Rotational Spectra: Why Absorption & Not Emission Lines?

[koustav]

why pure rotational spectrum is almost always seen as absorption lines,and not as emission lines?

 

[drvrm]

why pure rotational spectrum is almost always seen as absorption lines,and not as emission lines?

there exists both spectra absorption and emission but

people focus on absorption as its easier to handle and more info can

be derived...the intensities are good ...see for details

Reference:< http://www.pci.tu-bs.de/aggericke/PC4e_osv/Spectroscopy050119/node18.html>

 

[Henryk]

why pure rotational spectrum is almost always seen as absorption lines,and not as emission lines

That's probably because people are most likely to look into the absorption lines.
However, at one point I was playing with a far-infrared laser. I didn't make much progress but I believe, these were emission lines of rotational spectra