Excited Atoms Research Articles

Temporal evolution of laser-induced ionization and recombination processes in argon-helium mixture

Preparation of metastable atoms (1s5) through laser-induced preionization holds the potential to mitigate the electromagnetic interference (EMI) issues associated with the large volume, atmospheric pressure discharge of traditional optically pumped rare-gas metastable laser (OPRGL). In this work, we conducted experimental investigations into the temporal evolution of the Ar 763.5 nm (2p6→1s5) spectral line in Ar-He mixture. These experiments unveiled the intricate interaction mechanism involving the laser, Ar atoms, He atoms, and free electrons within the laser-induced plasma. Our findings highlight the dual contributions of the multiphoton ionization and the inverse bremsstrahlung process to the initial plasma formation. Notably, the time-resolved atomic emission spectrum at 763.5 nm reveals two distinct regimes, namely Regime1 and Regime2. Regime1 primarily arises from the “excitation + radiation + collisional relaxation” process, wherein excited states Ar atoms, populated via multiphoton excitation and electron impact excitation, accumulate on the 2p6 level. Conversely, Regime2 is predominantly a result of the “ion-electron recombination” process. In this regime, highly excited states Ar atoms are generated through the recombination of ion and electron, subsequently populating the 2p6 level through a combination of radiation and collisional relaxation channels. The differences in the temporal evolution between 763.5 nm and 811.5 nm spectral lines can be attributed to the distinct radiation and collisional relaxation channels in the two aforementioned processes.

Influence of Voltage, Pulselength and Presence of a Reverse Polarized Pulse on an Argon–Gold Plasma during a High-Power Impulse Magnetron Sputtering Process

This work aims to provide information about the deposition of gold via bipolar high-power impulse magnetron sputtering (HIPIMS) in order to identify suitable process parameters. The influences of voltage, pulse length and the kick-pulse on an argon–gold plasma during a bipolar high-power impulse magnetron sputtering deposition process were analysed via optical emission spectroscopy (OES) and oscilloscope. The voltage was varied between 700 V and 1000 V, the pulse length was varied between 20 µs and 100 µs and the process was observed once with kick-pulse and once without. The influence of the voltage on the plasma was more pronounced than the influence of the pulse width. While the intensity of several Au I lines increased up to 13-fold with increasing voltages, only a less-than linear increase in Au I brightness with time could be identified for changes in pulse length. The intensity of excited argon is only minimally affected by changes in voltages, but follows the evolution of the discharge current, with increasing pulse lengths. Contrary to the excited argon, the intensity emitted by ionized argon grows nearly linearly with voltage and pulse length. The reverse polarised pulse mainly affects the excited argon atoms in the plasma, while the influence on the ionized argon is less pronounced, as can be seen in the the spectra. Unlike the excited argon atoms, the excited gold atoms appear to be completely unaffected by the kick-pulse. No ionization of gold was observed. During the pulse, a strong rarefaction of plasma takes place. Very short pulses of less than 50 µs and high voltages of about 1000 V are to be preferred for the deposition of gold layers. This paper offers a comprehensive overview of the gold spectrum during a HIPIMS process and makes use of optical emission spectroscopy as a simple measuring approach for evaluation of the reverse polarized pulse during a bipolar process. Future uses of the process may include the metallization of polymers.

Zeno Regime of Collective Emission: Non-Markovianity beyond Retardation.

To build up a collective emission, the atoms in an ensemble must coordinate their behavior by exchanging virtual photons. We study this non-Markovian process in a subwavelength atom chain coupled to a one-dimensional (1D) waveguide and find that retardation is not the only cause of non-Markovianity. The other factor is the memory of the photonic environment, for which a single excited atom needs a finite time, the Zeno regime, to transition from quadratic decay to exponential decay. In the waveguide setup, this crossover has a time scale longer than the retardation, thus impacting the development of collective behavior. By comparing a full quantum treatment with an approach incorporating only the retardation effect, we find that the field memory effect, characterized by the population of atomic excitation, is much more pronounced in collective emissions than that in the decay of a single atom. Our results maybe useful for the dissipation engineering of quantum information processings based on compact atom arrays.

Deep core level hard X‐ray photoelectron spectroscopy for catalyst characterization

AbstractHeterogeneous catalysts are the crucial element for many catalytic processes. In most of the cases, the pivotal structure consists of catalytic metals/alloy particles supported by oxides. Knowledge of the interaction between metal and oxide is central to understand the structure–performance relationship of such systems. X‐ray photoelectron spectroscopy provides access to the chemical–physical properties of metal and oxide interface as well as polarization effects. The results are usually derived from changes of the measured binding energies based on initial state analysis. We propose to extend the analysis using photoelectron as well as Auger transition to include final state effects (Wagner/Hohlneichner plots). This gives additional information on the specific chemical neighborhood of the excited atom. Three archetypal systems are investigated by hard X‐ray photoelectron spectroscopy (HAXPES) to introduce two approaches to this analysis for the most common support elements Al and Si.

Simulation study on influence of oxygen concentrations on atmospheric-pressure streamer in oxygen-rich nitrogen–oxygen mixture

This study aims to investigate the behaviour of positive streamer discharge in various mixtures with different concentrations via two-dimensional simulations. The proposed model considers the effects of photoionisation caused by the photons generated by excited oxygen atoms , which enables the simulation of streamers in oxygen rich conditions. The simulation results indicate that the propagation velocity and diameter of the streamer increase with the oxygen concentration, while the emission intensity decreases. The result conforms to the experimental results of our previous work. Moreover, the simulation results suggest that changes in the ionisation rate caused by variations in the oxygen concentration affect the streamer propagation, while the impact of photoionisation is limited.

Reactive species variation in cold atmospheric pressure plasma jet discharge under the influence of intrinsic parameters and its effect on E. coli inactivation.

Cold atmospheric pressure plasma jet (CAPJ) has piqued the interest of researchers for various antimicrobial applications such as disinfection, wound decontamination, etc. In the current context, a deeper understanding of the correlation between CAPJ's intrinsic parameters, discharge characteristics, species composition, and antimicrobial activity is required for any successful application. This research evaluated the effect of intrinsic operational parameters such as voltage, frequency, gas flow rate, and operating gas on the reactive species composition of an in-house-developed CAPJ discharge along with the antimicrobial activity. It was observed that the identified excited atoms (Ar I, He I, N2, and O I), ions (Ar+, N2+, N+, H2O+, H3O+, etc.), radical reactive oxygen and nitrogen species (RONS) (OH•), and nonradical RONS (O I, O+, OH+, NO+, O2+, O2-, NO2-, N2O2-, NO3-, N2O3-, etc.) might play a synergistic role in bacterial inactivation via oxidative and electrostatic stress. The variation in voltage, frequency, gas flow rate, and operating gas influenced the discharge chemistry, leading to variation in bacterial inactivation. The reactive species in the discharge responsible for such variation was evaluated extensively. This investigation into various operational parameters would aid in determining the most effective settings for a developed CAPJ to achieve high productivity.

Multiphoton excitation and ionization of hydrogen atoms in two-color laser fields

Multiphoton excitation and ionization of hydrogen atoms in two-color laser fields

Excitation of 87Rb Rydberg atoms to nS and nD states (n≤68) via an optical nanofiber

Cold Rydberg atoms are a promising platform for quantum technologies, and combining them with optical waveguides has the potential to create robust quantum information devices. Here, we experimentally observe the excitation of cold rubidium atoms to a large range of Rydberg S and D states through interaction with the evanescent field of an optical nanofiber. We develop a theoretical model to account for experimental phenomena present such as the AC Stark shifts and the Casimir–Polder interaction. This work strengthens the knowledge of Rydberg atom interactions with optical nanofibers and is a critical step toward the implementation of all-fiber quantum networks and waveguide quantum electrodynamics (QED) systems using highly excited atoms.

Excitation of 87Rb Rydberg atoms to nS and nD states (n≤68) via an optical nanofiber

Cold Rydberg atoms are a promising platform for quantum technologies, and combining them with optical waveguides has the potential to create robust quantum information devices. Here, we experimentally observe the excitation of cold rubidium atoms to a large range of Rydberg S and D states through interaction with the evanescent field of an optical nanofiber. We develop a theoretical model to account for experimental phenomena present such as the AC Stark shifts and the Casimir–Polder interaction. This work strengthens the knowledge of Rydberg atom interactions with optical nanofibers and is a critical step toward the implementation of all-fiber quantum networks and waveguide quantum electrodynamics (QED) systems using highly excited atoms.

Computational Analysis of Excited State Intramolecular Hydrogen Atom Transfer and Microsolvation Studies on 8-Acetyl-7-hydroxy-4-methylcoumarin using DFT and TDDFT

The computational calculations were carried out on 8-acetyl-7-hydroxy-4-methyl coumarin (AHM) and its water complex AHM+(H2O)4- [AHMH] at ground and excited states by employing density functional theory (DFT)/specific state time-dependent density functional theory (SS-TDDFT). In AHM and AHMH molecules, there is an intramolecular hydrogen bond between hydroxyl group and acetyl group along with inter-molecular hydrogen bonds in the hydrated molecule. The computational studies of molecular structural parameters, molecular electrostatic potential, natural bond orbital (NBO) analysis, the molecular orbital’s and UV-Vis spectra of both the molecules under polar solvents were explored by B3LYP/cc-pVDZ/PCM/EFP1 method. The intramolecular hydrogen atom transpired between hydroxyl to acetyl group in AHM/AHMH molecules from S0→S1 state but not in S0→S3/S0→S2 states even though the S3/S2 states have significant oscillation strengths. This indicates that intramolecular chage transfer (ICT) occurs within the molecules and it confirmed using potential energy surface (PES) scan studies.

Tunable atom lithography scheme with high resolution in atomic system

We theoretically propose a scheme that utilizes the excited state of a Λ-type three-level atom system for atom lithography. A standing-wave coupling field and a probe field provide localized probability distribution of the excited atoms. Unlike the previous studies in atom localization, we focus on splitting the single-peak structure localized on the node of the standing-wave field to a double-peak structure. The shift distance of double-peak related to the parameters of external laser fields is investigated. The period of lithography pattern could reach λ/4, while the resolution of lithography is promising to be smaller than 0.02λ by adjusting system parameters. This scheme can be extended to form a two-dimensional pattern for lithography.

Hyperfine-induced effects on linear polarization following electron-impact excitation of heliumlike Tl79+ ions with nuclear spin

A most general density-matrix formalism is presented to investigate linear polarization of characteristic lines following electron-impact excitation of atoms or ions with arbitrary nuclear spin, which can account for depolarization of energy levels and multipole mixing of radiation fields. It is then applied to the linear polarization of the line radiated from heliumlike ions with nuclear spin . As an example, detailed calculations are performed for Tl79+ ions using the multi-configurational Dirac-Fock method and relativistic distorted-wave theory. It is found that the effect of the hyperfine interaction on the linear polarization depends dominantly on impact electron energy. For low impact energies close to the excitation threshold, the hyperfine interaction results in an enhancement of the linear polarization, especially for those photons emitted perpendicularly to the impact electron beam. In contrast, such a hyperfine-induced effect diminishes quickly with increasing impact energy and vanishes at medium and high energies, which is very different from the results for the case of radiative electron capture (Surzhykov et al 2013 Phys. Rev. A 87 052507). The present study is experimentally accessible at both electron-beam ion traps and ion storage rings and, thus, accurate polarization measurements at low energies can be utilized to probe the hyperfine interaction in highly charged few-electron ions.

Investigation on transient process of dense hydrogen microdischarge by dual-wavelength laser interferometry

In this paper, the transient process of single-pulse hydrogen microdischarge with peak discharge current of at constant pressure of is investigated by dual-wavelength laser interferometry. It is found that the phase shift signals related to electrons and excited H (n = 2) atom appear almost simultaneously after breakdown, with a delay of 700 ns compared with the current signal. The velocity of the ionization wave is thus estimated to be on the order of . Subsequently, the current and phase signals increase rapidly, indicating that the energy deposited is applied to heat electron and gas, and strong molecular ionization and atomic excitation process occur in microdischarge. The phase signal for excited H (n = 2) atoms shows the maximum value at and then decreases rapidly, while the current reaches the peak at . In the duration between these two peaks the electronic degrees of freedom (DOF) continue to gain energy from the external field, but the energy transferred to the atomic DOF decreases. After , the current drops first rapidly, and then slowly decreases until the end of the discharge. In discharge attenuation process, the energy flowing into all DOF decreases. The phase-shift dispersion curve is obtained by changing laser frequency. Through the global optimized fitting of the phase shift curve, the average line-of-sight density of excited H (n = 2) atoms is estimated to be in microdischarge, and the corresponding broadening information is given.

Wavelength Dependence of Atomic Excitation for Ar Subject to Intense Midinfrared Laser Pulses

We report experimental and theoretical investigations of wavelength dependence of Rydberg state excitation (RSE) process of Ar subject to intense laser fields. By simultaneously measuring ionization and RSE yields of Ar atoms subject to strong laser fields at a series of wavelengths, we obtain the wavelength scaling law of the ratio of Ar* over Ar+ with respect to the laser intensity, and this result can be well reproduced by a nonadiabatic model, but not by the classical-trajectory Monte Carlo model. Our results indicate that the nonadiabatic corrections of the photoelectron tunneling exit and tunneling probability play a significant role at shorter wavelengths. Analysis shows that the wavelength dependence phenomenon is due to the interplay of the nonadiabatic effect, wave-packet diffusion and Coulomb focusing effect of the liberated electron.

Distant entanglement generation and controllable information transfer via magnon–waveguide systems

The generation of entanglement and information distribution among macroscopic systems which are spatially separated are of paramount importance in the realm of quantum networks and quantum communication. In this paper, we propose a scheme that enables the preparation of long-distance entanglement between two magnons and controllable information transfer from a giant atom to either magnon, where the two magnons can be positioned symmetrically or asymmetrically in relation to a giant atom, and all of the magnons and the atom are coupled to a microwave cavity-array waveguide. By considering significant frequency detuning, we can virtually excite photons and establish an indirect interaction between the giant atom and the two magnons. Through the modulation of atom–waveguide coupling rates, we can not only evenly transfer atomic excitation to both magnons, thereby generating maximally entangled states, but also selectively transmit the atomic state to a single magnon. It is our hope that our proposed scheme will prove valuable in the field of quantum communications.

Theoretical study of antihydrogen formation reactions in the three body <span class="inline-formula"><span class="math">\({{e}^{ + }}{{e}^{ - }}\bar {p}\)</span></span> system via Faddeev–Merkuriev equations in total orbital momentum representation

The results of calculations of low-energy reaction in the three body \({{e}^{ + }}{{e}^{ - }}\bar {p}\) system with the emphasis on the process of the antihydrogen formation from the ground and excited states of the positronium are presented. This reaction is important for some of the current antimatter experiments. The multi-channel scattering calculations are based on a new highly efficient method of solving the Faddeev–Merkuriev equations in total orbital momentum representation. We discuss the effects that originate from the long-range dipole interaction between the excited atom and the spectator particle.

A hybrid mechanism of 3D optical trapping of particles immersed in a cold buffer gas

A novel mechanism of the formation of a deep optical superlattice (OSL) for three-dimensional (3D) trapping of resonant impurity atoms immersed in a transparent cold buffer gas is proposed and investigated theoretically. This mechanism is associated with the joint action of two factors of different physical nature: rectification effect of the gradient force of light pressure and effect of the light induced drift (LID) of atoms (a consequence of the difference in gas-kinetic cross sections of excited and unexcited atoms). Such a situation can be realised when a gas mixture is irradiated by a bichromatic combined light beam which is a special coaxial superposition of cosine-Gaussian optical beams. Here, the RcGF creates a periodic 1D array of deep potential wells (OSL) along the beam axes, and an effective force, causing LID, pulls resonant particles into the combined light beam and provides their transverse (with respect to the beam axes) confinement in OSL. As a result, there can occur a giant accumulation of impurity particles in the OSL cells, accompanied by a strong spatial particle localization far away from the walls of the reservoir, in which the gas mixture is located.

In-plasma analysis of plasma–surface interactions

During deposition, modification, and etching of thin films and nanomaterials in reactive plasmas, many active species can interact with the sample simultaneously. This includes reactive neutrals formed by fragmentation of the feed gas, positive ions, and electrons generated by electron-impact ionization of the feed gas and fragments, excited states (in particular, long-lived metastable species), and photons produced by spontaneous de-excitation of excited atoms and molecules. Notably, some of these species can be transiently present during the different phases of plasma processing, such as etching of thin layer deposition. To monitor plasma–surface interactions during materials processing, a new system combining beams of neutral atoms, positive ions, UV photons, and a magnetron plasma source has been developed. This system is equipped with a unique ensemble of in-plasma surface characterization tools, including (1) a Rutherford Backscattering Spectrometer (RBS), (2) an Elastic Recoil Detector (ERD), and (3) a Raman spectroscopy system. RBS and ERD analyses are carried out using a differentially pumped 1.7 MV ion beam line Tandetron accelerator generating a beam at grazing incidence. The ERD system is equipped with an absorber and is specifically used to detect H initially bonded to the surface; higher resolution of surface H is also available through nuclear reaction analysis. In parallel, an optical port facing the substrate is used to perform Raman spectroscopy analysis of the samples during plasma processing. This system enables fast monitoring of a few Raman peaks over nine points scattered on a 1.6 × 1.6 mm2 surface without interference from the inherent light emitted by the plasma. Coupled to the various plasma and beam sources, the unique set of in-plasma surface characterization tools detailed in this study can provide unique time-resolved information on the modification induced by plasma. By using the ion beam analysis capability, the atomic concentrations of various elements in the near-surface (e.g., stoichiometry and impurity content) can be monitored in real-time during plasma deposition or etching. On the other hand, the evolution of Raman peaks as a function of plasma processing time can contribute to a better understanding of the role of low-energy ions in defect generation in irradiation-sensitive materials, such as monolayer graphene.

Feasibility study of nuclear excitation by electron capture using an electron beam ion trap

The nuclear excitation by electron capture (NEEC) mechanism is considered to be one of the most effective ways to excite nuclear isomers. Despite being proposed over 50 years ago, direct experimental evidence of NEEC is yet elusive. In this study, we propose an experimental scheme to examine the NEEC process using an electron beam ion trap (EBIT). In an EBIT, highly charged ions are bombarded by an electron beam, which can result in excitation of the nuclei in the trap through mechanisms such as NEEC and Coulomb excitation (CE), etc. Our calculations show that the total rates of NEEC production for some nuclei can reach over 0.1/s, which is higher than these from other mechanisms like CE. The proposed scheme may result in the confirmation of the existence of NEEC, and can also be used to study atomic nuclear excitation related processes.

Electromagnetically induced grating in a nonlinear optomechanical cavity

Abstract We investigate theoretically the Fraunhofer diffraction pattern of the output field in a nonlinear optomechanical cavity with a degenerate optical parametric amplifier (OPA) and a higher order excited atomic ensemble. Studies show that the higher-order-excitation atom, which is similar to the degenerate OPA that acts as a nonlinear medium, induces an electromagnetically induced grating in the output spectrum of the probe field. The coherence of the mechanical oscillator leads to transfer of the probe energy in different diffraction orders of the probe field spectrum such that the phenomenon of optomechanically induced grating is generated from the output probe field. In particular, the presence of nonlinearities with the degenerate OPA and the higher order excited atoms can significantly affect the efficiency of the diffraction pattern providing an additional flexibility for controlling optical properties.