Excitation, Ionization, and Fragmentation in Dimethylmercury

Unlike the situation in metal carbonyls where M⁺ ions result from multiphoton ionization (MPI) of the photoproduced metal atoms, the MPI of Hg(CH₃)₂ proceeds through the parent molecular ion Hg(CH₃)₂⁺ which then photofragments to yield Hg⁺. This is demonstrated by the absence of Hg(I) atomic lines in the MPI spectrum and also by mass spectrometry wherein a considerable quantity of molecular parent ion is observed at low laser flux while at high flux this is replaced totally by Hg⁺. The MPI spectrum of Hg(CH₃)₂ in a jet-cooled, seeded molecular beam displays a two-photon-allowed/one-photon-forbidden Rydberg excitation (A1’ → E”) in the 51000-56000 cm^-1 region. The Jahn-Teller splitting (14 cm^-1) of the v₁₁(1,1) sequence bands in the upper state of this transition confirms its E” spin-orbit assignment. Clustering in the beam removes the sharp-line MPI features totally, as appropriate for a Rydberg excitation. The inelastic electron-impact spectrum of Hg(CH₃)₂ also was recorded up to 30-eV loss, and the transitions were assigned with due regard for the strong spin-orbit splitting within the one-electron configurations and the Rydberg term-value concept.