Why does irradiating a low pressure gas result in a continuous spectrum?

[SciencePF]

Hi!
The question is
Suppose we are irradiating a sample of a low pressure gas and some of the radiation is absorbed. Some books say we will obtain a discontinuous spectrum!
But if we are irradiating a sample of a low pressure gas we will have absorption and emission at the same time and the spectrum must be continuous; statistically some atoms are absorbing and others are emitting!
Thanks in advance for your interest and help.

 

[Renge Ishyo]

A photon is emitted from an excited gas molecule in a random direction when it relaxes back to its ground state. So if you are illuminating the gas with light in a narrow particular direction and you are observing the spectrum directly opposite the direction of the illuminating light on the other side of the sample then an absorption spectrum appears, because the probability of the photon being emitted in *exactly the same direction* as that of the incident light every single time an emission takes place is low.