Observation of H2 Aggregation onto a Doubly Charged Anion in a Temperature-Controlled Ion Trap

Hydrogen is the second most difficult gas to be condensed due to its weak intermolecular interactions. Here we report observation of H-2 aggregation onto a doubly charged anion, -O2C(CH2)(12)CO2-(DC2-). Weakly bound DC2- (H-2)(n) clusters were formed in a temperature-controlled ion trap and studied using photoelectron spectroscopy. The onset of clustering was observed at 30 K, whereas extensive condensation was observed at 12 K with n up to 12. Photoelectron spectra were obtained for DC2-(H-2)(n) (n = 0-6) at 193 and 266 nm. The spectra of DC2- (H-2)(n) were observed to be identical to that of the bare DC2- dianion except a slight blue shift, indicating the weak interactions between H, and the parent dianion. The blue shift on average amounts to similar to 34 meV (3.3 kJ/mol) per H-2, which represents the lower limit of the H-2 binding energy to DC2-.