High Resolution Threshold Photodetachment Spectroscopy of Negative Ions

In recent years, negative ion photoelectron spectroscopy (PES) has emerged as a versatile technique well-suited to the study of a variety of exotic chemical species. These species include reactive free radicals [ 11, size-selected clusters [ 2 1, and the unstable collision complexes formed in chemical reactions [ 31. In a typical experiment, a mass-selected negative ion beam is photodetached with a fixed-frequency laser, and some fraction of the ejected photoelectrons is collected and energy-analyzed. The experiment can be used to study the vibrational and electronic spectroscopy of both the negative ion and the neutral resulting from photodetachment, and, for example, can probe electronic states of the neutral which are inaccessible to more conventional optical spectroscopic methods [ 1,4]. A significant drawback to this technique is the relatively poor (compared to optical spectroscopy) resolution of the electron energy analyzer. The energydispersive and time-of-flight analyzers used in fixed frequency PES generally have a resolution limit of 5-l 0 meV, This is sufficient to observe some vibration and electronic structure in small molecules. However, for many systems of interest, particularly clusters of moderate size, this limit prevents the ob-