Excitation of Rydberg states of HgCl2 and HgBr2 by electron impact

We have examined the electronic structure of HgCl2 and HgBr2 from 5 eV to 14 eV by high-resolution electron energy-loss spectroscopy. Our measurements include the near-UV region, which has not been examined by any previous technique and which is found to be rich in Rydbery states. In particular, in each molecule we identify members of two optically allowed Rydberg series and one forbidden series with electronic structures sigma/sub g/2 sigma/sub u/2 π/sub u/4 π/sub g/3 [npπ/sub u/, npsigma/sub u/ and ndπ/sub g/, or (n+1)ssigma/sub g/] that converge to the 2PI/sub g/ ground ionic state. In addition, other structures that form the dominant energy-loss mechanisms in our spectra are identified as arising from optically allowed Rydberg states associated with the excited ionic states 2PI/sub u/, 2Σ/sub u/, and 2Σ/sub g/. Measurements at energy losses below 9 eV confirm previous valence states observed in photoabsorption and suggest the existence of two new valence states in each molecule. Angular measurements facilitate identification of many of the newly observed structures in our electron energy-loss spectra