Studies of Ar and N2 using threshold photoelectron spectroscopy by electron attachment (TPSA)

Threshold photoelectron spectra of Ar and N2 are studied by the technique of attachment of the threshold electron to SF6 followed by detection of the SF6−. Studies on Ar provide a measure of the rejection ratio for nonthreshold electrons of this technique. This ratio is compared to that for two other types of electron detectors. Collisional ionization of high Rydberg-state electrons by SF6 is also observed and discussed. In N2 direct ionization to vibrational levels of the X 2Σg+, A 2Πu, and B 2Σu+ states of N2+ are clearly resolved. Moreover, resonant autoionizations are found to greatly enhance the population of the vibrational levels of the X state and the v′=2 level of the A state over that expected from direct photoionization alone. Extra peaks lying between the vibrational levels are detected which correspond to autoionization into high-J levels of the N2+ ion. From intensity considerations a rotational propensity rule is given which states that an autoionizing N2 state prefers to decay to N2+ states with a minimum change in rotational angular momentum. Normalization of the threshold spectra to the absolute cross section scale is also discussed.