Probing dissociative molecular dications by mapping vibrational wave functions

We present high-resolution photoelectron–Auger-electron coincidence spectra of methane (${\mathrm{CH}}_4$). Since the vibrational structure in the photoelectron spectrum is resolved, the Auger spectra corresponding to different vibrational levels can be separated. The seven final states of ${\mathrm{CH}}_4^{2+}$ are either dissociative or metastable, but in any case are populated in a repulsive part of their potential-energy curve via the Auger decay. The Auger line shapes can therefore be obtained by mapping the vibrational wave functions of the core-hole state into energy space. We have implemented this connection in the data analysis. By simultaneously fitting the different Auger spectra, detailed information on the energies of the dicationic states and their repulsive potential-energy curves is derived.

Figure 1

The C $1s$ photoelectron–Auger-electron coincidence map of ${\mathrm{CH}}_4$ excited by a photon energy of $h\nu =304$ eV. The upper panel shows the photoelectron spectrum and the right panel the corresponding Auger spectrum. See text for details.

Figure 2

The C $1s^{-1}$ Auger spectra with $v=0$ (upper panel) and $v=1$ (lower panel) as initial states. The solid lines through the data points represent the fit result. The colored subspectra indicate the contributions of the individual Auger transitions. The assignment is based on a comparison with the calculations of Ortenburger and Bagus [35] and Kvalheim [36].

Figure 3

Schematic picture of the present model. The upper panel (a) indicates a harmonic oscillator representing the potential-energy curve of the bound core ionized state and $|{\chi }_{\text{vib}}(Q){|}^2$ for the two lowest vibrational substates. The lower left panel (b) shows the potential-energy curve of a dissociative dicationic final state and the right panel (c) the Auger intensities obtained within the Condon reflection approximation.

Figure 4

Final-state energies for the ${\mathrm{CH}}_4^{2+}$ dicationic states according to different authors. Panel (a) shows the four states below the most intense Auger line, panel (b) shows the remaining states on a different energy scale. See the description of Table for details. Relative energies (the three rightmost data sets) have been aligned to our value for the $2a_1^{-1}1t_2^{-1}({}^1{T}_2)$ state. Error bars are explained in the caption of Table .