Neutral Perturbers Research Articles

Temperature and density dependence of line profiles of sodium perturbed by helium

Context. Ultracool stellar atmospheres and hot exoplanets show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. The effects of Na–He collision broadening are also central to understanding the opacity of cool DZ white dwarf stars. Aims. In order to be able to construct synthetic spectra of brown dwarfs and cool DZ white dwarfs, where helium density can reach several 1021 cm−3, Na–He line profiles of the resonance lines have been computed over a wide range of densities and temperatures. Methods. Unified line profiles that are valid from the core to the far wings at high densities are calculated in the semiclassical approach using up-to-date molecular data including electronic spin-orbit coupling from the sodium atom. Far wings are extended to more than 4000 cm−1 from the line center when the helium density can reach 1021 cm−3 at 5000 K. Results. We present a comprehensive study of Na–He collisional profiles at high density, and temperatures from 5000 K, which is the temperature prevailing in the atmosphere of ultra-cool DZ white dwarf stars, down to 1 K, which is the temperature in liquid helium clusters. Collision broadening and shift parameters within the impact approximation obtained in the semiclassical and quantum theory using our new accurate molecular data are presented.

Probing an Electron Scattering Resonance using Rydberg Molecules within a Dense and Ultracold Gas.

We present spectroscopy of a single Rydberg atom excited within a Bose-Einstein condensate. We not only observe the density shift as discovered by Amaldi and Segrè in 1934, but a line shape that changes with the principal quantum number n. The line broadening depends precisely on the interaction potential energy curves of the Rydberg electron with the neutral atom perturbers. In particular, we show the relevance of the triplet p-wave shape resonance in the e^{-}-Rb(5S) scattering, which significantly modifies the interaction potential. With a peak density of 5.5×10^{14} cm^{-3}, and therefore an interparticle spacing of 1300 a_{0} within a Bose-Einstein condensate, the potential energy curves can be probed at these Rydberg ion-neutral atom separations. We present a simple microscopic model for the spectroscopic line shape by treating the atoms overlapped with the Rydberg orbit as zero-velocity, uncorrelated, pointlike particles, with binding energies associated with their ion-neutral separation, and good agreement is found.

Recent Results for Widths of Lines Important in the Spectra of Cool Stars

Abstract The broadening of spectral lines by neutral perturbers is an important factor in the interpretation of the observed spectra of cool stars and is often included in the analysis by using a simple van der Waals formula. Detailed calculations are carried out for selected lines and are used to examine the validity of this formula for a range of temperatures.

Observation of the far wing of Lyman α due to neutral atom and ion collisions in a laser-produced plasma

The time-resolved spectrum of a laser-produced plasma in ${\mathrm{H}}_{2}$ exhibits a continuum that we attribute to radiative collisions of H atoms in a $2p$ state with ground-state neutral atoms and protons. This binary collision Lyman-\ensuremath{\alpha} line wing observed from 1150 \AA{} to 1700 \AA{} is emitted by the shock dissociated gas that surrounds the focal region. The observed temperature, electron density, and shock front speed confirm models of the shock that predict that Lyman \ensuremath{\alpha} arises from a shell of atomic hydrogen in which the neutral atom and ion densities are sufficient to create the observed line broadening. The experimental far wing spectrum agrees with unified theory calculations of the Lyman-\ensuremath{\alpha} line, which allow for the dependence of the radiative dipole moment on internuclear separation during a radiative collision. Broad neutral atom effects become evident at 1180 \AA{}, 1260 \AA{}, and 1600 \AA{} when the density of neutral perturbers is of the order of ${10}^{20}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3},$ while features at 1230 \AA{}, 1240 \AA{}, and 1400 \AA{} appear when the plasma is highly ionized. These satellites result from free-free transitions of a H atom during collisions with other neutral atoms and protons and are correlated with potential curves and radiative dipole moments of ${\mathrm{H}}_{2}$ and ${\mathrm{H}}_{2}^{+}.$

Absolute cross-section measurements for ionization of He Rydberg atoms in collisions with K.

Absolute cross sections for ionization of 1.5--10.0 keV/amu Rydberg helium atoms in principal quantum states 12\ensuremath{\le}n\ensuremath{\le}15 due to collisions with potassium have been measured. The data are compared with the free-electron cross section at equal velocity. Our results for the collisional ionization cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$) agree both in shape and absolute magnitude with the data available for the total electron-scattering cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{e}}$) and support recent theoretical models for ionization of Rydberg atoms with neutral perturbers.

Broadening and shifts of the lines of molecular ions by collisions with neutral perturbers.

We present a theoretical model for studying the shape of the rotational lines of molecular ions perturbed by collisions with neutral species. This model shows that the interaction between the ionic monopole and the multipole moments of the perturber has no role in the collisional relaxation of the lines. The effect of the ionic monopole is restricted to that of inducing a polarization of the neutral perturbers that can cause relaxation by interaction with the other permanent moments of the absorbing ion. We also present, for ${\mathrm{HCO}}^{+}$ perturbed by argon, an experimental study of the pressure broadening and shift of the rotational lines J=1\ensuremath{\leftarrow}0, 2\ensuremath{\leftarrow}1, and 4\ensuremath{\leftarrow}3. The agreement between theory and experiment is satisfactory.

Collisional ionization and destruction cross sections for hydrogen and helium Rydberg atoms on Ar, Xe, and N2: Failure of the independent-particle model.

Absolute cross sections for the collisional ionization and the total destruction of hydrogen and helium atoms in low-lying Rydberg states colliding with Ar, Xe, and ${\mathrm{N}}_{2}$ have been measured over the energy range from 1.0 to 30 keV/amu. The independent-particle model fails to explain the behavior of the Rydberg atom's outer electron in ionization collisions below 3 keV/amu for He (12\ensuremath{\le}n\ensuremath{\le}15) on Xe and ${\mathrm{N}}_{2}$, but not on Ar. This represents an observation of the model's failure in collisions of Rydberg atoms with neutral perturbers in this energy range and shows that at low energies, where a molecular picture of the core-target scattering is valid, the Rydberg electron is sensitive to details of the molecular interactions between the target and the Rydberg atom's core.

The equation of state for stellar envelopes. II - Algorithm and selected results

A free-energy-minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed. The adopted free energy includes terms representing the translational free energy of atoms, ions, and molecules; the internal free energy of particles with excited states; the free energy of a partially degenerate electron gas; and the configurational free energy from shielded Coulomb interactions among charged particles. Internal partition functions are truncated using an occupation probability formalism that accounts for perturbations of bound states by both neutral and charged perturbers. The entire theory is analytical and differentiable to all orders, so it is possible to write explicit analytical formulas for all derivatives required in a Newton-Raphson iteration; these are presented to facilitate future work. Some representative results for both Saha and free-energy-minimization equilibria are presented for a hydrogen-helium plasma with N(He)/N(H) = 0.10. These illustrate nicely the phenomena of pressure dissociation and ionization, and also demonstrate vividly the importance of choosing a reliable cutoff procedure for internal partition functions.

Erratum: Interaction between a Rydberg atom and neutral perturbers

Erratum: Interaction between a Rydberg atom and neutral perturbers

Interaction between a Rydberg atom and neutral perturbers.

A simple combination of quantum defect theory with a nonperturbative Fermi-type analysis is shown to reliably determine a class of molecular Born-Oppenheimer potential curves. Applications to alkali-metal-atom--rare-gas-atom dimers verify that their potential curves are determined to semiquantitative accuracy using two pieces of information: the alkali-atom quantum defects and the electron--rare-gas scattering phase shifts.

Master-equation approach to collisionally induced absorption and emission.

We study the collisionally induced excitation and photon emission of a neutral radiator atom surrounded by a gas of neutral foreign perturbers within the binary-collision approximation. We derive an equation of motion for the reduced density matrix of the radiator and obtain the spectrum of the spontaneously emitted photons including components due to collisionally induced radiation and to Rayleigh scattering, which is valid in both impact and quasistatic regimes. In our formulation single and subsequent collisional contributions are taken into account, and various coherent and incoherent corrections are examined.

Collisional Inversion of the Populations of Λ-Doublets in CH and OH: a Critical Study

Astronomical studies of the CH and OH masers have been based on the hypotheses of both radiative and collisional pumping. The mechanism of collisional inversion of the ground state Λ-doublets by neutral perturbers was first described by Bertojo et al. (1976) who made a semi-quantitative analysis of the collision process, based on calculations of potential energy curves. More recent work (Kaplan and Shapiro, 1979; Dixon and Field, 1979; Flower, 1980) has, however, been critical of some of the assumptions made by Bertojo et al. Consideration of the symmetry and the form of the interaction of CH and OH with the spherically symmetric perturbers para-H2 and He shows that current knowledge of the interaction potential is insufficient for quantitative results to be obtained.

Quasi-molecular treatment of pressure broadening of spectral lines by neutral perturbers

Quasi-molecular treatment of pressure broadening of spectral lines by neutral perturbers

Quantal studies of alkali-metal-rare-gas collisions. III. Broadening of Na D lines by He

For pt.II see ibid., vol.8, no.14, p.2393 (1975). Fully quantal calculations of the width and shift of the Na D double due to collision with He(1S) in the temperature range 400K<or=T<or=3500K are presented. This is the first time that Baranger's fully quantal impact theory (1962) has been applied to the broadening of spectral lines by neutral perturbers. The complex cross section is opacity analysed and the complex opacity functions are studied as a function of l. The statistical approximation can be used at low l and can reproduce the total width with the cut-off impact parameter decreasing with increasing temperature. Inelastic processes contribute significantly to the width. The shift is an order of magnitude smaller than the width.

Line broadening calculations for some infrared solar Fraunhofer lines

Theoretically calculated damping constants for the solar atmosphere at different optical depths, are compared with empirical values for three infrared multiplets of C i, O i and Si i resp. A test-calculation was also performed for the NaD-doublet broadened by a gas of He particles, for which many theoretical and experimental values are known. In the case of neutral perturbers (H and He) we assumed that the interatomic potential was not a Van der Waals potential, but that a Smirnov-Roueff potential was the main cause of the line broadening. The latter description produces an enchancement of (γ H + γ He) with somewhat more than 20% in comparison with the pure Van der Waals assumption. Colliding electrons turn out to give a significant contribution γ e to the line broadening, especially towards the center of the solar disk (increasing values of the optical depth τ0). Using the Harvard Smithsonian Reference Atmosphere, assuming that the line broadening was caused almost solely by Van der Waals interaction and adopting abundance values A of Muller (1966), De Jager and Neven (1970) stated that the empirical values of Aγ were systematically larger than the theoretical ones, with factors ranging from 1.4 to 4.3. Using the same values for the abundance and the same model atmosphere we established the same discrepancy, but now with a factor ranging only from 0.8 (theoretical value somewhat higher than the empirical one) to 3.1. The discrepancy in the case of the C i multiplet (10700 A) is almost completely removed. The calculations show further a decreament of γ with decreasing optical depth.