Time-of-flight photoelectron spectroscopy of atoms and molecules

Following the advent of third-generation synchrotron-radiation sources with ultra-high brightness and X-ray pulse widths of a few picoseconds, the technique of time-of-flight (TOF) electron spectroscopy has experienced a dramatic enhancement in energy resolution. Using soft-X-ray beams focused to 100 ?m or less, a new gas-phase TOF-photoelectron apparatus in operation at the Advanced Light Source (ALS) has demonstrated electron energy resolution as high as 8000 (E/?E), comparable to some of the best electrostatic analyzers, while maintaining the traditional efficiency of the TOF technique. This apparatus is being used to probe limitations of basic approximations in X-ray photoemission: (1) the independent-particle approximation, and (2) the dipole approximation. In both cases, new limits of these approximations have been discovered in unexpected photon-energy regimes. This paper includes an overview of the TOF technique as well as a summary of results from the ALS on limits of these basic approximations.