Excited Helium Research Articles

The effect of wave function orthogonality on the simultaneous ionization and excitation of helium

Within the framework of the first-order Born approximation, the triple differential cross sections (TDCSs) for simultaneous ionization and excitation of helium are calculated. The wave function of the ejected electron is chosen to be orthogonal or non-orthogonal to the wave function of the bound electron before ionization. It is found that the orthogonality has a strong effect on the TDCS, especially when plane waves and Coulomb waves are used to describe the projectile and the ejected electron.

The fine structure of microwave-induced magneto-oscillations in photoconductivity of the two-dimensional electron system formed on a liquid-helium surface

The influence of the inelastic nature of electron scattering by surface excitations of liquid helium (ripplons) on the shape of magnetoconductivity oscillations induced by resonance microwave (MW) excitation is theoretically studied. The MW field provides a substantial filling of the first excited surface subband which sparks off inter-subband electron scattering by ripplons. This scattering is the origin of magneto-oscillations in the momentum relaxation rate. The inelastic effect becomes important when the energy of a ripplon involved compares with the collision broadening of Landau levels. Usually, such a condition is realized only at sufficiently high magnetic fields. On the contrary, the inelastic nature of inter-subband scattering is shown to be more important in a lower magnetic field range because of the new enhancement factor: the ratio of the inter-subband transition frequency to the cyclotron frequency. This inelastic effect affects strongly the shape of conductivity oscillations which acquires an additional wavy feature (a mixture of splitting and inversion) in the vicinity of the level-matching points where the above noted ratio is close to an integer.

Direct analysis in real time—a critical review on DART-MS

Direct analysis in real time mass spectrometry (DART-MS) has become an established technique for rapid mass spectral analysis of a large variety of samples. DART-MS is capable of analyzing the sample at atmospheric pressure, essentially in the open laboratory environment. DART-MS can be applied to compounds that have been deposited or adsorbed on to surfaces or that are being desorbed therefrom into the atmosphere. This makes DART-MS suitable and well-known for analysis of ingredients of plant materials, pesticide monitoring on vegetables, forensic and safety applications such as screening for traces of explosives, warfare agents, or illicit drugs on luggage, clothes, or bank notes, etc. DART can also be used for analysis of either solid or liquid bulk materials, as may be required in quality control, or to quickly investigate the identity of a compound from chemical synthesis. Even living organisms can be subjected to DART-MS. Driven by different needs in analytical practice, the combination of the DART ionization source and interface can be configured in multiple geometries and with various accessories to adapt the setup as required. Analysis by DART-MS relies on some sort of gas-phase ionization mechanism. In DART, initial generation of the ionizing species is by use of a corona discharge in a pure helium atmosphere which delivers excited helium atoms that, upon their release into the atmosphere, will initiate a cascade of gas-phase reactions. In the end, this results in reagent ions created from atmospheric water or (solvent) vapor in the vicinity of the surface subject to analysis where they effect a chemical ionization process. DART ionization processes may generate positive or negative ions, predominantly even-electron species, but odd-electron species do also occur. The prevailing process of analyte ion formation from a given sample is highly dependent on analyte properties.

Generalized Landau superfluidity criterion

An analysis of experimental data shows that, in addition to phonon-roton excitations in superfluid helium, there necessarily exist at least one branch of elementary excitations whose energy spectrum strongly depends on temperature. On this basis, the Landau superfluidity criterion is generalized for several branches of elementary excitations, taking into account that the critical velocity should vanish during the phase transition of liquid helium from the superfluid state to the normal state.

Line Spectra Observation of the Rydberg Helium Atoms due to Volumetric Recombination in the RF Plasma Source DT-ALPHA

Keeping compatibility between steady state gas puffing and stable radio frequency (RF) discharge, helium recombining plasma production was achieved in an RF plasma device. In this experiment, axial position of orifice, which suppresses backflow of secondary gas, was modified to increase electron density at a test region. Changing neutral pressure at the test region from 11 Pa to 21 Pa, the electron temperature, the electron density and the wavelength spectrum were measured. The electron temperature decreased with increasing neutral pressure and finally becomes about 3 eV. The electron density shows similar pressure dependence as the electron temperature. When the neutral pressure increases to 15 Pa, the line spectra from highly excited helium atoms were clearly observed. The electron temperature estimated from these line spectral intensities is about 0.05 eV, which indicates that the electron density reduction is caused by volumetric recombination occurring at the periphery of the plasma column.

Criterion of superfluidity, elementary excitations, and heat capacity singularity in superfluid helium

We proceed from the premise that the spectrum of elementary excitations in the normal component in the Landau theory of superfluidity should depend on the superfluid helium temperature. This leads to generalization of the Landau superfluidity criterion. On this basis, taking into account available experimental data on inelastic neutron scattering, it is shown that, in addition to phonon–roton excitations, there is another type of elementary excitation in superfluid helium, which we called helons. The energy spectrum with such momentum dependence was first proposed by Landau. The helon energy spectrum shape and its temperature dependence make it possible to explain the singular behavior of the heat capacity of superfluid helium near its phase transition to the normal state.

Nonmonotonic radial distribution of excited atoms in a positive column of pulsed direct currect discharges in helium

Nonmonotonic radial distributions of excited helium atoms have been experimentally observed in a positive column of pulsed helium discharges using planar laser induced fluorescence. Computational analysis of the discharge dynamics with a fluid plasma model confirms the experimental observations over a range of pressures and currents. The observed effect is attributed to the peculiarities of electron population-depopulation of the excited states during the “dynamic discharge” conditions with strong modulations of the electric field maintaining the plasma.

Gaussian basis sets for highly excited and resonance states of helium

A consistent method for optimizing Gaussian primitives for Rydberg and multiply excited helium states is designed. A novel series for the "exponentially tempered Gaussians" is introduced, which is markedly more efficient than the commonly used series of even tempered Gaussians. The optimization is made computationally feasible due to an approximate calculation of excited states using the effective one-electron Hamiltonian that is defined as Fockian from which the redundant Coulomb and exchange terms are dropped. Finally, ExTG5G and ExTG7F Gaussian basis sets are proposed. They enable calculations of the helium spectrum all the way from the ground state up to the (5, 4)(5) (1)S(e) and (6, 5)(7) (1)S(e) doubly excited resonances, respectively, mostly in the spectroscopic accuracy of 1 cm(-1).

Characterization of an atmospheric helium plasma jet by relative and absolute optical emission spectroscopy

The characteristics of plasma temperatures (gas temperature and electron excitation temperature) and electron density in a pulsed-dc excited atmospheric helium plasma jet are studied by relative and absolute optical emission spectroscopy (OES). High-resolution OES is performed for the helium and hydrogen lines for the determination of electron density through the Stark broadening mechanism. A superposition fitting method composed of two component profiles corresponding to two different electron densities is developed to fit the investigated lines. Electron densities of the orders of magnitude of 1021 and 1020 m−3 are characterized for the center and edge regions in the jet discharge when the applied voltage is higher than 13.0 kV. The atomic state distribution function (ASDF) of helium demonstrates that the discharge deviates from the Boltzmann–Saha equilibrium state, especially for the helium lower levels, which are significantly overpopulated. Local electron excitation temperatures T13 and Tspec corresponding to the lower and upper parts of the helium ASDF are defined and found to range from 1.2 eV to 1.4 eV and 0.2 eV to 0.3 eV, respectively. A comparative analysis shows that the Saha balance is valid in the discharge for helium atoms at high excited states.

Long-range interactions between excited helium and alkali-metal atoms

The dispersion coefficients for the long-range interaction of the first four excited states of He, i.e., He(2 S-1,S-3) and He(2 P-1,P-3), with the low-lying states of the alkali-metal atoms Li, Na, K, and Rb are calculated by summing over the reduced matrix elements of the multipole transition operators. For the interaction between He and Li the uncertainty of the calculations is 0.1-0.5%. For interactions with other alkali-metal atoms the uncertainty is 1-3% in the coefficient C-5, 1-5% in the coefficient C-6, and 1-10% in the coefficients C-8 and C-10. The dispersion coefficients C-n for the interaction of He(2 S-1,S-3) and He(2 P-1,P-3) with the ground-state alkali-metal atoms and for the interaction of He(2 S-1,S-3) with the alkali-metal atoms in their first P-2 states are presented in this Brief Report. The coefficients for other pairs of atomic states are listed in the Supplemental Material.

Spacer-less, decoupled granular L1 FePt magnetic media using Ar–He sputtering gas

Spacer materials are often used to fabricate granular L10 FePt media and reduce the grain size, however, at the expense of reduced out-of-plane coercivity. Here, we demonstrate a spacer-less method in which adding 1% helium to argon sputtering gas leads to a substantial improvement in the chemical ordering, as well as the magnetic and microstructural properties of FePt. This change is attributed to the modification in the ion current density of the plasma caused by the excited metastable helium species. Helium plays a pivotal role in providing the Fe and Pt atoms optimal adatom mobility, thereby, producing well-ordered L10 FePt media. This leads to an enhancement of the out-of-plane coercivity from 15 to 22 kOe. Furthermore, exchange decoupled grains with a twofold reduction in their diameter to ∼24 nm are also achieved. These properties are crucial for magnetic media of the future for data storage recording densities beyond 1 Tb/in2.

The character of resonant charge exchange involving highly excited atoms

We study the process of resonant charge exchange involving excited helium atoms with the principal quantum number n = 5 colliding with the helium ion in the ground state in the collision energy range from thermal up to 10 eV. This information may be important for the analysis of planet atmospheres containing helium, in particular, for Jupiter’s atmosphere, but our basic interest is the transition from the quantum to classical description of this process, where, due to large cross sections, evaluations of the cross sections are possible. For the chosen process, quantum theory allows determining the cross section as a result of a tunnel electron transition, while classical theory accounts for over-barrier electron transitions. The classical theory additionally requires effective transitions between states with close energies. The analysis of these transitions for helium with n = 5 shows that electron momenta and their projections are mixed for a part of the states, while for other states, the mixing is absent. A simple criterion to separate such states is given. In addition, the main contribution to the cross section of resonant charge exchange follows from tunnel electron transitions. As a result, the quantum theory is better for calculating the cross sections of resonant charge exchange than the classical one and also allows finding the partial cross sections of resonant charge exchange, while the classical approach gives the cross section of resonant charge exchange in a simple manner with the accuracy of 20%.

Two-electron dynamics in nonlinear double excitation of helium by intense ultrashort extreme-ultraviolet pulses

We present a theoretical study for multiphoton double excitation of He atoms exposed to an intense ultrashort extreme-ultraviolet (EUV) pulse, where the photon energy coincides with the transition energy from the ground state to a Rydberg state, by solving the time-dependent Schr\odinger equation in the hyperspherical coordinates. Photoelectron spectra under the conditions comparable with a recent experiment [Hishikawa et al., Phys. Rev. Lett. 107, 243003 (2011)] are calculated and analyzed. We identify the mechanism of the enhanced three-photon absorption probability which is more than one order of magnitude larger than that for the two-photon process in accordance with the experiment. The enhancement is attributed to a propensity rule for double excitation in a two-step mechanism, in which a one-photon absorption by one electron to a Rydberg state is followed by a two-photon absorption by the other electron to an excited orbital, while the first electron remains at nearly the same principle quantum number. Based on the time-dependent perturbation theory, the three-photon absorption probability exhibits peculiar cubic dependence on the pulse duration due to the propensity rule, in contrast to the linear dependence of the two-photon absorption probability. Thus a crossover between the two- and three-photon absorption probabilities takes place for sufficiently intense and long pulses. We also study the time evolution of a doubly excited two-electron wave packet created by an intense ultrashort EUV pulse efficiently using the same enhancement scheme, opening up the possibility of visualizing the correlated motion of two electrons in the time domain.

R-matrix Floquet study of simultaneous electron-photon excitation of helium in a CO2 laser field

R-matrix Floquet theory is used to study simultaneous electron-photon excitation of helium in a CO2 laser field at intensities between 107 and 108 W cm−2, for collision energies near the 1s2ℓ excitation thresholds. The results are compared with those of two simple approximations as well as with experiment.

Energy harvesting in doped helium nano-droplets

We report the observation of sequential Penning ionization of dopants by metastable helium atoms in helium nano-droplets resulting in doubly charged ions. Strong charge induced dipole-interaction between the excited helium atom and the target ion provides a high probability for the transfer of the internal energy of the excited helium atom to the dopant ion. This process may also lead subsequently to a Coulomb explosion of molecular or cluster dopants.

Influence of excited state spatial distributions on plasma diagnostics: Atmospheric pressure laser-induced He-H2 plasma

Atmospheric pressure plasmas in helium-hydrogen mixtures with H2 molar concentrations ranging from 0.13% to 19.7% were investigated at times from 1 to 25 μs after formation by a Q-switched Nd:YAG laser. Spatially integrated electron density values are obtained using time resolved optical emission spectroscopic techniques. Depending on mixture concentration and delay time, electron densities vary from almost 1017 cm−3 to about 1014 cm−3. Helium based results agree reasonably well with each other, as do values extracted from the Hα and Hβ emission lines. However, in particular for delays up to about 7 μs and in mixtures with less than 1% hydrogen, large discrepancies are observed between results obtained from the two species. Differences decrease with increasing hydrogen partial pressure and/or increasing delay time. In mixtures with molecular hydrogen fraction of 7% or more, all methods yield electron densities that are in good agreement. These findings seemingly contradict the well-established idea that addition of small amounts of hydrogen for diagnostic purposes does not perturb the plasma. Using Abel inversion analysis of the experimental data and a semi-empirical numerical model, we demonstrate that the major part of the detected discrepancies can be traced to differences in the spatial distributions of excited helium and hydrogen neutrals. The model yields spatially resolved emission intensities and electron density profiles that are in qualitative agreement with experiment. For the test case of a 1% H2 mixture at 5 μs delay, our model suggests that high electron temperatures cause an elevated degree of ionization and thus a reduction of excited hydrogen concentration relative to that of helium near the plasma center. As a result, spatially integrated analysis of hydrogen emission lines leads to oversampling of the plasma perimeter and thus to lower electron density values compared to those obtained from helium lines.

Attosecond cascades and time delays in one-electron photoionization

We present time-resolved ab initio study of the attosecond dynamics of electron-electron correlation during single-electron, single XUV-photon ionization of an excited helium atom. We identify multiple time scales in these dynamics, caused by the interplay of the initial shake-up excitation with the excitation cascades induced by the postionization interaction of the outgoing electron with the core. The time scales and the structure of these excitation cascades are directly linked to the time it takes the ``active'' continuum electron, lifted from a deeper bound orbital, to traverse the orbit of the ``passive'' electron residing in the outer orbit.

Volume element structure and roton-maxon-phonon excitations in superfluid helium beyond the Gross-Pitaevskii approximation

We propose a theory which deals with the structure and interactions of volume elements in liquid helium II. The approach consists of two nested models linked via parametric space. The short-wavelength part describes the interior structure of the fluid element using a non-perturbative approach based on the logarithmic wave equation; it suggests the Gaussian-like behaviour of the element's interior density and interparticle interaction potential. The long-wavelength part is the quantum many-body theory of such elements which deals with their dynamics and interactions. Our approach leads to a unified description of the phonon, maxon and roton excitations, and has noteworthy agreement with experiment: with one essential parameter to fit we reproduce at high accuracy not only the roton minimum but also the neighboring local maximum as well as the sound velocity and structure factor.

Single photoionization with excitation and double photoionization of He@C36, He@C60 and He@C82

In anticipation of future experimental investigations of two-electron processes in endohedral atoms, the double photoionization and single photoionization with excitation of helium confined in the fullerenes C36, C60 and C82 have been studied using the time-dependent close-coupling method. It is found that both He@C60 and He@C82 display strong confinement resonances in the double photoionization cross section, whereas for He@C36, the confinement resonances are suppressed. For single photoionization leaving the He+ ion in its ground state, we found not only that the magnitudes of the cross sections for He@C36, He@C60 and He@C82 are similar to the case of helium but that confinement resonances are also apparent. For single photoionization with excitation to the n = 2 shell, the endofullerene cross sections showed a reduction as compared to bare He atoms, while for photoionization with excitation to the n = 3 shell, the cross sections for the endofullerenes are enhanced, with a particularly strong enhancement evident for He@C36.

Resonant charge-exchange involving excited helium atoms and reactive transport of local thermodynamic equilibrium helium plasma

The cross sections for charge-exchange and charge-transfer processes are evaluated for collisions of helium ions with parent-atoms in ground and excited states, with the principal quantum number n = 1–5, in the collision energy range from thermal up to 10 eV. Corresponding diffusion-type collision integrals are derived, and the role of “abnormal” transport of electronically excited states on the reactive thermal conductivity of equilibrium helium plasma, at atmospheric pressure, estimated in the frame of a simplified approach.