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

• I
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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DrClaude
Mentor
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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Astronuc, Charles Link, Klystron and 2 others
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

DrClaude
Mentor
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.

amilton
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)?

Staff Emeritus
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.

Astronuc
DrClaude
Mentor
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.

amilton
if you keep pouring energy to the system we will arrive to a plasma state and so no spectrum

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?

shjacks45
Staff Emeritus
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.