Isotope enrichment in neon clusters grown in helium nanodroplets

Abstract

Neon cluster ions Nes + grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope 22 Ne that depends on cluster size s and the experimental conditions. For small sizes the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of 22 Ne in the cluster ion and its corresponding depletion factor F in the vapor phase. The value thus found for F is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size si of the precursor of the observed Nes + . The value is in satisfactory agreement with the size estimated by modeling the growth of Nes + , and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than for bare clusters.

Phenanthrene: establishing lower and upper bounds to the binding energy of a very weakly bound anion

“…The selected precursor ions are accelerated and sent into a cell where they collide with a thermal gas of argon; product ions are then analyzed in the TOFMS. 32,33,40 Two CID spectra are presented in Fig. 2.…”

SF6+: Stabilizing Transient Ions in Helium Nanodroplets

“…Why? The initial speed of the HNDs and, more importantly, the thermal motion of the collision gas provide an offset of ∼0.05 eV to the average energy transfer E * per collision , that exceeds the dissociation energy of the parent ion (see below). Values of E * are indicated along the upper abscissa in Figure c.…”

“…The charged HNDs pass through a pickup cell containing SF 6 gas at ∼250 μPa, and an “evaporation cell” that contains helium at low, variable pressure P evap . Each collision will transfer, on average, 0.05 eV to the HND, ∼80 times the evaporation energy of bulk helium. , Multiple collisions will lead to partial or complete evaporation of helium from the doped HND. The ions are guided by a radiofrequency field into the extraction region of a time-of-flight mass spectrometer (TOFMS) equipped with a reflectron in W configuration.…”

Adsorption of helium on a charged propeller molecule: hexaphenylbenzene

“…For simplicity, we discuss ions that are singly charged. The fate of multiply charged HNDs is discussed elsewhere [33,34].…”

“…The charged HNDs pass through a pickup cell into which vapor of HPB (obtained from Sigma-Aldrich, stated purity 98 %) is introduced from an external oven kept at 260 • C, and next an "evaporation cell" that contains helium at ambient temperature and low, variable pressure P He . Each collision will transfer, on average, 0.05 eV to the HND, about 80 times the evaporation energy of a single helium atom from bulk helium [34]. Multiple collisions will lead to partial or complete evaporation of helium from the doped HND.…”