What happens to two hydrogen atoms colliding with each other in the above mentioned condition??
During the collision a H₂ molecule is tentatively formed. However this tentative molecule is not stable. It can discard the excess energy produced by it's formation. Unless a 3rd body takes it away or the molecule spontaneously emitting a photon. This probabilities that a collision, if we can assume that the relative velocity of the hydrogen atom of 0.3 km s-1 is at 10 kelvin according to above the equation during collision is roughly estimated to be 0.3 pascal. There is size of the hydrogen atom is approximately 10-⁷ s-¹ for density of 10⁴ cm-³, and hence the probability that third body collision occurred during binary collision is as low as 3× 10-²⁰. Similarly the probability that transient molecule emissions a photon during the collision is very low. For the stabilization, IR vibration - Rotation transitions allowed by the electric quadrupole and moment play important role in the hydrogen molecule case. However the Einstein a coefficient for the transition is roughly 10-⁶−10−⁷ s−¹.The probability of radiation stabilization is also on the orderly 3× 10−¹⁹− 3× 10−²⁰. Because the molecules formed during the collision is eventually dissociated back into two hydrogen atoms. The hydrogen molecule is not formed directly the binary collision of hydrogen atoms as far as energy conservation holds during the collision. In interstellar clouds, that Hydrogen molecule is formed on dust grains from two hydrogen atoms absorbed and the dust grains acting as third body.
The all situation is different in terrestrial conditions, where the density is much higher. At atmospheric pressure the collision rate of two hydrogen atoms is a high as 10⁸s-¹ and the resultant probability of the third - body collision is as the high as 3× 10-⁵
The collision between two hydrogen atoms proceeds from step 1 to 3 but doesn't form Hydrogen molecule unless stabilized by the third body or photon emissions occurs.
A binary reaction is forming a single product such as,
A +B→ C .... (1)
In this reaction doesn't proceed efficiently, but it's should be noted, however the radiative association process that stability the product by emitted photons becomes efficient for larger molecules.... The electron recombination reaction that given a single neutral species such as
A⁺ +e → A......... (2)
Its normally slow, although it is important for electron recombination of atomic ions, thus most general type of binary reaction that are effective in interstellar conditions is:
A+ B→ C+ D...... (3)
Which is giving two or more product species in this case the redundant energy produced by the reaction can be taken away as the kinetic energy of the two products moving in opposite direction. For example :
OH+ C →CO +H....... (4)
He⁺ + CO →C⁺ + O +He..... (5) and
OH₃⁺ + e → H₂O + H........ (6)
This above the equation constraint mainly originates from the low density of interstellar clouds and which is prohibited tertiary reactions.
The another constraint on reaction in interstellar clouds and that the reaction is highly exothermic and doesn't having energy barriers and towards products. Because the gas and kinetic temperature of the interstellar clouds is as a very low as 10 - 100 kelvin. The most cases of endothermic reactions required to external energy input don't proceed. Evenly for exothermic reactions and some reactions if the barriers is sufficiently higher than the thermal energy of the reactants. Such reaction don't proceed efficiently despite their exothermicity. And importantly exception is shocked regions, and where gas kinetic temperature is temporarily increased up to few 100 kelvin.... In this case endothermic reactions as well as reaction with energy barriers can occurred to the some extent.
The laboratory experiments, various additional factors such as the third body effects (it's means three body reactions) catalytic effect on the surface and temperature effects making chemical processes much more the complex. This complex of the nature is sometimes useful for the new producing new types of molecule or acceleration reactions. However the complexity also hampers our understanding of the chemical processes in a microscopic way like a slower motion movie. Thus Chemistry in interstellar clouds in inextricably linked to the laboratory experiments and theoretical studies of the molecular dynamics. In this sense, it can be that an interstellar could is an ideal laboratory for the chemical reactions.
Nice explaination
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