Statistical analysis of fragmentation channels of small multicharged molecular ions

Laser-induced multiple ionization and Coulomb explosion of small molecules may lead on average to more than one fragmentation event per laser pulse even when experiments are performed at very low gas pressures in the 10(-8)-10(-9) mbar range. In order to bypass the failure of coincidence techniques in this case, Frasinski et al (1989 Science 246 1029) proposed an alternative technique denoted covariance mapping based on correlated fluctuations of digitized ion signals coming from identical fragmentation channels. A probabilistic approach of the detected signals shows that covariance eliminates false coincidences due to fragments from different fragmentation channels, and false coincidences due to fragments from the same fragmentation channel, but which do not originate from the same unimolecular fragmentation events. The analysis is extended to devices which do not accept more than one count per time-of-flight bin such as multichannel scalers or time-to-digital converters. It is still possible to remove false coincidences using covariance but with a formulation which is noticeably different from the initial proposal for digitized signals.