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.

 

[astrokreng]

I would like to clarify that the statement about obtaining a discontinuous spectrum from irradiating a low pressure gas is not entirely accurate. In fact, irradiating a low pressure gas will result in a continuous spectrum due to the nature of the process.

When a low pressure gas is irradiated, the atoms in the gas will absorb certain wavelengths of radiation based on their energy levels. This absorption will result in the emission of photons at different wavelengths, which will create a spectrum. However, since the gas is at a low pressure, there are not enough atoms to absorb all the possible wavelengths of radiation. This means that there will be gaps or "missing" wavelengths in the spectrum, giving the appearance of a discontinuous spectrum.

However, if we were to increase the pressure of the gas, more atoms would be present and able to absorb a wider range of wavelengths. This would result in a more continuous spectrum, as there would be fewer gaps in the wavelengths absorbed and emitted.

In short, the reason for a continuous spectrum from irradiating a low pressure gas is due to the limited number of atoms available to absorb radiation, resulting in gaps in the spectrum. This is a statistical effect and does not necessarily indicate a true discontinuous spectrum.