Many thanks to all who replied to my previous question concerning the
polarizability of D2. I got the answer I needed plus plenty of other
very helpful info regarding D2 and H2.
I now have a second question: In a paper by Sheldon Green (NASA
Tech. Memorandum 83869, 1981) in which he reports calcuations of
cross sections for NH3-He collisions he states," Excitation by
collisions with cold H2 (in its lowest, J=0, rotational level) is
expected to be similar to excitation by He ... Owing to lower mass,
at a given temperature H2 molecules have a 40 percent higher velocity
than He atoms. To account for this and for the slightly larger
observed pressure broadening cross sections, it is recommended that
the (He) rates ... be increased by 50 percent for excitation by cold
H2."
My question is this: Is this a general rule, that all things being
equal (e.g. electronic structure, rotational state) the lighter
molecule /atom will give rise to larger collision cross sections? I
ask because we recently performed low temp. (20-40 K) pressure
broadening experiments of NH3 by J=0, D2 and found cross sections
which were larger than our previously measured cross sections for
J=0, H2 (D. Willey, et al, ApJ Supp. 139, 191, 2002) which seems to
be counter to the expectation expressed above.
Thanks,
-Dan
--
Dan Willey
Chairman
Department of Physics
Allegheny College
520 N. Main St.
Meadville, PA 16335
814-332-5368 or 332-5364
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