Metastable Atoms Research Articles

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases. While preferential n-type doping was observed in graphene domains in nitrogen-containing plasmas, preferential p-type doping was observed at grain boundaries in boron-containing plasmas.

Narrow-line magneto-optical trap for europium

We report on the realization of a magneto-optical trap (MOT) for europium atoms using a narrow-line cooling transition with a natural linewidth of 97 kHz. Our starting point is continuous capturing and cooling of optically pumped metastable europium atoms. We have employed simultaneous MOT for the metastable and ground-state atoms. The trapped metastable atoms are successively pumped back to the ground state and then continuously loaded to the narrow-line MOT, where up to $4.7\times10^7$ atoms are captured. A spin-polarized sample at a temperature of $6\,\mathrm{\mu K}$ and with a peak number density of $2.2\times10^{11}\,\mathrm{cm^{-3}}$ is obtained through the compression process, resulting in a phase space density of $3\times10^{-5}$.

Rapid generation of metastable helium Bose-Einstein condensates

We report the realisation of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed beam optical dipole trap. A novel quadrupole-Ioffe configuration (QUIC) magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10G bias, without compromising optical access. The bias enables in-trap 1D doppler cooling to be used, which is the only cooling stage between the magneto-optic trap (MOT) and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 seconds, with essentially pure BECs observed with $\sim 10^6$ atoms after evaporative cooling in the dipole trap.

Systematic investigation of the pulsed barrier discharges in flowing and stationary gas: From differences to similarities

Pulsed dielectric barrier discharges in flowing and stationary helium at atmospheric pressure are comparatively studied. The discharges are generated in plane-parallel configuration and driven by positive voltage pulses. The intensified charge coupled device imaging and electrical measurements are used for temporally, spatially, and spectrally resolved discharge emission. The populations of excited states are investigated by optical emission and absorption spectroscopy. The discharge operating in flowing helium is characterized by a single current pulse per each voltage flank and a “residual current peak.” In stationary helium or in flowing helium with low velocity (<10 cm/s), multiple current pulses are observed. The appearance of the subsequent current pulse(s) is related to the disappearance of the “residual current peak.” Besides these differences between the two discharge regimes, some similarities are also registered. The maximum emission intensity is reached at the cathode, even for the subsequent current pulses formed in stationary helium, confirming the generation of the glow and multi-glow discharges. Also, the most important similarities between the discharges working in stationary helium and in flowing gas mixture, in relation to flowing helium mode, are discussed. Finally, the subsequent current pulses formed in multi-glow discharges are due to re-ignition of the discharge dominantly by direct electronic excitation and ionization processes, not by Penning ionization, because of the decrease in metastable atoms density and their lifetime. In addition, the metastable lifetime reaches a critical value, for which the discharge transits from glow to multi-glow mode, when the gas velocity or its composition changes.

A study on the E-H mode transition with the line integrated He [formula omitted] metastable atom density in an inductively coupled plasma

Helium (He) inductively coupled plasma (ICP) was generated by a right-helical-type RF antenna. At a fixed He pressure of 20 mTorr, the optical emission spectrum from the He plasma was measured with RF power. The E-H mode transition and the characteristic hysteresis of the mode transition were confirmed by a change in the spectral line intensity of the 33D−23P transition at 587.6 nm. By laser absorption spectroscopy using a laser light with a wavelength of 1083 nm resonant to the 23S→23P absorption transition, the line-integrated He 23S metastable density was measured and found to show consistent behavior with the spectral line intensity of the 33D−23P transition during the E-H mode transition. The line-integrated He 23S metastable atom density exhibited distinctly different longitudinal distributions in the E and H modes.

Current-voltage characteristics of a magnetron with a hot titanium target in chemically active environments

This work analyzes the results of original experimental studies focused on the discharge current-voltage (I-V) characteristics of a magnetron with a hot titanium target in environments containing reactive gases. Our work presents a fundamental study of a direct current gas discharge with a hot cathode in chemically active environments with a magnetic field applied. The results obtained are useful for practical applications in the development of technologies for the deposition of oxide, nitride and other films. It is found that regardless of the gas environment composition, the I-V characteristics reflect the physical processes in the discharge with a hot target. At low currents, there is a competition of two processes on the target surface, namely, the formation of a titanium compound film and its removal due to sputtering with argon ions and evaporation. A distinctive feature of the hot target discharge is the influence of the thermionic emission of the target, the quenching of metastable argon atoms by atoms and molecules of reactive gases and possibly an additional increase in the rarefaction of the gas near the target due to its heating not only by sputtered atoms but also by evaporated ones on the I-V characteristics. Furthermore, calculations using the Richardson–Dushman equation show that the presence of reactive gases entails a change in the electron work function of titanium.

Optically pumped argon metastable laser with repetitively pulsed discharge in a closed chamber

The optically pumped rare gas laser (OPRGL) is an innovative type of gas laser, which has the potential to be a candidate for high-energy lasers (HELs), in which metastable rare gas atoms generated by gas discharge are used as active particles. To realize these kinds of lasers as HELs in field application, high efficiency and low cost are necessary. For this purpose, we demonstrated a closed-chamber operation with repetitively pulsed gas discharge for an OPRGL of argon metastables. The pulsed discharge was powered by a home-made LC generator, which provided a high efficiency for the deposition of electric energy. The metastable density produced was higher than 1012 cm−3, quite enough for efficient laser operation. The dependencies of laser radiation on the parameters of discharge voltage, gas pressure, and argon concentration were studied, which showed an optimized argon concentration of 1%. Long-time operations were also performed, which demonstrated the practicability of the operation of OPRGL in a closed chamber, with the avoiding of the running away of rare gases in the open cycle of flowing operation.

Continuous control of CuBr laser pulse energy

The paper presents the results of the research and development of the CuBr laser with continuous control of each pulse energy. The prototype consists of a large volume CuBr active element with an unstable cavity, two high voltage pulses generators and control system. The HBr additive was used to increase the laser energy-frequency characteristics. Control of laser pulses energy has been provided by high-voltage preionization pulses. The linear control of output power from 5 W to 0 have been demonstrated. The metastable atom population is the major factor in the energy control. The dependence of the preionization pulse energy for full suppression on the active media operating mode (CuBr concentration, pumping energy etc.) was studied. The ratio between excitation pulse energy and preionization pulse energy for full suppression of the lasing changing from 18 to 25.

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.

Surface ionization of metastable calcium atoms

We report on an experimental study of surface ionization of metastable calcium atoms on a hot polycrystalline tungsten surface in vacuum. We implemented a hollow-cathode discharge to excite a fraction of calcium atoms in an atomic beam to metastable states and collected the resulting calcium ions. We observed that metastable calcium atoms are ionized with a significantly greater efficiency than ground-state atoms, and the results suggest that virtually every metastable atom impacting the hot surface is ionized. These results demonstrate the potential of metastable atom surface ionization as a means of enriching calcium isotopes for applications in medicine, metrology, and fundamental science.

A bright and stable beam of slow metastable helium atoms

A stable high-intensity atomic beam source plays a key role in many precision measurements. The precision spectroscopy of slow metastable (<inline-formula><tex-math id="M6">\begin{document}$2^3{\rm S}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M6.png"/></alternatives></inline-formula>) helium atoms is of great interest in testing quantum electrodynamics and determining the fine structure constant. By improving the source cavity structure and using laser cooling method, the beam flux is greatly enhanced. The added Zeeman slower reduces the longitudinal velocity of atoms, and at the same time increases the beam brightness of atoms at one single speed. Near the back end of Zeeman slower, a two-dimensional magneto-optical trap is added to collimate and focus the atomic beam. In addition, A beam stabilizing system is developed by using feedback control method. By changing the frequency of transverse cooling laser to change the cooling efficiency, the fluctuation of atomic beam intensity can be compensated in real time, and then the beam intensity can be stabilized at the target number. Experiments show that the continuous beam of metastable helium atoms at a velocity of <inline-formula><tex-math id="M7">\begin{document}$(100\pm 3.6)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M7.png"/></alternatives></inline-formula> m/s has an intensity of <inline-formula><tex-math id="M8">\begin{document}$5.8\times10^{12}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M8.png"/></alternatives></inline-formula> atoms/s/sr and a relative stability of 0.021%. In the experiment of precise spectral measurement based on atomic beam, the narrow longitudinal velocity distribution reduces the lateral Doppler broadening effect, and the lower longitudinal velocity also reasonably reduces the systematic error caused by the first-order Doppler effect. The atomic beam with such high intensity and stability in a single momentum and quantum state obviously improves the signal-to-noise ratio of the spectrum, and further reduces the statistical error of the results in the same detection time. Using this atomic beam, we demonstrated spectroscopy of the <inline-formula><tex-math id="M9">\begin{document}$2^3{\rm S}-2^3{\rm P}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M9.png"/></alternatives></inline-formula> transition of <inline-formula><tex-math id="M10">\begin{document}$^4{\rm{He}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M10.png"/></alternatives></inline-formula> under the condition of only 0.1% of the saturated intensity. At this time, the full width at half maximum of the spectral peak is almost close to the natural line width, but the spectral signal-to-noise ratio is still better than 400 and the frequency shift caused by the detection laser power can be less than 1 kHz. This kind of spectral detection at low power can effectively reduce the power-dependent frequency shift, thus obtaining more reliable detection results. This metastable helium atom beam experimental system can also be used as a reference for similar precision measurement experiments.

Чисельне моделювання нестаціонарних потоків холодної плазми при роботі плазмового актуатора

The numerical simulation of unsteady flows of cold plasma is considered in this article. A low-temperature non-equilibrium ideal plasma is formed when the plasma actuator interacts with the air. The mathematical model has been developed to describe the behavior of low-temperature plasma. It is based on non-stationary equations describing the dynamics of charged particles and plasma electrodynamics equations. The 14 types of particles: metastable and excited nitrogen and oxygen atoms, positive and negative ions, electrons and atomic oxygen are considered. Volumetric and surface chemical reactions describing processes in a barrier discharge that occur above the dielectric surface are considered. For non-stationary equations of plasma dynamics, an implicit numerical algorithm with pseudo-time iteration has been developed, which is based on a finite-volume approach. The equation for the electrostatic potential with sources was solved using the generalized minimal residual method with incomplete LU preconditioning. In non-stationary equations for the density of plasma particles, the drift derivatives were approximated using the TVD scheme with the MinMod limiter function. The derivatives in the equation for the electric potential were calculated using finite-volume relations taking into account the upwind approximation of the concentration of charged plasma particles. The numerical results of the generation, propagation and destruction of a streamer during a dielectric barrier discharge are obtained. The unsteady plasma characteristics in the region above the dielectric surface are analyzed, including the distribution of the particles density, electric potential and the Lorentz force components. The results of numerical simulation of unsteady flows of low-temperature plasma are in good agreement with the available experimental data.

Characterization of an RF-driven argon plasma at atmospheric pressure using broadband absorption and optical emission spectroscopy

Atmospheric pressure plasmas in argon are of particular interest due to the production of highly excited and reactive species enabling numerous plasma-aided applications. In this contribution, we report on absolute optical emission and absorption spectroscopy of a radio frequency (RF) driven capacitively coupled argon glow discharge operated in a parallel-plate configuration. This enabled the study of all key parameters including electron density and temperature, gas temperature, and absolute densities of atoms in highly electronically excited states. Space and time-averaged electron density and temperature were determined from the measurement of the absolute intensity of the electron-atom bremsstrahlung in the visible range. Considering the non-Maxwellian electron energy distribution function, an electron temperature (Te) of 2.1 eV and an electron density (ne) of 1.1×1019m−3 were obtained. The time-averaged and spatially resolved absolute densities of atoms in the metastable (1s5 and 1s3) and resonant (1s4 and 1s2) states of argon in the pure Ar and Ar/He mixture were obtained by broadband absorption spectroscopy. The 1s5 metastable atoms had the largest density near the sheath region with a maximum value of 8×1017m−3, while all other 1s states had densities of at most 2×1017m−3. The dominant production and loss mechanisms of these atoms were discussed, in particular, the role of radiation trapping. We conclude with comparison of the plasma properties of the argon RF glow discharges with the more common He equivalent and highlight their differences.

Spatio-temporal dynamics of a nanosecond pulsed microwave plasma ignited by time reversal

In the present paper, a detailed investigation of the spatio-temporal dynamics of the recently developed time reversal microwave plasma source is presented. This novel source allows to ignite a plasma at a desired location in a reverberant cavity by focusing the electromagnetic energy in time and space. An important feature is the possibility to control the plasma position only by changing the input microwave waveform. The source is operated in a repetitive pulsed mode with very low duty cycle (typically 5 × 10−2%). Nanosecond pulses have rise time lower than 1 ns. The generated plasmas have typical sizes in the millimeter range and are observed using imaging for dozens of nanoseconds. The plasma behavior is investigated for different pressures and repetition frequencies. A strong dependence is observed between each discharge pulse suggesting the existence of an important memory effect. The latter is probably due to argon metastable atoms and/or residual charges remaining in the post-discharge and allowing the next breakdown to occur at a moderate electric field.

Argon/dust and pure argon pulsed plasmas explored using a spatially-averaged model

The properties (densities of electrons and metastable argon atoms, effective electron temperature and dust charge) of argon/dust and pure argon pulsed plasmas are studied using a spatially-averaged model. The calculated time-dependencies for the densities of electrons and metastable atoms are compared with the experimental measurements and are found to be in a good qualitative agreement. It is analyzed how the plasma properties depend on the shape of the electron energy probability function (EEPF), the pulsing frequency and the duty cycle for both dust-free and dusty plasma. The analysis reveals that the agreement between theory and experiment is better with Druyvesteyn EEPF than the Maxwellian EEPF. Further, the variation in the pulsing frequency ν p differently affects the metastable density n m in a dust-free and in a dusty plasma. For large ν p, the metastable density in the dust-free pulsed plasma is larger than in the continuous-wave (CW) discharge, while the opposite is obtained in the presence of dust particles. This difference probably arises because of faster variation in the effective electron temperature in the dusty plasma due to collection of electrons by dust particles. Our calculations also show that dust particles may affect the behavior of electron density in the beginning of the on-period due to an enhancement in electron collection by dust particles.

Temperature measurements of the background gas and metastable argon atoms in N2/Ar nanosecond repetitively pulsed discharges

Characterization of plasma properties that underpin kinetic processes in nanosecond repetitively pulsed discharges (NRPDs) is necessary to understand and manipulate the behavior of these discharges for a wide variety of applications. Here, the neutral gas temperature in N2 and 50% N2/50% Ar NRPDs during the discharge is determined by characterizing the rotational temperature of rovibrational spectra from the N2 2nd positive system. At the conditions investigated, it is shown that the timescale for rotational–translational relaxation is shorter than the effective lifetime of the N2(C) state, thereby, rendering the rotational temperature measurements a reasonable representation of the background gas temperature. The measurements show that the translational temperature of ground state nitrogen molecules does not increase significantly above ambient temperature during the discharge generated at a constant pressure of 20 Torr, 10 kHz pulse repetition frequency, and pulse energy of 50 μJ. An absorption based detection technique with a 2 ns time-resolution used to measure the translational temperature history of the metastable argon atoms (Ar(1s3)) in the N2/Ar NRPDs shows that the Ar(1s3) atoms and the neutral background gas are in thermal non-equilibrium during the discharge. Furthermore, the addition of nitrogen gas is shown to significantly reduce the translational energy enhancement of the metastable argon atoms produced in the N2/Ar discharges compared to that in pure argon discharges.

Ionization waves in the PK-4 direct current neon discharge

The PK-4 system is a micro-gravity dusty plasma experiment currently in operation on-board the International Space Station. The experiment utilizes a long DC discharge in neon or argon gases. We apply our 2D particle-in-cell with Monte Carlo collisions discharge simulation to compute local plasma parameters that serve as input data for future dust dynamics models. The simulation includes electrons, Ne+ ions, and Ne m metastable atoms in neon gas and their collisions at solid surfaces including secondary electron emission and glass wall charging. On the time scale of the on-board optical imaging, the positive column appears stable and homogeneous. On the other hand, our simulations show that on microsecond time scales the positive column is highly inhomogeneous: ionization waves with phase velocities in the range between 500 m s−1 and 1200 m s−1 dominate the structure. In these waves, the electric field and charged particle densities can reach amplitudes up to 10 times of their average value. Our experiments on ground-based PK-4 replica systems fully support the numerical findings. In the experiment, the direction of the DC current can be alternated, which has been found to favor dust particle chain formation. We discuss possible mechanisms for how the highly oscillatory plasma environment contributes to the dust particle chain formation.

Hanbury Brown and Twiss Bunching of Phonons and of the Quantum Depletion in an Interacting Bose Gas.

We report the realization of a Hanbury Brown and Twiss (HBT)-like experiment with a gas of interacting bosons at low temperatures. The low-temperature regime is reached in a three-dimensional optical lattice and atom-atom correlations are extracted from the detection of individual metastable helium atoms after a long free fall. We observe, in the noncondensed fraction of the gas, a HBT bunching whose properties strongly deviate from the HBT signals expected for noninteracting bosons. In addition, we show that the measured correlations reflect the peculiar quantum statistics of atoms belonging to the quantum depletion and of the Bogoliubov phonons, i.e., of collective excitations of the many-body quantum state. Our results demonstrate that atom-atom correlations provide information about the quantum state of interacting particles, extending the interest of HBT-like experiments beyond the case of noninteracting particles.

The role of metastable atoms in atomic excitation process of magnesium in microwave-assisted laser plasma

Study on the excitation process of magnesium atom has been conducted in microwave-assisted laser plasma. Experimentally, an Nd:YAG laser (532 nm, 5 mJ) was directed into the magnesium oxide (MgO) sample in various gases of helium, argon, and air at a reduced pressure to induce a luminous plasma. The plasma emission was significantly enhanced when a microwave (2.45 GHz, 400 W) was introduced into the laser plasma. Results revealed that the emission intensity of the ionic magnesium lines that have relatively low excitation energy (around 4 eV) is high only in helium and argon ambient gases. On the contrary, the intensity of ionic magnesium lines having high excitation energy (8–11 eV) is high only for helium gas. The results suggested that a particular excitation process takes place in the microwave-assisted laser plasma induced in helium and argon gases, namely the excitation mechanism proceeds through metastable atoms overwhelmingly produced in the microwave-assisted laser-induced plasma.