Photoelectron Spectroscopy of Dilute Species

Photoelectron spectroscopy (PES) is a valuable tool for elucidation of the structure and the dynamics of molecules. The fact that ionization potentials generally lie in the VUV part of the spectrum, where few sources exist with bandwidths narrow enough to resolve rotational splitting, has severely limited the resolution attainable for these experiments. Multiphoton absorption techniques and the generation of VUV photons through harmonic processes have overcome this limitation to some extent. The combination of harmonic generation with threshold photoelectron analysis permits resolution in PES measurements at the level of individual rotations (on the order of a cm−1).

In general, intensities from harmonic generation sources are low, and the tunability is restricted. The first limitation essentially removes from analysis a broad range of extremely important molecules, particularly radicals and weakly bound complexes that can be produced only in small quantities in molecular beams. Radicals are involved in some of the most important chemical processes, including in combustion and atmospheric chemistry. Studies of weakly bound complexes at the rovibronic level provide information not only about structure but also about chemical dynamics, including energy transfer and reaction processes.

A Universal Detector

A new application for a high-power vacuum ultraviolet source is as a universal detector. In many current studies of reactive and photodissociative chemical events, the products are detected by electron bombardment ionization. Not only is this process extremely inefficient, but it also offers only limited selectivity. A tunable, high-power VUV source should ionize products with nearly 10% efficiency. In addition, by tuning the source to the threshold energy for the species of interest, one might avoid ionizing background species of higher ionization potential. Such a detector would have many applications, but two are to molecular reaction dynamics and surface chemistry.

Pump-Probe Photoemission

Past pump-probe experiments have used an intense pump beam to put a significant density of electrons into a long-lived excited state (e.g., a band-gap state in semiconductors or an image state in metals). The probe beam then ionizes these electrons, and the resulting photoelectrons are detected, providing information about the energy, momentum, and lifetime of the excited state. The photon energies of the probe beam have ranged from 2 to 20 eV, but recent developments in high-order harmonic generation have pushed the energy to 50 eV, with an upper limit envisioned beyond 100 eV. This development makes it possible to detect the shift of core levels



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