Molecular Beams of Macroions

TLDR

Electrospraying a dilute polymer solution into an evaporation chamber creates a supersonic molecular beam of macroions, which is sampled by a Kantrowitz–Gray nozzle‑skimmer system and monitored with a Faraday cage after light ions are repelled by a negative‑voltage repeller grid. The experiment shows that macroions can be formed in distinct mass/charge states, with 51 000‑amu polystyrene forming dimers/trimers and 411 000‑amu forming multiply charged singles, that their velocities are narrowly distributed around 743 m s⁻¹, and that macroions become highly concentrated relative to solvent and nitrogen during beam transit.

Abstract

By means of electrospraying a dilute polymer solution into an evaporation chamber, negative macroions can be produced and a molecular beam formed by sampling the gaseous mixture of macroions, solvent, and nitrogen molecules with a nozzle-skimmer system of the Kantrowitz–Gray type. The macroion current can be detected by a Faraday cage after the light ions have been repelled from the beam by negative voltages on a repeller grid. Theoretical repeller voltages which best agree with the observed are those calculated by assuming a macroion velocity within 2% of the estimated supersonic beam velocity of 743 m sec−1. Polystyrene macroions of 51 000 weight-average amu tend to form dimers and trimers in the beam while larger polystyrene macroions of 411 000 weight-average amu appear mostly to be multiply charged single species. The results demonstrate that definite mass/charge states can be formed by the electrospray technique, that a considerable monochromatization of macroion velocities in the beam takes place, and that the macroions become highly concentrated relative to low-molecular-weight solvent and nitrogen ions during the transit time in the supersonic beam.