Collisional energy dependence of molecular ionization by metastable rare gas atoms

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Abstract
The collisional energy dependence of several molecular total ionization cross sections by metastable rare gas atoms was studied over the thermal energy region using the crossed molecular beam time‐of‐flight method. Results are reported for the collision systems He, Ne, and Ar ionizing the geometric isomers cis‐ and trans‐dichloroethylene and ortho‐ and para‐dichlorobenzene. The He ionization cross sections oscillate about an energy dependence of E−1/2 over the energy range 0.004–1.0 eV, and the Ar*+para‐dichlorobenzene cross section oscillates about an energy dependence of E−2/5 over the energy range 0.011–0.64 eV. The remaining systems are characterized by ’’bent’’ E−m dependences with m values of 0.56–0.70 at low energies changing to 0.07–0.29 at higher energies. Comparison with the slopes of the He* systems and the Ar*+para‐dichlorobenzene system shows that the ’’bent’’ and ’’oscillating’’ energy dependences are similar except for the form of the cross section functions at the lowest energies. No systematic differences are found between the cross section energy dependences for ionization of different geometric isomers or for ionization by the different metastable rare gas atoms.