Double photoionization of methane.

Abstract

Double photoionization (DPI) of ${\mathrm{CH}}_{4}$ has been studied in the photon-energy range 35--52 eV by the photoion-photoion coincidence (PIPICO) method. Throughout this energy range the measured DPI cross section is found to be much smaller than that of the corresponding process in the isoelectronic neon atom. Unlike the case of rare-gas atoms, where the ratio of the double- to single-photoionization cross section attains a constant value at higher energies, this ratio for methane passes through a maximum at about 47 eV. Comparison of experimental with calculated energies of ${\mathrm{CH}}_{4}$${\mathrm{}}^{2+}$ electronic states gives evidence that both triplet ${(}^{3}$${\mathrm{T}}_{1}$) and singlet ${(}^{1}$E) states are populated by ejection of two electrons from the 1${t}_{2}$ orbital of ${\mathrm{CH}}_{4}$. In agreement with the calculations of Siegbahn, our results show that the Franck-Condon factors for excitation of the $^{3}\mathrm{T}_{1}$ state are spread over a wide energy range (\ensuremath{\simeq}2 eV). ${\mathrm{CH}}_{4}$${\mathrm{}}^{2+}$ in its $^{3}\mathrm{T}_{1}$ state rapidly dissociates (via an indirect process) into ${\mathrm{CH}}_{3}$${\mathrm{}}^{+}$(X\ifmmode \tilde{}\else \~{}\fi{}${\mathrm{}}^{1}$${\mathrm{A}}_{1}^{\mathcal{'}}$)+H $^{+}$(${\mathrm{}}^{1}$S), which is the most favorable dissociation pathway, both energetically and dynamically. At higher excitation energies, ${\mathrm{CH}}_{4}$${\mathrm{}}^{2+}$ in its $^{1}\mathrm{E}$ state dissociates into the three products ${\mathrm{CH}}_{2}$${\mathrm{}}^{+}$(X\ifmmode \tilde{}\else \~{}\fi{} $^{2}\mathrm{A}_{1}$)+${\mathrm{H}}^{+}$(${\mathrm{}}^{1}$S)+H (${}^{2}$S) with a 5-eV kinetic-energy release. Comparison of experimental PIPICO curves with simulated curves derived on the basis of various fragmentation models leads us to conclude that the ionic fragments ${\mathrm{CH}}_{2}$${\mathrm{}}^{+}$ and ${\mathrm{H}}^{+}$ are ejected at 180\ifmmode^\circ\else\textdegree\fi{} from each other and that they take almost all the available kinetic energy. This would indicate that the Coulomb repulsion between positive charges plays a dominant role in the process of dissociation into these three fragments.