Excited Helium Research Articles

Local charge exchange of He+ ions at Aluminum surfaces

We report on experiments designed to observe the correlation between the autoionization of doubly excited helium atoms and the Auger decay of 2p vacancies in Al. The autoionizing states are formed when incident He+⁎ and He++ are neutralized by resonant electron capture at the surface. 2p excitation in Al occurs in dielectronic charge transfer during the close encounter of an excited helium ion and an Al atom. These results clarify the mechanism for Al-2p excitation in the case of singly charged ground state He+(1s) ion impact, where the dielectronic transition occurs after promotion of the 1s electron of incoming ions.

Radiation transport in kinetic simulations and the influence of photoemission on electron current in self-sustaining discharges

A kinetic description for electronic excitation of helium for principal quantum number n 4 has been included into a particle-in-cell (PIC) simulation utilizing direct simulation Monte Carlo (DSMC) for electron-neutral interactions. The excited electronic levels radiate state-dependent photons with wavelengths from the extreme ultraviolet (EUV) to visible regimes. Photon wavelengths are chosen according to a Voigt distribution accounting for the natural, pressure, and Doppler broadened linewidths. This method allows for reconstruction of the emission spectrum for a non-thermalized electron energy distribution function (EEDF) and investigation of high energy photon effects on surfaces, specifically photoemission. A parallel plate discharge with a fixed field (i.e. space charge neglected) is used to investigate the effects of including photoemission for a Townsend discharge. When operating at a voltage near the self-sustaining discharge threshold, it is observed that the electron current into the anode is higher when including photoemission from the cathode than without even when accounting for self-absorption from ground state atoms. The photocurrent has been observed to account for as much as 20% of the total current from the cathode under steady-state conditions.

Theoretical framework for thin film superfluid optomechanics: towards the quantum regime

Excitations in superfluid helium represent attractive mechanical degrees of freedom for cavity optomechanics schemes. Here we numerically and analytically investigate the properties of optomechanical resonators formed by thin films of superfluid 4He covering micrometer-scale whispering gallery mode cavities. We predict that through proper optimization of the interaction between film and optical field, large optomechanical coupling rates and single photon cooperativities are achievable. Our analytical model reveals the unconventional behaviour of these thin films, such as thicker and heavier films exhibiting smaller effective mass and larger zero point motion. The optomechanical system outlined here provides access to unusual regimes such as and opens the prospect of laser cooling a liquid into its quantum ground state.

THE X-RAY THROUGH OPTICAL FLUXES AND LINE STRENGTHS OF TIDAL DISRUPTION EVENTS

ABSTRACT We study the emission from tidal disruption events (TDEs) produced as radiation from black hole accretion propagates through an extended, optically thick envelope formed from stellar debris. We analytically describe key physics controlling spectrum formation, and present detailed radiative transfer calculations that model the spectral energy distribution and optical line strengths of TDEs near peak brightness. The steady-state transfer is coupled to a solver for the excitation and ionization states of hydrogen, helium, and oxygen (as a representative metal), without assuming local thermodynamic equilibrium. Our calculations show how an extended envelope can reprocess a fraction of soft X-rays and produce the observed optical fluxes of the order of 1043 erg s−1, with an optical/UV continuum that is not described by a single blackbody. Variations in the mass or size of the envelope may help explain how the optical flux changes over time with roughly constant color. For high enough accretion luminosities, X-rays can escape to be observed simultaneously with the optical flux. Due to optical depth effects, hydrogen Balmer line emission is often strongly suppressed relative to helium line emission (with He ii-to-H line ratios of at least 5:1 in some cases) even in the disruption of a solar-composition star. We discuss the implications of our results to understanding the type of stars destroyed in TDEs and the physical processes responsible for producing the observed flares.

Extreme ultraviolet radiation emitted by helium microwave driven plasmas

The extreme ultraviolet radiation emitted by helium microwave-driven (2.45 GHz) plasmas operating at low-pressure conditions was investigated. Novel data regarding emitted spectral lines of excited helium atoms and ions in the 20–33 nm wavelength range and their intensity behavior with variation of discharge operational conditions are presented. The intensity of all the spectral emissions was found to strongly increase with the microwave power delivered to the plasma. Furthermore, the intensity of the ionic spectral emissions decreases by nearly one order of magnitude as the pressure was raised from 0.2 to 0.5 mbar.

Computer-aided modeling of physicochemical characteristics of various energy sources

The authors’ review presents original methodical development works and integrates the research on the modeling of processes and estimation of physicochemical characteristics of natural (hydrocarbons) and chemical (energetic materials) energy sources. A fundamentally new method of energy generation based on transition of excited helium atoms to the superstable state in superstrong magnetic fields is described. A hypothetical possibility for these states to exist was predicted by one of the authors and confirmed by calculations.

Laser cooling and control of excitations in superfluid helium

Superfluidity is a quantum state of matter that exists macroscopically in helium at low temperatures. The elementary excitations in superfluid helium have been probed with great success using techniques such as neutron and light scattering. However, measurements of phonon excitations have so far been limited to average thermodynamic properties or the driven response far out of thermal equilibrium. Here, we use cavity optomechanics to probe the thermodynamics of phonon excitations in real time. Furthermore, strong light–matter interactions allow both laser cooling and amplification. This represents a new tool to observe and control superfluid excitations that may provide insight into phonon–phonon interactions, quantized vortices and two-dimensional phenomena such as the Berezinskii–Kosterlitz–Thouless transition. The third sound modes studied here also offer a pathway towards quantum optomechanics with thin superfluid films, including the prospect of femtogram masses, high mechanical quality factors, strong phonon–phonon and phonon–vortex interactions, and self-assembly into complex geometries with sub-nanometre feature size. It takes extreme sensitivity to measure the elementary excitations in liquid helium-4. An optomechanical cavity with a thin film of superfluid inside can be used to both observe and control phonons in real time.

Dissipation of the kinetic energy of a tuning fork immersed in superfluid helium at different oscillation frequencies

The drag coefficient characterizing the dissipation of the energy of oscillating tuning forks immersed in liquid helium is studied experimentally. The experiments are done at temperatures from 0.1 to 3.5 K, a range that covers both hydrodynamic flow and the ballistic transport of thermal excitations in superfluid helium below 0.6 K. It is found that a frequency dependence of the drag coefficient exists in the hydrodynamic limit, where the main dissipation mechanism is viscous friction of the liquid against the surface of the oscillating object at temperatures above 0.7 K. In this case, the drag coefficient is proportional to the square root of the oscillation frequency and its temperature dependence in He II is determined by the corresponding relationships between the density of the normal component and the viscosity of the liquid. At lower temperatures, there is no frequency dependence of the drag coefficient and the magnitude of the dissipative losses is determined only by the temperature dependence of the density of the normal component. At the same time, over the entire range of temperatures studied here, the magnitude of the dissipative losses depends on the geometrical dimensions of the oscillating object.

Improvement in ionization efficiency of direct analysis in real time-mass spectrometry (DART-MS) by corona discharge.

Herein it is shown that a combination of direct analysis in real time (DART) with a corona discharge system consisting of only a needle electrode easily improves DART ionization efficiency. Positive and negative DC corona discharges led to a formation of abundant excited helium atoms as well as the reactant ions H3O(+)(H2O)n and O2˙(-) in the DART analyte ionization area. These phenomena resulted in an increase in the absolute intensities of (de)protonated analytes by a factor of 2-20 over conventional DART. The other analyte ions detected in this corona-DART system (i.e., molecular ions, fragment ions, oxygenated (de)protonated analytes, dehydrogenated deprotonated analytes, and negative ion adducts) were quite similar to those obtained from DART alone. This indicates a lack of side reactions due to the corona discharge. The change in the relative intensities of individual analyte-related ions due to the combination of a corona discharge system with DART suggests that there is no effect of the abundant excited helium in the analyte ionization area on the fragmentation processes or enhancement of oxidation due to hydroxyl radicals HO˙. Furthermore, it was found that the corona-DART combination can be applied to the highly sensitive analysis of n-alkanes, in which the alkanes are ionized as positive ions via hydride abstraction and oxidation, independent of the type of alkane or the mass spectrometer used.

Relaxation dynamics of helium nanodroplets after photodissociation of a dopant homonuclear diatomic molecule. The case of Cl2@((4)He)(N).

To investigate the quantum dynamics of the relaxation process of excited helium nanodroplets, (4)HeN, arising from the photodissociation of Cl2 embedded molecules (B ← X electronic transition), here we have performed a time dependent density functional theory (TDDFT) study considering nanodroplets of different sizes (N = 50, 100, 200, 300 and 500), extending a previous study which was centered on the photodissociation step. The relaxation process takes place in the timescale of several hundred picoseconds and a simple dependence of this process on time has been found. The results have been satisfactorily analyzed in terms of a phenomenological model proposed here and also by applying the Rice-Ramsperger-Kassel (RRK) statistical chemical kinetic model for unimolecular reactions. From what we know, this is the first time that the dynamics of these de-excitation processes has been studied, opening up a window for understanding them. We expect that this work will encourage further research on this little known but interesting phenomenon.

Time-resolved line emission spectroscopy and the electrical currents in the plasma jet generated by dielectric barrier discharge for soft ionization

The spatially and temporally resolved optical emission from the helium capillary dielectric barrier discharge driven by a square wave high voltage was measured simultaneously with the discharge currents during the positive and negative voltage periods. The He and N2+ emission intensities were monitored at 501nm and 391nm, respectively. The plasma jet was found to be formed only during the positive voltage period, and it preceded the discharge ignition in the capillary. Thus, the measurements of the difference between the capillary discharge currents during the positive and negative voltage periods enabled quantification of the charge, which was transported through the plasma jet. The analysis of the spatiotemporal behavior of the He emission intensities in the plasma jet yielded the velocity of the helium excitation propagation and the effective plasma jet length. The combination of these measurements enabled determination of the absolute values of the plasma jet currents. The linear relationship between the He emission intensities in the plasma jet and the plasma jet currents was found to exist over the emission intensity range of two orders of magnitude. The N2+ emission intensity was also found to be linearly dependent on the plasma jet current. The fast electrons, producing the excited He atoms, were found to be located mostly within the estimated effective plasma jet length. In this region a cascade of processes involving excited He atoms, N2+ and H2O molecules lead to establishment of the conditions necessary for the plasma jet to be used as soft ionization source. The production of reactive species can be related to the plasma jet current, which can serve as a measure of the soft ionization efficiency.

The collective-excitation spectrum in superfluid helium

On the basis of the Landau quasiparticle concept and the exact relation for the “density–density” response function, an explicit expression for the energy spectrum of collective excitations in superfluid helium is obtained with the strong-interaction effects taken into account.

Ultrafast two-photon Rabi oscillations in excited He driven by femtosecond intense laser fields

SynopsisWe demonstrate ultrafast multiphoton coherent excitation in near-infrared intense laser fields (≤10 TW/cm2) using excited helium in the 1s2p (1P) state as the target. Strong two-photon coupling between the 2p state and the 5f and 6f Rydberg states drives ultrafast Rabi oscillations in tens of femtosecond. Theoretical calculations solving the time-dependent Schrodinger equation were carried out to elucidate the strong field effects.

Excited state distribution and spin-effects in strong-field excitation of neutral Helium

SynopsisWe investigated the principal quantum number n distribution of excited states resulting from the interaction of Helium with strong, short laser pulses. We find excellent agreement with predictions of the semiclassical frustrated tunneling ionization (FTI) model [1] as well as fully quantum mechanical calculations. Furthermore, the excitation process directly populates triplet excited states due to the breakdown of the Russel-Saunders coupling scheme for high orbital angular momentum l states of Helium, which are predominantly populated in the strong laser field.

The realization of the dipole(γ, γ) method and its application to determine the absolute optical oscillator strengths of helium

SynopsisThe dipole(γ, γ) method based on the inelastic x-ray scattering (IXS) technique is proposed and realized to measure the absolute optical oscillator strength (OOSs) for the valence-shell excitations of atoms and molecules. The OOSs of valence- shell excitations of helium are determined, and the good agreement with the previous results is observed.

Non-perturbative B-spline R-matrix with pseudo-states calculations for electron-impact excitation-ionization of helium to the n=3 states

SynopsisWe present fully-differential cross sections for electron-impact ionization plus simultaneous excitation of helium obtained from a non-perturbative close-coupling formalism with our B-spline R-matrix approach. Using a large number of pseudo-states we obtain excellent agreement with directly measured cross-section ratios for ionization leaving the residual He+ ion in either the 1 s ground state, the n = 2 (2s + 2p) excited states, or the n = 3 (3s + 3p + 3d) excited states.

Spectrum of quasistable states in a strong infrared field

The stability of highly excited hydrogen and helium atoms subject to an intense infrared laser field is studied, and similarities to the case of microwave driving are discussed.

Time- and spatially resolved emission spectroscopy of the dielectric barrier discharge for soft ionization sustained by a quasi-sinusoidal high voltage

A helium capillary dielectric barrier discharge was investigated by means of time-resolved optical emission spectroscopy with the aim of elucidating the process of the formation of the plasma jet. The helium emission line at 706 nm was utilized to monitor spatial and temporal propagation of the excitation of helium atoms. The discharge was sustained with quasi-sinusoidal high voltage, and the temporal evolution of the helium atomic emission was measured simultaneously with the discharge current. The spatial development of the plasma was investigated along the discharge axis in the whole region, which covers the positions in the capillary between the electrodes as well as the plasma jet outside the capillary. The high voltage electrode was placed 2 mm from the capillary orifice, and the distance between the ground and high voltage electrode was 10 mm. The complete spatiotemporal grid of the development of the helium excitation has shown that during the positive half-period of the applied voltage, two independent plasmas, separated in time, are formed. First, the early plasma that constitutes the plasma jet is formed, while the discharge in the capillary follows subsequently. In the early plasma, the helium atom excitation propagation starts in the vicinity of the high voltage electrode and departs from the capillary towards the ground electrode as well as several millimeters outside of the capillary in the form of the plasma jet. After relatively slow propagation of the early plasma in the capillary and the jet, the second plasma starts between the electrodes. During the negative voltage period, only the plasma in the capillary between the electrodes occurs.

Efimov Physics and the Three-Body Parameter for Shallow van der Waals Potentials

Extremely weakly-bound three-boson systems are predicted to exhibit intriguing universal properties such as discrete scale invariance. Motivated by recent experimental studies of the ground and excited helium trimers, this work analyzes the three-body parameter and the structural properties of three helium atoms as the s-wave scattering length is tuned artificially. Connections with theoretical and experimental studies of the Efimov scenario as it pertains to cold atom systems are made.

Real-Time Analysis of Ambient Organic Aerosols Using Aerosol Flowing Atmospheric-Pressure Afterglow Mass Spectrometry (AeroFAPA-MS).

Organic compounds contribute to a major fraction of atmospheric aerosols and have significant impacts on climate and human health. However, because of their chemical complexity, their measurement remains a major challenge for analytical instrumentation. Here we present the development and characterization of a new soft ionization technique that allows mass spectrometric real-time detection of organic compounds in aerosols. The aerosol flowing atmospheric-pressure afterglow (AeroFAPA) ion source is based on a helium glow discharge plasma, which generates excited helium species and primary reagent ions. Ionization of the analytes occurs in the afterglow region after thermal desorption and produces mainly intact quasimolecular ions, facilitating the interpretation of the acquired mass spectra. We illustrate that changes in aerosol composition and concentration are detected on the time scale of seconds and in the ng m(-3) range. Additionally, the successful application of AeroFAPA-MS during a field study in a mixed forest region is presented. In general, the observed compounds are in agreement with previous offline studies; however, the acquisition of chemical information and compound identification is much faster. The results demonstrate that AeroFAPA-MS is a suitable tool for organic aerosol analysis and reveal the potential of this technique to enable new insights into aerosol formation, growth, and transformation in the atmosphere.