Isomer-specific photofragmentation of CH at the carbon K-edge.

Individual fingerprints of different isomers of CH cations have been identified by studying photoionization, photoexcitation, and photofragmentation of CH near the carbon K-edge. The experiment was performed employing the photon-ion merged-beams technique at the photon-ion spectrometer at PETRA III (PIPE). This technique is a variant of near-edge X-ray absorption fine-structure spectroscopy, which is particularly sensitive to the 1s → π* excitation. The CH primary ions were generated by an electron cyclotron resonance ion source. CH product ions with = 0, 1, 2, and 3 were observed for photon energies in the range of 279.0 eV to 295.2 eV. The experimental spectra are interpreted with the aid of theoretical calculations within the framework of time-dependent density functional theory. To this end, absorption spectra have been calculated for three different constitutional isomers of CH. We find that our experimental approach offers a new possibility to study at the same time details of the electronic structure and of the geometry of molecular ions such as CH.