Laser spectroscopic studies of molecular ions using fast ion beam photofragment spectroscopy

Spectroscopic information on molecular ions is generally much more difficult to obtain than similar information on neutral molecules, due primarily to the relatively low densities of ions that can be obtained. Ion densities in typical experiments are in the range from 104 to 108 particles/cm3; densities exceeding 109/cm3 can be obtained only with great difficulty. Thus experiments which depend on directly detecting the absorption of radiation by molecular ions are, practically speaking, not feasible, and those which directly detect emission from molecular ions are generally difficult. The ion density problem is further complicated by the usual spread of the ion beam population over a large number of rotational, vibrational and metastable electronic states. For basically these reasons, spectroscopic knowledge of molecular ions has lagged far behind such knowledge of neutral molecules. The particular case of photodissociation is especially favorable for ion studies, since photofragment ions produced by the interaction of a laser with an ion beam can be detected with nearly unit efficiency. Advantage was taken of this fact in the pioneering studies of von Busch and Dunn, who crossed a beam of H2+ ions with photons from a powerful arc lamp, and measured the total cross section for photoproduction of H+ ions, with a resolution of 200. Unfortunately, arc lamps are not powerful enough for general use in studies of ions in fast beams. However, they have proven to be very useful in ion studies using ion cyclotron resonance techniques