Electronic Excitation and High‐Energy Reactions Originate From Anionic Microdroplets Formed by Electrospray or Pneumatic Nebulization

Abstract Formation of energetic species at the surface of aqueous microdroplets, including abundant hydroxyl radicals, oxidation products, and ionized N 2 and O 2 gas, has been previously attributed to the high electric field at the droplet surface. Here, evidence for a new mechanism for electronic excitation involving electron emission from negatively charged water droplets is shown. Droplet evaporation can lead to the emission of ions and droplet fission, but unlike positively charged droplets, negatively charged droplets can also shed charge by electron emission. With nanoelectrospray, no anions or negatively charged droplets are produced with a positive electrospray potential. In contrast, abundant O 2 +• and H 3 O + (H 2 O) are formed with negative electrospray. When toluene vapor is introduced with negative electrospray, abundant toluene radical cations and fragments are produced. Both O 2 +• and toluene radical cations are produced with pneumatic nebulization. The electrons produced from evaporating negatively charged droplets can be accelerated by an external electric field in electrospray, or by the field generated between droplets with opposite polarities produced by pneumatic nebulization. This electron emission/ionization mechanism leads to electronic excitation >10 eV, and it may explain some of the surprising chemistries that were previously attributed to the high intrinsic electric field at the surface of aqueous droplets.