Double photoionisation spectra of methane, ammonia and water

Complete double photoionisation spectra of methane, ammonia and water measured by the TOF-PEPECO method are reported. The electron pair energy distributions and low quantum yields suggest that for methane and ammonia the double photoionisation is mainly direct. In addition to the direct process, water undergoes two indirect dissociative double ionisation processes, one involving autoionisation of an oxygen atom in course of, or after, dissociation. The existence of these processes explains discrepancies between previous determinations of the ionisation threshold. Results on these and related species lead to a general conclusion that molecular double photoionisation in small molecules is mainly direct, whereas the same process in complex atoms is mainly indirect.