Neon Atoms Research Articles

Диэлектрический барьерный разряд в смеси He-Ne низкого давления. Спектроскопия послесвечения

The paper considers the possibility of using a low-pressure dielectric barrier discharge (DBD) as a plasma source for the active medium of a He-Ne laser. The results of a spectroscopic study of the decay stage of a DBD plasma of a cylindrical configuration with a pronounced inverse population of the upper level of the 2p55s configuration, which makes the line of 632.8 nm one of the brightest in the visible region of the spectrum, are presented. It is shown that in the early stage of the DBD afterglow, the distribution of the populations over the 2p55s and 2p54d levels of the neon atom is due to the excitation transfer mechanism Не(21S0) +Ne -- > He(11S0) + Ne*. In the late afterglow with the departure of helium atoms Не(21S0), emission in the visible region of the spectrum is formed mainly by transitions from levels of the 2p53p configuration, the population of which is associated with Не(23S1) atoms. At this stage, the population of the 2p55s and 2p54d states by electron-ion recombination processes is ineffective and does not lead to the formation of population inversion. As an optimal solution in terms of the 632.8 nm line brightness in the afterglow, it is proposed to use a discharge with electrodes along the outer surface of a cylindrical discharge tube, initiated at frequencies that exclude the recombination stage of the afterglow.

Dielectric barrier discharge in a low-pressure He-Ne mixture. Afterglow spectroscopy of 2p-=SUP=-5-=/SUP=-5s-> 2p-=SUP=-5-=/SUP=-3p transitions

The results of a spectroscopic study and simulation of the processes of population and destruction of the levels of the 2p55s configuration of a neon atom in a decaying plasma of a low-frequency barrier discharge in a He-Ne mixture are presented. Experimental conditions: helium pressure 0.08-22 Torr, neon pressure ≤ 3 mTorr, electron density less than 1011 cm-3. On the basis of data on the evolution of the populations of the 3si levels (in Paschen's notation) with a change in helium pressure, data on the rate constants of collisional processes that determine the kinetics of these levels in He-Ne plasma are obtained more accurate than those available in the literature. Keywords: dielectric barrier discharge, helium-neon plasma, excitation transfer, afterglow, inelastic collisions.

Auger Transitions in a Quasi-Molecule in Collisions of Neon Atoms in the Kiloelectonvolt Energy Range

Auger Transitions in a Quasi-Molecule in Collisions of Neon Atoms in the Kiloelectonvolt Energy Range

Revisiting the oscillator strengths and cross sections of atomic neon by fast electron scattering

The generalized oscillator strengths of the valence-shell excitations from the ground state to the 2p5(3s, 3s′, 3p, 3p′, 4s, 4s′) states of atomic neon have been determined by means of fast electron scattering, with an incident electron energy of 1500 eV and an energy resolution of about 80 meV. The adopted relative flow technique improved accuracy of the experimental measurements prominently. The validity condition of the first Born approximation is clarified by comparing the present measured data with previous experimental and theoretical results. We found that the first Born approximation is not applicable for the monopolar transition in the whole momentum transfer region even at the incident electron energy of 1500 eV. However, the high-order Born terms are found dominating only at the minimum for the dipolar transitions. The good agreement of the present extrapolated optical oscillator strengths with previous available data confirms the reliability of the measurement. With the cross-checked generalized oscillator strengths, the state-resolved integral cross sections in a wide collision energy range have been obtained by adopting the BE-scaling method. The analysis of the feasibility of the BE-scaling method demonstrates that the scaling law applies for the typical dipole-allowed transitions while it would overestimate the cross sections for the dipolar transitions with significant dipole-forbidden components. Present data compilation constitutes a benchmark for developing and testing the theoretical methods or computational codes.

Controlling the atomic-orbital-resolved photoionization for neon atoms by counter-rotating circularly polarized attosecond pulses.

We theoretically investigate the atomic-orbital-resolved vortex-shaped photoelectron momentum distributions (PMDs) and ionization probabilities by solving the two-dimensional time-dependent Schrödinger equation (2D-TDSE) of neon in a pair of delayed counter-rotating circularly polarized attosecond pulses. We found that the number of spiral arms in vortex patterns is twice the number of absorbed photons when the initial state is the ψm=±1 state, which satisfy a change from c2n+2 to c2n (n is the number of absorbed photons) rotational symmetry of the vortices if the 2p state is replaced by 2p+ or 2p- states. For two- and three-photon ionization, the magnetic quantum number dependence of ionization probabilities is quite weak. Interestingly, single-photon ionization is preferred when the electron and laser field corotate and ionization probabilities of 2p- is much larger than that of 2p+ if the proper time delay and wavelength are used. The relative ratio of ionization probabilities between 2p- and 2p+ is insensitive to laser peak intensity, which can be controlled by changing the wavelength, time delay, relative phase and amplitude ratio of two attosecond pulses.

A novel crossed-molecular-beam experiment for investigating reactions of state- and conformationally selected strong-field-seeking molecules

ABSTRACT The structure and quantum state of the reactants have a profound impact on the kinetics and dynamics of chemical reactions. Over the past years, significant advances have been achieved in the control and manipulation of molecules with external electric and magnetic fields in molecular-beam experiments for investigations of their state-, structure- and energy-specific chemical reactivity. Whereas studies for neutrals have so far mainly focused on weak-field-seeking species, we report here progress towards investigating reactions of strong-field-seeking molecules by introducing a novel crossed-molecular-beam experiment featuring an electrostatic deflector. The new setup enables the characterisation of state- and geometry-specific effects in reactions under single-collision conditions. As a proof of principle, we present results on the chemi-ionisation reaction of metastable neon atoms with rotationally state-selected carbonyl sulfide (OCS) molecules and show that the branching ratio between the Penning and dissociative ionisation pathways strongly depends on the initial rotational state of OCS.

Stereo-dynamical effects in chemi-ionization reactions of atmospheric O2 and N2 molecules promoted by collisions with Ne*(3P2,0) atoms

This letter focuses on general aspects of the stereodynamics of chemi-ionization reactions, triggered by collisions of O2 and N2 molecules with metastable neon atoms. Basic characteristics of the long-range intermolecular forces involved, some of chemical and others of physical origin, suggest that the weakly bound precursor state, determining the reaction transition state structure and stability, is rather different. Fundamental reaction selectivities are clarified, highlighting differences in the: i) optical potential whose real and imaginary parts must be interdependent; ii) polarizability, quadrupolar moments and orbital representation of the molecules involved, which allow to lay the foundations for future more in-depth investigations.

Infrared predissociation spectroscopy of protonated methyl cyanide, CH3CNH+

The gas phase vibrational spectrum of CH3CNH+ is investigated using a messenger infrared predissociation (IRPD) action spectroscopic method. Vibrational bands were recorded in the 300-1700 cm−1 and 2000-3300 cm−1 regions making use of the widely tunable free electron laser for infrared experiments, FELIX, coupled to a cryogenic ion trap instrument. Band assignments were aided by high-level quantum-chemical calculations, which showed excellent agreement with the experimental data. Effects of the neon atom used as messenger in the IRPD method are investigated in detail. The data presented here will support astronomical searches for the CH3CNH+ ion in space, and provide a basis for high-resolution ro-vibrational and pure rotational studies in vibrationally excited states.

Thermophysical properties of low-density neon gas from highly accurate first-principles calculations and dielectric-constant gas thermometry measurements.

New interatomic potential energy and interaction-induced polarizability curves for two ground-state neon atoms were developed and used to predict the second density, acoustic, and dielectric virial coefficients and the dilute gas shear viscosity and thermal conductivity of neon at temperatures up to 5000 K. The potential energy curve is based on supermolecular coupled-cluster (CC) calculations at very high levels up to CC with single, double, triple, quadruple, and perturbative pentuple excitations [CCSDTQ(P)]. Scalar and spin-orbit relativistic effects, the diagonal Born-Oppenheimer correction, and retardation of the dispersion interactions were taken into account. The interaction-induced polarizability curve, which in this work is only needed for the calculation of the second dielectric virial coefficient, is based on supermolecular calculations at levels up to CCSDT and includes a correction for scalar relativistic effects. In addition to these first-principles calculations, highly accurate dielectric-constant gas thermometry (DCGT) datasets measured at temperatures from 24.5 to 200 K were analyzed to obtain the difference between the second density and dielectric virial coefficients with previously unattained accuracy. The agreement of the DCGT values with the ones resulting from the first-principles calculations is, despite some small systematic deviations, very satisfactory. Apart from this combination of two virial coefficients, the calculated thermophysical property values of this work are significantly more accurate than any available experimental data.

Cryogenic Messenger-IR Ion Spectroscopy Study of Phenol & Aniline Molecular Ions and of the common Fragment Ion [C5H6][rad]+ formed by EI-MS

Molecular ions of phenol and aniline as well as the fragment ion at m/z 66 formed by electron ionization are examined with cryogenic messenger-IR ion spectroscopy and theory. According to the recorded IR spectra aromatic ground state molecular ions of phenol and aniline are clearly identified. Similar to earlier reports our new set of spectroscopic and computational results supports the assumption that tautomeric molecular ions of phenol and aniline are only short-lived intermediates in the reaction path to the ultimate CO loss reaction product of phenol as well as the HNC loss product of aniline. Even the present cryogenic messenger-IR ion spectroscopy approach failed to collect spectroscopic evidence of the intermediate presence of these elusive tautomers of phenol and aniline molecular ions. However, our study clearly shows that the electron ionization mass spectrometry dissociation product ion of phenol and aniline at m/z 66 is the cyclopentadiene radical cation [C5H6]+ for the presence of which convincing spectroscopic evidence could be collected proving the structure assignment and identification. All messenger-IR ion spectroscopy experiments were conducted on a cryogenic 22-pole ion trap with the use of neon atoms for tagging. The MS instrument was coupled to a free electron laser delivering wavelength tunable IR radiation for spectroscopy. Harmonic and anharmonic force field calculations complement the experiments and support band assignments of the recorded IR spectra and general data interpretation.

Excitation energy transfer model for Ne-CO2 and Ne-N2 mixtures

The excited states of neon are capable of ionizing the CO2 and N2 molecules through Penning energy transfer mechanisms. Probability of such additional ionizations can be calculated from the measured gas gain fits. In the present work, Penning ionization processes for Ne-CO2 and Ne-N2 mixtures have been investigated with a newly developed model. The decreases in the transfer rates observed at high molecular gas concentrations indicate the existence of a remarkably interesting transfer mechanism for the excited neon atoms.

Clocking Auger electrons

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive at XFELs due to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here, we develop a new approach termed self-referenced attosecond streaking, based upon simultaneous measurements of streaked photo- and Auger electrons. Our technique enables sub-femtosecond resolution in spite of jitter. We exploit this method to make the first XFEL time-domain measurement of the Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical description, retrieve a lifetime of $2.2^{ + 0.2}_{ - 0.3}$ fs for the KLL decay channel. Importantly, our technique can be generalised to permit the extension of attosecond time-resolved experiments to all current and future FEL facilities.

Диссоциативная рекомбинация в послесвечении барьерного разряда в неоне низкого давления. Заселение атомов конфигурации 2p-=SUP=-5-=/SUP=-3d

The decaying neon plasma produced by the dielectric barrier discharge (DBD) in a cylindrical tube at a pressure of 0.1–40 Torr has been spectroscopically investigated to analyze the dissociative recombination (DR) of molecular ions with electrons as a mechanism for the formation of excited atoms. It is shown that, at the electron density in the afterglow less than 5•1010 cm-3 the DR is the dominant source of population of 3d levels at pressures PNe ≥ 0.6 Torr. At lower pressures, the optical properties of the decaying plasma are formed to a greater extent by the collisional-radiative recombination of Ne+ ions. A significant variation of the relative intensities of the 3d -> 3p transition lines in the afterglow with a change in gas pressure was found, reflecting the effect of inelastic collisions on the formation of the spectrum of decaying plasma in the near infrared region. From measurements carried out at a pressure of 0.6 Torr, the relative values of the partial DR coefficients for the 3dj levels of the neon atom were found. Comparison of these data with measurements in the near ultraviolet region, containing the lines of 4p -> 3s transitions, indicates the need to take into account the cascade 4p -> 3d transitions to correctly solve the problem of the final products of dissociative recombination.

Strengthening the case for interatomic Coulomb decay as a subdominant reaction channel in slow O3+−Ne2 collisions with independent-atom-model coupled-channel calculations

We report on electron removal calculations for 2.81 keV/amu Li$^{3+}$ and O$^{3+}$ ion collisions with neon dimers. The target is described as two independent neon atoms fixed at the dimer's equilibrium bond length, whose electrons are subjected to the time-dependent bare and screened Coulomb potentials of the classically moving Li$^{3+}$ and O$^{3+}$ projectile ions, respectively. Three mutually perpendicular orientations of the dimer with respect to the rectilinear projectile trajectories are considered and collision events for the two ion-atom subsystems are combined in an impact parameter by impact parameter fashion and are orientation-averaged to calculate probabilities and cross sections for the ion-dimer system. The coupled-channel two-center basis generator method is used to solve the ion-atom collision problems. We concentrate the ion-dimer analysis on one-electron and two-electron removal processes which can be associated with interatomic Coulomb decay, Coulomb explosion, and radiative charge transfer. We find that the calculated relative yields are in fair agreement with recent experimental data for O$^{3+}$-Ne$_2$ collisions if we represent the projectile by a screened Coulomb potential, but disagree markedly for a bare Coulomb potential, i.e., for Li$^{3+}$ impact. In particular, our calculations suggest that interatomic Coulomb decay is a significant reaction channel in the former case only, since capture of a Ne($2s$) electron to form hydrogenlike Li$^{2+}$ is unlikely.

Theoretical study of electron impact single ionization of neon by multicenter three distorted-wave method at low projectile energy: an overall agreement with experiment

Universal theoretical description of electron impact single ionization process for complex atoms and molecules is a long-standing challenge. A perturbative approach named multicenter three distorted wave method (MCTDW) has recently been developed by our group, aiming at calculating (e, 2e) fully differential cross sections for molecular target. In MCTDW method, the incident, scattered and ejected electrons are described by distorted waves using single center expansion technique. The continuum wave functions of the incident and two outgoing electrons are solved in the potential of neutral molecule and ion, respectively. The accuracy and generality of this method has been demonstrated for water molecule as well as simple two-electron atom helium (2018 Phys. Rev. A 98 042710) . Here MCTDW is extended to investigate more complex multi-electron atom neon of electron impact ionization from its 2p orbital at projectile energy of 100 eV and 65 eV. The results are compared with the internormalized experiments reported in the literatures (2013 Phys. Rev. Lett. 110 153202; 2015 Phys. Rev. A. 91 032707). An overall agreement has been achieved with experimental data for different scattering angles and energies, as well as theoretical calculations employing nonperturbative B-spline R-matrix with pseudostates approach.

Mapping Resonance Structures in Transient Core-Ionized Atoms

The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry. (Less)

Consistency of splitting frequency difference with longtudinal modes spacing variation in Zeeman dual-frequency laser

Phase anisotropy in laser resonant cavity will bring about an influence on laser frequency and polarization, such as laser frequency splitting, of which the frequency difference is determined by their introduced phase retardation. For a helium-neon laser with a small phase retardation in the cavity, the two split modes are very close to each other whose burned holes are overlapped. Then only one mode oscillates while the other is always in lock-in state due to strong mode competition, which forms hidden frequency split. Meanwhile the spacing between adjacent longitudinal modes deviates from original value and produces a certain variation equal to twice the hidden splitting frequency difference. As a result the longitudinal modes spacing variation is dominated by the phase retardation. On the other hand, by applying transverse magnetic field to a laser tube along the polarization direction, the neon atoms will undergo transverse Zeeman effect and be divided into two groups to provide the gain for polarized light beams parallel to the magnetic field and perpendicular to the magnetic field respectively. Then the laser mode competition is greatly weakened so that the two split modes can oscillate simultaneously to obtain the frequency difference. In order to make profound study of the consistency between longitudinal mode spacing variation and splitting mode frequency difference in the presence of transverse magnetic field, the samples of tilted quartz plate or half wave plate is placed into laser cavity to produce phase retardation. By the two mentioned methods, the splitting frequency difference varying with phase retardation of samples is deduced to make a comparison. Two measurements show that the average relative deviation is less than 1%, while the experimental results accord with theoretical analyses quite well. In this way splitting frequency difference of Zeeman dual-frequency laser can be determined accurately, and a new method to measure the phase retardation of half wave plate is provided.

The dynamics of the spatiotemporal distribution of excited atoms in a nanosecond discharge with a slot cathode

The work is devoted to the study of the spatiotemporal dynamics of the production of excited atoms in an extended nanosecond discharge with a slot cathode in neon at medium gas pressures. The authors experimentally investigated the frame-by-frame dynamics of the spatiotemporal formation of the discharge optical irradiation with an exposure time of 5 ns and time intervals between frames of 2 ns. Under similar conditions, the concentration of excited neon atoms at metastable levels with a time resolution of about 10 ns was measured by laser absorption spectroscopy, and the current-voltage characteristics of the discharge were experimentally determined in a wide range of amplitudes of voltage pulses and gas pressures in the range of 1–60 Torr. To analyze the kinetic processes in the plasma source under study, numerical simulation of ionization processes in the Comsol Multiphysics software environment was performed using a special Plasma module.

Spin polarization of photoelectrons in bichromatic extreme-ultraviolet atomic ionization

Spin polarization of photoelectrons is considered in the ionization of atoms by a laser field and its second harmonic, concentrating on the effects of interference between the two ionization paths. The possibility for a crucial increase of the spin polarization by manipulating the relative phase of the harmonics is demonstrated. Observation of these effects, exemplified by the ionization of neon atoms, is accessible by modern free-electron lasers working in the extreme-ultraviolet wavelength regime.

Theoretical determination of polarizability and magnetic susceptibility of neon

We report theoretical determination of the dipole polarizability of the neon atom, including its frequency dependence. Corrections for the relativistic, quantum electrodynamics, finite nuclear mass, and finite nuclear size effects are taken into account. We obtain the value $\alpha_0=2.66080(36)$ for the static polarizability, and $\alpha_2=2.850(7)$ and $\alpha_4=4.932(14)$ for the first two polarizability dispersion coefficients (Cauchy moments); all values are in atomic units (a.u.). In the case of static polarizability, our result agrees with the best experimental determination [C. Gaiser and B. Fellmuth, Phys. Rev. Lett. 120, 123203 (2018)], but our estimated uncertainty is significantly larger. For the dispersion coefficients, the results obtained in this work appear to be the most accurate to date overall compared to published theoretical and experimental data. We also calculated the static magnetic susceptibility of the neon atom, needed to obtain the refractive index of gaseous neon. Our result, $\chi_0 = -8.484(19) \cdot 10^{-5}$ a.u., is about 9% larger in absolute value than the recommended experimental value [CRC Handbook of Chemistry and Physics, CRC Press, 2019, p. 4-145].