Excited Helium Research Articles

Excitation of Helium by Proton and Antiproton Impact

The electron transitions in atoms caused by charged particle impact are benchmarks for the study of electron dynamics. In the present paper we focus on the excitation of helium by proton and antiproton impact. We perform both ab initio and perturbational calculations, revealing the importance of electron correlations and higher-order effects. The influence of the projectile charge sign on the excitation cross section is also studied.

Observation of Interatomic Coulombic Decay Induced by Double Excitation of Helium in Nanodroplets.

Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound complexes exposed to intense or high-energy radiation. So far, neutral or ionic atoms or molecules have been prepared in singly excited electron or hole states that can transfer energy to neighboring centers and cause ionization and radiation damage. Here we demonstrate that a doubly excited atom, despite its extremely short lifetime, can decay by ICD; evidenced by high-resolution photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find that ICD proceeds by relaxation into excited He^{*}He^{+} atom-pair states, in agreement with calculations. The ability of inducing ICD by resonant excitation far above the single-ionization threshold opens opportunities for controlling radiation damage to a high degree of element specificity and spectral selectivity.

Bright continuously tunable vacuum ultraviolet source for ultrafast spectroscopy

Ultrafast electron dynamics drive phenomena such as photochemical reactions, catalysis, and light harvesting. To capture such dynamics in real-time, femtosecond to attosecond light sources are extensively used. However, an exact match between the excitation photon energy and a characteristic resonance is crucial. High-harmonic generation sources are advantageous in terms of pulse duration but limited in spectral tunability in the vacuum ultraviolet range. Here, we present a monochromatic femtosecond source continuously tunable around 21 eV photon energy utilizing the second harmonic of an optical parametric chirped pulse amplification laser system to drive high-harmonic generation. The unique tunability of the source is verified in an experiment probing the interatomic Coulombic decay in doped He nanodroplets across the He absorption bands. Moreover, we achieved intensities sufficient for driving collective processes in multiply excited helium nanodroplets, which have been previously observed only at free electron lasers.

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium.

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the 2p^{2} ^{1}D^{e} double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined R-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general.

Efficient vibrational excitation of molecular nitrogen in low-pressure plasma with ultralow electron temperature

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Active species evolution in the presence of different targets impacted by a helium plasma jet at atmospheric pressure

A low-temperature plasmas jet is generated by a dielectric barrier discharge powered by a pulsed high voltage in helium flow (3 L min−1) at atmospheric pressure in the presence of different targets (a glass slide, ultra-pure water and a grounded metal plate) positioned perpendicular to the plasma propagation axis. Experimental electrical characterizations such as discharge current, voltage and power, and optical characterizations such as intensified camera ICCD, Schlieren imaging and emission spectroscopy to follow specific excited species have been achieved. The transition from laminar to turbulent regime was observed during the discharge ignition with a larger spreading of the plasma on the surface target with lower dielectric permittivity. The generation of two discharges during each voltage pulse is highlighted during the propagation of the ionization wave which has a variable speed along the plasma axis not depending on the target kind. The evolution of some active species (such as OH, O and excited nitrogen and helium) are investigated using time resolved mapping of the emissions of radiative excited species propagating in ambient air between the plasma jet output and the target. For a low relative permittivity target (glass), the volume ionization wave at its arrival on the target spreads on its surface thus behaving as a surface ionization wave. For the highest relative permittivity (metal), a conductive channel appears between the target surface and the plasma jet during the first discharge, followed by a diffuse plasma plume from the target surface towards the plasma jet after the impact of the ionization wave on the target. A hybrid behavior is highlighted for the ultra-pure water which leads to a short spreading of the ionization wave on the target surface, the formation of a conductive channel in ambient air between tube output and target and the formation of a plasma plume on the target surface.

Effects of pulse voltage rising edge on discharge evolution of He atmospheric pressure plasma jet in dielectric tube

In this work, we employ pulse voltage to drive an atmospheric pressure plasma jet (APPJ) in Helium, and consider mainly the evolution of discharge inside tube. Specifically, the effects of rising edge on the discharge evolution are studied through the simulation and experiment. The spatiotemporal evolution of electron density, ionization source, electron temperature and excited helium atom are evaluated. Besides, the mechanism affecting the rise time is analyzed by the parameters such as discharge current, sheath thickness and surface charge density distribution. In the considered cases, the ionization wave propagates to the ground electrode and downstream of the active electrode in the dielectric tube. The plasma with faster rising edge has larger electron temperature, discharge current, electron density and electric field strength. With the change of voltage rising edge, there occur two discharge modes: hollow mode and solid mode in dielectric barrier discharge (DBD) area. When the rising edge is of nanosecond and sub microsecond, it develops into hollow mode, and changes into solid mode after the rising edge has continued to increase. Both discharge modes are essentially affected by the sheath thickness, the electric field distribution, and the surface charge density inside the tube. When the sheath thickness is less than 1.8 mm, the plasma usually propagates in hollow mode, and when the sheath thickness is equal to 1.8 mm, the radial propagation range of the plasma is limited and changes into solid propagation. In the DBD region, when the electric field is mainly axial component, the plasma propagates in the mode at the beginning of discharge; inside the ground electrode, owing to the fact that the applied electric field is deviated from the radial direction, and that the positive charge deposited on the tube wall forms a radial self-built electric field, the strong radial electric field formed by the superposition of the two fields causes the discharge to propagate in hollow mode.

Relaxation dynamics in excited helium nanodroplets probed with high resolution, time-resolved photoelectron spectroscopy.

Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of nanoscopic bubbles or excimers, and autoionization channels generating low-energy free electrons. Here, we discuss the full spectrum of ultrafast relaxation processes observed when helium nanodroplets are electronically excited. In particular, we perform an in-depth study of the relaxation dynamics occurring in the lowest 1s2s and 1s2p droplet bands using high resolution, time-resolved photoelectron spectroscopy. The simplified excitation scheme and improved resolution allow us to identify the relaxation into metastable triplet and excimer states even when exciting below the droplets' autoionization threshold, unobserved in previous studies.

Polarization in strong-field ionization of excited helium

We analyze how bound-state excitation, electron exchange and the residual binding potential influence above-threshold ionization (ATI) in helium prepared in an excited p state, oriented parallel and perpendicular to a linearly polarized mid-IR field. Using the ab initio B-spline algebraic diagrammatic construction, and several one-electron methods with effective potentials, including the Schrödinger solver Qprop, modified versions of the strong-field approximation (SFA) and the Coulomb quantum-orbit strong-field approximation, we find that these specific physical mechanisms leave significant imprints in ATI spectra and photoelectron momentum distributions. Examples are changes of up to two orders of magnitude in the high-energy photoelectron region, and ramp-like structures that can be traced back to Coulomb-distorted trajectories. The present work also shows that electron exchange renders rescattering less effective, causing suppressions in the ATI plateau. Due to the long-range potential, the electron continuum dynamics are no longer confined to the polarization axis, in contrast to the predictions of traditional approaches. Thus, one may in principle probe excited-state configurations perpendicular to the driving-field polarization without the need for orthogonally polarized fields.

Simultaneous Excitation of Helium by Means of an Electron and a Photon: A Joined Experimental and Theoretical Study

We report on a joined experimental and theoretical study of differential cross-sections resulting from inelastic scattering of a monoenergetic electron by helium atoms in the presence of an intense carbon dioxide laser. In particular, we measured the signals of the scattered electrons during the simultaneous electron–photon excitation of He 21P state for the first three microseconds of the laser pulse. The signals were measured for an incident electron energy of 45 eV and showed a structure that emerged at small scattering angles. The latter was found to be sensitive to the nature of the transferred photons, as well as the intensity of the laser field. The experimental findings were supported by quantum calculations based on the second-order Born approximation in which the correlated electron–electron interactions were taken into account.

Estimation and control of low (

We propose a method for estimation of the relatively low values (less than 100 V) of the steady-state negative potential built-up on a dielectric target irradiated by an electron beam at fore-vacuum pressure. The method is based on comparing the intensity of the emission lines of excited helium to similar emission lines for the known potential of a metal target. At low values of potential, we measured the plasma parameters in the region of the target and determined the contribution to ionization processes from each of the electron components: fast beam electrons, secondary electrons from the target, and plasma electrons.

Micro atmospheric pressure plasma jets excited in He/O2 by voltage waveform tailoring: a study based on a numerical hybrid model and experiments

A hybrid simulation code is developed to treat electrons fully kinetically by the particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm, while ions and neutral species are handled by a fluid model, including a time slicing technique to reduce the computational expenses caused by the responses of various species on different time scales. The code is used to investigate a capacitively coupled COST reference micro atmospheric pressure helium plasma jet with 0.1% oxygen admixture excited by a valley-type tailored voltage waveform with a fixed peak-to-peak voltage of 400 V, and a fundamental frequency of 13.56 MHz. The computational results are compared to experiments based on several sophisticated diagnostics, showing good agreement in the electron impact helium excitation rate, the helium metastable density, and the atomic oxygen density. The spatio-temporal electron heating dynamics, are found to be asymmetrical due to the specific shape of the driving voltage waveform. Tailoring the voltage waveform is shown to enable to control the electron energy probability function (EEPF) in distinct spatio-temporal regions of interest. As a consequence, the generation of reactive neutral species can be enhanced by increasing the number of consecutive harmonics. Based on a simplified two dimensional neutral transport model in the hybrid code, it is demonstrated that the transport between the electrodes, as well as the gas flow have different effects on various neutral species distributions due to the relevant chemical reaction rates for the generation and destruction of species.

Ultrafast Resonant Interatomic Coulombic Decay Induced by Quantum Fluid Dynamics

When weakly-bound complexes are multiply excited by intense electromagnetic radiation, energy can be exchanged between neighboring atoms through a type of resonant interatomic Coulombic decay (ICD). This decay mechanism due to multiple excitations has been predicted to be relatively slow, typically lasting tens to hundreds of picoseconds. Here, we directly measure the ICD timescale in resonantly excited helium droplets using a high resolution, tunable, extreme ultraviolet free electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as fast as 400 femtoseconds, and to only present a weak dependence on the density of the excited states. Using a combination of time dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles which drastically accelerates ICD.

Neutralization of excited antiprotonic helium ion in collisions with helium atoms

We consider the neutralization of an excited antiprotonic helium ion which is accompanied by antiproton transitions to lower states during collisions with helium atoms $$({\bar{p}}\mathrm {He}^{2+})_{nl} + \mathrm {He} \rightarrow \left[ ({\bar{p}}\mathrm {He}^{2+})_{n_f l_f} e\right] _{1s} + \mathrm {He}^+$$ in a low-temperature medium. Interactions in the input and output channels are taken to be similar to the potentials of the $$\mathrm {H}^+ - \mathrm {He}$$ ( $$X^1\varSigma ^+$$ ) and $$\mathrm {H} - \mathrm {He}^+$$ ( $$A^1\varSigma ^+$$ ) systems, respectively. The potentials in the output channels are shifted by an amount equal to the dexcitation energy of the antiproton. Therefore, the potentials in the input and output channels can intersect for the some antiproton quantum numbers leading to appreciable cross sections of the transitions. We have found that this process can increase the effective annihilation rates of highly excited antiprotonic helium ions in helium targets of low atomic density and temperature that were used in recent experiments.

Optical and mass-spectral characterization of mixed-gas flowing atmospheric-pressure afterglow sources

Plasma-based ambient desorption/ionization (ADI) sources for mass spectrometry (MS) often use helium as the primary discharge gas due to the large reaction cross section of excited helium species with atmospheric gases, which ultimately leads to efficient reagent-ion formation. However, some studies have shown that mixed-gas plasmas provide unique advantages. For instance, our group showed that a helium‑oxygen flowing atmospheric-pressure afterglow (He:O2-FAPA) source yielded enhanced ion signals for small polar analytes, but compounds with aromatic rings underwent chemical modification to produce pyrylium species. Here, it is shown that the addition of H2 to the helium FAPA gas significantly decreased the reagent-ion signals, but analyte-ion signal was not affected to the same degree. In addition, mass spectra obtained with He:H2-FAPA were chemically cleaner with fewer ions stemming from analyte species and produced less oxidation of analytes. Addition of nitrogen increased the abundance of negative reagent ions (e.g., NO2− and NO3−) which led to enhanced ion signal for RDX. To better understand these unique advantages of a mixed-gas FAPA, the impact of molecular-gas (O2, N2, or H2) addition on the optical-emission spectra from the discharge was also measured. Spatially resolved emission was obtained from the anode- and negative-glow regions. In general, addition of small fractions (~0.1%) of molecular gases decreased helium and OH emission intensities. Atomic oxygen and hydrogen emission increased with the addition of O2 and H2, respectively. Furthermore, emission characteristics of N2, N2+, and NO with respect to molecular gas composition on He-FAPA were measured. Fundamental plasma parameters, such as OH rotational temperature (Trot), were also calculated for these mixed-gas systems.

Kinetic Isotope Effect in Low-Energy Collisions between Hydrogen Isotopologues and Metastable Helium Atoms: Theoretical Calculations Including the Vibrational Excitation of the Molecule.

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (4He(23S) and 3He(23S)) colliding with the hydrogen isotopologues (H2, HD, D2) in the ground and first excited rotational and vibrational states at subkelvin regime. The calculations are performed using the current best ab initio interaction energy surface, which takes into account the nonrigidity effects of the molecule. The results confirm a recently observed substantial quantum kinetic isotope effect (Nat. Chem. 2014, 6, 332–335) and reveal that the change of the rotational or vibrational state of the molecule can strongly enhance or suppress the reaction. Moreover, we demonstrate the mechanism of the appearance and disappearance of resonances in Penning ionization. The additional model computations, with the morphed interaction energy surface and mass, give better insight into the behavior of the resonances and thereby the reaction dynamics under study. Our theoretical findings are compared with all available measurements, and comprehensive data for prospective experiments are provided.

Monitoring damage of self-assembled monolayers using metastable excited helium atoms.

The breaking of molecular bonds during exposure to ionizing radiation and electron beams creates irreversible damage in the molecular structure. In some cases, such as lithography, controlled damage of a molecular resist is a desirable process and is the basis for the entire semiconductor industry. In other cases, such as environmental exposure or probing of the molecular structure, the induced damage is a major problem that has limited advances in science and technology. We report here the use of an in situ probe that is minimally invasive to detect real-time damage induced in organic materials. Specifically, we use metastable excited helium atoms in the 3S1 state to characterize the damage caused by a low-energy electron beam ∼30 eV on an organic self-assembled monolayer of 11-bromo-1-undecanethiol on a gold substrate. We were able to monitor the damage caused by the electron beam without introducing any additional observed damage by the probing metastable atoms.

Use of hexapole magnet and spin flipper combined with time-of-flight analysis to characterize state-selected paramagnetic atomic/molecular beams.

In the past, the Stern-Gerlach experiment has been used as a standard method for analyzing the population of magnetic substates contained in spin-polarized and/or state-selected atomic/molecular beams. However, this experiment is quite demanding due to its low signal intensity and difficulty in beam alignment. The present study shows that the use of a hexapole magnet and a spin flipper, together with the time-of-flight analysis, allows us to conduct an almost equivalent analysis while greatly improving the signal intensity. Applications to the analysis of spin-polarized triplet excited helium and state-selected O2( 3 Σg -) beams are presented.

Frozen-planet resonances in doubly excited helium atom; adiabatic approach

An improved adiabatic method is applied to study the highly excited asymmetric two-electron configurations of helium atom known as frozen-planet resonances. It is shown that our approach provides much better agreement with numerically calculated resonance positions than the previously used Born–Oppenheimer type approximation. Wide range of states were studied, related to N = 7 − 15 first-ionization thresholds. We show that estimates of tunneling widths of these states however are not reliable, because of the breakdown of adiabatic approximation in the underbarrier region of configuration space. We also provide computational evidence that a single-potential approximation in hyperspherical coordinates would be inferior to our approach.

Mobility of a spatially modulated electron liquid on the helium surface

We present the results of large-scale numerical simulations of the mobility of a two-dimensional electron liquid on the helium surface in the presence of a one-dimensional periodic potential. Even where the potential is much weaker than the electron-electron interaction, it can strongly change the mobility. The effect depends on the interrelation between the potential period and the mean interelectron distance. It is most pronounced where the period is close to the period of the Wigner crystal that would form if the liquid were cooled to a lower temperature. The results suggest, in particular, that the correlation length in the electron liquid can be found by measuring the mobility in a weak periodic potential. The simulations are based on the microscopic model of the electron scattering by the excitations in helium.