# Absorption spectra of molecular hydrogen

I've been looking everywhere for information on the absorption spectra of molecular hydrogen. I need some pretty exact numbers. If anybody can point me in the right direction it would be greatly appreciated.

SpectraCat
Thanks SpectraCat!

Just a quick one, brace yourself, this question mught be a bit retarded...
How do those tables work?
I was hoping to find something like a list of constants associated with a list of frequencies.

Here's the formula i was hoping to plug the values into:
If= Io exp(-k(f)nx)
If=incident intensity
Io=original intensity
n=atoms per unit vol
x=distance
k(f) = the stuff i've been searching everywhere for

SpectraCat
Thanks SpectraCat!

Just a quick one, brace yourself, this question mught be a bit retarded...
How do those tables work?
I was hoping to find something like a list of constants associated with a list of frequencies.

Here's the formula i was hoping to plug the values into:
If= Io exp(-k(f)nx)
If=incident intensity
Io=original intensity
n=atoms per unit vol
x=distance
k(f) = the stuff i've been searching everywhere for
Ok ..that's quite a bit more basic than what I thought you were looking for. That table gives very accurate values for the relative energies and molecular constants for all of the measured electronic states of hydrogen. Based on your first post, that's what I thought you wanted.

However, the equation you just posted is a general expression for figuring the absorption from a gas phase sample ... what you need is the cross-section for the molecular absorption at the frequency of interest, which unfortunately isn't in that table I linked .. it only provides constants to help you predict the frequencies at which the transitions will appear.

So, which transition are you actually interested in?

Thanks again SpectraCat

I'm not really sure how to answer that question, maybe if i just tell you what i'm planning on doing with the information you can help me figure it out?

Pretty much what i want to do is have a laser that works at a frequency that is well absorbed by hydrogen pointed, via a whole lot of almost pure hydrogen gas (from a pipeline), at a photodetector. So, looking at that formula, I can get readings for Io and If, I'll also know the distance and k(f). I want to have a way of getting n.

It's an exercise in electronics more than anything else (i need to make sure that there is no way in heck the associated circuit would spark or heat up too much).

I dont really know much about transitions...

SpectraCat
Thanks again SpectraCat

I'm not really sure how to answer that question, maybe if i just tell you what i'm planning on doing with the information you can help me figure it out?

Pretty much what i want to do is have a laser that works at a frequency that is well absorbed by hydrogen pointed, via a whole lot of almost pure hydrogen gas (from a pipeline), at a photodetector. So, looking at that formula, I can get readings for Io and If, I'll also know the distance and k(f). I want to have a way of getting n.

It's an exercise in electronics more than anything else (i need to make sure that there is no way in heck the associated circuit would spark or heat up too much).

I dont really know much about transitions...
What is the frequency of your laser?

I haven't decided yet, something infra-red would be best i think. I can get a tunable laser if need be

SpectraCat
I haven't decided yet, something infra-red would be best i think. I can get a tunable laser if need be
I thought you said you wanted it at a frequency at which hydrogen absorbs? That means UV .. in fact, the whole discussion we have been having is pretty much moot in terms of the experiment you describe. According to that table I linked, the lowest lying transition of H2 is in the vacuum UV at around 112 nm .. that means that even if you could get a laser operating at that frequency (expensive and tricky), you wouldn't be able to pass the beam through the air, since the air molecules themselves absorb at those frequencies.

Passing a laser through high-pressure hydrogen can generate Raman shifted lines under the right conditions, but I am not sure how to relate the intensities of those lines to the number density of hydrogen molecules.

Ah, i didn't read the table properly... I see now that the only wavelengths examined were in the ultraviolet. My mistake. Looking at available uv lasers, i can go down to 255nm without too much of a problem. Can you help me find a cross section for a wavelength atleast that?

In terms of Ramen scattering, I dont think it will really cause a problem. As far as i know it only effects about one photon in 10 million. I think that the accuracy of the measuring device will cause more of an error. I know the beam will be passing through a fair number of molecules but my path length is quite small.

I'll do some calculations to see how much scattering i should expect under ideal conditions by considering the width of the laser beam, path length, and expected temperature and pressure to find out how many atoms the beam would likely pass through. I was thinking i could just use the ideal gas equation for this, i know that the fact that there is high pressure will screw with the numbers a bit but it'll give me a good idea of what to expect.

I really appreciate your patience with this, it's been pretty difficult to find help

And i just read that the smallest IR wavelength that can be absorbed by hydrogen is 823.5nm. I'm not really sure of the maths because i've never done the calculation myself, if you want to check it out it's here:

http://www.goiit.com/posts/list/modern-physics-largest-wavelength-in-the-uv-region-of-hydrogen-921737.htm [Broken]

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SpectraCat
And i just read that the smallest IR wavelength that can be absorbed by hydrogen is 823.5nm. I'm not really sure of the maths because i've never done the calculation myself, if you want to check it out it's here:

http://www.goiit.com/posts/list/modern-physics-largest-wavelength-in-the-uv-region-of-hydrogen-921737.htm [Broken]
That has nothing to do with your question .. it is a calculation for the H-atom first of all, and second of all, it is only valid for emission (or absorption at extremely high temperatures).

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Ah, i didn't read the table properly... I see now that the only wavelengths examined were in the ultraviolet. My mistake. Looking at available uv lasers, i can go down to 255nm without too much of a problem. Can you help me find a cross section for a wavelength atleast that? If smaller is the only option I'll find a light source that will work.

In terms of Ramen scattering, I dont think it will really cause a problem. As far as i know it only effects about one photon in 10 million. I think that the accuracy of the measuring device will cause more of an error. I know the beam will be passing through a fair number of molecules but my path length is really quite small. Also i have ways of lowering the pressure if i need to, that wont be a problem.

I'll do some calculations to see how much scattering i should expect under ideal conditions by considering the width of the laser beam, path length, and expected temperature and pressure to find out how many atoms the beam would likely pass through. I was thinking i could just use the ideal gas equation for this, i know that the fact that there is high pressure will screw with the numbers a bit but it'll give me a good idea of what to expect.

I really appreciate your patience with this, it's been pretty difficult to find help