Spitzer Space Telescope - Archive Research Proposal #40645


Computing the Temperature Dependent Rovibrational Spectrum of Ammonia

Principal Investigator: Timothy Lee
           Institution: NASA Ames Research Center

     Technical Contact: Timothy Lee, NASA Ames Research Center

Co-Investigators: 
David Schwenke, NASA Ames Research Center                             
Xinchuan Huang, NASA Ames Research Center                             

Science Category: brown dwarfs/very low mass stars 
Dollars Approved: 71074 

Abstract:
Molecules with large amplitude motions possess significant complexity in their
rovibrational and purely rotational spectra. Because of this complexity, they
are ideal molecules to be used to characterize the physical conditions of the
celestial objects and galactic environments in which they are observed, which
was noted more than two decades ago by Ho and Townes. The simplest and best
characterized molecule with a large amplitude motion of interest in astronomy is
ammonia. The available experimental data for even this molecule, however, is not
sufficient to generate a synthetic spectrum that compares well with observations
from the Spitzer Space Telescope of one of the coolest known T dwarfs Gl570D,
and this is likely to be the case for any celestial environments above 400K.
The IRS instrument on the Spitzer Space Telescope has already recorded spectra
that cannot be well modeled and interpreted using the available experimental
data for ammonia. In response to the urgent need for better line lists,
including intensities, we propose to use the tools of theoretical spectroscopy,
combined with refinement using the available experimental data, to obtain highly
accurate line lists for ammonia and its isotopomers. Through the use of high
accuracy electronic structure calculations, and refinement of the resulting
potential energy surface (PES) with the available experimental data, Schwenke
has already constructed highly accurate lists for all isotopomers of the water
molecule. The accuracy of the resulting PES, transition energies, and
intensities has been demonstrated by later spectroscopic studies. We propose to
use a similar approach for ammonia. Our approach will be to use high-level
electronic structure calculations to construct a highly accurate, isotope
independent PES and dipole surface for the ammonia molecule. The resulting
highly accurate PESs and dipole moment surfaces will then be used to solve the
nuclear Schroedinger equation to generate the necessary line lists.