Excited Atomic Systems Research Articles

Modeling Time‐Resolved Kinetics in Solids Induced by Extreme Electronic Excitation

AbstractThe authors present a concurrent Monte Carlo (MC)–molecular dynamics (MD) approach to modeling matter response to excitation of its electronic system at nanometric scales. The two methods are combined on‐the‐fly at each time step in one code, TREKIS‐4. The MC model describes the arrival of irradiation (a photon, an electron, or a fast ion). It traces induced cascades of secondary electrons and holes, and their energy exchange with atoms due to scattering. The excited atomic system is simulated with an MD model. An efficient way is proposed to account for nonthermal effects in the electron‐atom energy transfer in covalent materials via the conversion of the potential energy of the electronic ensemble into the kinetic energy of atoms. Such a combined MC–MD approach enables a time‐resolved tracing of the excitation kinetics of both, the electronic and atomic systems, and their simultaneous response to a deposited dose. As a proof‐of‐principle, it is shown that the proposed method describes atomic dynamics after X‐ray irradiation in good agreement with tight‐binding MD. The model also allows gaining insights into the atomic system behavior during the energy deposition from a nonequilibrium electronic system excited by an ion impact.

Spectra of a Rydberg Atom in Crossed Electric and Magnetic Fields

Contemporary spectroscopic studies of astrophysical and laboratory plasmas frequently deal with extremely large values of principle quantum numbers of atomic systems. These atomic states are very sensitive to electric and magnetic fields of the surrounding medium. While interpreting the spectra of such excited atomic systems, one faces the problem of a huge array of radiative transitions between highly excited atomic levels. Moreover, external electric and magnetic fields significantly complicate the problem because of the absence of standard selection rules typical for the spherical quantization. The analytical expression in the parabolic representation for dipole matrix elements obtained by Gordon contains hyper-geometric series and it has a very complex structure. The matrix elements that involve the presence of electric and magnetic fields are calculated while using a representation closely related to the parabolic quantization on two different axes. This matrix element depends in a complex way on the transition probabilities in the parabolic coordinate system (Gordon’s formulas) and the Wigner d-functions. This circumstance leads to even greater computational difficulties. A method of simplification of these complicated expressions for transition probabilities is demonstrated. The semiclassical approximation for coordinate matrix elements (Gulayev) and recurrence properties of the Wigner d-functions are used. The Hnβ line is under consideration. Specific calculations for the transition 10–8 in the case of parallel and perpendicular fields are presented.

Polarization transfer from the twisted light to an atom

When polarized light is absorbed by an atom, the excited atomic system carries information about the initial polarization of light. For the light that carries an orbital angular momentum, or the twisted light, the polarization states are described by eight independent parameters, as opposed to three Stokes parameters for plane waves. We use a parameterization of the spin-density matrix of the twisted light in terms of vector and tensor polarization, in analogy with massive spin-1 particles, and derive formulae that define atom’s response to specific polarization components of the twisted light. It is shown that for dipole (S → P) atomic transitions, the atom’s polarization is in one-to-one correspondence with polarization of the incident light; this relation is violated, however, for the transitions of higher multipolarity (S → D, S → F, etc) We pay special attention to contributions of the longitudinal electric field into the matrix elements of atomic transitions.

Magnetic-field control of subradiance states of a system of two atoms

A method is proposed for the creation of an entangled metastable (subradiance) excited state in a system of two closely spaced identical atoms. The system of unexcited atoms is first placed in a magnetic field that is directed at a magic angle of $${\alpha _0} = {\text{arccos}}\left( {1/\sqrt 3 } \right) \approx 54.7^\circ $$ to the line connecting the atoms and has a transverse gradient. The gradient of the field results in the detuning of frequencies of an optical transition of the atoms. Then, the resonant laser excitation of an atom with a higher transition frequency is performed with the subsequent adiabatic switching-off of the gradient of the magnetic field. It is shown that the excited atomic system in this case transits with overwhelming probability to an entangled subradiance state. Requirements on the spectroscopic parameters of the transitions and on the rate of varying the gradient of the magnetic field necessary for the implementation of this effect are analyzed.

Nondipole ionization dynamics of excited atoms in intense femtosecond pulses

The photoionization of the $4s$ Rydberg state in hydrogen by an intense femtosecond $400\text{\ensuremath{-}}\mathrm{nm}$ light pulse is studied beyond the dipole approximation. We show that a recently predicted nondipole, high-frequency ionization mechanism [M. F\o{}rre et al., Phys. Rev. Lett. 97, 043601 (2006)] has a low-frequency counterpart in excited atomic systems. This opens the possibility that the effect could be measured in the near future.

Two-photon cooperative decay in a cavity in the presence of a thermalized electromagnetic field

We study the cooperative two-photon spontaneous decay of an excited atomic system in a microcavity whose size is of the order of several wavelengths of atomic radiation. We show that a thermalized electromagnetic field in the microcavity strongly affects the two-photon cooperative spontaneous emission of radiation. The increase in the rate of spontaneous cooperative decay is due to the presence of a small number of thermalized photons in a microcavity mode. At low temperatures, the two-photon absorption probability is found to be a linear function of the two-photon flux, and photon superbunching is observed.

Auger decay of multiply excited atomic systems

By studying the branching ratios in the Auger decay of many triply excited three-electron atomic systems, we try to discover the physical reason behind the relative magnitude of the partial widths. It appears that Auger decay is most likely to happen when the two interacting electrons are in close proximity and the overlap of their charge distribution is significant in coordinate space. It is also pointed out that when an ${\mathrm{nl}}_{1}{\mathrm{nl}}_{2}{n}^{\ensuremath{'}}{l}_{3}{(n}^{\ensuremath{'}}gng~2)$ state decays into an ${n}^{\ensuremath{''}}{l}_{1}{n}^{\ensuremath{'}}{l}_{3}(ng{n}^{\ensuremath{''}})+\ensuremath{\epsilon}{l}_{2}$ channel, the ${n}^{\ensuremath{'}}{l}_{3}$ orbital of the initial state is more diffused than that of the two-electron target. Therefore, some ${n}^{\ensuremath{''}}{l}_{1}{\mathrm{ml}}_{3}(mg{n}^{\ensuremath{'}})+\ensuremath{\epsilon}{l}_{2}$ channels will also contribute significantly to the Auger width of this state. The argument presented in this work is also applied to two- and four-electron systems. Explicit examples are presented.

Cooperative bistability in dense, excited atomic systems.

We report a new mechanism of intrinsic bistability, operative in excited atomic systems with substantial atom-atom coupling. Predictions and observations of bistable luminescence from ${\mathrm{Yb}}^{3+}$ pairs in crystalline ${\mathrm{Cs}}_{3}$${\mathrm{Y}}_{2}$${\mathrm{Br}}_{9}$: ${\mathrm{Yb}}^{3+}$ are in good agreement and have interesting implications for local field effects in solids and dense, ultracold gases.

Energy and fine structure of 1s2nd and 1s2n f states for the lithium isoelectronic sequence

The non-relativistic energies of 1s2nd (n=3,4 and 5) and 1s2nf (n=4 and 5) states for the lithium isoelectronic sequence from Li I to Ne VIII are calculated with a full-core plus correlation method using multiconfiguration interaction wavefunctions. Relativistic and mass-polarization effects on the energy are evaluated perturbatively as the first-order corrections. The fine structure is calculated by using spin-orbit and spin-other-orbit interaction operators. The authors results are in excellent agreement with experimental data in the literature. Possible misidentifications in the observed optical spectra are pointed out. They show that this full-core plus correlation method is useful for excited atomic systems with a 1s2 core.

Structure of Stark Broadened Hβ Lines at low Electron Densities

Abstract The profile of the hydrogen Hβ line emitted by a hydrogen low-temperature afterglow plasma has been measured at electron densities 1.8·1015 , 1·1015 and 5·1014 cm-3 by means of a 10-channel analyzer system with a spectral resolution of 0.1 Å. All profiles showed weak plasma satellites suddenly appearing at wavelength distances which correspond to the electron plasma frequency and showing “satellite line wings” which slowly smooth out at larger wavelength distances. In the line core-where all quasi-static and ion dynamic theories predict a minimum - appeared an intensity maximum well separated from the closely spaced intensity shoulders. This new feature not yet predicted by line broadening theories and not yet observed by other experimentalists is probably due to low-frequency ion oscillations which couple with the excited atomic system. Such an effect would explain discrepancies between available Stark broadening theories and the mass-dependent structure of line profiles recently observed in other experiments. The effect may possibly be useful for measuring collective low-frequency oscillations in plasmas under very special conditions.