Triple ionisation of methane by double Auger and related pathways

Abundance of molecular triple ionization by double Auger decay

Systematic measurements of electron emission following formation of single 1s or 2p core holes in molecules with C, O, F, Si, S and Cl atoms show that overall triple ionization can make up as much as 20% of the decay. The proportion of triple ionization is observed to follow a linear trend correlated to the number of available valence electrons on the atom bearing the initial core hole and on closest neighbouring atoms, where the interatomic distance is assumed to play a large role. The amounts of triple ionization (double Auger decay) after 1s or 2p core hole formation follow the same linear trend, which indicates that the hole identity is not a crucial determining factor in the number of electrons emitted. The observed linear trend for the percentage of double Auger decay follows a predictive line equation of the form DA = 0.415 · N_ve + 5.46.

Relative extent of double and single Auger decay in molecules containing C, N and O atoms

We show that the proportion of double Auger decay following creation of single 1s core holes in molecules containing C, N and O atoms is greater than usually assumed, amounting to about 10% of single Auger decay in many cases. It varies from molecule to molecule, where the size of the molecule has a positive correlation to the amount of double Auger decay. In neon, examined as a related benchmark, the proportion of double Auger decay is similar to that in methane, and is in the order of 5%.

Field ionization of high-Rydberg fragments produced after inner-shell photoexcitation and photoionization of the methane molecule

We have studied the production of neutral high-Rydberg (HR) fragments from the CH4 molecule at the C 1s → 3p excitation and at the C 1s ionization threshold. Neutral fragments in HR states were ionized using a pulsed electric field and the resulting ions were mass-analyzed using an ion time-of-flight spectrometer. The atomic fragments C(HR) and H(HR) dominated the spectra, but molecular fragments CH(x)(HR), x = 1-3, and H2(HR) were also observed. The production of HR fragments is attributed to dissociation of CH4(+) and CH4(2+) ions in HR states. Just above the C 1s ionization threshold, such molecular ionic states are created when the C 1s photoelectron is recaptured after single or double Auger decay. Similar HR states may be reached directly following resonant Auger decay at the C 1s → 3p resonance. The energies and geometries of the parent and fragment ions have been calculated in order to gain insight into relevant dissociation pathways.

Multi-electron spectroscopy: Auger decays of the argon 2s hole

Auger decay of an inner shell hole is an efficient way to create multiply charged ions in the gas phase. We illustrate this with the example of the argon 2s decay, and show that multi-electron coincidence spectroscopy between the 2s photoelectron and all released Auger electrons leads to a complete reconstruction of the Ar 2s decay cascade. Spectra of the intermediate and final Ar(n+) states are obtained and are compared with a theoretical model.

Double core hole creation and subsequent Auger decay in NH3 and CH4 molecules

Energies of the hollow molecules CH(4)(2+) and NH(3)(2+) with double vacancies in the 1s shells have been measured using an efficient coincidence technique combined with synchrotron radiation. The energies of these states have been determined accurately by high level electronic structure calculations and can be well understood on the basis of a simple theoretical model. Their major decay pathway, successive Auger emissions, leads first to a new form of triply charged ion with a core hole and two valence vacancies; experimental evidence for such a state is presented with its theoretical interpretation. Preedge 2-hole-1-particle (2h-1p) states at energies below the double core-hole states are located in the same experiments and their decay pathways are also identified.

Spectra of the triply charged ion CS(2)(3+) and selectivity in molecular Auger effects

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.