Excited Atomic Levels Research Articles

Resonance lifetime and energy of an excited Cs state on Cu(111)

Using time-resolved two-photon photoemission spectroscopy, we investigated the resonance lifetime and energy of an excited Cs state on Cu(111) in the low-coverage case. We found a pronounced lifetime increase of up to 11\ifmmode\pm\else\textpm\fi{}3 fs on a copper surface covered with cesium as compared to a clean copper surface in the energy range of an excited electronic Cs state in the spectrum. This result is in agreement with recent theoretical predictions of hybridization among excited atomic levels of alkali atoms near metal surfaces.

Stochastic kinetics of electron transitions in nonideal plasmas

The time evolution of the population of excited atomic and ionic levels for an ensemble of reacting particles in dense plasmas is studied on the basis of a master equation. Many-body effects taken into account via energy-level shifts that are determined by the interaction parts of the chemical potentials of free and bound particles. These chemical potentials are calculated from an approximation for free partially ionized plasmas accounting for exchange and correlation as well as for short-range repulsion due to bound shell electrons. Using semi-empirical cross sections the method is applied to investigate the recombination of dense plasma containing carbon and hydrogen. The master equation can be combined with a temperature equation to account for different thermodynamic constraints. We compare the isentropic, isothermal, and isoenergetic recombination of dense hydrogen plasma.

Atoms and ions in superfluid helium

Optical spectra, level population and nonradiative deexcitation processes of atomic impurities in liquid helium have been investigated. The recombination of electrons and positive metal ions are described by the tunneling model. This model explains the population gap of 1.8 eV below the ionisation limit which was observed for several neutral defect atoms in liquid helium. In the framework of a pseudopotential theory excited singlet levels of Ba, Ca and Na atoms are recalculated and compared to experimental data. Non-spherical atomic defects for excited p-states are treated also. Quadrupole vibrations of these distorted defect structures are assumed to be responsible for inducing multiphonon transitions between excited atomic states.

Electron impact coherence parameters for the (...6s5d1D2) to (...6s6p1P1) excitation in 138Ba determined by superelastic scattering measurements

Superelastic electron scattering involving the de-excitation of laser-excited 138Ba (...6s6p1P1) to the lower-lying (...6s5d1D2) metastable level has been investigated. These measurements have been used to extract the electron impact coherence parameters which describe excitation out of an isotropic (...6s5d1D2) level to the higher-lying (...6s6p1P1) state. Studies have been carried out for an electron impact energy of 20 eV and a range of scattering angles up to 45 degrees . This investigation represents a significant extension of coherence parameter measurements to inelastic transitions between excited atomic levels.

Two-beam linear magneto-optical spectroscopy of atomic transitions between short lived states

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7S1/2 level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted.

Strong-Field Phenomena in Atoms - Quasiclassical Approach

A quasiclassical (WKB) approach is used to construct a theory of atomic transitions induced by a strong light field. This approach is used to find the Coulomb-Volkov solutions of the Schrodinger equation in which both the Coulomb and light fields are taken into account. These solutions are shown to be applicable in . a region of low light frequencies, low electron energies and angular momenta.. The found solutions are used to describe two kinds of processes: strong-field photoionization from highly excited (Rydberg) atomic levels, and field-assisted electron-ion scattering. In the photoionization problem the strong-field complex quasi-energies of an atom are found. A problem' of the strong-field stabilization of an atom, as well as the expected behavior of the ionization time in its dependence on the light field strength are discussed.

Method of investigation of electron impact induced transitions between excited atomic levels

A method for the measurement of rate constants for inelastic collisions between thermal electrons and excited atoms is presented. The measurements were performed in a mixture of saturated caesium and sodium vapor. Electrons were generated by ionization of caesium vapour by intense resonant dye laser pulses. Pulses of second dye laser, delayed with respect to the first one, excited the sodium atoms to the 3P level. Subsequently Na(3P) atoms were excited to higher levels by collisions with electrons. The fluorescence intensity from these levels served as a measure of the rate constant for these transitions. The method was used to investigate the electron impact induced 3P–4P excitation.

UV multiphoton dissociation of SbBr 3

SbBr 3 molecules in the gas phase were dissociated by 193 and 248 nm laser radiation. The fluorescence was dispersed by a monochromator. Highly excited atomic levels of Sb were found to be significantly populated for three- or four-photon dissociation. A region of continuous fluorescence in the visible range around 550 nm was attributed to the diatomic SbBr radical resulting from a two-photon dissociation. Only weak lines of atomic bromine, but from high levels, were found. A broad weak continuous fluorescence in the UV remains unassigned.

Scattering of radio waves by high atomic Rydberg states

The scattering coefficient of highly excited atomic levels formed by recombination from a plasma and energetically broadened by plasma field fluctuations is determined. For this purpose, the population of the levels is calculated from a detailed balance equation which is solved consistently together with the corresponding equation for the free electron spectrum. Numerical results are obtained for parameters appropriate for ionospheric conditions. The solution proves to be in no way related to LTE situations, therewith invalidating usual treatments of the problem, which are not consistently based on a detailed balance approach. The theoretical result for the equilibrium coefficient for resonant scattering of radio waves by Rydberg atoms is consistent with experimental data obtained from the ionosphere. This suggests that the considered mechanism is exclusively responsible for the scattering of radio waves in particular and electromagnetic radiation in general.

A one-dimensional model of dc glow discharges

A one-dimensional continuum model for a dc-glow-discharge plasma between parallel-plate electrodes is presented, which is based on the local balance equations for particle densities and fluxes of electrons and ions, as well as on the local electron energy balance and the Poisson equation. In addition to similar treatments, the inertia term in the ion momentum balance equation leading to a singularity at the ion sound velocity, as well as the electron energy losses by excitation of atomic levels, have been taken into account. The system of nonlinear first-order differential equations supplemented by suitably chosen boundary conditions has been solved by the numerical procedure Henyey method ordinary differential equation solver, which allows also for the occurrence of removable singular points. Calculations for an argon plasma have been performed and the influence of parameter choices on the solution has been investigated. In general, the features typical for dc discharges are reproduced and agreement with the spatial profiles obtained by other authors with different methods is found.

An efficient excitation scheme for resonance ionization of tin in a laser ion source

An efficient three-colour, three-step resonant excitation/ionization scheme has been found for tin that leads from the 5 p 2 3 P 2 level of the ground-state multiplet via two excited atomic levels (λ 1 = 317.51 nm and λ 2 = 811.40 nm) to an autoionizing state 9 s 3 P 2(λ 3 = 823.49 nm) at 59375.9 cm −1. This excitation path permits the saturation of all transitions with the limited power available from a copper vapour laser pumped dye laser system ( E sat ≈ 1.5 mJ cm 2 for λ 3). The high repetition rate of such a laser system is essential for a highly efficient laser ion source.

Emission spectroscopy on a supersonically expanding argon/silane plasma

Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A, V=80 V, Ar flow=60 scc/s and pressure 4 × 104 Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018 m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+ particles. The emission spectrum of the SiH radical, the A 2Δ–X 2Π electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.

Quantum optics of localized light in a photonic band gap.

We describe the quantum electrodynamics of photons interacting with hydrogenic atoms and molecules in a class of strongly scattering dielectric materials. These dielectrics consist of an ordered or nearly ordered array of spherical scatterers with real positive refractive index and exhibit a complete photonic band gap or pseudogap for all directions of electromagnetic propagation. For hydrogenic atoms with a transition frequency in the forbidden optical gap, we demonstrate both the existence and stability of a photon-atom bound state. For a band gap to center frequency ratio \ensuremath{\Delta}\ensuremath{\omega}/${\mathrm{\ensuremath{\omega}}}_{0}$\ensuremath{\sim}5%, the photon localization length ${\ensuremath{\xi}}_{\mathrm{loc}}$\ensuremath{\ge}10L, where L is the lattice constant of dielectric array. This strong self-dressing of the atom by its own localized radiation field leads to anomalous Lamb shifts and a splitting of the excited atomic level into a doublet when the transition frequency lies near a photonic band edge. We estimate the magnitude of this splitting to be ${10}^{\mathrm{\ensuremath{-}}6}$ at the vacuum transition energies.

Kinetics of ionization-recombination processes in nonideal hydrogen plasmas

The time evolution of the relative populations of excited atomic levels in a reacting dense H-plasma is studied on the basis of rate equations. Many-particle effects are taken into account via energy level shifts. It is found that the occupation of the bound state levels is essentially smaller than for the ideal case. It is shown that the mechanism of the relaxation to the thermal equilibrium depends on the density. Inversions of the level populations can be observed.

Energy shifts and broadening of atomic levels near metal surfaces.

We present calculations of the lifetime broadening and the shifts of hydrogenlike atomic levels (ground and excited states of H, Li, Na, K, Rb, and Cs) near jellium metal surfaces (Al and Na). The energies and widths of the atomic resonances are obtained from the Schr\odinger equation with use of the complex scaling technique; the electron potential in the surface region is calculated with use of density-functional theory. We find that the energy shifts of the atomic levels are influenced mainly by the properties of the surface potential close to the atom. The widths of the atomic levels, on the other hand, depend on the surface potential in the whole surface region. We show that in order to obtain accurate widths of atomic levels it is important to incorporate nonlocal effects, particularly the image potential. We also find that the widths are strongly influenced by hybridization among the near-degenerate atomic levels. The excited atomic levels shift differently with distance from the surface leading to multiple level crossings. At such positions the hybridization can be strongly enhanced, resulting in states that are oriented towards or away from the surface, with very different lifetimes that may vary in a nonexponential manner with distance from the surface.

A relativistic C.I. study on theJ=1/2 even spectrum of Sr II

We present a method for performing relativistic CI calculations in ground and excited atomic and ionic levels. An electron occupying a relativistic shellnκ in a given electronic configuration is described by a single numerical four-component Dirac-Fock orbital having the samen and κ quantum numbers to those of the shellnκ. Application of this method yields estimates for the I.P. (88 741 cm−1) and the core correlation energy (−30916 cm−1) for Sr II and for the total correlation energy in Sr III (−30916 cm−1). Core-valence correlation energies for the |core 5s〉 (−4379 cm−1), |core 6s〉 (−1191 cm−1) and |core 13s〉 (−32 cm−1) levels have been calculated for Sr II. Estimates for the total relativistic, Breit, vacuum polarization and self energy corrections for these levels are also reported. Configurations in which the core is not fully occupied have been found to yield significant contributions to the correlation energies of both ground and excited levels. Our results show that full scale relativistic CI calculations using numerical four-component Dirac-Fock orbitals are feasible and provide a useful ab-initio tool for the investigation of atomic properties within the framework of fully relativistic theories.

Quantum electrodynamics near a photonic band gap: Photon bound states and dressed atoms.

It is shown that in dielectrics exhibiting a complete photonic band gap, quantum electrodynamics predicts the occurrence of bound states of photons to hydrogenic atoms. When the atomic transition frequency lies near a photonic band edge, the excited atomic level experiences an anomalous Lamb shift and splits into a doublet. One member of this doublet exhibits resonance fluorescence whereas the other level is dressed by the emission and reabsorption of near-resonant photons whose amplitude decays exponentially from the vicinity of the atom.

Role of Beam Foil Spectroscopy in Understanding Basic Plasma Processes on the Sun

Beam-Foil spectroscopy(BFS) has proved to be a valuable technique for the determination of radiative lifetimes of excited atomic levels leading to the evaluation of the transition probabilities. The time- resolved nature of the decay process in a collisionless environment is a unique characterstic of the beam-foil light source. The relevance of BFS to astrophysics comes from the importance of radiative transition probabilities in the quantitative analysis of optical spectra. Stellar abundances are obtained from the intensity of a spectral line which essentially is a product of the abundance of the element in the source and the probability of the transition. Thus the evaluation of accurate values of transition probabilities contribute significantly to stellar abundance analysis.

COLRAD—A program to calculate population densities of the excited atomic levels of hydrogen-like ions in a plasma

COLRAD—A program to calculate population densities of the excited atomic levels of hydrogen-like ions in a plasma

Molecular enhanced four-wave parametric generation in sodium atom

A number of resonances have been observed in the excitation spectrum of the uv resonance lines of sodium. The mechanism proposed includes absorption by molecular states, energy transfer to excited atomic levels and four-wave parametric generation enhanced by radiative decay of the excited states.