Covariance Mapping of Ammonia Clusters: Evidence of the Connectiveness of Clusters with Coulombic Explosion

Direct evidence is reported of the connectiveness of charged clusters with highly charged species (N 4+, N 3+, and N 2+ ) produced upon the interaction of molecular ammonia clusters with an intense femtosecond laser beam (∼10 15 W/cm 2 at 120 fs). The value of covariance analysis as a general technique for studying dynamical processes in clusters is demonstrated through elucidating the details of various Coulomb explosion events. Positive covariance determinations identify concerted processes such as the concomitant explosion of protonated cluster ions of unsymmetrical size, while anticovariance mapping is exploited to distinguish competitive reaction channels such as the production of highly charged nitrogen atoms formed at the expense of the protonated members of the cluster ion ensemble. The present study demonstrates the great potential which covariance analysis offers in identifying the precursors and products of dynamical events in clusters and in the present case provides further support to the ignition model as a mechanism contributing to the initial ionization events in clusters leading to highly charged atomic species.