Millimeter Spectroscopy
Molecules in gas clouds collide with each other and tumble, at
various speeds and in various directions. The energy and orientation of a molecule's
tumbling motion is described as a rotational state and these are quantized.
A molecule can drop spontaneously from its current rotational energy state to
the one immediately below, radiating away the excess energy as a photon. Such
rotational transitions of most molecules lie in the radio, mm and sub-mm range.
Density and temperature of the emitting gas determine how the
molecules are distributied among the various rotational states. Higher density
means there are more number of particles within a volume leading to more frequent
collisions. Higher temperature means faster motion of a particle and therefore
larger volume sampled in unit time and consequently more frequent collisions!
This way the particle distribution among the various rotational states is determined
by the combination of temperature and density.
The intensities of the spectral lines measure the number of molecules
in each rotational state. By seeing how the molecules are distributed among
the rotational energy states, the temperature and density of the gas can be
deduced. If the densities are high enough, so that mean collisional interval
is much shorter than the spontaneous decay time, then the distribution is primarily
decided by the collisional process. Then, the ladder population will be according
to the Boltzmannian distribution appropriate to the kinetic temperature of the
gas.
The sum of all the line intensities, which can often be deduced
from observations of just a few, gives the total number of molecules along the
line of sight. The Doppler shift and the line-shapes yield information about
the gas motions. Thus, using spectral lines, one can know about the density,
temperature, relative-velocity and internal motions of the emitting gas and
the spatial distribution of these quantities! That's a lot, isn't it?
-- Ramesh Balasubramanyam
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