Photoelectron Spectroscopy on HCl and DCl : Synchrotron Radiation Based Studies of Dissociation Dynamics

Abstract: Dissociation dynamics of the ionized molecules HCl and the deuterated system DCl has been studied in gas-phase using synchrotron based photoelectron spectroscopy (PES).The inner-valence "(4σ)-1" photoionization band for DCl and HCl was recorded using maximum resolution in order to probe an interference pattern between a dissociative and a bound electronic state. For HCl+, we clearly observed distorted Fano-type peaks even for modest resolution, whereas for DCl+, the pattern was hardly discernible. The observation in HCl+ has been explained by a coupling between two adiabatic electronic states, where the bound state was populated through non-adiabatic curve-crossing. The nuclear motion of HCl+ is too fast for the Born-Oppenheimer approximation to be fully valid in this case. Whereas for DCl+, with larger reduced mass and therefore slower nuclear motion, the non-adiabatic coupling is less pronounced, and the vibrational progression vanishes.A comparative study between PES and threshold photoelectron spectra (TPES) of the inner-valence bands of HCl and DCl has been performed, showing differences in intensities and shapes of the vibrational bands. These differences were attributed to the fact that the sudden approximation, which can be assumed to be valid for PES, is violated in the case of TPES.A resonant Auger electron spectroscopy study of HCl and DCl has been performed, which shows an interference pattern between atomic and molecular Auger- and photoelectron channels. The atomic features are associated with ultra-fast dissociation of the molecules, on the same time scale as the Auger decay. The observation shows that the excited molecular system has to be regarded as a superposition of fragmented and molecular states.A study of the X-state of HCl+, populated via a core-excited state, shows a selective population of the final state. The explanation was shown to be that the magnetic orientation of the core-hole is transferred to the final state of the molecule.A setup for data acquisition of Photo-Electron Photo-Ion Photo-Ion COincidence (PEPIPICO) measurements using a Time-Of-Flight (TOF) spectrometer has been developed. A Time-to-Digital Converter (TDC) card has been linked together with the data treatment program Igor as a user interface. Furthermore, the PEPIPICO spectrometer has been characterized to provide a solid basis for the analysis of experimental data.