Durham Atomic and Molecular Physics:
Part of the JQC Durham–Newcastle
Rovibrational Excitation of Interstellar Molecules at Low Energies
Molecular astrophysics; linking the very small with the very big!
Atomic and molecular systems play an important role in Astronomy and Astrophysics. In general:
There are currently over 120 different types of molecule which have been observed in interstellar space.
Understanding atomic and molecular processes allows us to detect, interpret and predict astrophysical observations.
Far Right: Possible Glycine detection in the Orion KL region .
Astronomical H2 Spectra
The image (near left) shows the ρ Ophiuchi region . The spectra (far left) show pure rotational transitions of H2 for ortho and para species. The transitions are in the ultra-violet part of the spectrum and are viewed via space observatories .
The astrophysical importance of H2
Molecular hydrogen is the most abundant molecule in the universe and plays a fundamental role in many astrophysical contexts:
The rovibrational scattering problem
To understand the observed spectra we need various theoretical quantities:
where R is the distance between the atom and centre of mass of the molecule, μ is the reduced mass, ν and j are the vibrational and rotational angular momentum quantum numbers of the molecule respectively, kνj is the wavenumber in the centre of mass frame relative to level νj of the molecule, l is the atom-molecule relative angular momentum number, and J = j + l is the total angular momentum of the system.
The matrix elements of the system's interaction potential are given by:
where fλ is a Percival-Seaton coefficient and,
where χ(R), and vλ(r,R) are the expansion coefficients of the interaction potential,
and r is the internuclear axis vector.
We can derive from the wavefunction solution the T matrix , which determine the cross sections, σ(νj ← ν' j') .
These cross sections are used to derive the thermally averaged collision rate coefficients.
The radiative and collision rate coefficients are used in radiative transfer models to calculate fractional population densities for the observed object. The population densities are related to ratios of observed line intensities.
The observed line ratios and calculated population densities can be used to determine/constrain physical parameters of the observed object (e.g. temperature, density, velocity fields).
The H2 + H system: a not so simple problem.
H2 may collide with other atomic and molecular species within the medium.
Some species are more abundant than others and hence are more commonly observed.
H2 + H is an important collision system, and being the simplest (in terms of its constituents) one would assume it is well understood and modeled. This may not be the case.
We need the following to proceed with calculations:
What we are doing in Durham
At present we are in the process of determining:
We are also interested in:
|Department of Physics, Durham University||Tel +44 (0)191 33 43520|
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