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

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SUMMARY

When irradiating a low pressure gas, both absorption and emission occur simultaneously, leading to a continuous spectrum rather than a discontinuous one. The discussion highlights that while some atoms absorb photons, others emit them, resulting in a mix of emitted light. Observing the spectrum from a direction opposite to the incident light reveals an absorption spectrum due to the low probability of emitted photons traveling in the same direction as the incident light. This phenomenon clarifies the misunderstanding regarding the spectral output of low pressure gases under irradiation.

PREREQUISITES
  • Understanding of photon emission and absorption processes
  • Familiarity with low pressure gas behavior
  • Knowledge of spectral analysis techniques
  • Basic principles of light-matter interaction
NEXT STEPS
  • Research the principles of gas spectroscopy
  • Explore the concept of emission and absorption spectra in detail
  • Learn about the statistical mechanics of photon interactions with gases
  • Investigate the effects of pressure on gas spectral output
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Physicists, chemists, and students studying spectroscopy, as well as anyone interested in the behavior of gases under irradiation.

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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.
 
Physics news on Phys.org
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.
 

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