Why does a hydrogen gas tube produce a hydrogen atomic spectrum?

In summary: Hydrogen molecules do have emission bands. Such as Fulcher bands in visible and Werner bands in vacuum UV.But we do see a spectrum.
  • #1
amilton
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To measure the atomic hydrogen spectrum people often uses hydrogen gas tubes as light source.
Since the gas in the tube is the molecule ##H_2## , why we obtain the spectrum of atomic hydrogen?
My guess is that because the voltage is so high, so that the molecules are totally dissociated.
If you could give me some reference I would be grateful.
 
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  • #2
The H2 molecule has a binding energy of 4.52 eV. Visible spectra of H, the Balmer series, requires excitation to at least ##n=3##, which requires 12 eV from the ground state. The energy required for excitation is more than enough to break apart the molecule.
 
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  • #3
DrClaude said:
The H2 molecule has a binding energy of 4.52 eV. Visible spectra of H, the Balmer series, requires excitation to at least n=3, which requires 12 eV from the ground state. The energy required for excitation is more than enough to break apart the molecule.
Is there any justification based on quantum mechanics that for higher energy the process of dissociation of hydrogen molecule is more probable than that of excitation of the electron of this molecule
 
  • #4
In such a tube, you are not exciting one molecule once and looking at what happens, so the branching ratios between the different outcomes are not relevant.

The molecules end up continuous being hit by energetic electrons and quickly break apart, and you are left with atoms being hit by electrons and emitting light.
 
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  • #5
DrClaude said:
In such a tube, you are not exciting one molecule once and looking at what happens, so the branching ratios between the different outcomes are not relevant.

The molecules end up continuous being hit by energetic electrons and quickly break apart, and you are left with atoms being hit by electrons and emitting light.
How do we know that the rate of dissociation is greater that of recombing (production of molecules)?
 
  • #6
amilton said:
How do we know that the rate of dissociation is greater that of recombing

When you start out with 100% molecules there is only one direction the system can evolve in.
 
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  • #7
Vanadium 50 said:
When you start out with 100% molecules there is only one direction the system can evolve in.
Also, you keep pouring energy in the system.
 
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  • #8
if you keep pouring energy to the system we will arrive to a plasma state and so no spectrum
 
  • #9
Hydrogen molecules do have emission bands. Such as Fulcher bands in visible and Werner bands in vacuum UV.
What determines the branching ratio between Balmer lines and Fulcher bands in visible, or Lyman lines and Werner bands in UV?
 
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  • #10
amilton said:
if you keep pouring energy to the system we will arrive to a plasma state and so no spectrum

But we do see a spectrum.

More to the point, we see emission lines, so clearly energy is leaving the system through at least that one process.
 

1. Why does a hydrogen gas tube produce a hydrogen atomic spectrum?

A hydrogen gas tube produces a hydrogen atomic spectrum because when an electric current is passed through the tube, the electrons in the hydrogen atoms become excited and jump to higher energy levels. When these electrons return to their original energy levels, they release energy in the form of light. This light is then dispersed into different wavelengths, creating a unique spectrum for hydrogen.

2. What causes the different colors in a hydrogen atomic spectrum?

The different colors in a hydrogen atomic spectrum are caused by the different energy levels that the excited electrons return to. Each energy level corresponds to a specific wavelength of light, and these wavelengths are what we perceive as different colors.

3. Can other elements produce atomic spectra like hydrogen?

Yes, other elements can produce atomic spectra. Each element has its own unique set of energy levels and thus, will produce a unique spectrum. However, hydrogen is often used as a reference for other elements because its atomic spectrum is relatively simple and well-understood.

4. How is the hydrogen atomic spectrum used in scientific research?

The hydrogen atomic spectrum is used in scientific research in a variety of ways. It can be used to identify the presence of hydrogen in a substance, to determine the composition of stars and other celestial bodies, and to study the behavior of atoms and subatomic particles. It is also used in fields such as astronomy, chemistry, and physics.

5. Is the hydrogen atomic spectrum the same for all sources of hydrogen?

No, the hydrogen atomic spectrum can vary slightly depending on the source of the hydrogen. Factors such as temperature, pressure, and the presence of other elements can affect the energy levels of the hydrogen atoms and thus, the resulting spectrum. However, the overall pattern and wavelengths of the lines in the spectrum remain consistent and can be used for identification purposes.

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