The gas phase L2,3VV Auger electron spectra of chlorine in XCl (X=H, D, Li, Na, K) molecules

Abstract

The Auger electron spectra (AES) following the chlorine 2p ionization in the gas phase alkali-chlorides as well as in the HCl and DCl molecules were studied theoretically and experimentally. Nonrelativistic ab initio calculations based on quantum chemical methods and the one-center approximation were used to compute the Cl L2,3VV AES. The vibrational band structure in the AES was simulated by full life time vibrational interference (LVI) theory and a more approximate moment method. Calculations were compared with the corresponding experimental electron or photon impact excited spectra. Overall features and changes in the series of the experimental AES are correctly predicted by the theory. For the most intense transitions in these spectra a qualitative explanation of the energies is given on the basis of a model that includes electrostatic interactions, Pauli repulsion, and polarization. This explains that the substantial changes of the spectra with X are due to a R−3 dependence of the electrostatic interaction on the X–Cl bond length. A comparison of the two theoretical methods shows that the LVI vibrational band breadths are well reproduced by the moment method in the AES of HCl, DCl, and LiCl, whereas the moment breadths are underestimated in the AES of NaCl and KCl. The LVI band breadths for HCl, DCl, and KCl are in good agreement with experimental data. In contrast, the experimental spectra for LiCl and NaCl show almost two times broader vibrational bands than predicted by the LVI theory. This contradiction indicates that the LiCl and NaCl vapors are contaminated by the dimer form of these substances. A large contribution of Li2Cl2 (74%) and Na2Cl2 (29%) was measured in the LiCl and NaCl vapors by time-of-flight mass spectroscopy. The Auger peaks in the spectra of the dimers lie very close to the monomer peaks, and thus make it very difficult to distinguish dimer and monomer contributions.