The versatility of anion PES is, however, limited by its energy resolution, which is typically on the order of 10-30 meV, or 100-300 cm -1 . At this resolution, one can readily resolve electronic structure, but the observation of vibrational structure in clusters and polyatomic molecules can be more challenging, particularly in the presence of low-frequency vibrations and Franck-Condon (FC) activity in multiple vibrational modes. These considerations have led to efforts in our laboratory and elsewhere to improve the resolution of anion PES while retaining its versatility. Our first attempts at higher resolution experiments focused on anion zero electron kinetic energy (ZEKE) spectroscopy, inspired by the work by Muller-Dethlefs and Schlag on the photoionization of neutral molecules. In their demonstration of this technique, NO was ionized via resonant two-color ionization, and electrons with nearly zero electron kinetic energy were detected as the photon energy was scanned. The underlying idea is that whenever the ionizing laser energy is just enough to access a cation state, one observes zero energy electrons. In this way, they were able to resolve individual rotationally resolved transitions between NO and NO + , achieving an energy resolution on the order of 1 cm -1 . The mechanism of ZEKE electron production of neutrals is more complex than first believed and involves field ionization of high-lying Rydberg states rather than direct ionization. Negative ions, on the other hand, do not have Rydberg states. Nonetheless, we initiated analogous experiments on negative ions starting in the late 1980's based on the selective detection of near-zero energy electrons produced by photodetachment with a tunable laser, and were able to obtain ZEKE spectra with an energy resolution of 1-2 cm -1 for a series of semiconductor clusters, open-shell van der Waals clusters, and, by photodetachment of IHIˉ, the transition state of the I + HI reaction in which reactive resonances were spectroscopically resolved for the first time. Anion ZEKE experiments with similar resolution were also carried out by Boesl on Iˉ(H2O), FeOˉ, and other anions. These experiments proved very challenging and were limited in generality by the Wigner threshold law, 1/ 2 () eKE  +  (1) which dictates that at low electron kinetic energy (eKE), the photodetachment cross section  scales with (eKE) l+1/2 , where l is the orbital angular momentum of the photoelectron. For anion ZEKE spectroscopy, where only electrons with near zero kinetic energies are detected, the near-threshold photodetachment cross section thus becomes vanishingly small with increasing angular momentum, so the technique works only for s-wave (l=0) detachment.