hydrogen gas emission spectrum

papernuke

If a hydrogen spectrum tube is filled with H2 gas and powered on, why do online sources show the tube emitting the spectrum of atomic single H hydrogen (the spectrum defined by the Rydberg formula)? Why not the H2 spectrum?

For example, here: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Simon Bridge

Because those are the one's you can see.

When you switch the tube on, the molecules break up when the hydrogen ionizes. You get atomic spectra when protons and electrons recombine.

Molecular Hydrogen spectra:
http://kicp.uchicago.edu/~odom/molec...tabulation.pdf

How a gas discharge tube works:
http://en.wikipedia.org/wiki/Gas-discharge_lamp

sophiecentaur

This may not be a satisfactory answer that properly explains the fact that emission and absorption spectra are the same. There is no ionisation in the process of absorption with low levels of power.

Simon Bridge

Yah - you want to provide the rest?
I think it is a good question.

Why does absorption spectra show atomic H lines in H₂ gas?

The H₂ molecular spectra is not visible like the famous Balmer lines.
Presumably there is also atomic H mixed in with the gas anyway.

This is certainly something not covered in text books for basic physics courses.
(Arn't there usually fewer lines in the absorbtion spectrum?)

sophiecentaur

I'll pass .

I guess the main reason is that the 'other' half of the molecule must have only a small effect on the energy level of each of the electrons. Strange, as with covalent bonding, we regard the electrons as being 'shared'. Would we really expect a high proportion of atomic H in H gas?
Too hard for me guv - I'm an Electronic Engineer when asked questions of this depth. I tend to be a Physicist when asked Elecronics questions . . . . .

That would be reasonable because only one frequency will be absorbed per atom / molecule for each interaction - electrons would be very unlikely to be 'pumped up' in two stages to a higher level, whereas an incident electron could produce any energy level and produce two or more photons on the way down.

papernuke

I was under the impression that by the time plasma had formed, nearly all of the H-H bonds would have been broken already because of its low bond energy (only like 4.5eV).
The first excitation of atomic H takes 10.2eV (n=1 to 2)

Is that right or wrong?

sophiecentaur

I can't disagree with that but how does that account for the absorption spectrum being similar to the atomic emission spectrum?

papernuke

Wait, the absorption spectrum of what is similar to the H atomic emission spectrum?

Simon Bridge

The molecular electron bound-state energy levels are linear combinations of atomic levels.

You'd expect that the electrons in a H2 molecule would spend most of their time between the two nuclei but when you solve the schodinger equ for it, you get a state with a big hump near one or the other and the time evolution sloshes back and forth.

Of course, you also get vibrational and rotational modes to a diatomic molecule.

Back to college :) - this is Dr Sadoway(?) lecture on covalent bonding.
http://videolectures.net/mit3091f04_sadoway_lec10/
... 1st 5mins is all about the test.
... 20mins in: this is an awesome lecture! I love the way he draws many connections with other ideas.
... 30mins - H2 energy level diagram.

Of the top of my head ... possibilities:
* hydrogen gas contains a mixture of H and H2
* the absorption process measures the atomic state of the electron out of the superposition
* the observed absorbtion spectra of hydrogen gas is actually very dissimilar to the emmission spectra of the same gas in a discharge tube.
* the spectra we observe is only approximately the atomic H spectra
* something else

OP refers to discharge tube though. And it is quite reasonable to assert that the discharge emmission spectra is from atomic hydrogen recombining from electrons and protons. Presumably there are molecular lines also present. The reason text books don't normally mess about with them is because they are hard to see.

All this suggests to me:
The molecular spectra is like the atomic one - but slightly doubled.
The actual lines you see are split very slightly into two lines due to the superposition involved in becoming a molecule.
We see only a single line on our spectrograph because the machine available to students is not precise enough. The width of the line depends on the width of the slit - if the slit is wide enough to see the image with the naked eye, two lines close together will just overlap and look like one line corresponding to the atomic transition.
These are further split for the rotational and vibrational modes.

This is taking me back quite a way :)

Simon Bridge

You are OP. Since that was your impression - didnt you already know the answer to the question you posed in post #1?

Sophiecentaur and I are off on a tangent :)

Simon Bridge

Ahah: found it ...

... the H2 energy level structure.

from Farley D. R. et al Modeling of hydrogen ground state rotational and vibrational temperatures in kinetic plasmas; Journal of Quantitative Spectroscopy and Radiative Transfer Volume 112, Issue 5, March 2011, Pages 800–819

sophiecentaur

Afaik, the absorption spectrum of Hydrogen gas (Isn't that all molecular?) has lines which correspond to the emission spectrum from an H discharge tube.
I suppose the explanation based on "line broadening or splitting" is the best one.

Simon Bridge

Who else can we ask?