Fundamental to the modern conception of chemistry is the transition state (TS), the transient species located near a saddle point in the reaction potential energy surface (PES) where chemical bond formation and cleavage occur. As the TS controls not only the reaction rate, but also the dynamics of the chemical transformation, it is essential to probe this key region of the PES in order to gain insight into the fundamental interactions that govern chemical reactivity. Such motivation led to the development of crossed molecular beam experiments of increasing complexity to determine properties of the TS, including the barrier height and geometry. While these experiments have greatly improved our understanding of chemical reactions, they provide only an indirect probe of the TS region of the PES. Complementary experiments utilizing anion photoelectron spectroscopy, however, have yielded vibrationally-resolved structure characteristic of the TS region of the PES for both unimolecular and bimolecular reactions. In these experiments, a stable anion whose geometry is similar to the TS undergoes photodetachment, projecting the vibrational wavefunction of the anion vertically onto the neutral PES. The ejected photoelectrons carry with them information on the evolution of the wave-packet created by photodetachment, resulting in a photoelectron spectrum that reveals vibrational structure of complexes near the transition state region. By improving the resolution of the photoelectron spectrometer, one can observe sharp structure associated with resonances that are quasi-bound along the reaction coordinate.