NASA NTRS · 19870027450
The C3H2 2(20)-2(11) transition - Absorption in cold dark clouds
Abstract
The first observations of the 2(20)-2(11) transition of cyclopropenylidene (C3H2) at 21.6 GHz are described. The most significant finding is that the 2(20)-2(11) transition line is always seen in absorption, in contrast to the 18.3-GHz 1(10)-1(01) resonance line of the ortho species which always appears in emission in cold dust clouds. Thus the former must have an excitation temperature less than the brightness temperature of the universal microwave background and becomes only the second molecule to exhibit such 'refrigeration' below this temperature in cold, dark dust clouds.
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Matthews, H. E., Avery, L. W., Madden, S. C., Irvine, W. M.. 1986-08-15. The C3H2 2(20)-2(11) transition - Absorption in cold dark clouds. https://ntrs.nasa.gov/citations/19870027450
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