Photo-ionisation co-incidence techniques

The technique of photoelectron-photoion coincidence spectrometry enables study of the unimolecular decomposition of polyatomic ions with precisely known internal energies. A monochromatic photon source is used to photoionise the target molecule: an energy dispersive hemispherical analyser permits energy analysis of the photoemitted electron whilst a time-of-flight mass spectrometer, time-gated against the detection of the photoelectron, is used to identify the coincident cation.

Coincidence techniques may further employ the threshold mode of photoelectron collection, in which case a continuously tuneable photon source (synchrotron radiation) is required. The threshold method has two distinct advantages: threshold photoelectrons are collected with near 100% efficiency and the method also enables photoionisation to occur in the Franck-Condon gaps, thus providing a continuous distribution of parent ion energies.

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