in highly excited states), produced by impact of 20-200eV electrons were allowed to collide with H2O and D2O molecules. It was confirmed that ions H+, HeH+, OH+ and H2O+ were produced by impact of He* with H2O. An ion produced by, a rearrangement chemi-ionization, HeH+, was identified for the first time in the He*+H2O system. Another process, where He** is ionized into He+ by collision with H2O, was also observed. These processes were discriminated by the difference in the threshold energies of the electrons used for exciting the helium atoms. The corresponding deuterated ions were observed when D2O was used for the target.

Investigation of Excited States of Helium Atoms in a Stationary PIG-Discharge

Abstract Spectral line intensities emitted by a quiescent PIG-discharge have been measured and the population densities up to a principal quantum number n = 24 have been derived from them. The experimentally determined population densities have been compared with theoretical ones calculated on the basis of a collisional-radiative model in which one accounts for electron and atom collisions. I t is shown that even in the case of different electron and atom temperatures, Te and Ta, an evaluation of the Boltzmann plot at medium and moderately high quantum numbers always leads to the electron temperature, whereas the Saha-Eggert equation for the same states may yield incorrect electron densities. The theoretical calculations predict an inflection of the slope of the Boltzmann plot from Te to Ta for very highly excited states. For the plasma para­ meters under which the PIG-discharge was operated (ne ≅ 2.5 × 1012 cm-3, Te ≅ 1200°K, n0 ≅ 1.5 × 1015 cm-3, 300°K) the change of the slope should become visible for states having principal quantum numbers n &gt; 18. Due to the large experimental error bars it was not possible to check this behaviour.