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 oute r valence ionization region under good resolution conditions (3-6 meV) usin g synchrotron radiation and employing the penetrating-field electron detect ion 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 r esonance autoionization of Rydberg states lying in this energy range. The R ydberg 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 iso topic vibrational Dunham equation have led to greatly improved spectroscopi c 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. A utoionization is found not to be important in the formation of the A(2 Sigm a+) 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 diag ram of the relevant states of HF and HF+. Partially rotationally resolved t hreshold photoelectron spectra of HF+ and DF+ over the nu(+) = 0 vibrationa l band of the X (2)Pi(i) system show pronounced effects due to autoionizati on processes.