Reactions of Ions with Known Internal Energies

REACTIONS OF IONS WITH KNOWN INTERNAL ENERGIES 

The photoionization cross-section curve of the NH3 molecule near threshold is shown by the open circles of Fig. 10. The steplike character of this curve indicates predominantly direct ionization which produces the NH3 ion with successively higher vibrational excitation in the out-of-plane bending mode of the planar ion (as explained in Section 3.7.2). In absorption, Rydberg series converging to each vibrationally excited state of the ion are observed However, these Rydberg states do not au-toionize significantly, but very likely predissociate so that the steplike thresholds are clearly apparent. 

The NH3 ions produced in the chamber shown in Fig, 1(a) were allowed to undergo the exothermic reaction 

The apparent photoionization cross section for the NH4 ion was also measured. The variation of the cross section for the reaction as a function of the vibrational state of the reactant NH3 ion was determined by comparing the relative step heights of the curves for NH3 and NH4 after the ordinate scales of both curves were adjusted so that the data points of the first plateau at about 10.2 eV coincide. Then the ratio of a pair of corresponding step heights is proportional to the ratio of the reaction cross section to that of the NH3 ion in its vibrational ground state. The step heights used to determine the relative reaction cross section of NH3 with t = 5 are shown in Fig. 10. Similar data were measured for various values of the repeller voltage, and the relative 

As explained in Sections 3.2.2 and 3.7. the photoionization of the H2 molecule in the threshold region is dominated by autoionization by vibration, The H2 ions produced by the autoionization are predominantly in the highest energetically possible vibrational state. 

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