Molecular Hydrogen Ionization

[Misha Kuznetsov]

The ionization energy of H2 is 1488 kJ/mol. How strong of a DC electrostatic field would be need to ionize H2 molecules all at a distance 'd' from the source of the field? What I am asking is how to convert the known unit (joule) into a measure of an electrostatic field since electric fields are expressed in Newtons per coulomb or volts per meter.

Would this be easier to do if the ionization energy was in eV rather than in joules? (It is approximately 15.4 eV)

I need to know how to do this for a personal project, not for homework.

Thank you.

 

[DrClaude]

You can't convert the electric field to an energy. You have to calculate the probability of ionization for the molecule placed in a given electric field. See tunnel ionization.

As far as I know, there is no way you can create in practice a DC field strong enough to actually ionize H₂.

 

[Misha Kuznetsov]

In that case, what would be a practical way to do it? I read something about using microwaves, could that work to do it?

 

[blue_leaf77]

If what you read seems trustworthy, that is it presents experimental data, then it worths a try. Just one thing for sure, you will need a system that is able to produce very strong microwave radiation because the ionization energy you mentioned above cannot be overcome by photon energy of microwave field.

 

[Misha Kuznetsov]

Can anyone recommend a way to ionize very small volumes of hydrogen gas that is both effective, and more importantly, not too energy consuming? By small volume, I mean no more than a few hundred molecules at STP. In the case of multiphoton ionization, does each photon have to have lots of energy?

 

[blue_leaf77]

In the case of multiphoton ionization, does each photon have to have lots of energy?

On the contrary, multiphoton ionization takes place when photon energy ($\hbar \omega$) is less than ionization energy. Therefore it takes multiple of photons for them to add up their individual energy so that the sum can surpass the ionization energy. However the ionization rate of multiphoton ionization strongly depends on the light's intensity, that's why when using laser, short pulse and tight focusing are required.

One photon ionization is another way to ionize thing as long as one photon energy is sufficient to exceed the ionization potential, in the case of 15.4 eV potential you need to provide radiation within XUV range. However working in UV region down to shorter wavelengths requires evacuating the system (such as using vacuum chamber) because air molecules heavly absorbs photons with short wavelengths.

 

[Quantum Defect]

Can anyone recommend a way to ionize very small volumes of hydrogen gas that is both effective, and more importantly, not too energy consuming? By small volume, I mean no more than a few hundred molecules at STP. In the case of multiphoton ionization, does each photon have to have lots of energy?

In the olden olden days, people studied H2+ molecular ion. Looking at Herzberg's Diatomics yields: H. Beutler and H. O. Juenger, Z. Physik 101, 304 (1936); O. W. Richardson, Proc. Roy. Soc. London 152, 503 (1935); O. W. Richardson, Nuovo cimento, 15, 232 (1938).

A very nice review article on the spectroscopy of molecular ions has references for experimental work on H2+ as well as HD+. The original papers will describe how they produced the ions -- generally an electric discharge is used to produce the ions (electron impact ionization). It is very difficult to make ca. 15eV photons, but not so hard to make 15eV electrons with which to ionize the H2.

http://www.cchem.berkeley.edu/rjsgrp/publications/papers/1980-1983/13_saykally_1981.pdf