Threshold photoelectron spectroscopy of HF and DF in the outer valence ionization region

We have recorded threshold photoelectron spectra of HF and DP over the outer valence ionization region under good resolution conditions (3-6 meV) using synchrotron radiation and employing the penetrating-field electron detection technique. The spectra show extensive vibrational structure in the X(2 Pi(i)) system in the first Franck-Condon gap region that is attributed to resonance autoionization of Rydberg states lying in this energy range. The Rydberg states responsible for these effects are identified as [A((2)Sigma(+))]ns sigma (1)Sigma(+), with n = 4-6. Analyses of all the vibrational data contained in these spectra for the X ((2)Pi(i)) state using a modified isotopic vibrational Dunham equation have led to greatly improved spectroscopic constants for this state in HF+ and DF+. The same method was also used to determine improved spectroscopic constants for the A((2)Sigma(+)) state. Autoionization is found not to be important in the formation of the A(2 Sigma+) state due to the absence of Rydberg states in the energy region of the A state. Based on the observations and interpretations of the present work, combined with literature data, we have constructed a potential-energy diagram of the relevant states of HF and HF+. Partially rotationally resolved threshold photoelectron spectra of HF+ and DF+ over the nu(+) = 0 vibrational band of the X (2)Pi(i) system show pronounced effects due to autoionization processes.