Measure ionization energy (Hydrogen/Deuterium)

[neilparker62]

Just wondering if there's any accurate method for direct measurement of the ionization energy of atomic Hydrogen and/or Deuterium ?

There's lots of measurements on transitions between various hydrogen/deuterium energy levels but not (it seems) to the limit where n = infinity.

Alternatively could one measure the difference between ionization energies of hydrogen and deuterium since you could (I think) derive the separate ionization energies from calculations based on the difference. See for example Problem 5.72 in the linked reference.

 

[TeethWhitener]

Photoelectron spectroscopy:
https://ac.els-cdn.com/000926147080...t=1519327482_764aa7202ec427c2efe418aa086a9ad0

 

[neilparker62]

Thanks for this but if this technique is applied to Hydrogen gas, the photo energy supplied will have to first break the H-H bond. So I'm not sure how this would lead to a determination of ionization energy of atomic hydrogen? Also how accurate (in general) is this technique? I am wondering if ionization energy of atomic hydrogen can be measured to the same precision as (for eg) 1s-2s (Lymann Alpha) transition. Or more recently the 2s - 4p (Balmer-Beta) transition which has apparently gone some way towards resolving the proton radius puzzle.

http://www2.mpq.mpg.de/~haensch/hydrogen/index.php/Research/H2SnP?from=H1s2sResearch.RydbergProject

In principle then we fire high energy photons separately at atomic hydrogen and deuterium and we measure the difference in KE of photoelectrons produced. Does that work and if so how accurate would it be ? (I have no idea how you first produce 'atomic' hydrogen but I presume that is something that must be done in the above mentioned transition measurements).

 

[TeethWhitener]

Thanks for this but if this technique is applied to Hydrogen gas, the photo energy supplied will have to first break the H-H bond. So I'm not sure how this would lead to a determination of ionization energy of atomic hydrogen?

No:

The atoms were produced in each case by an electrodeless 2450 MHz microwave discharge of the parent gas in a flow tube.

The molecules are homolytically split into atoms basically via dielectric breakdown of the gas under a large electric field (this is how discharge tubes typically operate).

Also how accurate (in general) is this technique?

In the paper it's accurate to 0.01eV, but I'm not sure of the ultimate limit of accuracy. In this setup there will be Doppler broadening (both from the exciting helium and the absorbing hydrogen), but one could possibly imagine using optical traps to mitigate that. I don't know what the state of the art is right now. Bound-to-bound spectroscopic transitions generally give far more accurate measurements than bound-to-free transitions.

 

[ZapperZ]

I'm a bit puzzled here. Didn't everyone do something like using a spectrometer and a diffraction grating to look at various gasses in discharge tubes? Didn't we determine at least a few of the visible transition lines in those simple, general physics lab experiments?

So why can't one just use a discharge tube of hydrogen, but use a better photodetector than the human eye to detect the UV range? Shouldn't you get the Rydberg-like states for the hydrogen atom? After all, we could see a few of the Balmer series lines even with the naked eye!

Zz.

 

[neilparker62]

Yup - Gerhard Herzberg did something like this in the 1950s. He measured Lyman Alpha (deuterium) and subtracted from the calculated Dirac energy to obtain a first measure of so-called 'ground state lamb shift'. But I don't think such a technique could get you a spectral line for n=infinity.

 

[ZapperZ]

Yup - Gerhard Herzberg did something like this in the 1950s. He measured Lyman Alpha (deuterium) and subtracted from the calculated Dirac energy to obtain a first measure of so-called 'ground state lamb shift'. But I don't think such a technique could get you a spectral line for n=infinity.

Why not? In principle, there's nothing that says that a keV electron bombarding such gasses can't ionize the gas. In fact, if you put a magnet close to such a discharge tube, you'll see changes to the gas, meaning that you actually have a plasma in there. You have a lot of recombination, sure, but should be able to detect such ionization.

Baring that, photoionization technique is a popular and well-established method.

Zz.

 

[neilparker62]

 

[neilparker62]

How will you tell the line at n=infinity ? Diffraction grating techniques reached their limits when it came to fine structure (Herzberg hoped he might eventually manage to distinguish 2s from 2p) and that's why precision measurements of transitions are now all based on laser spectroscopy which has improved resolution and hence level of accuracy by several orders of magnitude. Although I have wondered if ever precision diffraction gratings might make a comeback!

 

[ZapperZ]

How will you tell the line at n=infinity ? Diffraction grating techniques reached their limits when it came to fine structure (Herzberg hoped he might eventually manage to distinguish 2s from 2p) and that's why precision measurements of transitions are now all based on laser spectroscopy which has improved resolution and hence level of accuracy by several orders of magnitude. Although I have wondered if ever precision diffraction gratings might make a comeback!

I didn't say that it is the most accurate method to determine the ionization energy. It's why we don't use it when we want to make an actual determination, and why photoionization is still the most direct method. But the light from the discharge tube should have such a transition, and if we buy the Rydberg picture, one may also determine the ionization energy indirectly from other the other transitions made.

Zz.

 

[neilparker62]

I think that is exactly what they are trying to do with laser spectroscopy - ie measure the Rydberg constant (already the most accurately measured of all physical constants or so I am led to believe) and hence - theoretically anyway - the ionization energy of hydrogen. Along with proton radius. I was just wondering if there might be a way to measure H+ ionization energy directly and to an equal or better level of precision. And I speculated that might best be done not by absolute measurement but rather by the difference between H+ IE and D+ IE.

 

[neilparker62]

Level of accuracy would need to be somewhere near the number supplied at NIST. As far as I know this is a calculated rather than measured quantity.

 

[TeethWhitener]

Level of accuracy would need to be somewhere near the number supplied at NIST.

Yeah I seriously doubt you'll get anywhere near that precision from photoelectron spectroscopy. Some papers on ZEKE claim to get a resolution of ~0.01 cm^-1 (~1 μeV) (example), but I'm not too familiar with those results.

 

[neilparker62]

At a first attempt: D+ (IE) - H+ (IE) = 3.700086 meV (894.6769 GHz) with a correction for difference in ground state lamb shift. Any takers to verify/trash via measurement ?!

 

[neilparker62]

The 'challenge' is to measure a difference in ionization energies to about 1 neV. Is that perhaps a little past current 'state of the art' in measurement ? I was hoping that a difference (rather than absolute) measurement to this level of accuracy would be easier to achieve.

 

[ZapperZ]

The 'challenge' is to measure a difference in ionization energies to about 1 neV. Is that perhaps a little past current 'state of the art' in measurement ? I was hoping that a difference (rather than absolute) measurement to this level of accuracy would be easier to achieve.

Why do you need that kind of accuracy?

Zz.

 

[ZapperZ]

The ultimate aim is to directly measure the ionization energy of Hydrogen/Deuterium to the same level of accuracy as 'bound to bound' transitions such as 1s-2s. As mentioned in a previous post here "Bound-to-bound spectroscopic transitions generally give far more accurate measurements than bound-to-free transitions."
I would say we need H+ IE to the same level of precision - it is the most important 'transition' of all (IMHO).

And at what accuracy is the 1s-2s transition measured to?

Zz.

 

[neilparker62]

And at what accuracy is the 1s-2s transition measured to?

Zz.

Somewhere in this region: https://arxiv.org/abs/1107.3101

 

[ZapperZ]

The ultimate aim is to measure H+ ionization energy to the same level of precision as (for eg) 1s-2s if this is possible noting a previous post here:
I would say we need a precision measurement of H+ IE. It is the most important transition of all (IMHO of course!).

You are avoiding giving me actual numbers. You seem to have arbitrarily put out this neV accuracy requirement, and yet, you are not able to show that this is the same level of precision that the transition has been measured.

I did spectroscopy measurements (x-ray, photoemission, etc.). I know how difficult it was going from 100 meV resolution down to 10 meV resolution, and now meV resolution. The effort and expense isn't linear! So there has to be ample and convincing justification to want to drive the precision even more.

So, if you wish to have the same accuracy of such atomic transition, I would appreciate that you cite to me where this transition has been measured at neV level of accuracy.

Zz.

 

[neilparker62]

To my knowledge it has not been measured anywhere even remotely close to the accuracy of 1s-2s - that's the whole point. Measuring to 1neV would be just a start and we would still have a long, long way to go evidently. But from what you are saying it seems I am asking for the impossible!

The most accurate number I can find Ei (H) = 109 678.771 742 6 (10) cm−1 comes from the reference below but I think they are calculating rather than measuring.

https://research.vu.nl/ws/portalfiles/portal/2485538

 

[ZapperZ]

Somewhere in this region: https://arxiv.org/abs/1107.3101

Wait, I thought we were talking about measurement using photoionization/photoemission here?

Zz.

 

[neilparker62]

Quoting myself at the beginning of this thread:

Just wondering if there's any accurate method for direct measurement of the ionization energy of atomic Hydrogen and/or Deuterium ?.

We can use any possible method of measurement. Question is whether there is in fact a "possible method". I would guess you would need to photoionize because after all you have to apply at least enough energy to ionize the hydrogen atom. And then somehow measure precisely what that energy is. I thought a difference method might be best - so as to take advantage of "common mode rejection" if I may borrow a term from electronics.

 

[neilparker62]

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