Auger Electron Emission from Fixed-in-Space CO

Abstract

We have measured the angular distribution of Carbon K-Auger electrons from fixed in space, core-ionized, CO molecules in coincidence with the kinetic energy release of the C + and O + fragments . We find a very narrow ejection of Auger electrons in the direction of the oxygen and an oscillatory diffraction pattern. Even for similar electron energies, the angular distribution strongly depends on the symmetry of the final state. Our results do not support an earlier study (Guillemin et al [1]) which claimed observation of a breakdown of the two-step model of Auger and photoelectron emission.The study of Auger decay from molecules still pursues many open questions. One of the challenges results from the number and complexity of the final states. The many very broad overlapping structures in the Auger energy distribution often do not allow for a clear assignment of the decay channel [2]. A second challenge results from the interaction of the Auger electron with the molecular potential. Similar to photoelectrons [3,4] the Auger electron will be multiply scattered in the molecule hence modifying Auger rates and angular distributions. A third challenge was posed recently by the claim [1] that even off-resonance the creation of a core hole by photo-ionization and its subsequent Auger decay cannot be treated as two independent steps (two-step model), as has been commonly assumed [3,5].In the present letter we address these three challenges by reporting an experiment on the Auger decay of carbon K-shell ionized CO + . We have measured the Auger electron energy and angle in coincidence with the energy and angle of both fragment ions of the CO 2+ . Such complete monitoring of the process results in a qualitatively new level of insight into the molecular Auger decay. First the high resolution in electron energy and kinetic energy release (KER) allows determination of the final electronic states of the C + and O + fragments which in turn helps to identify the molecular decay channel. Second and more importantly, the measurement of the direction of fragmentation often, a posteriori, determines the molecular axis at the instant of Auger emission. We therefore obtain Auger electron angular distributions in * Electronic address: doerner@hsb.uni-frankfurt.de the molecular frame. These have, as we show below, a very rich structure. It has been emphasized from the theory side that the angular distributions from fixed in space molecules are a key to deeper understanding of the molecular Auger process [3,7,8]. Zähringer et al. have shown that the Auger electron angular distribution can be understood as resulting from two processes acting together. The symmetry of the molecular states involved and their nonspherical electron density lead to a coarse structure. On top of this a diffraction pattern from the interaction of the Auger electron wave with the molecular potential has been seen in the calculations. None of these effects have been observed experimentally until now [1,9].The experiment was performed at Bl 4.0 [10] of the Advanc...

Auger decay and subsequent fragmentation pathways of ethylene followingK-shell ionization

“…However, the higher angular momentum found here in the fits with spherical harmonics of the MFAADs can be reproduced by the coherent sum of p waves (|B 1 sin φ| + |B 2 cos φ|) 2 emitted perpendicular to and along the molecular orientation (not shown here) and hence suggests that the observed MFAADs are mainly caused by multiple scattering of the outgoing Auger electron wave in the molecular potential. This seems plausible since contributions from higher angular momentum in the scattering process of the Auger electron were found in small molecules before (such as N 2 and CO [4,47,48,56]). …”

“…The spectrometer is similar to the one used in Refs. [47,51]. The retarding field, however, leads to the rejection of low-energy electrons including the photoelectrons from reaching the detector.…”

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“…Above PCI, most discussions so far were using the two-step model to address angular correlations between both outgoing electrons. In that sense the two-step model was found to be valid [42], with LVI [3] proving to be the main violating channel.…”

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