Synopsis The photo-fragmentation of ammonia molecular clusters ionized with soft x-rays is studied for photon energies near the N-1s threshold. The fragmentation is studied with a 3D momentum mass spectrometer to access the energy and angular correlations between fragments. By choosing the cluster sizes below or above the critical size of stable dication, we investigate the coulomb explosion dynamics and the interplay between charge delocalization and mobility of molecules in the clusters.
Atomic van der Waals clusters have been the subjects of many studies to investigate matter in the soft X-ray regime. While the complexity of nano-scale objects makes the comparison between experiments and ab-initio theories challenging, simpler models can be used, for instance, to unravel phase transitions between atomization and fission. Ammonia clusters are of central interest as prototype systems for studying the transition from isolated ions to solvation in the liquid phase. The inter atomic/molecular forces encountered in the bounding of these clusters such as, the dispersion interaction, the dipole-dipole interaction and hydrogen interaction are expected to play a major role in redistributing internal energies. However experimental and theoretical studies on these weakly-bound molecular clusters are scarce. Singly charged clusters undergo fragmentation and mass spectra exhibit a simple pattern with a high abundance of protonated (NH3)nNH+ 4 species compared to the homogeneous species (NH3) + n. Delayed Coulomb fission of doubly charged stable ammonia cluster has recently been studied, and the fission mechanism is found to be well described within.
