The Experimental Physical Chemistry Program is providing support for Philip M. Johnson of the State University of New York-Stony Brook to develop a new method of laser spectroscopy, called photoinduced Rydberg ionization spectroscopy (PIRI), which promises a substantial improvement in the resolution attainable in the spectra of molecular cations. This technique relies upon the fact that the optical properties of neutral molecules with one electron excited to a high Rydberg state are almost identical to those of the corresponding ion. The method proceeds by preparing a neutral molecule in a high-n Rydberg state just below the first ionization limit. The ion cores of these Rydberg molecules are then electronically excited by a tunable laser, resulting in autoionization when an ion transition occurs. The production of ions with respect to wavelength produces spectra which are equivalent to photoelectron spectra but are not restricted by the resolution of electron energy analyzers. PIRI is able to examine the excited electronic states of ions which are not accessible to previous high resolution threshold ionization techniques such as zero kinetic energy electron spectroscopy and mass analyzed threshold ionization spectroscopy. This new technique will be applied to several small molecules of atmospheric interest in order to explore the parameters and capabilities of the method while gathering new information about these important species. Photoinduced Rydberg ionization spectroscopy or PIRI will enable the electronic spectroscopy of molecular cations to be observed with a resolution previously unattainable. For the first time, rotational and vibrational information on congested excited electronic states of ions will be acquired. Initial experiments will focus on ionic states of oxygen, water, carbon dioxide, hydrogen chloride and sulfur dioxide.
