Excited Atomic States Research Articles

Time-resolved formation of excited atomic and molecular states in XUV-induced nanoplasmas in ammonia clusters.

High intensity XUV radiation from a free-electron laser (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron-ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron-ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from -0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen.

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone.

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Two-photon laser-assisted electron scattering on Hydrogen atom

SynopsisWe considered the influence of light-dressed-atom effect on two-photon laser-assisted electron-atom scattering in the Born approximation. We performed calculations for the ground state of a Hydrogen atom. For small scattering angles, the main contribution to the cross section is made by the combined process, when one of the photons is absorbed by the atomic electron and the second one by the scattered electron. However, in the UV region, when two-photon resonances with highly excited atomic states arise, the main contribution is determined by the inverse polarization Bremsstrahlung absorption.

Subradiance in multiply excited states of dipole-coupled V-type atoms

We generalize the theoretical modeling of collective atomic super- and subradiance to the multilevel case including spontaneous emission from several excited states towards a common ground state. We show that in a closely packed ensemble of N atoms with N –1 distinct excited states each, one can find a new class of non-radiating dark states, which allows for long-term storage of N –1 photonic excitations. Via dipole-dipole coupling only a single atom in the ground state is sufficient in order to suppress the decay of all N –1 other atoms. By means of some generic geometric configurations, like a triangle of V-type atoms or a chain of atoms with a transition, we study such subradiance including dipole-dipole interactions and show that even at finite distances long lifetimes can be observed. While generally hard to prepare deterministically, we identify various possibilities for a probabilistic preparation via a phase-controlled laser pump and decay.

Vibrational modes and quantum zero-point energy of hydrogen in ZrH0.0155 and ZrH2

We report on an inelastic neutron scattering study of the proton dynamics in ZrH0.0155 and ε-ZrH2. In particular, we present measurements of the incoherent dynamic structure factor, generalized vibrational density of states, and proton momentum distribution of these two materials. Our results are generally consistent with theoretical predictions of Elsässer et al. [Mat. Res. Soc. Symp. Proc. 453 221–226 (1997)]. They argued that the effective Born-Oppenheimer potential experienced by the hydrogen atoms in ε-ZrH2 is nearly isotropic and harmonic at energies below 0.3 eV, but becomes anisotropic and anharmonic for higher energies. At low temperatures, the proton momentum distribution is dominated by the quantum-mechanical ground state of the protons. We find that it assumes a Gaussian shape, consistent with the concept that the potential surface is approximately harmonic for small displacements of the hydrogen atoms. However, the anharmonicity of the potential becomes readily apparent in the excited states of the hydrogen atoms, as the harmonic approximation breaks down in the description of the multiphonon bands.

Sub-Doppler Spectroscopy of Excited Atoms in a Multilayer Gas Cell

The potential for spectroscopy in a multilayer cell with rarefied gas medium is theoretically investigated where said cell can be used as a compact analog of a large number of plane-parallel beams of atoms (or molecules) in an optically excited quantum level. In particular, this situation occurs for metastable and highly excited long-lived Rydberg states of atoms and molecules. Velocity selection of atoms (molecules) excited to said quantum level by broadband optical pumping was found to be efficient because of their characteristic transit and collisional relaxation in said cell. Sub-Doppler resonances at the optical transition frequencies from a given level were narrow as a result of the selection when the monochromatic probe light beam was absorbed.

Cathode spot formation possibly explained by cathode electron emission from impact of excited state atoms

A theory is given for electron emission from electric arcs with non-thermionic cathodes, for example the iron cathodes used in arc welding, proposing that the principal process for electron emission is from the impact of excited atomic states at the cathode. For plasmas in contact with a metal cathode, a sheath of positive ions is formed over the cathode surface because electrons move away from the cathode. A sheath thickness of only 0.1 µm causes secondary electrons produced at the cathode surface to gain ~12 V. The electron transit through this thin sheath is collision-less. These 12 V electrons produce excited states of atoms to a distance of approximately ~10 µm from the cathode. Diffusion and direct transfer of these excited atoms back to the cathode in the almost collision free region next to the cathode is calculated to be in times of ~10−8 s. This time is less than the radiative lifetimes of almost all atomic transitions except for some resonance transitions where there would be an enhancement of lifetimes due to imprisonment of radiation. If these transfer times are less than the radiative lifetimes, secondary emission coefficients of electrons from the cathode are likely to be approximately 1, the measured coefficients for metastable atoms. The energies of these excited atoms are ~12 V or more, much larger than the usual work function of ~4.5 V required to eject electrons. If such secondary electrons, together with secondary electrons produced from the impact of positive ions and photons, replace or exceed the original number of electrons, amplification is possible in successive avalanches to cause electron densities to grow to supply the total arc current. The mechanism would then explain why observed cathode sheath voltages are independent of arc current for currents varying from 10 to 20 000 A for various low melting point electrodes and arc gases. Electron emission from cold cathodes, when temperatures are too low for thermionic emission, would also be explained. Solutions of the densities of electrons, positive ions and excited state atoms as a function of time and distance from the cathode are presented which give a physical explanation for the formation of cathode spots.

Photodissociation dynamics of dichlorodifluoromethane (CF2Cl2) around 235 nm using time-sliced velocity map imaging technology

Photodissociation dynamics of dichlorodifluoromethane (CF2Cl2) around 235 nm has been studied using the time-sliced velocity map imaging technology in combination with the resonance enhanced multi-photon ionization technology. By measuring the raw images of chlorine atoms which are formed via one-photon dissociation of CF2Cl2, the speed and angular distributions can be directly obtained. The speed distribution of excited-state chlorine atoms consists of high translation energy (ET) and low ET components, which are related to direct dissociation on 3Q0 state and predissociation on the ground state induced by internal conversion, respectively. The speed distribution of ground-state chlorine atoms also consists of high ET and low ET components which are related to predissociation between 3Q0 and 1Q1 states and predissociation on the ground state induced by internal conversion, respectively. Radical dissociation channel is confirmed, nevertheless, secondary dissociation and three-body dissociation channels are excluded.

Complete Study of Nature and Importance of Spectral Bands Contained in Infrared Spectra of Leaves of Maize Inbred Lines with Significant Breeding Properties

In this article we used the method of IR spectroscopy to identify bioactive organic compounds and their structural characteristics in the leaves of maize inbred lines ZPPL 186, ZPPL 225 and M1-3-3 Sdms based on a standard study of high intensity high-amplitude spectral bands and based on attempt-study of hypothetically influential low intensity low-amplitude spectral bands. The made assumption was that numerous spectral bands, not analyzed yet, but occurring in different kinetic forms should be systematically analyzed and the dynamics of their formation caused by different oscillations and vibrations of molecular bonds of organic compounds and their functional groups should be proven. Low intensity spectral bands imply unstable state of the biological system (leaf), which is a consequence of the excited state of molecules, radicals, atoms or ions in tissues, cells or biomembranes. Similar transport processes occur when biological systems are rhythmically stimulated, as well as in the case of a complex ion transport across the excited thylakoid membrane. These bands most frequently occurred in the wave number range of 500–1600 cm–1; they appeared in several spots in wave number ranges up to 3000 cm–1. Amines, primary and secondary amides, proteins, amino acids, alkanes, alkenes, aldehydes, ketones, aromatic compounds, carbohydrates, carboxylic acids, ethers, alcohols, sulfides, mono- and polysaccharides were identified in leaves of inbred lines. The analysis of spectral bands showed that the most intensive was the inbred line ZPPL 186, then the ZPPL M1-3-3 Sdms, while the lowest intensity was detected in ZPPL 225. Thus, by overall study it is possible to identify not only organic compounds and their structures in leaves of inbred lines, but also to indicate the possibility of causing an unstable and conformational state in them.

Search for Anderson localization of light by cold atoms in a static electric field

We explore the potential of a static electric field to induce Anderson localization of light in a large three-dimensional (3D) cloud of randomly distributed, immobile atoms with a degenerate ground state (total angular momentum $J_g = 0$) and a three-fold degenerate excited state ($J_e = 1$). We study both the spatial structure of quasimodes of the atomic cloud and the scaling of the Thouless number with the size of the cloud. Our results indicate that unlike the static magnetic field, the electric field does not induce Anderson localization of light by atoms. We explain this conclusion by the incomplete removal of degeneracy of the excited atomic state by the field and the relatively strong residual dipole-dipole coupling between atoms which is weaker than in the absence of external fields but stronger than in the presence of a static magnetic field. A joint analysis of these results together with our previous results concerning Anderson localization of scalar waves and light suggests the existence of a critical strength of dipole-dipole interactions that should not be surpassed for Anderson localization to be possible in 3D.

Two-photon laser-assisted electron scattering on hydrogen atom

We considered the influence of light-dressed-atom effect on two-photon laser-assisted electron-atom scattering in the Born approximation. We performed calculations for the ground state of a Hydrogen atom. We demonstrated that for small scattering angles, the main contribution to the cross section is made by the combined process, when one of the photons is absorbed by the atomic electron and the second one by the scattered electron. However, in the UV region, when two-photon resonances with highly excited atomic states arise, the main contribution to the cross section is determined by the inverse polarization Bremsstrahlung absorption.

Accurate numerical method for the calculation of doubly excited states in atoms

We report in this paper computed values of the energy positions and widths of the lowest 1Po singlet doubly excited states of the helium atom. The results are obtained by a direct numerical solution of the time-independent Schrödinger equation using a discretization technique with B-spline functions combined with complex rotation method. The present approach has the numerical advantage to generate accurate energy positions and widths in a single calculation. The computed data are in very good agreement with results from other theoretical approaches.

Time-resolved measurements of extreme ultraviolet (EUV) emission, from EUV-induced He, Ne, and Ar plasmas

AbstractIrradiation of gases with intense pulses of extreme ultraviolet (EUV) can result in the formation of low-temperature plasmas. During the time of irradiation, various non-thermal processes driven by the EUV photons and photoelectrons take place, leading to the creation of excited states of atoms and ions. Fast relaxation of these states should result in EUV emission within a time comparable to the driving EUV pulse. On the other hand, from our earlier works, a time duration of the emission in an optical range is over an order of magnitude longer. It can be thus expected that the time of EUV emission can be also relatively long. In this work, time-resolved measurements of the EUV emission from low-temperature plasmas induced in He, Ne, and Ar gases were performed. Due to a low intensity of the emitted radiation, a specially prepared detection system, based on an EUV collector and an EUV sensitive photodiode, was employed. In all cases, a time duration of the EUV emission was much longer compared with the driving EUV pulse. Time profiles of the corresponding signals were specific for particular gases. In case of He and Ne plasmas, these time profiles varied with initial densities of gases to be ionized. The corresponding dependence was especially visible in case of plasmas induced in helium. In case of Ar plasmas, such dependence was not revealed.

“Hypervalency” and the chemical bond

Bonding in chemistry refers to the stability of molecules, therefore it is of paramount importance to natural sciences and certainly to all aspects of life. Yet bonding is a chameleon–like fuzzy concept, leaving enough space to the sprouting of a variety of “theories” supposedly explaining the structure and bonding of molecular systems. Admittedly some of these “theoretical ideas” played a constructive and useful role to the development of chemistry in the last one hundred years, however they are no longer needed. Intertwined with binding the hypervalency definition introduced half a century ago, started to be used not only for communication purposes as intended, but as an “explanation” of the structure and bonding. In the present communication and within the spirit of nonproliferation of redundant concepts, we have tried to prove that bonding between atoms can be understood and clarified based solely on the tenets of quantum mechanics as applied to molecules. More specifically the indisputable role of the excited state of atoms in the bonding process is emphatically projected. Clearly chemistry is an excited state rather than a ground state field.

Synergistic Effect of Plasma Discharge and Substrate Temperature in Improving the Crystallization of $$\hbox {TiO}_2$$ TiO 2 Film by Atmospheric Pressure Plasma Enhanced Chemical Vapor Deposition

Three deposition modes of red atmospheric pressure plasma enhanced chemical vapor deposition were applied to study the influences of plasma discharge and the external substrate temperature on improving its crystallization: deposition in plasma red zone with post-heat annealing [Mode (a)], deposition outside plasma zone with a heated substrate [Mode (b)] and deposition inside plasma zone with a heated substrate [Mode (c)]. $$\hbox {TiO}_2$$ film obtained with Mode (c) is anatase and the other two are amorphous. Four more substrate temperatures (100 $${^{\circ }}$$ C, 200 $${^{\circ }}$$ C, 300 $${^{\circ }}$$ C, 400 $${^{\circ }}$$ C) were investigated in Mode(c). The OES peaks of Ti excited atomic state shows out in Mode (c) and their intensity increases with the substrate temperature. The crystalline degree increases and the structure becomes more compact with the substrate temperature. The film obtained at 400 $${^{\circ }}$$ C presented the highest photocatalytic activity.

Super- and subradiance of clock atoms in multimode optical waveguides

The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole–dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super- and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of π/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom–atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cut-off for the interaction range emerging from the dephasing of different fiber contributions.

Preliminary results of optical emission spectroscopy by line ratio method in the RF negative ion source at ASIPP

Optical emission spectroscopy (OES) using the trace rare gases of Ar and Xe have been carried out in a radio frequency (RF) driven negative ion source at Institute of Plasma Physics, Chinese Academy of Science (ASIPP), in order to determine the electron temperature and density of the hydrogen plasma. The line-ratio methods based on population models are applied to describe the radiation process of the excited state particles and establish their relations with the plasma parameters. The spectral lines from the argon and xenon excited state atoms with the wavelength of 750.4 and 828.0 nm are used to calculate the electron temperature based on the corona model. The argon ions emission lines with the wavelength of 480 and 488 nm are selected to calculate the electron density based on the collisional radiative model. OES has given the preliminary results of the electron temperature and density by varying the discharge gas pressure and RF power. According to the experimental results, the typical plasma parameters is Te ≈ 2–4 eV and ne ≈ 1 × 1017–8 × 1017 m−3 in front of plasma grid.

Morphological and structural evolution of Si-Cu nanocomposites by an instantaneous vapor-liquid-solid growth and the electrochemical lithiation/delithiation performances

Polymorphic Si-Cu nanocomposites of Si@Cu3Si nanowires, Si@Cu3Si nanorods, and Si@Cu3Si(Cu) nanocapsules are synthesized via the high-energy arc-discharge plasma. Electrochemical performances of these materials as anodes for lithium-ion batteries are also investigated. It is found that the morphologies and structures of above Si-Cu nanocomposites are alterable by the composition of the raw target and synthetic conditions. Optical emission spectroscopy is adopted to reveal the energetic states of excited atoms in plasma; thus, the temperature of working plasma as well as the evaporation rate of each element can be evaluated, in which both favor to control the composition of Si-Cu nanopowder product and the aborative nanostructures. Formation of multifarious Si-Cu nanostructures is understood from the in situ nucleation and anisotropic growth processes, induced by an instantaneous vapor-liquid-solid mechanism within the robust plasma. The optimal composition and microstructure of Si@Cu3Si nanorods are found for the excellent electrochemical behaviors, typically a stable discharge capacity of 783 mAh g−1 with the coulombic efficiency of 98.51% at 100 mA g−1 after 100 cycles. Good performances are attributed to one-dimensional Si-Cu nanostructure, which favors to promote Li+ ion diffusion. Metallic Cu component released from Cu3Si precursor enhances the conductivity, buffers the volume change, and facilitates the stabilization in cycling.

Intracavity cold atomic ensemble with high optical depth.

We describe the apparatus of an optical cavity loaded with cold 85Rb atoms of high optical depth (OD) in the weak coupling regime. The relevant cavity-atom parameters are the single-photon Rabi frequency g0 = 2π × 0.25 MHz, the cavity power decay rate κ = 2π × 9.0 MHz, and the atomic excited state decay rate Γ = 2π × 5.75 MHz. In this bad-cavity configuration where the atomic natural linewidth (Γ/2π) is less than the cavity linewidth (κ/2π), the cavity enhancement factor for the longitudinal OD is about 188. We obtain a cavity enhanced OD up to 7600, corresponding to an atomic ensemble with a bare single-pass OD of 40, coupled to the cavity mode. Our intracavity cold atomic ensemble with high OD may have many applications in studying collective atom-light interaction inside an optical cavity.

Symmetry and coherent control of the quantum dynamics of Rydberg atoms

Transitions probabilities for highly excited Rydberg states of atoms in electromagnetic traps under the influence of an external electromagnetic fields are calculated. We used the method of groups of dynamical symmetry for hydrogenlike atoms and the technique of relevant coherent states. This allows us to calculate the transition probabilities between atomic levels outside the framework of perturbation theory. The problem of coherent controlling the quantum dynamics of atoms in cavities and some possible applications for quantum information theory are briefly discussed.