Radiation less transitions in gas phase molecular ions

Abstract

Open shell molecular ions are particularly interesting species for studying radiationless transitions. With respect to closed shell species, there are both structural and instrumental advantages. The important experimental parameters to determine are fluorescence decay patterns and quantum yields. These are measured using a technique in which the ions are produced by photoionization and are detected in delayed coincidence with the photons emitted from their excited electronic states. A brief survey is given of the theory of radiationless transitions between radiant |s> and quasi-nonradiant |l> levels, paying particular attention to the following three cases: 1) resonance limit, where the density ρ l of {|l>} states is very small; 2) statistical limit, where the {|l>} states form a quasi continuum; 3) intermediate case. Experimental results on [Math] are interpreted in terms of the resonance limit, while those on a series of polyfluorobenzene ions are considered in terms of the statistical limit. Emission quantum yields and multiexponential decay behaviour of mono- and di-chloroacetylene ions are interpreted in terms of the strong coupling theory of the intermediate case. Coupling energies, densities of states and level decay rates are determined in each case. Vibrationally hot ground electronic state ions can result from the electronic relaxation process; their fate may involve intramolecular nonradiative vibrational redistribution and fast infrared emission. Problems for further study are identified in the conclusion.