Auger and double-charge-transfer spectra of polyconjugated hydrocarbons: Butadiene, hexatriene, and polyacetylene.

Abstract

The doubly ionized states of butadiene, hexatriene, and polyacetylene have been studied by use of x-ray-excited Auger spectroscopy and double-charge-transfer spectroscopy with both ${\mathrm{H}}^{+}$ and ${\mathrm{OH}}^{+}$ as projectile ions. Ab initio calculations by a Green's function method have been performed to assign the observed structures. Substantial hole mixing takes place in all parts of the normal Auger spectra. Together with the Auger spectrum of ethene, the spectra of polyconjugated hydrocarbons represent an oligomer-to-bulk sequence showing a reasonable convergence of binding energies. State interference determines the localization of the final states and the onset of intensity in the polymer spectrum. Shake-up Auger transitions can be identified at the high-kinetic-energy part of the Auger spectrum of butadiene. The first singlet doubly ionized states for butadiene are found at 25.43\ifmmode\pm\else\textpm\fi{}0.05 eV in the Auger experiment and at 25.9\ifmmode\pm\else\textpm\fi{}0.5 eV in the double-charge-transfer experiment, with ${\mathrm{H}}^{+}$ as the projectile ion. The triplet states are probed in the double-charge-transfer experiment with ${\mathrm{OH}}^{+}$ as projectile ion giving the first triplet double-ionization energies at 26.5\ifmmode\pm\else\textpm\fi{}0.5 and 24.3\ifmmode\pm\else\textpm\fi{}0.5 eV in butadiene and hexatriene, respectively. The first singlet double-ionized state of hexatriene is found at 23.3\ifmmode\pm\else\textpm\fi{}0.5 eV in the double-charge-transfer experiment with ${\mathrm{H}}^{+}$ as projectile ion and at 22.88\ifmmode\pm\else\textpm\fi{}0.05 eV in the Auger experiment.