Electron states of the HCl dimer

The He i photoelectron spectrum of HCl dimer has been recorded and the observed ionizations at 12.3, 13.6, 15.1 and 17.0 eV have been assigned to different orbitals on the basis of molecular orbital calculations. The observed first adiabatic IE (11.9 eV) agree well with the value of 11.91 eV from a photoionization study of the dimer. The two HCl units in the dimer are nonequivalent and there is significant interaction between the σ orbitals. Both the calculations and the experiment show that the geometry undergoes considerable distortion on ionization.