Metastable Noble Gas Atoms Research Articles

A Kr*–Rb cold collision apparatus based on atom trap

A cold collision between atoms and molecules (<1 K) is one of the hot research fields in atomic and molecular physics. At low temperatures, the number of partial waves participating in the collision process decreases dramatically, and quantum phenomena start to emerge. The reaction is often dominated by quantum tunneling, and pronounced resonances can exist on collision cross sections. Here, we report on an apparatus designed for studying cold collisions between metastable noble gas atoms and alkali atoms. Our apparatus features a combined Magneto-Optical-Trap (MOT) and velocity map imaging (VMI) system. The center of a Rb MOT is overlapped with the VMI system. Cold Kr* atoms are launched toward the Rb atoms to induce Kr* + Rb reactions. The collision energy between the two species can be varied from 100 mK to 20 K. With this setup, we are planning to explore the quantum phenomena in Kr* + Rb cold collisions, including the shape resonance and stereodynamics in the reaction.

Coherent dynamics in a five-level atomic system

The coherent control of multi-partite quantum systems presents one of the central prerequisites in state-of-the-art quantum information processing. With the added benefit of inherent high-fidelity detection capability, atomic quantum systems in high-energy internal states, such as metastable noble gas atoms, promote themselves as ideal candidates for advancing quantum science in fundamental aspects and technological applications. Using laser-cooled neon atoms in the metastable 3P2 state of state 1s 22s 22p 53s (LS-coupling notation) (Racah notation: 2 P 3/23s[3/2]2) with five m J -sublevels, experimental methods for the preparation of all Zeeman sublevels |m J ⟩ = |+2⟩, |+1⟩, |0⟩, |−1⟩, |−2⟩ as well as the coherent control of superposition states in the five-level system |+2⟩, …, |−2⟩, in the three-level system |+2⟩, |+1⟩, |0⟩, and in the two-level system |+2⟩, |+1⟩ are presented. The methods are based on optimized radio frequency and laser pulse sequences. The state evolution is described with a simple, semiclassical model. The coherence properties of the prepared states are studied using Ramsey and spin echo measurements.

Maximizing optical production of metastable xenon.

The wide range of applications using metastable noble gas atoms has led to a number of different approaches for producing large metastable state densities. Here we investigate a recently proposed hybrid approach that combines RF discharge techniques with optical pumping from an auxiliary state in xenon. We study the effect of xenon pressure on establishing initial population in both the auxiliary state and metastable state via the RF discharge, and the role of the optical pumping beam power in transferring population between the states. We find experimental conditions that maximize the effects, and provide a robust platform for producing relatively large long-term metastable state densities.

Spectroscopic study of excited molecular nitrogen generation due to interactions of metastable noble gas atoms

This work provides an insight into the generation of excited nitrogen species by allowing noble gases to interact both with one another and ambient air. He and Ar were utilized to generate the optimum selectivity process to create reactive nitrogen species. An optimum setting for the generation of excited molecular nitrogen species, based on their excited energy levels, was obtained when using a mixture of Ar‐He at a ratio of 10:1. At that point, when a voltage of 27 kV is applied to the system, it reached the maximum efficiency for selectivity processes to occur which allowed for a greater non‐radiative transfer of energy through the mixture of noble gas atoms and into the molecular nitrogen present in ambient air.

Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings.

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Metastable noble gas atoms in strong-field ionization experiments

The interactions of strong-field few-cycle laser pulses with metastable states of noble gas atoms are examined. Metastable noble gas atoms offer a combination of low ionization potential and a relatively simple atomic structure, making them excellent targets for examining ionization dynamics in varying experimental conditions. A review of the current work performed on metastable noble gas atoms is presented.

The Possible Role of Penning Ionization Processes in Planetary Atmospheres

In this paper we suggest Penning ionization as an important route of formation for ionic species in upper planetary atmospheres. Our goal is to provide relevant tools to researchers working on kinetic models of atmospheric interest, in order to include Penning ionizations in their calculations as fast processes promoting reactions that cannot be neglected. Ions are extremely important for the transmission of radio and satellite signals, and they govern the chemistry of planetary ionospheres. Molecular ions have also been detected in comet tails. In this paper recent experimental results concerning production of simple ionic species of atmospheric interest are presented and discussed. Such results concern the formation of free ions in collisional ionization of H2O, H2S, and NH3 induced by highly excited species (Penning ionization) as metastable noble gas atoms. The effect of Penning ionization still has not been considered in the modeling of terrestrial and extraterrestrial objects so far, even, though metastable helium is formed by radiative recombination of He+ ions with electrons. Because helium is the second most abundant element of the universe, Penning ionization of atomic or molecular species by He*(23S1) is plausibly an active route of ionization in relatively dense environments exposed to cosmic rays.

All-optical production and trapping of metastable noble-gas atoms down to the single-atom regime

The determination of isotope ratios of noble-gas atoms has many applications, e.g., in physics, nuclear-arms control, and Earth sciences. For several applications, the concentration of specific noble-gas isotopes (e.g., Kr and Ar) is so low that single-atom detection is highly desirable for a precise determination of the concentration. As an important step in this direction, we demonstrate the operation of a krypton atom trap trace analysis (ATTA) setup based on a magneto-optical trap (MOT) for metastable Kr atoms excited by all-optical means. Compared to other state-of-the-art techniques for preparing metastable noble-gas atoms, all-optical production is capable of overcoming limitations regarding minimal probe volume and avoiding cross-contamination of the samples. In addition, it allows for a compact and reliable setup. We identify optimal parameters of our experimental setup by employing the most abundant isotope 84Kr, and demonstrate single-atom detection within a 3D MOT.

Nanoscale Imaging of Neutral Atoms with a Pulsed Magnetic Lens

We present a scheme for imaging of neutral atoms to the nanoscale with a pulsed magnetic lens and show its viability through numerical calculations. This scheme achieves focal lengths on the order of several centimeters and focal spots of less than 10 nm. With these results, it is possible to create sub-10 nm structures on surfaces in a parallel and time-efficient manner. When used with metastable noble gas atoms, and in combination with electron spectroscopy, this scheme can create a chemically sensitive microscope which can probe surfaces on the nanometer scale.

Negative oxygen ions in a pulsed RF-discharge with inductive coupling in mixtures of noble gases and oxygen

We report on the spatio-temporal behaviour of negative oxygen ions in a pulsed inductively coupled RF excited discharge in oxygen noble gas mixtures. With a high fraction of noble gas admixtures and at high power an increase in the negative ions is observed in the early phase of the afterglow. This indicates a generation channel of negative ions that becomes effective with decreasing electron temperature. Dissociative attachment of electrons to highly excited metastable oxygen molecules is suggested as a negative oxygen ion source. Comparative measurements of the temporal behaviour of the negative ions in Ar/O2, Kr/O2 and Ne/O2 mixtures indicate that metastable noble gas atoms do not play an essential role in the formation process of negative ions during the afterglow. A simple global model for the negative oxygen ion density supports the experimental results. The model is based on sound experimental data such as the atomic oxygen density and the quench rates by molecular oxygen of the metastable noble gas atoms.

Reaction of Metastable Ar*(3P2) and Kr*(3P2) Atoms with Water Vapor: Excitation Functions for Electronic Quenching Collisions

In this paper, we examine the functional nature of the state-specific cross section for electronic quenching σQ(E) in collisions of metastable noble gas atoms [Ar*(3P2) and Kr*(3P2)] with ground-state water molecules confined to a scattering-gas cell. The relative kinetic energy range studied is E = 0.0463−0.772 eV for Ar*(3P2) and 0.0432−0.692 eV for Kr*(3P2). These beam-gas-luminescence experiments incorporate a novel state-specific monitor system employing Xe gas. Photon emission from a state-specific Xe state produced in quenching collisions with the respective metastable noble gas (Ng*) atoms is a direct measure of the total disappearance of the metastable state under scrutiny in the reaction with water vapor. We use the classical orbiting and absorbing sphere models with empirically derived parameters [London dispersion Cdisp, induction Cind (spherically averaged or aligned dipole), and absorbing sphere radius ras] to predict energy-dependent cross sections for the total quenching reactions. The underlying premise of the models is that although the quenching process itself is envisioned to be a short-range two-electron exchange interaction, it is the long-range attractive forces between the respective collision partners that are critical in determining whether such short-range processes ultimately occur. Values of σQ determined from the models are in very good agreement with our experiment. This work supersedes our previous study for Ar*(3P2,0) [Novicki, S.; Krenos, J. J. Chem. Phys. 1988, 89, 7031].

Molecular beam study of the collisions of state-monitored, metastable noble gas atoms with O2(X 3Σg−)

We describe a new molecular beam-luminescence method for measuring state-resolved cross sections for the quenching of metastable noble gas atoms, and report values for “dark” collisions of Ng*(3P2,3P0) with O2(X 3Σg−), where Ng=Ar, Kr, and Xe. Cross sections for quenching σQ and, in some cases, cross sections for excited products σ* are also given for a number of state-specific, luminescent monitor reactions. The elastic reaction of Ng* with He or Ne is employed to correct the total disappearance cross section QT for viewing losses caused by nonquenching processes. The velocity-averaged, quenching cross section QQ is obtained by subtracting the nonquenching cross section QN from QT. Values of QQ measured at average relative velocity ḡ (average relative kinetic energy Ē) are deconvoluted to yield σQ(ḡ). For Ar* collisions with O2, we find σQ[3P2] values of 35.6±1.8 Å2 for ḡ (Ē) between 690 and 2000 m/s (50 and 350 meV) that gradually decrease above 2000 m/s, and σQ[3P0] values of 46±4 Å2 between 690 and 830 m/s (50 and 70 meV). For Kr* collisions, we report σQ[3P2] values of 38.1±2.5 Å2 between 575 and 810 m/s (46 and 87 meV) with no apparent velocity dependence, and a σQ[3P0] value of 56±7 Å2 at 576 m/s (46 meV). For Xe* collisions, we find σQ[3P2] values of 48±3 Å2 at 535 m/s (44 meV) and 38±2 Å2 at 697 m/s (73 meV), and a σQ[3P0] value of ∼125 Å2 at 535 m/s (44 meV). Comparisons with σQ values obtained with other techniques that do not require a viewing loss correction are excellent. We also use the Ionic-Intermediate-Curve-Crossing Model (IICCM) to calculate cross sections for the Ar*(3P2)+O2→Ar+O*(1D)+O(3P) reaction. In our application of the model, the product state dissociative continuum is coupled to Ar+O2− through the predissociating O2*(E 3Σu−) state that is valence Rydberg in character. Values of σQ derived from the model are in good agreement with our experiment.

Diffraction of metastable helium atoms by a transmission grating

We report on the diffraction of metastable helium atoms (de Broglie wavelength λ dB ≊ 1A) passing a free-standing gold grating with a periodicity of 0.5 μm. The observed positions and intensities of the different diffraction peaks are in good agreement with a theoretical model for the grating shape. The overall transmission of the grating for the excited state atoms is about 30%, mainly determined by the grating geometry. Our result indicates that microfabricated transmission gratings can be used as efficient and coherent beam splitters for rare gas atoms in long-lived excited states. This fact offers interesting possibilities in view of atom interferometry with metastable noble gas atoms.

Metastable noble gas apparatus for laser photoionisation studies

Metastable noble gas atoms are produced by crossing a low-energy electron sheet beam with a jet of gas injected by an automotive fuel injector. A laser beam is directed at right angles through the wide dimension of the electron beam, and the gas jet is directed at a small (19 degrees ) angle to the laser beam. An electric field is used to extract photoionised atoms from the interaction region which are then detected in a time-of-flight (TOF) mass spectrometer. The apparatus has two novel features. First the metastable atoms are created directly in the interaction region where they are photoionised instead of being created in a discharge cell and transported as a molecular beam to the interaction region. This simplification eliminates at least one level of differential pumping and creates a relatively high density of metastable atoms. Because the low-energy electron gun which creates the metastable atoms can also be operated at higher energies, it can be used to ionise any atom or molecule present. In this mode the apparatus is self-diagnostic for system alignment and can readily detect the presence of impurities in the target gas.

Auger neutralization spectroscopy of clean and adsorbate-covered transition-metal surfaces by deexcitation of metastable noble-gas atoms.

The extreme surface sensitivity of metastable noble-gas atom deexcitation spectroscopy is demonstrated with clean Pd(111), Cu(110), and W(polycrystalline) surfaces as well as for the adsorption systems Pd(111)/H, W(polycr.)/H, Pd(111)/O, Cu(110)/O, and W(polycr.)/O. Proper analysis of the data leads to information on the electronic surface density of states not available with spectroscopic techniques of finite sampling depth such as uv photoelectron spectroscopy. The results are compared with calculated electronic surface densities of states if available. Chemisorption of hydrogen and oxygen strongly affects the electronic surface density of states because surface states and resonances are shifted to higher binding energies or lose their surface state character due to the formation of the adsorbate---metal bond. In addition, occupied bonding as well as antibonding adsorbate-metal states are observed. Based on the specific sensitivity of this spectroscopic technique for the outermost atomic layer, it can be shown that H on Pd(111) and O on Cu(110) adsorb at 300 K above the first metal layer, in contrast to earlier conclusions which favored subsurface sites.

The interaction of cesium with oxygen

Ultraviolet photoelectron spectroscopy (UPS) and electron spectroscopy by deexcitation of metastable noble gas atoms (MDS) were used to follow the continuous oxidation of Cs films. While UPS has a finite information depth, MDS probes only the properties of the outermost atomic layer. Small doses of O2 cause the formation of a ‘‘monolayer’’ of Cs11O3 in which the O2− ions are incorporated below the surface, while the surface itself consists merely of metallic Cs atoms. Further uptake of oxygen leads to a continuous transformation of the subsurface layer into Cs2O2, while a small concentration of adsorbed O atoms builds up on the surface which act as nucleation centers for subsurface oxidation. Finally, the surface layer is completely oxidized into CsO2 which then retards further oxygen penetration into the bulk. The chemical state of the surface can be directly correlated with the associated change of the work function. A monolayer of Cs adsorbed on a Cu(110) surface exhibits quite different properties.

Interaction of metastable noble-gas atoms with transition-metal surfaces: Resonance ionization and Auger neutralization.

Electron emission due to deexcitation of metastable noble-gas atoms occurs at transition-metal surfaces which are either clean or covered by atomic adsorbates via resonance ionization of the metastable atom and subsequent Auger neutralization. The resulting electron-energy distributions contain information on the local surface density of states. For analysis, the electron-energy distribution is approximated by a self-convolution function. The validity of this approximation is discussed in detail. The deconvolution of the experimental data yields an effective transition density function reflecting the main features of the electronic surface density of states. The deconvolution technique is described in the Appendix. Furthermore, results are presented on the angular distribution of electrons emitted by Auger neutralization as well as on the lowering of the ionization potential of meta- stable and ground-state noble-gas atoms at surfaces which provides information on the distance range where resonance ionization, Auger neutralization, and Auger deexcitation occur.

Electron spectroscopy of surfaces by de-excitation of metastable noble gas atoms

Electronically excited, metastable noble gas atoms A* (for example H e*21S, excitation energy E* — 20.6 eV) with thermal kinetic energy are very efficiently quenched upon collision with a surface S, i.e. A* + S -> A + S+ + e~. De-excitation proceeds through two competing mechanisms: (i) Auger de-excitation (equivalent to Penning ionization), or (ii) resonance ionization followed by Auger neutralization. The energy distribution of the emitted electrons is governed by the overlap between the wavefunctions of the target and the unoccupied (ground-state) level of the impinging atoms. As a consequence, this technique is extremely sensitive to the density of valence electronic states of the outermost atomic layer. Results for clean and adsorbate-covered surfaces are presented in comparison with data recorded by ultraviolet photoelectron spectroscopy, to demonstrate the capabilities of this method.

Penning ionization electron spectroscopy of molecular adsorbates on Pd and Cu surfaces

The electronic properties of CO, PF 3, NH 3, C 2H 2 and C 6H 6 adsorbed on Pd(111), Pd(110), and Cu(110) surfaces were studied by Auger deexcitation (AD) of metastable noble gas atoms (Penning ionization) and by ultraviolet photoelectron spectroscopy (UPS). Electron emission via AD is restricted to the outmost levels localized at the adsorbed particles. The AD process competes with reasonance ionization of the metastable atoms, and its probability depends on the geometry of the adsorbed species and on its adsorption sites as well as on coverage. The differences in kinetic energy of electrons emitted by AD or by photons reflect modifications of the electron affinities of the adsorbed molecules. The extreme surface sensitivity of AD spectroscopy allows in particular to probe the local density of states at the adsorbate in the energy range close to the Fermi level which arises from π “back donation” as well as σ antibonding contributions.

Peening ionization electron spectroscopy of molecular adsorbates on Pd and Cu surfaces

Abstract The electronic properties of CO, PF 3 , NH 3 , C 2 H 2 and C 6 H 6 adsorbed on Pd(111), Pd(110), and Cu(110) surfaces were studied by Auger deexcitation (AD) of metastable noble gas atoms (Penning ionization) and by ultraviolet photoelectron spectroscopy (UPS). Electron emission via AD is restricted to the outmost levels localized at the adsorbed particles. The AD process competes with reasonance ionization of the metastable atoms, and its probability depends on the geometry of the adsorbed species and on its adsorption sites as well as on coverage. The differences in kinetic energy of electrons emitted by AD or by photons reflect modifications of the electron affinities of the adsorbed molecules. The extreme surface sensitivity of AD spectroscopy allows in particular to probe the local density of states at the adsorbate in the energy range close to the Fermi level which arises from π “back donation” as well as σ antibonding contributions.