Hydrogen molecule third-order energy

In Cl plasma, all the ions are liable to associate with polar molecules to form adducts, a kind of gas-phase solvation. The process is favoured by the possible formation of hydrogen bonds. For the adduct to be stable, the excess energy must be eliminated, a process which requires a collision with a third partner. The reaction rate equation observed in the formation of these adducts is indeed third order. Ions resulting from the association of a reagent gas molecule G with a protonated molecular ion MH+ or with a fragment ion F+, of aprotonated molecular ion MH+ with a neutral molecule, and so on, are often found in Cl spectra. Every ion in the plasma may become associated with either a sample molecule or a reagent gas molecule. Some of these ions are useful in the confirmation of the molecular mass, such as... 

As the product molecule AB becomes more complex, the value of k, decreases because the combination energy is distributed among more and more vibrational modes. The concentration of the third body, [M], is usually related directly to the pressure since in the atmosphere M is the sum of N2 and 02. The concentration of M at which the reaction rate behavior changes from third-order to second-order is lower the more complex the product molecule. Combination of two hydrogen atoms to form H2 is third-order all the way up to 104atm. On the other hand, addition of the OH radical to the alkene, 1-butene, C4H8, is second-order at all tropospheric pressures. 

The first RE is of limited interest, as many molecules cannot absorb an energy of the order of 54 eV in an ionization process. This is probably the case for all hydrocarbons, as they are built up of carbon atomic orbitals with orbital energies of the order of 300, 24, and 13 eV, and hydrogen atomic orbitals at 13 eV. As the third RE is too low for ionization of most molecules, the electron will enter the 2s or 2p orbital of He if partial charge exchange takes place when He meets a molecule. The effective RE will then be somewhat lower than 13.6 eV. 

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