Ultrafast Molecular Three-Electron Auger Decay

Three-electron Auger decay is an exotic and elusive process, in which two outer-shell electrons simultaneously refill an inner-shell double vacancy with emission of a single Auger electron. Such transitions are forbidden by the many-electron selection rules, normally making their decay lifetimes orders of magnitude longer than the few-femtosecond lifetimes of normal (two-electron) Auger decay. Here we present theoretical predictions and direct experimental evidence for a few-femtosecond three-electron Auger decay of a double inner-valence-hole state in ${\mathrm{CH}}_3$F. Our analysis shows that in contrast to double core holes, double inner-valence vacancies in molecules can decay exclusively by this ultrafast three-electron Auger process, and we predict that this phenomenon occurs widely.

Figure 1

Spectra showing the summed energy of electron pairs from ${\mathrm{CH}}_3\mathrm{F}$, (a) in coincidence with C $1s^{-1}$ ionization at the photon energy $h\nu =360\mathrm{eV}$, (b) in coincidence with F $1s^{-1}$ ionization at $h\nu =770\mathrm{eV}$, (c) pairs alone at $h\nu =150\mathrm{eV}$ and (d) pairs as components of electron triples at 150 eV. The intensity scales of the different spectra have been adjusted for presentation and are not related, as the data come from different experiments. The strong onset in spectrum (d) probably represents the molecular triple ionization energy (i.e., photon energy minus kinetic energy sum of electron triples) at about 70 eV, in agreement with the calculated value in Table 1.

Figure 2

Left panel: Energy levels of ionized ${\mathrm{CH}}_3\mathrm{F}$ involved in the three-electron transition after initial creation of a single vacancy in the C $1s$ and F $1s$ orbital, respectively. The detection of the three electrons in coincidence proves the existence of the final step where two electrons fill the double hole in F $2s$ while a third is ejected. Right panel: Potential energy curves (PECs) along the C–F bond for the states relevant for the C $1s$ Auger decay route. The black arrow corresponds to vertical excitation from the ground to the core-ionized state at the equilibrium distance. The green arrow shows at which geometry the doubly inner-valence ionized state is populated assuming classical dynamics on the C $1s^{-1}$ PEC and a C $1s$ Auger lifetime ${\tau }_{\mathrm{C}}=8.7\mathrm{fs}$. Uppermost panel shows the decay width of the ${\mathrm{CH}}_3{\mathrm{F}}^{2+}(2s^{-2})$ state with the collective Auger decay lifetime $\tau$ indicated for the significant geometries.

Figure 3

Experimental triple ionization spectra acquired from the C $1s$ (upper panel) and F $1s$ (lower panel) initiated cascade (green and red error bars, respectively). The black curves show the theoretical spectra calculated at fixed equilibrium geometry. The blue spectra were obtained with the inclusion of nuclear motion in the ${\mathrm{CH}}_3{\mathrm{F}}^{2+}(2s^{-2})$ state, assuming this state is populated by the Auger decay at $R_{(\mathrm{C}\text{−}\mathrm{F})}=1.315\AA$ (C $1s$ case) and at $R_{(\mathrm{C}-\mathrm{F})}=1.4\AA$ (F $1s$ case), respectively, as detailed in the Supplemental Material [30]. The magenta spectra were calculated in the same way but with the ab initio decay width scaled down by a factor of 5. $\overline{\tau }$ is the fixed-geometry decay lifetime averaged over the classical C–F bond dynamics analogously to Eq. (2). In all theoretical spectra, only the decay channels accessible exclusively by the three-electron Auger decay were taken into account.