Infrared spectroscopy of ions and ionic clusters upon ionization of ethane in helium droplets

Helium droplets are unique hosts for isolating diverse molecular ions for infrared spectroscopic experiments. Recently, it was found that electron impact ionization of ethylene clusters embedded in helium droplets produces diverse carbocations containing three and four carbon atoms, indicating effective ion-molecule reactions. In this work, similar experiments are reported but with the saturated hydrocarbon precursor of ethane. In distinction to ethylene, no characteristic bands of larger covalently bound carbocations were found, indicating inefficient ion-molecule reactions. Instead, the ionization in helium droplets leads to formation of weaker bound dimers, such as (C₂H₆)(C₂H₄)⁺, (C₂H₆)(C₂H₅)⁺, and (C₂H₆)(C₂H₆)⁺, as well as larger clusters containing several ethane molecules attached to C₂H₄ ⁺, C₂H₅ ⁺, and C₂H₆ ⁺ ionic cores. The spectra of larger clusters resemble those for neutral, neat ethane clusters. This work shows the utility of the helium droplets to study small ionic clusters at ultra-low temperatures.