Vibrationally resolved decay spectra of CO at the C and O K-edges: experiment and theory

Abstract

The resonant Auger spectrum of CO has been measured at both the C and O K-edges. At the C resonance the decay spectrum was recorded selectively at the energies of the = 0, 1 and 2 vibrational states. Vibrational fine structure was not only resolved on the participator but also on the spectator lines. For the O resonance the photon energy bandwidth was sufficiently low that different, vibrationally distinct regions of the absorption profile could be selected. Ab initio calculations and their detailed analysis are presented for both the C and O decay spectra. At the C resonance the calculation explicitly accounts for the vibrational fine structure including vibrational-lifetime interference effects. The appearance of the spectator part of the spectrum is shown to be strongly influenced by the existence of unbound states and diabatic interactions. The nodal structure of the vibrational wavefunction of the core-excited state is reflected in the decay spectrum.

Molecular-Frame Angular Distributions of ResonantCO:C(1s)Auger Electrons

“…The spectrum is in good agreement with published noncoincident spectra [32][33][34]. The three most energetic lines, ''a,'' ''b,'' and ''c,'' belong to participator Auger transitions which correspond to singly ionized final states [32][33][34][35] and are coincident with nonfragmenting parent ions [36]. The broader, partially overlapping groups at lower kinetic energies, labeled ''d'' to ''i,'' consist of several spectator transitions, where the excited electron stays in the 2 orbital and the decay leads to two holes in the valence orbitals.…”

“…Ã (h ¼ 287:4 eV) resonant Auger spectrum detected at 54.7 with respect to the polarization vector in coincidence with C þ and CO þ ions, collected in 4. The spectrum is in good agreement with published noncoincident spectra [32][33][34]. The three most energetic lines, ''a,'' ''b,'' and ''c,'' belong to participator Auger transitions which correspond to singly ionized final states [32][33][34][35] and are coincident with nonfragmenting parent ions [36].…”

“…While the transitions in the groups ''d'' to ''g'' create C þ fragment ions with low kinetic energies peaked at less than 0.2 eV, the transitions in the groups ''h'' and ''i'' lead to C þ ions with much higher kinetic energies [36]. In a previous theoretical study [37], group ''h'' was assigned to a 2 Å state with the leading configuration 4 À1 5 À1 2, while the nature of group ''i'' is still unclear [33,38,39].…”

Nonlinear resonant Auger spectroscopy in CO using an x-ray pump-control scheme

“…Specifically, we study this pump-probe scheme in the CO molecule, where the weak pump pulse is tuned at ω 1 = 287.4 eV to excite the ground electronic state I (X 1 + ) to the carbon core-excited state R (C 1s −1 π * 1 ), and the control pulse tuned at around ω 2 = 277.6 eV couples state R to the valence excited state F (1π −1 π * I 1 − ). The core-excited state R predominantly decays via an Auger process to the ground ionic state A (1π −1 2 ) with a lifetime of about 8.2 fs [30]. The role of the weak pump pulse is only to create the initial wave packet in the core-excited state, while the manipulation of the wave-packet dynamics is performed with the help of the strong control pulse.…”

Interface originated modification of electron-vibration coupling in resonant photoelectron spectroscopy

“…Instead we compare the RPES data on CuPc to studies of smaller molecules for which RPES calculations are feasible. In several studies of small molecules [14,15,28,29] (CO, N 2 , O 2 , and OCS) a quite similar hν-dependent line-shape evolution of the molecular orbital signals (usually termed participator decays in these publications) is observed. The case of CO constitutes the simplest scenario of one electronic transition in the NEXAFS spectrum and one vibration coupling to it.…”