the integral electron spectrum (shaded area), together with a shoulder at the higher electron energy. This indicates that the break-up is preferentially induced by excitation of the 1s shell of C${}_{\mathrm{F}}$. The spectra at $h\nu = 299.0$ eV and $h\nu = 286.9$ eV support this finding. In the former one, the strong peak has disappeared, in the latter one, no sign of a photoelectron is visible. The relatively high yield at 299.0 eV might be due to the fact that the energy is very close to the threshold of C${}_{\mathrm{F}}$. These results show that a specific fragmentation pathway can strongly correlate to the selective excitation of one carbon atom.\
In the case of halothane, various shells from the halogen atoms contribute to the ionization, thus preventing a truly selective excitation. At the photon energies used, the cross sections for photoionization from the chlorine 2p and the bromine 3d level are even higher than for the targeted carbon 1s. For oxygen and nitrogen, in contrast, the photoionization cross section at 300 eV is more than an order of magnitude lower than for carbon [@Yeh1985]. The probability for alternative ionization pathways is thus expected to decrease significantly for organic species that contain 