Direct evidence is reported of the connectiveness of charged
clusters with highly charged species (N4+,
N3+,
and N2+) produced upon the interaction of molecular
ammonia clusters with an intense femtosecond laser
beam (∼1015 W/cm2 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.
The studies presented herein elucidate details of the Coulomb explosion event initiated through the interaction of heterocyclic clusters with an intense femtosecond laser beam (⩾1 PW/cm2). Clusters studied include 7-azaindole and pyridine. Covariance analysis verifies that the fragmentation channels are competitive. Kinetic-energy analyses, from experiment and simulation, suggest that Coulomb exploded fragments are created with varying amounts of energy and have a strong mass-to-charge relationship. Backward-ejected protons are found to impact the repeller and eject adsorbed protons from the surface. Moreover, delayed fragmentation is suggested by fast-Fourier transformation of a proton time-of-flight mass spectrum and confirmed by deconvoluting the aforementioned signal through intensity decrements. Voltage gradient, laser power, and microchannel plate detector studies yield insight into the solvation effect of clusters in the Coulomb explosion event. Conceptually, the dynamic charge resonance enhanced ionization (Dynamic CREI) model best explains these results of heterocyclic Coulomb explosion.
Recent femtosecond pump–probe experiments have suggested that a stepwise dissociative mechanism is operative for acetone excited to Rydberg states and upper regions of the mixed singlet/triplet state. The present work focuses on the excitation of acetone and acetone clusters to the 3d (or perhaps 4s) electronic intermediate state in order to further explore the operative dissociation mechanisms and the effects of solvation (clustering). As reported herein, results from femtosecond pump–probe experiments suggest that the availability of additional vibrational modes in clusters, where internal energy may be dispersed, increases the fraction of acetyl intermediates which remain behind the barrier to dissociation into methyl and CO fragments. At progressively higher laser fluences, multiply charged elemental carbon and oxygen ions abruptly appear. Interestingly, the extent of their formation is observed to depend on both laser intensity and the relative time delay between the pump and probe laser beams responsible for their occurrence.
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