Optical–optical double resonance spectroscopy of molecular ions

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Abstract
Three‐level double resonance spectroscopy is employed in a laser‐ion coaxial beams photofragment spectrometer to study highly excited molecular ion states. A probe laser is used to label a bound molecular ion state in a single set of its quantum numbers (v″, J″, Ω″) by pumping from it into a discrete, predissociated state. The charged photofragments produced from this dissociated state are monitored as a measure of the population in the labeled level while a second laser interacts with the molecular ion beam upstream from the probe laser. When this second laser is tuned to transitions accessing either radiating or dissociating levels which originate from the labeled lower level, the resulting depletion of ions in this level is reflected by a decrease in the probe laser photofragment intensity. The technique is applied to the O+2b 4Σ−g(v′ = 4) ← a 4Πu(v″ = 5) and f 4Πg(v′ = 1,2,3) ← a 4Πu(v″ = 5) systems. Transitions into the v′ = 1 level and the Ω = 1/2 and −1/2 substates of the f 4Πg are observed here for the first time. The OODR technique promises wide application in molecular ion spectroscopy and in half‐collision studies. A profound simplification of molecular spectra is achieved and nonpredissociated states of molecular ions can now be observed with the very high resolution which accompanies the coaxial laser‐fast beam technique.