Excited Helium Research Articles

Roton-Type Mode Softening in a Quantum Gas with Cavity-Mediated Long-Range Interactions

Long-range interactions in quantum gases are predicted to give rise to an excitation spectrum of roton character, similar to that observed in superfluid helium. We investigated the excitation spectrum of a Bose-Einstein condensate with cavity-mediated long-range interactions, which couple all particles to each other. Increasing the strength of the interaction leads to a softening of an excitation mode at a finite momentum, preceding a superfluid-to-supersolid phase transition. We used a variant of Bragg spectroscopy to study the mode softening across the phase transition. The measured spectrum was in very good agreement with ab initio calculations and, at the phase transition, a diverging susceptibility was observed. The work paves the way toward quantum simulation of long-range interacting many-body systems.

Measurement of Helium Metastable Atom Densities in a Plasma-Based Ambient Ionization Source

We describe direct imaging of the densities of helium metastable atoms in the afterglow of a helium dielectric-barrier discharge (He-DBD) using collisionally assisted laser-induced fluorescence (LIF). For the conditions tested, comparison of fluorescence images of a He-DBD with analogous maps of emission from highly excited helium atoms revealed that helium metastable atom densities did not correlate well with emission from the plasma. Fluorescence images also showed that helium metastable atom densities increased substantially when a glass slide was placed 10.0 mm from the discharge capillary in a geometry typical for desorption-ionization experiments. We also studied the effect hydrogen has on the helium metastable atom densities. The hydrogen severely quenched the metastable state leaving it virtually undetectable. Emission was quenched as well, but to a lesser extent. The addition of 1% H(2) to the helium in the source provided nearly a factor of 2 improvement in the sensitivity of the signal for coumarin 47 when the plasma was used to ionize the dye under ambient conditions, despite the quenching of the helium metastable atom population.

Solar chromosphere in the D3 helium line from spectra of the eclipse of March 29, 2006

We present the processing results of slitless chromospheric spectra in a D3 helium line obtained during the solar eclipse of March 29, 2006. A “Cascade-Photometrics 650” CCD matrix photodetector with a rate of 16 frames/s was used for the observations. Altitude distribution of absolute brightness of the helium chromosphere has been mapped in a latitude interval 30 N-10 S on the western limb. A correlation between absolute brightness of the helium chromosphere and a solar cycle phase has been confirmed. It has been revealed that the altitude distribution of helium emission has one maximum at an altitude of approximately 2000 km. In the lower chromosphere, helium emission is undetectable. This confirms our assumption about absence of an additional maximum at 300 km in the D3 line brightness distribution in the chromosphere during sunspot maximum periods. The results also prove that X-ray coronal radiation with λ < 6 nm is the source of helium excitation in the lower chromosphere.

Nonperturbative Treatment of Ionization with Excitation of Helium by Electron Impact

We present cross sections for electron-impact ionization and simultaneous ionization plus excitation of helium by electron impact. The results are obtained from a fully nonperturbative close-coupling formalism using our B-spline R-matrix approach. A large number of pseudostates in the expansion of the wave function represent the coupling to the ionization continuum. We obtain excellent agreement with the directly measured experimental cross section ratios (Bellm et al., Phys. Rev. A 75, 042704 (2007)) for ionization leaving the residual He⁺ ion in either the 1s ground state or the n = 2 (2s + 2p) excited states.

Simultaneous electron–photon excitation of helium in a CO2 laser field

The R-matrix Floquet method is used to study the electron-impact excitation of helium in the presence of a CO2 laser field, for collision energies between 19 and 21.5 eV and laser intensities of 107 and 108 W cm−2. Collision geometries with the electron incident at various angles to the laser polarization axis are considered. The signal for the production of metastable states is in good agreement with the results of a low-frequency approximation and a semi-classical approach, while agreement with experiment is reasonable.

Elucidation of Reaction Mechanisms Responsible for Afterglow and Reagent-Ion Formation in the Low-Temperature Plasma Probe Ambient Ionization Source

The development of ambient desorption/ionization mass spectrometry has shown promising applicability for the direct analysis of complex samples in the open, ambient atmosphere. Although numerous plasma-based ambient desorption/ionization sources have been described in the literature, little research has been presented on experimentally validating or determining the desorption and ionization mechanisms that are responsible for their performance. In the present study, established spectrochemical and plasma physics diagnostics in combination with spatially resolved optical emission profiles were applied to reveal a set of reaction mechanisms responsible for afterglow and reagent-ion formation of the Low-Temperature Plasma (LTP) probe, which is a plasma-based ionization source used in the field of ambient mass spectrometry. Within the dielectric-barrier discharge of the LTP probe, He(2)(+) is the dominant positive ion when helium is used as the plasma supporting gas. This helium dimer ion (He(2)(+)) has two important roles: First, it serves to carry energy from the discharge into the afterglow region in the open atmosphere. Second, charge transfer between He(2)(+) and atmospheric nitrogen appears to be the primary mechanism in the sampling region for the formation of N(2)(+), which is an important reagent ion as well as the key reaction intermediate for the formation of other reagent ions, such as protonated water clusters, in plasma-based ambient ionization sources. In the afterglow region of the LTP, where the sample is usually placed, a strong mismatch in the rotational temperatures of N(2)(+) (B (2)Σ(u)(+)) and OH (A (2)Σ(+)) was found; the OH rotational temperature was statistically identical to the ambient gas temperature (~300 K) whereas the N(2)(+) temperature was found to rise to 550 K toward the tail of the afterglow region. This much higher N(2)(+) temperature is due to a charge-transfer reaction between He(2)(+) and N(2), which is known to produce rotationally hot N(2)(+) (B (2)Σ(u)(+)) ions. Furthermore, it was found that one origin of excited atomic helium in the afterglow region of the LTP is from dielectronic recombination of vibrationally excited He(2)(+) ions.

Implementation of acoustic, radiofrequency and microwave rotating fields in analytical plasma sources

Four helium plasma sources operating at atmospheric pressure have been developed for analytical emission spectrometry by applying a synchronically rotating field with three or more phases operating at 1 kHz, 27 MHz or 2.45 GHz. The plasma takes the form of a disk and has minimum field strength at the axis. Thus, a channel is formed at the center through which the sample in the form of wet aerosol or a chemically generated vapor of halogen may be introduced. A dual-flow concentric ceramic injector was used to supply helium plasma gas and the sample to the plasma. The helium plasma operated at low power levels (40–300 W) and low gas flow rates of below 3 L min − 1 and was self-igniting. The acoustic, radio-frequency (rf) and microwave-driven plasmas can withstand wet aerosol loadings of 5, 30 and 100 mg min − 1 , respectively, generated by an ultrasonic nebulizer without a desolvation unit. The plasma physical characteristics were compared at these three frequencies under otherwise similar operating conditions. The helium excitation temperature, OH rotational temperature and electron number density increased with increasing frequency in ranges of 2800–4000 K, 1100–3200 K and 0.1–7 × 10 14 cm − 3 , respectively. To demonstrate the effect of frequency on the plasma excitation efficiency the emission intensity from halogen ions was evaluated using chemical vapor generation with continuous sampling without desiccation. Using 3-phase microwave, 6-phase microwave, 4-phase rf and 1 kHz helium plasma sources the detection limits (3σ) for chlorine at 479.40 nm were 26, 60, 230 and 1200 ng mL − 1 , respectively. The microwave-driven plasma was the densest and had the highest excitation potential toward chlorine and bromine ions.

Short-wave excitations in non-local Gross-Pitaevskii model

Abstract We show that a non-local form of the Gross-Pitaevskii equation describes not only long-wave excitations, but also the short-wave ones in Bose-condensate systems. At certain parameter values, the excitation spectrum mimics the Landau spectrum of quasi-particle excitations in superfluid helium with roton minimum. The excitation wavelength, at which the roton minimum exists, is close to the inter-particle interaction range. We determine how the roton gap and the effective roton mass depend on the interaction potential parameters, and show that the existence domain of the spectrum with a roton minimum is reduced if one accounts for an inter-particle attraction.

Doppler and Stark broadenings of spectral lines of highly excited helium atoms for measurement of detached recombining plasmas in MAP-II divertor simulator

The shapes of atomic helium spectral lines from various excited states were measured in the MAP (material and plasma)-II linear divertor simulator at the University of Tokyo, using a high-resolution visible–near-ultraviolet spectrometer with image magnification optics. A steady state pure helium plasma was produced by a direct current arc discharge in the volumetric recombination-dominated regime, where both the ionizing and the recombining components could be observed up to higher excited states. Highly excited states (principal quantum number n⩾6) of helium atoms are dominated by the recombining component, except the 1P states that are optically coupled with the ground state. Therefore, we propose a method to determine the neutral temperature, electron density, and ion temperature in the recombining plasmas from the spectral line shapes with Doppler and Stark broadenings of three different transitions, 21S–31P (501.567nm), 21S–71P (335.455nm), and 23P–73D (370.500nm).

Inelastic x-ray scattering study on the single excitations of helium

The inelastic x-ray scattering technique has been used to study the excitation mechanism of atomic helium in this work. The squares of the form factor for the single excitations of 1s2 → 1snl (n ⩽ 6) of helium have been determined by the high resolution (70 meV) inelastic x-ray scattering. The present squares of the form factor measured by inelastic x-ray scattering provide an independent cross-check to the ones measured by high-energy electron energy loss spectroscopy. It is found that the squares of the form factor for the single excitations of 1s2 → 1snl (n ⩽ 6) of helium measured by inelastic x-ray scattering are in excellent agreement with the ones measured by high-energy electron energy loss spectroscopy, which confirms the quality of these two experimental methods. The excellent agreement of the present measurements with the theoretical calculations also gives a rigorous test of the theoretical method. This work demonstrates that inelastic x-ray scattering is a powerful tool to study the excitation mechanism in atomic or molecular systems at the third-generation synchrotron radiation light source.

Development of double cylindrical dielectric barrier discharge ion source

This paper deals with the dielectric barrier discharge (DBD) ion source composed of the outer cylindrical dielectric tube and the inner grounded metallic tube electrode. The sample gas is supplied through the inner ceramic tube. In this ion source, the DBD plasma is localized in the DBD tube so that the sample gases can be ionized just outside of the ceramic tube by the DBD excited helium gas without being exposed in the plasma jet. Besides, ambient air does not take part in the ionization of the sample vapor because ionization takes place inside the DBD ion source. Thus, this method is totally free from contaminants in ambient air. It was found that this ion source is capable of soft, high-sensitivity, and reproducible ionization. Application of this technique to the analysis of methamphetamine, carbaryl and basil leaf was given.

Investigation of the Atmospheric Helium Dielectric Barrier Discharge Driven by a Realistic Distorted-Sinusoidal Voltage Power Source

The non-equilibrium atmospheric-pressure parallel-plate helium dielectric barrier discharge (DBD) driven by a realistic 20 kHz distorted-sinusoidal voltage waveform has been investigated by means of simulations and experiments. A self-consistent one-dimensional fluid modeling code considering the non-local electron energy balance was applied to simulate the helium DBD. The effect of selecting plasma chemistry was investigated by comparing simulations with experiments. The results show that the simulations, which include more excited helium, metastable helium and electron–ion-related reaction channels, can faithfully reproduce the measured discharged temporal current quantitatively. Based on the simulated discharge properties, we have found that there is complicated mode transition of discharges from the long Townsend-like to the “dark current”-like, then to the short primary Townsend-like and the short secondary Townsend-like for the helium DBD that is driven by a realistic distorted-sinusoidal voltage power source. Discharge properties in different periods of discharge are discussed in detail in the paper.

Double photoionization of excited lithium and beryllium

We present total, energy-sharing, and triple differential cross sections for one-photon, double ionization of lithium and beryllium starting from aligned, excited $P$ states. We employ a recently developed hybrid atomic orbital with numerical grid method based on the finite-element discrete-variable representation and exterior complex scaling. Comparisons with calculated results for the ground-state atoms, as well as analogous results for ground-state and excited helium, serve to highlight important selection rules and show some interesting effects that relate to differences between inter- and intrashell electron correlation.

Effect of plasma chemistry on the simulation of helium atmospheric-pressure plasmas

The effect on the selection of different plasma chemistries for simulating a typical dielectric barrier discharge (DBD) driven by quasi-pulsed power source (20 kHz) is investigated. The numerical simulation was performed by using the one-dimensional self-consistent fluid modeling solver. Our simulation result indicates that the computed temporal current density can be significantly improved by using a complex version of plasma chemistry module rather than the simple one and demonstrates an excellent agreement with the experimental data. The result suggests the metastable, excited and ionic helium related reaction channels, which are important in simulating a DBD, should be taken into account. Furthermore, it also reveals that the power absorption of ions is considerably higher than that of the electron.

Ultralong-range energy transfer by interatomic Coulombic decay in an extreme quantum system

Interatomic Coulombic decay is a recently discovered ionization process by which energy absorbed from incident radiation by one atom is rapidly transferred to another. A study of this process in helium now shows that it can operate over remarkably long distances of more than 45 atomic radii. When an atom is electronically excited, it relaxes by emitting a photon or an electron. These carry essential information on the electronic structure of their emitter. However, if an atom is embedded in a chemical environment, another ultrafast non-radiative decay process called interatomic Coulombic decay (ICD) can become operative1. As ICD occurs only in the presence of neighbours, it is highly sensitive to that environment. Therefore, it has the potential to become a powerful spectroscopic method to probe the close environment of a system. ICD has been observed experimentally in van der Waals clusters2,3,4,5 as well as in hydrogen-bonded systems6,7,8. A key feature of ICD is that the excited atom can transfer its excess energy to its neighbours over large distances. The giant extremely weakly bound helium dimer is a perfect candidate to investigate how far two atoms can exchange energy. We report here that the two helium atoms within the dimer can exchange energy by ICD over distances of more than 45 times their atomic radius. Moreover, we demonstrate that ICD spectroscopy can be used for imaging vibrational wavefunctions of the ionized–excited helium dimer.

Elastic and inelastic positron–helium scattering

Elastic and inelastic positron–helium scatterings have been investigated with the coupled-channel optical method (CCO). Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential. Calculations are reported of cross sections of elastic scattering, total excitation and n = 2, 3, and 4 excitations of ground-state helium for incident energies from 30 eV to 400 eV. The present calculation shows that the ionization and Ps-formation channels significantly affect the cross sections of elastic and inelastic positron–helium scatterings.

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.

Low-temperature mobility of surface electrons and ripplon-phonon interaction in liquid helium

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction of surface and volume excitations of liquid helium, which rapidly decreases with temperature. Thus, the temperature dependence of the low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V s), where n_e is the surface electron density. The relation T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and \omega/2\pi < 30 Hz.

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experiment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolution in both domains simultaneously. We show that resonant absorption is important in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660–94).

Development of a Remote-from-Plasma Dielectric Barrier Discharge Ion Source and Its Application to Explosives

The design of a new type of dielectric barrier discharge (DBD) ion source for ambient ionization is described. In this ion source, the DBD plasma is confined in the DBD tube so that the sample gases can be ionized just outside of the discharge tube by DBD excited helium gas without being exposed to the plasma jet. The findings indicate that the ion source is capable of the soft and high-sensitive ionization of explosives.