A combined high-resolution (6 meV) HeI photoelectron (PE) and ab initio theoretical study of the A 2Σ+ ion system of HI and DI has been conducted to elucidate the origin of a peculiar “holelike” feature in the vibrational distribution found in the high-resolution (6 meV) threshold photoelectron (TPE) spectrum of HI. The PE and TPE spectra were found to yield essentially identical results. Ab initio potential energy curves for the low-lying cationic states of HI have been calculated for the first time with and without spin–orbit contributions included. It has been found that the diabatic A 2Σ+ state of HI+ is strongly predissociated due to spin–orbit coupling with the Σ−4, Σ−2, and Π4 repulsive states leading to a complex set of adiabatic curves. It is shown that the adiabatic A 2Σ1/2+ state is only slightly bound (by 260 cm−1 after suitable adjustments of the positions of the various repulsive potentials relative to that of the A state are made based on observed atomic spectral data) and should support at most one vibrational level in both HI+ and DI+. However, using the complex rotational method, it was possible to calculate the energies, predissociation linewidths, and rotational constants of a number of nonstationary vibrational levels (or resonances) of the A 2Σ1/2+ state. Reasonably good agreement has been found between experiment and theory. The observed “hole” in the TPE and PE spectra is attributed to the fact that the stability of the v+=1 and 2 levels is notably less than for v+=0 in the A 2Σ1/2+ state of both HI+ and DI+.

We have recorded threshold photoelectron spectra of HF and DF 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(2i) 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+)]ns 1+, with n = 4-6. Analyses of all the vibrational data contained in these spectra for the X(2i) 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+) state. Autoionization is found not to be important in the formation of the A(2+) 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 v+ = 0 vibrational band of the X 2i system show pronounced effects due to autoionization processes.

The threshold photoelectron spectrum of SF6 has been recorded under good resolution conditions (9–17 meV) over the photon energy range 15.0–28.3 eV using synchrotron radiation and a penetrating-field electron spectrometer. Significant differences have been found between the threshold photoelectron spectrum and the Hell photoelectron spectrum that are ascribed to resonance autoionization processes in the former. The 6t1u shape resonance molecular orbital of SF6 appears to play a major role in this regard. This virtual molecular orbital is tentatively located 0.067 eV above the onset of the ionization continuum. The Rydberg series [2A1g] npx, y, z1F1u, for n = 4–7 of SF6 has been observed by means of resonance autoionization into, most probably, the E (2T1u) ion continuum.

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4-30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2-31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X ((2) Pi(i)) and A ((2) Sigma(+)) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X ((2) Pi(i)) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A ((2) Sigma(+))]ns sigma (1) Sigma(+) for n = 6-8. A vibrational progression in the A ((2) Sigma(+)) band system in HI+ leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A ((2) Sigma(+)) state of HI+ appears to be formed by an avoided crossing between the diabatic A ((2) Sigma(1/2)(+)) and (4) Pi(1/2) potentials. Spectroscopic constants are derived for the 'main' band system for 5s sigma(-1) inner-valence ionization of HI. (C) 1997 Elsevier Science B.V.

The threshold photoelectron spectrum of molecular fluorine has been recorded in the 5.6–21.9 eV photon energy range, at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In addition to observing the three known band systems of F2+ at higher resolution than previously achieved with conventional photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic systems of F2+. This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.