Theoretical simulation of molecular Auger spectra: The carbon and oxygen KLL Auger spectra of formaldehyde

A theoretical ab initio simulation of the carbon and oxygen KLL Auger spectra of formaldehyde is presented and discussed. The effects of nuclear vibrational motion on the energy position and broadening of the Auger peaks in the two very different spectra are explicitly accounted for using a method derived from the time-dependent theory of the nuclear dynamics of decaying states. The underlying vertical double ionization spectrum, comprising hundreds of relevant electronic states, is computed using Green’s function methods, while charge distribution effects and Auger intensities are estimated via a two-hole population analysis of the eigenstates. The resulting theoretical spectra reproduce accurately the experimental band shapes and positions, showing that the observed spectra are complex convolutions of a very large number of transitions, with strong correlation and intensity redistribution effects. The nuclear motion analysis is found to be particularly important for the correct reproduction of the spectra, accounting very well for the substantial differences in energy shifts and band widths in the two spectra.