On 27-Apr-2006, (Roy Smith) wrote:

f you mean monatomic hydrogen (just a single proton and electron
unbound to anything else), such a thing cannot exist at temperatures
short of a nuclear explosion of the inside of a star. It would
instantly recombine to form H2.

Not true...
I some a lot of research on atomic hydrogen (see for instance J.J. Berkhout,
E.J. Wolters, R. van Roijen, and J.T.M. Walraven, Physical Review Letters
57, page 2387 (1986)).
First: yes, recombining atomic hydrogen to molecular hydrogen releases a
tremendous amount of energy.
Second: it IS possible for atomic hydrogen to exist at low temperatures. I
myself have worked with H at temperatures as low as 60 mK (i.e. 0.060
degrees above absolute zero) and others at MIT have even reached
temperatures in the micro-Kelvin range.

Now for the bad parts:
Atomic hydrogen really, really LIKES to recombine, so you have to use tricks
to prevent this happening. Our research group investigated several ways of
storing it in some sort of (metal) matrix, and found that this indeed can
prevent recombination. Unfortunately, it does this by binding the atomic
hydrogen even tighter than it would be bound to another hydrogen atom, so
from an energetic point of view this solves nothing.
We prevented recombination by spin-polarizing the atomic hydrogen in a
strong magnetic field (several Tesla to more than 10 Tesla) and this
suppressed recombination at low densities. The magnets to create these sort
of magnetic field are either superconducting magnets with small (order of a
few centimeters) bore, or HUGE apparatus with the electrical energy demand
of a small city. Also, higher temperatures (more than about 600 mK) mean
more thermal excitation thus destroying the polarization. Creating and
maintaining these temperatures uses a lot of equipment. Higher densities
(i.e. more than that of a rather good vacuum) also means the spin-polarized
hydrogen sticks to the walls of the container and there recombines.
Conclusion: atomic hydrogen as an energy source is not feasible in a
Terrestrial environment.