Absolute cross sections for ethylene-dication production by electron impact

This work reports absolute cross-section measurements for the production of ethylene dications by electron impact, with the energy ranging between 25 and 800 eV. Separation of the entangled ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$ and ${\mathrm{CH}}_2^{+}$ fragments with the same mass-to-charge ratio, in the time-of-flight spectrum, is properly carried out using the DETOF technique. This separation shows that stable ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$ can be produced either by two primary vacancies or by a single primary vacancy in an inner valence shell, with its subsequent de-excitation through Auger-like decay. Our findings show that the latter process can be an important player in dication formation, prevailing in the case of ethylene at projectile energies above 500 eV.

Figure 1

Number of collisional products with an $m/q$ ratio of 14 (${\mathrm{CH}}_2^{+}$ and ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$) for different time delays, normalized to their respective number at the minimum delay time of 300 ns. The black line (total) represents the sum of all the distributions present in order to accomplish the best possible fit, using the DETOF technique. The purple line (MB) corresponds to the Maxwell-Boltzmann distribution, which accounts for the ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$. The remaining distributions, Expo (blue line) and $G_{0.8}$ (red line), represent different energy distributions of the ${\mathrm{CH}}_2^{+}$ fragment.

Figure 2

Absolute cross sections for the production of ${\mathrm{C}}_2{\mathrm{H}}_4^{+}$ (squares), species with $m/q$ = 14, comprising both ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$ and ${\mathrm{CH}}_2^{+}$ (circles), and the disentangled ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$ (blue triangles) as a function of the electron impact energy. Filled black or blue symbols, this work; open red symbols, Tian and Vidal [38]. The $m/q$ = 14 group is presented as a whole in order to compare it with the data of Tian and Vidal, being the sum of the cross sections of the three energy distributions presented in Fig. 1, whose relative weights are a function of the electron impact energy.

Figure 3

Ratio between double- and single-ionization cross sections for the unfragmented parent molecule as a function of the electron impact energy. It can be seen that there is a peaked structure centered at 100-eV impact energy and that the ratio behavior tends to a constant value for higher impact energies, which indicates that an Auger-like single-vacancy process contributes to the ethylene-dication formation. Error bars indicate calculated uncertainties below 11% for 40 eV, 8% for 50 eV, and up to 6% for the remaining electron impact energies.