K. Maier

A comprehensive photoabsorption, photoionization, and shake-up excitation study of the C 1s cross section of benzene

The absolute photoabsorption cross section of benzene (C6H6), encompassing the C 1s−1 π*e2u resonance, the C 1s threshold, the satellite thresholds, and extending up to 800 eV, has been measured using synchrotron radiation. Measurements of the discrete absorption structure from below the C 1s ionization threshold have been performed at high resolution. In order to unambiguously assign all structure present in the photoabsorption cross section, C 1s photoelectron spectra were measured from the C 1s threshold region up to 350 eV along with satellite spectra. The C 1s−1 single-hole and the satellite cross sections have been derived in absolute units, and their angular distributions have been determined. Resonant and normal Auger spectra were taken on the main features of the photoabsorption and single-hole cross sections. From the best resolved photoelectron spectra the underlying structure in the asymmetric benzene photoelectron peak can be partly disentangled. The experimental data show that at least two v...

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

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.

The resonant Auger spectra of formic acid, acetaldehyde, acetic acid and methyl formate

Abstract We have measured the resonant Auger spectra of four simple organic molecules containing a carbonyl (CO) group, following excitation of an oxygen 1s electron to an antibonding valence orbital. The full range of final state energies is presented. The spectra are analysed for similarities arising from similar chemical structures within the molecules. An ab initio calculation of the resonant Auger spectrum of the simplest molecule, formic acid, is shown to aid in the interpretation.

On the correct identification of shape resonances in NEXAFS

Using core level photoemission data from free molecules, it is shown that the putative shape resonances in NEXAFS can be identified unequivocally only by measuring the main line, or single hole, cross-section. This has important consequences for the characterisation of adsorbed molecules using NEXAFS where the technique is used to determine orientation and even intramolecular bondlengths.

VIBRATIONAL FINE STRUCTURE ON THE CORE LEVEL PHOTOELECTRON LINES OF SMALL POLYATOMIC MOLECULES

Abstract The C 1s and O 1s photoelectron spectra of CH 4 , CD 4 , CF 4 , C 2 H 6 and CO 2 have been measured with high photon energy resolution. Vibrational fine structure on the main line (single hole state) could be resolved for all molecules except CF 4 . Even in the case of C 2 H 6 where several modes are expected to couple, two prominent progressions are visible. The fine structure on the C 1s line of CO 2 results, as expected, from the totally symmetric stretching mode, whereas the O 1s main line is dominated by the antisymmetric stretching mode ofσ u symmetry due to vibronic coupling.

Angle-resolved electronic decay of the π*, σ* and Rydberg resonances below the O K-edge in O2

Abstract Angle-resolved electronic decay spectra of the O 2 molecule have been measured at photon energies corresponding to the excitation of core electrons into the π*, σ* and Rydberg levels. Asymmetry parameters have been determined for resonant Auger transitions and compared with previous data where available. The π* spectrum has also been studied with a narrow photon bandwidth in different regions of the vibrationally unresolved absorption feature.

Partial electron yield spectrum of N2: doubly excited states at the K-shell threshold

The partial electron yield spectrum of N2 has been measured at a kinetic energy corresponding to the Auger decay of doubly excited core–hole states. The spectrum reveals previously unresolved double excitations just below and above the K-shell ionization threshold. Their excitation energies and dissociative nature agree with calculated potential energy curves of the doubly excited states.

High-resolution study of resonant decay following the O 1s → π∗ excitation(s) in CO2: evidence for an overlapping Rydberg transition

Abstract Electronic decay spectra associated with the O 1 s → 2 π u (π ∗ ) transition(s) in CO 2 have been measured at several photon energies within the width of the primary excitation. Significant differences in the shape and relative intensity of the decay features allow the high-energy shoulder evident on the π ∗ absorption band to be identified as an overlapping Rydberg transition. This method requires the availability of a photon source with a very narrow bandwidth at the O K-edge.

Vibrational and shake-up excitations in the C 1s photoionization of ethane and deuterated ethane

The absolute photoabsorption spectra of ethane (C2H6), encompassing the C 1s-1 3p excitation, the C 1s threshold and the satellite thresholds is measured. Further, photoelectron spectra are taken in the region from the C 1s threshold region up to 380 eV, and the vibrational fine structure and C 1s-1 partial cross section studied. The complex vibrational fine structure for both ethane and deuterated ethane (C2D6) is analysed using a coupled least-squares fit procedure. While we cannot exclude the presence of the C-C stretching mode (3), the experimental data show that the C 1s photoelectron spectrum is dominated by two vibrational modes, the C-H stretch 1 and the C-H deformation mode 2. Compared with their values in the neutral ground state there is a significant increase in the vibrational energies to 407±5 meV and 176±5 meV for ethane and 291±10 meV and 130±10 meV for deuterated ethane, respectively. The lifetime broadening is determined to be 98±10 meV, which is in good agreement with calculations. The investigated shake-up structure closely resembles that of methane. The lowest-lying satellite band gains intensity towards threshold which may be attributed to conjugate shake-up processes.

Constant initial state (CIS) spectroscopy in the study of the decay of core-to-bound resonances in N2

Constant initial state (CIS) photoemission spectra of the N2 1u-1 and 3g-1 states have been measured in the photon energy range corresponding to the N 1s* core-to-bound transition, where they acquire intensity from participator decay. The resolution was sufficiently high that the spectra could be recorded individually for the first four vibrational components of the 1u-1 state and for the first two vibrational components of the 3g-1 state. Differences in the spectra for these two final states are discussed in terms of their potential curves compared to that of the intermediate neutral excited state. We point out the potential usefulness of the CIS method for elucidating the details of the molecular potential.