Doping with multiple cations and failure of charge transfer in large ionized helium droplets

We report experimental observations of aniline (A) cations and He₂ ⁺ when aniline is doped into ionized helium droplets. Large droplets containing 10⁸ atoms are bombarded by energetic electrons, resulting in more than one positive charge in one droplet. When aniline encounters the charged droplets, some are ionized via charge transfer, while others can remain neutral in the presence of He₂ ⁺ when the mass-to-charge ratio (m/z) of the droplet is sufficiently large. Upon resonant excitation of the dopant A_n or A_n ⁺ (n ≥ 1), He₂ ⁺ can be ejected. The excitation spectrum of He₂ ⁺ becomes a juxtaposition of the spectra of A_n and A_n ⁺. Moreover, an anticorrelation between the yields of He₂ ⁺ and A⁺ is observed with increasing energies of the ionizing electrons. We attribute this result to the combined effect of reduction in m/z of the droplets and the different locations of He₂ ⁺ and neutral A_n. Limited by the penetration depths of the ionizing electrons and further assisted by the Coulomb repulsion of coexisting cations, He₂ ⁺ is located within 20 nm of the surface, while neutral A_n has an average position inside a large droplet. Upon resonant excitation of the interior A_n, He₂ ⁺ is preferentially ejected. With increasing energies of the colliding electrons, the m/z of the droplets are reduced, leading to less effective charge shielding and more effective charge transfer, until ultimately, all He₂ ⁺ can be neutralized to form A⁺.