Theoretical Atomic and Molecular Physics
for Astrophysical and Laboratory Plasmas
The analysis of a diverse amount of astrophysical phenomena is hindered
by the incompleteness or limited accuracy of current atomic and
molecular (AM) data. As new, more powerful astronomical satellites
are deployed, the need for AM data will only be enhanced.
AM data are also crucial to the modeling of many types of laboratory
plasmas including tokamak fusion devices.
It is the aim of my
work to provide new or improved AM data for important species and
key processes through fully quantum-mechanical computations. In addition,
the results will be used in many cases to model
planetary nebulae, supernova remnants,
the early universe, and pre-galactic clouds.
Charge Transfer in Gaseous Nebulae and Fusion Plasmas
Charge transfer of H, He, and H_2 with singly and multiply charged ions is an important
process in planetary nebulae, supernova remnants,
interstellar medium, and laboratory plasmas.
In particular, charge transfer has been
shown to be a crucial reaction in establishing the ionization
balance and modeling of the emergent line spectra of late-time supernovae.
Models of the ejecta indicate that metals are highly ionized due to radioactive
decay of 56Co, but charge transfer with H tends to keep them singly
ionized. Charge transfer is also important in chemical models which attempt
to reproduce observed molecular abundances.
Fusion plasma impurity diagnostics which use neutral beam injection
techniques require state-dependent charge transfer cross sections to
extract impurity densities. Charge transfer and charge transfer isotope
effects may also be important in the edge region of tokamak devices.
Only semiclassical Landau-Zener calculations,
which are of limited quality, are available for the majority of collision
systems of interest. In collaboration with
B. Zygelman (Univ.
Nevada Las Vegas), N. Clarke, and
D. L. Cooper (Univ. Liverpool),
fully quantal, molecular-orbital, close-coupled calculations
of the charge transfer systems N^4+/H,
Si^4+/He, Si^3+/He, and Si^2+/H have been performed.
Manuscripts are currently in preparation.
It is proposed to extended the calculations
to many other systems such as Mg^2+/H.
Often for singly or doubly charged systems at low temperatures,
direct charge transfer
may be slow so that radiative charge transfer dominates. We have
just finished a study of Li/H+ collisions and plan to study other
systems in the future.
We are just beginning to
investigate electron capture during multiply-charged-ion/molecule collisions
and with K. Kirby (Harvard-Smithsonian Center for Astrophysics) we will
study N^2+/H_2 which was recently measured in the laboratory at UNLV.
Modeling of
various astronomical environments in collaboration with
S. Lepp (UNLV)
will be performed to investigate
the effects of the new rate coefficients.
Molecular Formation and Destruction in the Early Universe
Cross sections for radiative association
and photodissociation of LiH and LiH+ have been computed.
The calculations were
used to model the expansion of the early universe and
abundances of these molecules during the postrecombination
epoch.
A similar
set of computations for HD, HD+, and H_2D+ are being performed with
the assistance of A. Dalgarno
and S. Lepp.
We are also investigating the
effect of the cosmic background radiation field on molecule
formation through stimulated
radiative association.
Formation and Photo-destruction of Diatomic Molecules
in the Interstellar Medium and Cool Stellar Atmospheres
With K. Kirby, quantal
calculations of photodissociation of SiH+ and
its formation by radiative association are being investigated. The results
will be useful for modeling the silicon chemistry of interstellar clouds.
Similar calculations for other molecules such as SiO, SiO+, SiH, MgH,
etc. will be performed. Photodissociation of neutral hydrides may
provide an opacity source for missing flux in the blue region of cool
giant stars.
Fine-Structure and Hyperfine-Structure Changing Atom-Atom Collisions
With B. Zygelman, we are studying
collision-induced fine-structure transitions in
N-O collisions and
hyperfine-structure transitions in H-H (also with A.
Dalgarno, Harvard-Smithsonian CfA, and
M. Jamieson, Univ. Glasgow) and
alkali-alkali collisions. The former may be important for atmospheric
modeling while the latter are of interest to studies of cyrogenic
masers and Bose-Einstein
Condensation in dilute atomic gases.
Atoms in Magnetic White Dwarf Atmospheres
Progress in the analysis of H-deficient magnetic white dwarfs (MWD)
is hindered by insufficient AM data for many-electron
systems in strong magnetic fields. A computational investigation of
H-, He, C and possibly other atomic systems for B <
10^5 MG is needed. Variational calculations incorporating a mixed
Slater-Landau basis set can be used to determine energy levels and
oscillator strengths. Photodetachment and photoionization in strong
magnetic fields can also be investigated.
Applications to the identification of mysterious lines,
continuum and line flux modeling,
and polarization spectra modeling
of helium-rich MWDs will be pursued.
Still under construction ...