Dependence of ion kinetic energy and charge on cluster size in multi-photon ionization of xenon clusters

• Formation of multiply charged Xe atomic ions when Xe clusters are exposed to IR irradiation from an Nd:YAG laser.
• A rich mass spectrum of singly charged cluster ions (up to Xe80+) when Xe clusters are exposed to UV irradiation.
• Mean kinetic energy and mean charge state of fragment ions is estimated (under IR irradiation).
• Mean cluster ion size is estimated and is used as a proxy for neutral clusters (under UV irradiation).
• The mean kinetic energy and mean charge state of fragment ions increases with the mean cluster ion size.

Multi-photon ionization of xenon clusters created by nozzle expansion has been investigated by time-of-flight mass spectrometry. With infrared irradiation, cluster ions beyond the dimer are missing from the mass spectra, but there is a copious yield of multiply-charged atomic ions. With ultraviolet irradiation, multiple ionization is almost completely suppressed and singly ionized large cluster ions are detected. The mean cluster ion size under UV ionization shows an increasing trend with the stagnation pressure for nozzle expansion, as do the mean charge state and kinetic energy of atomic ions formed via IR irradiation. Interrogating the same cluster population by two different probes provides a corroboration between the effect of the size of the cluster in the ionization mechanism and the kinematic effects thereof. Furthermore, it provides evidence for the influence of the neutral cluster size on the formation of multiply charged ions by IR irradiation via the electron re-collision mechanism.

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