Only certain emission lines show up in absorption spectrum

In summary: This is why the 4 to 1 transition will show up in the emission spectrum, while the others might not.In summary, Eisberg's Modern Physics discusses emission and absorption spectra in the atomic spectra chapter. Absorption spectra are measured using a source emitting a continuous spectrum and a gas cell, which absorbs certain wavelengths. The emission spectrum of an element will have corresponding lines to its absorption spectrum, but not all emission lines will show up in the absorption spectrum. The transitions from higher energy levels to lower energy levels can happen directly or through intermediary states, and the probability of each transition is determined by branching factors.
  • #1
eprparadox
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I'm reading Eisberg's Modern Physics and in it, in the atomic spectra chapter, he says:

"We have been discussing the emission spectrum of an atom. A closely related property is the absorption spectrum. This may be measured with apparatus similar to that shown in figure (5-1) except that a source emitting a continuous spectrum is used and a glass-walled cell, containing the monatomic gas to be investigated, is inserted somewhere between the source and the prism. After exposure and development, the photographic plate is found to be darkened everywhere except for a number of unexposed lines. These lines represent a set of discrete wavelength components which were missing from the otherwise continuous spectrum incident upon the prism, and which must have been absorbed by the atoms in the gas cell. It is observed that for every line in the absorption spectrum of an element there is a corresponding (same wavelength) line in its emission spectrum. However, the reverse is not true. Only certain emission lines show up in the absorption spectrum. For hydrogen gas, normally only lines corresponding to the Lyman series appear in the absorption spectrum; but ,when the gas is at very high temperatures, e.g. at the surface of a star, lines corresponding to the Balmer series are found. "

There's a few of questions that come to mind:

1. For an emission spectrum, let's say, for example, it takes 10 eV to get the gas molecule to go from the n = 1 state to the n = 4 state. If I bombard a gas molecule with 10 eV of energy, then does it make a direct transition to that n=4 state or does it (for however small amount of time) stop by the intermediary states?

2. Let's say it's now in this n = 4 state. What happens? I assume it'll try and find it's way back to the n = 1 state. But it'll go from n = 4 to n =3 and emit? And then n = 3 to n = 2 and then emit? And then to n = 1 and emit again?

3. Maybe answers to #1 and #2 above will help, but I didn't get why this had to be true: Only certain emission lines show up in the absorption spectrum.

Thanks so much
 
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  • #2
eprparadox said:
1. For an emission spectrum, let's say, for example, it takes 10 eV to get the gas molecule to go from the n = 1 state to the n = 4 state. If I bombard a gas molecule with 10 eV of energy, then does it make a direct transition to that n=4 state or does it (for however small amount of time) stop by the intermediary states?
It makes a direct transition.
eprparadox said:
2. Let's say it's now in this n = 4 state. What happens? I assume it'll try and find it's way back to the n = 1 state. But it'll go from n = 4 to n =3 and emit? And then n = 3 to n = 2 and then emit? And then to n = 1 and emit again?
That is possible, but a direct transition back is possible as well. It can also go from 4 to 2 to 1 or from 4 to 3 to 1.

Transitions from 4 to 3 will show up in the emission spectrum if the source is hot enough. To show up in the absorption spectrum you need a large amount of atoms in state 3 - something you'll rarely have in these setups.
 
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  • #3
Thanks for the response.

Why will the transition from 4 to 3 show up only if the source is hot enough. Is it because if it's not hot enough, there won't be enough states in the n = 4 energy level to begin with?
 
  • #4
eprparadox said:
Why will the transition from 4 to 3 show up only if the source is hot enough. Is it because if it's not hot enough, there won't be enough states in the n = 4 energy level to begin with?
Right. On the other hand, cold hydrogen won't emit anything anyway, while it will still absorb something.
 
  • #5
The emission lines do exist for every absorption line, but sometimes these lines are very weak and therefore don't show up in the spectrum. This is because the population of the upper state is low. You can estimate the population using Boltzmann statistics. Higher energy states will barely be populated at lower temperatures.

eprparadox said:
2. Let's say it's now in this n = 4 state. What happens? I assume it'll try and find it's way back to the n = 1 state. But it'll go from n = 4 to n =3 and emit? And then n = 3 to n = 2 and then emit? And then to n = 1 and emit again?
They will all happen with some probability. There is a branching factor for each transition. Typically, the 4 to 1 transition has higher probability than the 4 to 2 or 4 to 3 transitions, if it is allowed by selection rules.
 

Related to Only certain emission lines show up in absorption spectrum

1. Why do only certain emission lines show up in an absorption spectrum?

The absorption spectrum is created when light passes through a material and certain wavelengths are absorbed by the atoms or molecules in the material. The absorbed wavelengths correspond to the energy levels of the atoms or molecules. Only the wavelengths that match the energy levels of the atoms or molecules will be absorbed, resulting in dark lines in the spectrum. This means that only certain emission lines, or wavelengths of light that are emitted by the atoms or molecules, will show up in the absorption spectrum.

2. Can all atoms and molecules produce an absorption spectrum?

Yes, all atoms and molecules have energy levels and can produce an absorption spectrum. However, the specific wavelengths that are absorbed will vary depending on the element or molecule and its unique energy levels. This is why each element or molecule has its own unique absorption spectrum.

3. How is the absorption spectrum different from the emission spectrum?

The absorption spectrum is created when light passes through a material and certain wavelengths are absorbed, resulting in dark lines in the spectrum. The emission spectrum, on the other hand, is created when atoms or molecules emit light at specific wavelengths, resulting in bright lines in the spectrum. The two spectra are complementary and can be used to identify the elements or molecules present in a material.

4. What is the significance of the absorption spectrum in scientific research?

The absorption spectrum is a valuable tool in scientific research as it can be used to identify the elements or molecules present in a material. It can also provide information about the energy levels and structure of these atoms or molecules. This is useful in fields such as astronomy, chemistry, and biology, where the composition and properties of materials are important to study.

5. Can the absorption spectrum be used in real-world applications?

Yes, the absorption spectrum has many practical applications in various industries. For example, in astronomy, it is used to analyze the composition of stars and galaxies. In environmental science, it can be used to detect pollutants in the air or water. It is also used in medical diagnostics to identify the presence of certain substances in the body. Additionally, the absorption spectrum is used in the development of new technologies, such as solar panels and LED lights.

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